Modulation of Potassium Channels by Organic Molecules Karnail S. Atwal Bristol-Myers Squibb Pharmaceutical Research Institute, P. 0. Box 4000, Princeton, New Jersey 08543-4000 I. Introduction ................................................ 11. ATP-Sensitive Potassium Channel (KATp) . . . . . . . . . ......... ....................... A. KATp Openers Pinacidil Analogs ............................... RP 52891 Analogs . . . . . . . . ............................... Other Openers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Are There Common Structural Features in Synthetic KATp Openers? ................................................. 6. Therapeutic Indications ......................... ....................... B. Blockers of K A T ~. . ....................... 1. Therapeutic Indi 111. Calcium-Activated Potassium Channel (k,) ............ .. ............................. A. Openers of &a . . . . . . . . . . . . . . . . 8. Blockers of Kc, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Delayed Rectifier Potassium Channel (IK) . . . . . . . . . . . . . . . V. Forskolin- and Phorbol Ester-Sensitive Potassium Channels . . . . . . . . . . . . . . VI. Fatty-Acid-Modulated Potassium Channel .............................. VII. Anesthetic-Activated Potassium Channel . . . . . . . . . ... VIII. Molecular Mechanism of Potassium Channel Gating ..................... IX. Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. 3. 4. 5.

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573 574 574 575 577 579 579 580 580 581 583 583 584 584 585 586

I. INTRODUCTION Potassium channels play a key role in the maintenance of resting membrane potential and repolarization of the action potential. Fluctuations in membrane potential control cell excitability, which regulates various physiological functions in smooth muscle, cardiac tissue, and the central nervous system. Potassium channels are largely regulated by voltage, cell metabolism, and calcium- and receptor-mediated processes. Unlike the relatively unique sodium and calcium channels, there are a variety of potassium channel types, often present in the same tissue. The diversity of potassium channels being unraveled by a combination of biochemical and electrophysiological techniques is increasing at an astounding rate. There is still much to be learned about the physiological function(s) associated with various channel types. However, it is already clear that modulation of potassium channels may form the basis of therapeutic approaches to various diseases of cardiac and smooth muscles and of neuronal tissues.lT2 Medicinal Research Reviews, Vol. 12, No. 6, 569-591 (1992) 0 1992 John Wiley & Sons, Inc.

CCC 0198-6325/92/060569-23

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Although introduction of sophisticated techniques (e.g., patch clamp) to measure the activity of various channel types has increased our ability to characterize existing pharmacological agents, these tools are not of a similar value to the medicinal chemist to design novel compounds. The synthesis of new compounds still relies heavily on the structures of existing prototypes. This is largely due to the lack of information about the three-dimensional structure of receptor proteints), which may or may not be part of the potassium channels. Efforts in medicinal chemistry have been largely focused on the synthesis of antihypertensive potassium channel openers related to cromakalim and antiarrhythmic potassium channel blockers related to sotalol. Some excellent review articles, dealing primarily with the medicinal chemistry of potassium channel openers, have recently appeared in the litera t ~ r e . Discussion ~-~ in this article is restricted to channel types known to be modulated by organic molecules, excluding those for which enough pharmacological data are not available at the present time. For a discussion of other channel types and their modulation, the reader is referred to reviews cited in Refs. 1-7. Referencing of this article is selective and far from complete. 11. ATP-SENSITIVE CHANNEL

(KATp)

These channels are regulated by ATP and are known to open when the internal ATP concentrations fall below physiological levels. KATp is known to exist in a variety of tissues, which includes cardiac muscle,8 skeletal m u ~ c l e , ~ and pancreatic p cells,lO and it is thought to exist in certain regions of the brain.11,12 By making use of patch clamp methods, K, has been shown to exist in arterial13and aortic1*smooth muscles. Recently, this channel has been shown to exist in rat liver inner mitochondrial membrane^.'^ Although the exact physiological function of this channel in mitochondria is not known, it is hypothesized to control water content of mitochondrial membranes. For a discussion of the mechanism of K,,, opening, the reader is referred to recent reviews on this s ~ b j e c t . l ~The , ' ~ inhibition of this channel is thought to involve specific binding of ATP to the channel.I6 The sensitivity of binding and the ratio of ATP molecules to binding site is dependent on the tissue type.16 Although much less potent than ATP, magnesium ions and certain nucleotides (ADP, AMP) also block the activity of this channel. KA,, may be regulated by adenosine receptors via G-protein coupling in rat ventricular myocytes.18Additionally, this channel may be regulated by CAMP-dependent protein kinase and/or protein kinase C in certain tissue^.'^ Although the channel has been hypothesized to be the receptor for the sulfonylurea class of ligands,17 definitive experiments using purified receptor proteints) need to be performed to demonstrate that the binding site for these compounds is the

Karnail S. Atwal obtained his graduate education at the University of New Brunswick under the guidance of professor Karel Wiesner, receiving a Pk.D. degree in Organic Chemistry. After postdoctoral work with Professor Gilbert Stork at Columbia University, he joined the Squibb Institute for Medical Research in 1982. He is currently a research group leader in the department of cardiovascular chemistry at the Bristol-Myers Squibb Pharmaceutical Research Institute and his research interests include the design and synthesis of biologically active compounds.

MODULATION OF POTASSIUM CHANNELS

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channel-forming protein(s). KATp is by far the most studied channel, due largely to the availability of pharmacological agents that modulate its activity. The discovery of KATp opening as a part of the mechanism of vasodilation by a diverse group of agents (1-6) has created a renewed excitement in studying this channel. A brief discussion on the agents that modulate this channel is provided below.

Ye

Cromakallm (1)

Mlnoxldll sulfate (4)

A. , K

Plnacldll (2)

Nlcorandll (5)

Dlazoxlde (3)

RP 52,891 (6)

Openers

Opening of KATp has recently been shown to play a role in the vasodilatory actions of antihypertensive drugs such as cromakalim (l),pinacidil (2), diazoxide (3), and minoxidil sulfate (4).1,2,7 The same mechanism also contributes to the vasodilator actions of nicorandil (5).20 Recently, a new structural type RP 52,691 (6)21 has been added to this growing list of agents. Although some of these compounds may also open other types of potassium channels (see Sec. III.A), they are grouped here under KATpopeners for a discussion of their structure-activity relationships (SAR). Their primary mechanism of action, uncovered after the discovery of these drugs as hypotensive agents, is believed to involve opening of potassium channels, which causes membrane hyperpolarization, resulting in the raising of the threshold for calcium entry through voltage-sensitive calcium channel^.^,^,^ Because these agents also inhibit agonist-induced vasoconstriction, the involvement of receptoroperated calcium channels in their mechanism of action cannot be ruled The past few years have seen an explosive growth in the synthesis of compounds related to the structure of the most potent of these agents, cromakalim (1).1-6,22--25 Most of the published structure-activity data has been described for oral antihypertensive activity. Consequently, it is difficult to rationalize the existing SAR data in terms of a hypothesis that involves binding affinities of these agents to a receptor protein(s). Since there is as yet no data to support receptor binding by these compounds, one can only assume that hypotensive effects of these molecules are mediated through binding to

572

CAMPBELL

specific receptor(s). Given below is a brief summary of the structure-activity relationships for the antihypertensive activities of cromakalim (1)/ pinacidil (2), and RP 52,891 (6),the three members of this class of agents that have seen the most synthetic activity over the past few years. 1. Cromakalim Analogs

Several review articles on the structure-activity relationships of cromakalim and its congeners have recently appeared in the l i t e r a t ~ r e . ~ Most -~ cromakalim analogs (7-12)26-31 differ with respect to substitution at C-4 of

G

+N-0

N

Mo 0

7

8

9

Mo

10

11

12

the benzopyran nucleus. The benzopyran is usually substituted at C-6 with an electron withdrawing group, although heterocyclic replacements (pyridine, thiophene) of the aryl ring have been reported to maintain potency.32 Most potent derivatives also have gem-dimethyl groups or a spirocarbocyclicring at C-2. The presence of a trans-hydroxyl does not appear to be critical for hypotensive activity in all series (e.g./ 7). The potent antihypertensive activity of the indane analog 11 demonstrates that the oxygen atom of the benzopyran nucleus is not a critical component of the pharmacophore. The C-4 substituent can be attached through a variety of linkers, which include nitrogen (1, 8, lo), oxygen (9), or carbon (7). Depending on the substitution at C-4, both sp3 and sp2 carbons are tolerated at 3- and 4-positions of benzopyran. In an interesting diversion from the benzopyran nucleus, benzoxazine derivatives (e.g., 12) are reported to be potent antihypertensive agents.31 This result, combined with the bioactivity of indane 11, indicates that a variety of changes are tolerated in this area of the molecule. The pharmacological effects of cromakalim and its analogs are stereoselective, with the 3S,4R-isomer being of structure-activity relationship the more potent e n a n t i ~ m e rA. ~summary ~ for antihypertensive activity is given in Fig. 1.

573

IVERMECTIN, AN ANTIPARASITIC AGENT

Iheteroatom with a uartial

1

negative charge in ;his genera area may be required

alkvl. awl erouu

tolerated

(lipophilic residud (required 1 Figure 1. Summary of structure-activity relationships for cromakalim (1) and its congeners.

2. Pinacidil Analogs

Since the original publication describing the structure-activity relationship of pinacidil (2).34there has not been a great deal of synthetic activity in this structural class. The disclosure of aryl cyanoguanidine (13) as a potent antihypertensive agent indicates that pyridine is not a prerequisite for the biological activity of p i n a ~ i d i lPrevious .~~ studies have shown that cyanoguanidine can be replaced with a thiourea without much loss of potency.36The presence of a branched alkyl group on the guanidine nitrogen appears to be important for optimal a ~ t i v i t y . As ~ ~ with , ~ ~ cromakalim (l), the biological activity of pinacidil (2) and its analogs is reported to reside predominantly in the R(-)-enantiomer, indicating their possible interaction with a receptor prot e i n ( ~ ) . The ~ ~ (ethenediamine ~) analog 14 of pinacidil is also reported to maintain p0tency.~7(b)

Plnacldll (2)

13

14

3. RP 52891 Analogs A publication describing the genesis of this compound has recently appeared in the l i t e r a t ~ r eStructure-activity .~~ studies indicate that the tetrahydrothiopyran ring is optimal for activity, as is the methyl group on the nitrogen of thioamide. Analogs replacing sulfoxide with an olefin (e.g., 15) or

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a substituted alkyl group (16)39and those replacing pyridine with aryl (17)40 or heteroary141 rings have also appeared in the patent literature.

15

16

17

4. Other Openers The mechanism of vasorelaxation by calcitonin gene-related peptide (CGRP) has been shown to involve, at least in part, the opening of K, in rabbit mesenteric artery.42The K, blocker glyburide, which by itself had very little effect, inhibited the hyperpolarization caused by CGRP.42 As shown by patch clamp studies, the hyperpolarization caused by CGRP is due to K, opening. Blockers of calcium-activated potassium channels had no effect on either vasorelaxation or K,, opening caused by CGRP. In a similar set of experiments, the endogenously present vasoactive intestinal peptide (VIP) and endothelium derived hyperpolarization factor were shown to possess KATP opening as a part of their mechanism of ~asodi1ation.l~ By measurement of s6Rb efflux in the insulinoma cells, it has been shown that the insulin-lowering hormone somatostatin activates K A T ~This . ~ effect ~ is inhibited by the K, blocker glyburide. The effect of somatostatin was blocked by pretreatment with pertussis toxin, indicating the involvement of a G protein in mediating the effects of this hormone.43 The hormone galanin, which elevates blood glucose by lowering insulin release, has also been shown to activate KATP in insulinoma cells.44 The effects of both somatostatin and galanin depend on the intracellular concentrations of CAMP.l9 In another study, 4~-phorbol-l2-myristate-l3-acetatewas shown to activate K, in rat insulinoma cells, suggesting the involvement of a second messenger system (for example, protein kinase C) in regulation of this ~ h a n n e 1 . Dilation l~ of coronary arteries by adenosine in the isolated heart has been hypothesized to involve, at least in part, opening of KATP.45 5. Are There Common Structural Features in Synthetic KATp Openers?

In order to ascertain whether cromakalim (1) and pinacidil (2) share common structural features, we evaluated the combination compounds 1335and 1846 and found them to be potent vasorelaxantiantihypertensive agents. These results, taken together with the reported antihypertensive activity of the pyridyl analog 1932(a)of cromakalim (l), suggest the presence of common pharmacophoric features in cromakalim (1) and pinacidil(2). It is of interest to note that most active analogs of RP 52,891 (6) contain a pyridine or an aryl ring substituted with electron withdrawing groups.40Thus there appears to be a general requirement of an aryl or a heteroaryl ring for optimum vas-

575

MODULATION OF POTASSIUM CHANNELS

13

18

19

orelaxant potency. Additionally, most potassium channel openers (e.g., 1-6) have a lipophilic residue and a potential site for hydrogen bond formation. Therefore these compounds may express their biological effects with similar structural requirements. Since there is no evidence to support receptor binding by these molecules, it is premature to speculate whether they bind to a single or multiple receptor sites.

6 . Therapeutic lndications Because of their vasodilator properties, the main clinical indication for potassium channel openers is for the treatment of hypertension.47 However, there does not seem to be any clear advantage of these agents over the more established antihypertensive drugs such as the calcium channel blockers and ACE inhibitors. There is extensive clinical data to support the use of pinacidil(2) for lowering blood pressure in hypertensive humans.48 The reduction in blood pressure is accompanied by reflexogenic increase in heart rate, which is attenuated by coadministration of a p adrenergic blocker. The other side effects (edema, headache, flushing) of pinacidil appear to be typical of most general vasodilator^.^^ The beneficial effect of pinacidil on blood lipids seems to diminish on cotreatment with the diuretic hydrochlorothiazide to control edema. Although the clinical experience with cromakalim (2) is limited, it also appears to have side effects typical of peripheral vasodilator^.^^,^^ The development of the racemic cromakalim has been terminated52in favor of a single enantiomer lemakalim (BRL 38,227), which has 3S,4R-stereo~hemistry.~~ The hemodynamic profile of lemakalim in various animal models is similar to that reported for ~romakalim.5~ The reduction in blood pressure by lemakalim is accompanied by reflex tachycardia and elevation in plasma-renin activity in renal hypertensive cats.53 The blood pressure lowering in all species is a consequence of reduction in total peripheral resistance. Aside from differences in their potencies, cromakalim and pinacidil have similar pharmacological profiles in animal models. In a study comparing hemodynamic profiles of cromakalim, pinacidil, and nicorandil in renal hypertensive cats, cromakalim was shown to have a beneficial effect on renal vascular r e ~ i s t a n c e . ~ ~ This difference has yet to be confirmed in clinical studies with cromakalim and pinacidil. Based on the established interaction of diazoxide with pancreatic p cells,55 there were some concerns that the newer compounds cromakalim, pinacidil, and RP 52,891 may also cause hyperglycemia by opening K, in insulinsecreting p cells.56Pinacidil has been shown to actually inhibit insulin release

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ATWAL

by increasing potassium efflux through K A T ~Studies . ~ ~ by Pratz et d . in normal and hyperglycemic rats show that, while diazoxide causes glyburidereversible hyperglycemia, cromakalim, RP 52,891, and nicorandil do not affect pl'asma glucose levels at concentrations much higher than those needed for lowering blood pressure.58 These studies indicate that there may be differences in regulation of KATP in various tissues and some degree of selectivity may already be built into the prototype potassium channel openers. The beneficial effects of prototype potassium channel openers on the heart are less well understood compared to their effects on the smooth muscle. By virtue of their potassium-channel-opening properties, these agents shorten the action potential d ~ r a t i o n . ~ They ~ - ~ lcan be hypothesized to have pro- or antiarrhythmic effects, depending on the type of arrhythmia.62 Studies in vitro show cromakalim and nicorandil inhibit triggered a ~ t i v i t y . Thus ~~,~ these agents can be expected to suppress ventricular tachyarrhythmias that occur following myocardial i n f a r ~ t i o n . However, ~ ~ , ~ ~ potassium channel openers may exacerbate arrhythmias of the reentrant type.67 Studies in globally ischemic perfused rat heats by Grover et aZ. show prototype KATp openers cromakalim, pinacidil, and RP 53,891 improve reperfusion function and reduce lactate dehydrogenase r e l e a ~ e . ~ - ~ These O effects are reversible by the K, blocker glyburide, indicating that KATp opening is involved in the cardioprotective effects of these agents. Advantageously, these compounds show cardioprotective activity at doses that cause relatively little cardiodepression compared to, for example, calcium channel blockers. In order to show efficacy in v i m , cromakalim had to be administered directly into the coronary artery.71 Because of their potent hypotensive and coronary dilating properties, prototype agents (e.g., 1, 2, 6) have a narrow margin of safety for the treatment of myocardial ischemia, although some reports demonstrating cardioprotection following intravenous administration have appeared in the 1iteratu1-e.~~ Studies with cromakalim and pinacidil in animal model^^^,^^ and with cromakalim in isolated human bronchioles suggest the clinical use of K, openers for asthma.75Cromakalim was shown to be effective in inhibiting the histamine-induced bronchoconstriction in healthy humans76 and it was found to be effective in patients with nocturnal asthma.77Although the hypotensive activity of cromakalim does not appear to be a problem in these studies, the identification of compounds selective for the bronchial tissue is desirable for these agents to succeed in the general patient population. Since cromakalim and pinacidil relax the bladder smooth m u s ~ l e ,K,~ ~ ,openers ~~ may find use in the treatment of urinary incontinence.80However, the observations in animal models have yet to be supported by well controlled clinical trials, as results of initial studies in man were rather disappointing.81 Recently, cromakalim has also been shown to prevent ischemia-induced damage in a rat skeletal muscle preparation.82 This result, and the observation that cromakalim restores the membrane potential of depolarized human skeletal muscle fibers,83 indicate that KATp openers may be useful for the treatment of peripheral vascular disease. Studies in vitro and in animal models indicate the potential clinical utility of KATp openers for diseases of the central nervous system (CNS). In in vitro experiments, KATP openers somatostatin and diazoxide were shown to pre-

MODULATION OF POTASSIUM CHANNELS

577

vent anoxia-induced depolarization of CA3 hippocampal neurons. 11rs4 These effects were inhibited by pretreatment with glyburide. The authors suggest that KATp may prevent anoxia-induced damage to hippocampal neurons by inhibiting the release of excitatory amino acids. The role of KATp opening in inhibiting the effects of anoxia in substantia nigra has also been discussed.85 Cromakalim and RP 52,891 have been shown to possess antiepileptic activity in a rat model of epilepsia.86In another study cromakalim prevented pilocarpineinduced behavioral response in mice.87 These studies are encouraging, but tissue-selective agents that cross the blood-brain barrier are needed to realize the potential clinical applications for CNS diseases. By virtue of their smooth-muscle-relaxingeffects, KATp openers may also be useful for the treatment of irritable bowel syndromes8 and premature labor.89 The use of minoxidil in treating male pattern baldness may indicate stimulation of hair growth to be a general property of KATp openers. Since the existing agents are rather promiscuous, it is clear that tissue-selective compounds need to be developed to explore the full clinical potential of K,,, openers.

B. Blockers of K,,, The most widely known compounds that block K,,, are the sulfonyl urea class of antidiabetic agentsg0Their mechanism of hypoglycemic activity involves blocking of KATp which causes membrane depolarization, leading to calcium entry through voltage-sensitive channels and, consequently, release of insulin.9* These compounds are useful for the treatment of non-insulindependent diabetes. The sulfonyl ureas are also known to block KATp in cardiac92 and brain tissues.93 In addition to their therapeutic value, these compounds have served as important tools for studying KATp in various tissues. Glyburide (20) and its analogs have been extensively employed to study the sulfonyl urea receptor and its coupling to KATP in various tissues. There is a good correlation between the potencies of various analogs to displace radiolabeled glyburide from its receptor site and hypoglycemic action in f3-cell tumor, indicating that the sulfonyl urea receptor may be the KATpforming protein(^).^^ However, the critical data showing reconstitution of KATp from the purified glyburide binding protein is still awaited. There appears to be a difference in the potencies of sulfonyl ureas in inhibiting 86Rb effluxes through K,,, in insulin-secreting f3 cells and substantia nigra, suggesting the sulfonyl urea receptors may not be identical in the various tissues.95 For a more detailed discussion of the pharmacology and biochemistry of the sulfonyl urea class of agents, the reader is referred to a recent review article by Lazdunski et a1.I7 The binding of 13H]-glyburideis biphasic, depending on the tissue, and it is inhibited by certain nucleotides in the presence of magnesium and d i t h i ~ t h r e i t o lThese . ~ ~ studies confirm earlier observations that there may be more than one nucleotide binding site97and that a sulfhydryl group is present at or near the K A T ~ ~ ~ The structures of some of the sulfonylurea analogs (20-25) are given below. The potencies of various hypoglycemic agents to inhibit glyburide binding has been described by Geisen and c o - w o r k e r ~A . ~nonsulfonyl ~ urea hypoglycemic agent linogliride (26) has been reported to inhibit KATp in f3 cells.*OO

578

ATWAL

OMo

Glyburlde (20)

Chlorpropamlde (23)

Gllplzlde (21)

Tolazamldg (24)

Hoe 036 (22)

Tolbutamlde (25)

0

Interestingly, this compound does not have an acid functionality, usually present in glyburide (20) and its congeners. Sodium 5-hydroxydecanoate (27) has also been shown to block K, in guinea pig ventricular cells.101 The same agent has been demonstrated to block the antiischemic effects of potassium channel opener cromakalim in globally ischemic rat Langendorff hearts.lo2 These effects of sodium 5-hydroxydecanoate are similar to those observed for glyburide.68 Recent studies suggest that phentolamine (28) type a-adrenoceptor blockers (yohimbine, antazoline, tolazoline) block the K,,p.103,104 Phentolamine (28) and its analog alinidine (29) inhibit the vasorelaxation and 86Rb effluxes caused by K, openers cromakalim (1) and pinacidil (2) in a number of smooth muscle preparations.105 However, the failure of other types of a-adrenoceptor blockers such as prazosin, rauwolscine, and phenoxybenzamine to block the effects of cromakalim indicates that this is not a general property of a-adrenoceptor b10ckers.l~~ Certain

Linogiiride (26)

Sodium 5-hydroxydecanoate (27)

Phentolamlne (28)

CI

Allnldlne (29)

Tedlsamll (30)

BRL 31660 (31)

MODULATION OF POTASSIUM CHANNELS

579

antiarrhythmic drugs such as the class I11 agent Tedisamil (30)Io6and the class I/IV agent BRL 31660 (31)Io7 can also be hypothesized to have some K, blocking activity, as they inhibit the pharmacological actions of cromakalim. 1. Therapeutic Indications

The single most important clinical use of sulfonyl ureas and other compounds that block K, in insulin-secreting p cells is for the treatment of noninsulin-dependent diabetes. *08 Although concerns over the development of tolerance to these agents remain, they are the most prescribed drugs for type I1 diabetes. Under ischemic conditions, opening of K, contributes to the loss of potassium from the myocardium. 109-111 This has been thought to contribute to ischemic damage leading to derangement of electrical activity, which results in lethal arrhythmias.l12 Thus blocking of KATp can be hypothesized to minimize myocardial injury and prevent ischemia-induced arrhythmias.l13 Recent experiments in the isolated perfused rat hearts show KATp blockers glyburide and sodium 5-hydroxydecanoate have no cardioprotective effects.lo2 In fact, potassium channel openers (e.g., cromakalim) reduce ischemia-induced damage in isolated hearts and their protective effects are completely abolished by K, b l o ~ k e r s . ~ ~These - ~ O studies support a beneficial role associated with K, opening in the ischemic myocardium and, more importantly, they indicate that potassium efflux through this channel during ischemia may be a physiological response of the heart to minimize injury. A cardioprotective role associated with K, opening during ischemia is also supported by studies by Gross et al. showing K,, blockers inhibit the beneficial effects of preconditioning, an episode of brief coronary artery occlusion.114 Experiments showing the utility of K, blockers as potential antiarrhythmic agents in the isolated heart115t116have been difficult to reproduce in uiuo. Recent studies by Lucchesi et al. show glyburide has no effect on ischemia-induced fibrillation in a canine model of myocardial ischemia. 117 These data nicely complement the results showing lack of effect of glyburide on ischemia-induced damage in isolated perfused rat hearts.68,102These studies call into question whether blocking myocardial K, is a viable mechanism to prevent ischemia-induced arrhythmias in humans. Obviously, myocardial specific agents need to be developed to explore the potential of K,, blockers as antiarrhythmic agents. 111. CALCIUM-ACTIVATEDPOTASSIUM CHANNEL (KCa)

This channel is activated by membrane depolarization and by increases in intracellular calcium. 118 Depending on the single channel conductance, this channel is termed high-conductance or maxi-K (100-250 pS), intermediateconductance (18-50 pS), or low-conductance (10-14 pS) K,,.l The highconductance K,, is present in neurones, cardiac cells, and various types of smooth muscles. 119-121 The intermediate-conductance channel has been shown to be present in red blood cells,122and in smooth muscle.123The lowconductance channel is present in a variety of cell types.12*The main function of K,, is thought to be its involvement in repolarization of the action potentials, secretion, and volume regulation. K,, may be modulated by other reg-

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ulatory mechanisms. For example, in rat brain K,, appears to be regulated by MgATP via a protein kinase-catalyzed pho~phorylation.'~~ Cyclic AMPdependent kinase (A-kinase) is reported to activate K,, in tracheal myocytes,lZ6aortic smooth muscle cells,127and in neurones. lZ8 Existing evidence also suggests that K,, may be modulated by guanosine-5'-monophosphatein vascular smooth muscle cells129and by a combination of inostol1,4,5-triphosphate and inostol 1,3,4,5-tetrakisphosphatein mouse lacrimal acinar ~ e 1 l s . l ~ ~

A. Openers of K,, There are very few organic molecules known to be specific openers of ha. Gelband et al. have reported the opening by cromakalim of what appears to be a high-conductance K,-, in rabbit aortic smooth muscle incorporated into planar lipid bilayers.131 This effect is dose dependent and it is reversed by g l y b ~ r i d e . 'Klockner's ~~ work also supports opening of K,, by cromakalirn.'33 Studies by Hermsmeyer suggest that pinacidil opens highconductance K,, in rat veins. 134 Cromakalim and a pinacidil analog (P-1060) have been shown to increase the open-state probability of maxi-K channels in smooth muscle cells enzymatically dissociated from rat portal vein. 135 These effects were blocked by glyburide and charybdotoxin, indicating that the maxi-K channel is sensitive to these blockers in the rat portal vein. Glyburide, but not charybdotoxin, inhibits the effects of cromakalim in several smooth muscle preparations. 136 Further work is obviously necessary to delineate whether &, opening by cromakalim and/or other purported KATp openers contributes to their vasorelaxant effects.

Nlguldiplne (32)

L-Alanine (33)

The calcium channel blocker niguldipine (32)has been shown to activate K,, in vascular smooth muscle.137 The contribution of K,, to the vasorelaxing properties of niguldipine is not clear as it also blocks voltage-sensitivecalcium channels and a-1 adrenoceptors. 13* Evidence also exists to support that the amino acid L-alanine (33) opens I(Ca in rat liver ~ e 1 1 s .The l ~ ~authors proposed that the effect of L-alanine in may involve inhibition of Na+-Ca2+ exchange.139Since the existing %, openers have additional actions, it is difficult to estimate the contribution of K,, opening to their pharmacological properties. The full potential of K,, openers or blockers cannot be appreciated until tissue- and channel-specific agents are developed.

B. Blockers of K,, The most important tools to distinguish between low- and high-conductance K,, are the toxins apamin140 and charybdotoxin.141While charybdotoxin

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specificallyblocks maxi-K, apamin is a potent blocker of the low-conductance However, in certain tissues, for example, rat brain, charybdotoxin may block lGaof all three types.142Although less potent than charybdotoxin, both tetraethyl ammonium (TEA) and Ba2 block maxi-K channel. Quinine and charybdotoxin, in addition to blocking maxi-K, weakly block the intermediate-conductance channel. For a more comprehensive discussion on the ability the reader is referred to a review of these and other agents to block K,, article cited in Ref. 1. Both apamine and charybdotoxin have high-affinity binding sites in a variety of tissues.la However, it is not clear at the present time whether the binding sites for these toxins are the channel-forming proteins. The charybdotoxin block of K,, has been proposed to involve electrostatic interaction between the positively charged toxin and an acidic residue at or near the mouth of the ~ h a n n e 1 . IIt~ remains ~ to be demonstrated whether a similar electrostatic interaction is involved in the inhibition of &a by other toxins. The solution structure of the 37-amino-acid charybdotoxin has been described ~,~~~ recently. 144,145 The three-dimensional structure of c h a r y b d o t ~ x i n land the proposed importance of critical amino acid(s) in blocking &a143 provide an excellent opportunity for the medicinal chemist to design small molecule openers/blockers for the charybdotoxin-sensitive The therapeutic potential of &, blockers remains unexplored due to the lack of tissue-specific agents. In a study by Tominaga, charybdotoxin at 0.15 mg/kg (iv) reduced the formation of ischemia-induced brain edema, as measured by water accumulation and ionic shifts.'& However, at a threefold higher dose, charybdotoxin exacerbated brain edema. The exact explanation of this non-dose-dependent effect is not clear but, as the authors pointed out, it may be related to the toxicity of charybdotoxin at the higher doses. Although further work is necessary to characterize this finding, it does show that blockers of K,, may find use in reducing brain injury due to ischemia. A peptide fragment, obtained by enzymatic digestion of charybdotoxin, was also found to retain protective effects against ischemic and traumatic injuries.147 There is no indication as to whether the effects of this peptide are still This result suggests some of the pharmediated through inhibition of Ga. macologic effects of charybdotoxin can be retained in small fragments, indicating the potential for structure-activity studies to arrive at a small peptidic or nonpeptidic modulator of &,.

&,.

+

IV. DELAYED RECTIFIER POTASSIUM CHANNEL (I,)

This channel is known to exist in cardiac'48 and noncardiac cell types.149It has a conductance of 5-50 pS and it is activated by depolarization above -40 mV.' The main function of this channel is thought to be its involvement in repolarization of the action p0tentia1.l~~ Recently, I, has been shown to be composed of two currents, a rapidly activating component blocked by certain class 111 antiarrhythmic drugs and a slowly activating component.151The organic compounds known to block this channel are some of the so-called class 111 antiarrhythmic agents. Class 111 agents suppress arrhythmias by prolongation of the cardiac action potential duration and effective refractory period. 152 Examples of these compounds include clofilium (34),153flecainide

ATWAL

582

Cloflllum (34)

Sotalol (36)

Flecalnlde (35)

E-4031 (37)

UK-66,914 (38)

40

UK-68,798 (39)

41

(35),154sotalol (36),151E-4031 (37),15' UK-68,798 (38),155UK-66,914 (39),156and 40.157 Additionally, tetraethyl ammonium, aminopyridines, quinine, bretylim, and strychnine may also block this current? While both d- and Ienantiomers of sotalol(36)are potassium channel blockers, the P-adrenoceptor blocking activity resides predominantly in the I-enantiomer.158 As is clear from the structures of compounds 37-41, sotalol(36) has served as a starting point for the synthesis of most of the benzene sulfonamide type of class I11 agents. No systematic structure-activity studies correlating the I, blocking potencies of class I11 agents with their antiarrhythmic efficacies have been reported to date. Interpretation of existing structure-activity data is complicated as some of these compounds, in addition to blocking I,, interact with other ion ~ h a n n e 1 s . IStudies ~~ by Baskin and co-workers show UK-68,798 (39) to be more potent than both E-4031 (37) and sotalol (36) in increasing myocardial refractory ~ e r i 0 d .Compound l~~ 40, arrived at by modification of another P-blocking agent 41, is reported to be 15-fold more potent than Pfizer's UK 68,798 (39). There is a large body of structure-activity data published on the prolongation of the action potential duration by benzene sulfonamide type compounds.160 However, it is not known at the present time if the effect on APD lengthening is solely due to blocking of I,. Based on

MODULATION OF POTASSIUM CHANNELS

583

their structural resemblance to the benzene sulfonamide type of antiarrhythmic agents (36-41), compounds 42-45161-16* may also block I,.

V. FORSKOLIN- AND PHORBOL ESTER-SENSITIVE POTASSIUM CHANNELS The diterpenoid forskolin (46) and the protein kinase C activator phorbol 12,13-dibutyrate have been reported to block what appears to be a delayed rectifier potassium channel in mouse neurones in ~ u 1 t u r e . IForskolin ~~ has also been shown to block the voltage-dependent potassium channels in several different cell types.165-169The effect of forskolin on potassium channels in a PC12 cell line appears to be independent of its CAMP-elevatingactivity, as its 1,9-dideoxy analog 47, which does not affect cAMP levels, also possesses

OCOMe

M Me" O H

Forskolin (46)

OCOMc

M

Me

OH

47

potassium-channel-blocking activity.166Similar conclusions about the lack of involvement of cAMP in mediating the effects of forskolin were drawn by Watanabe and Gola. 170 Using helix nerve cells, these authors have reported that the effect of forskolin cannot be mimicked by intracellular injections of cAMP or by CAMP-dependent phosphodiesterase inhibitors.

VI. FATTY-ACID-MODULATED POTASSIUM CHANNELS Arachidonic acid and certain related fatty acids can directly open potassium channels in cardiac171and smooth muscle cells.172These effects do not appear to be mediated through lipoxygenase- or cyclooxygenase-derived prod-

584

ATWAL

ucts.171 The effect of fatty acids on potassium channel opening is also independent of nucleotides (ATP, GTP) or calcium. These data indicate that fatty acids and the processes that control their release may be important physiological determinants in control of cell excitability. In smooth muscle they may antagonize agonist-induced contraction by membrane hyperpolarization and, consequently, limit calcium entry.l" Since free fatty acid levels rise during cardiac ischemia, opening of these channels may limit cell damage by inhibiting calcium entry and conserving ATP.171 This protective role may be similar to that played by cardiac KATP during i ~ c h e m i a . ~ +It~ lhas been recently shown that 5-lipoxygenase-derived products of arachidonic acid can open pertussis toxin-sensitive G-protein-gated muscarinic potassium channels in cardiac cells.173,174 Although further work is necessary to determine the physiological significance of these results, it does appear that arachidonic acid and some of its metabolites may mediate their effects via potassium channel modulation.

VII. ANESTHETIC-ACTIVATED POTASSIUM CHANNEL Evidence is accumulating that volatile anesthetics such as halothane (48), isoflurane (49),chloroform, and ether may be mediating their effects by open-

'>%..,

F F

Halothane (48)

F F

H

Isoflurane (49)

ing a neuronal potassium ~ h a n n e 1 . The l ~ ~ opening of this channel is reversible, and occurs at surgical pressures of these agents. Since a good correlation could be found between channel opening and inhibition of spontaneous firing in a given neuron, potassium channel opening may be relevant to the mechanism of action of these anesthetics.175Recent studies show the (+)-enantiomer of isoflurane (49)to be twofold more potent than the (-)enantiomer, indicating the probable binding of these compounds to a receptor ~ r 0 t e i n .However, l~~ it is not clear at the present time whether these agents act directly on the potassium channel protein(s) or if their effects are mediated through a second messenger system.

VIII. MOLECULAR MECHANISM OF POTASSIUM CHANNEL GATING Using techniques of molecular biology, protein chemistry, and electrophysiology, the molecular mechanisms of potassium channel gating are beginning to be unraveled. Much of this progress has been made possible by cloning of the shaker gene of D r o ~ o p h i l aThe . ~ ~cloning ~ of this gene has allowed the identification of a number of voltage-activated potassium channel proteins which appear to be related in structure.17sFour such proteins make up a single potassium channel. 179 Details of channel structure and gating mechanism are discussed in excellent review articles by Millerlm and Aldrich.lsl The six membrane-spanning domains of potassium channel proteins are similar to those described for sodium and calcium channels.ls2 Studies using

MODULATION OF POTASSIUM CHANNELS

585

deletion mutants show the ion-conducting pore is located between S5 and S6 membrane-spanning domains. 180,1s3 The inactivation process is hypothesized to obey the ball and chain model.lM According to this model, the channel contains an amino acid region (ball) on the cytoplasmic side near the amino terminal of the monomeric protein, which is connected to the membrane-spanning region through a small stretch (chain) of amino acids.180 Once it is open, the ball is hypothesized to provide a physical plug to close the channel. The inactivating region is shown to interact with the voltage-sensing region to cause charge immobilization and prevent it from returning to its original conformation.185 This simple model explains the voltage sensitivity of the channel opening and its block by charybdotoxin and tetraethyl ammonium by binding to the pore-forming region between S5 and S6 domains.ls0 Is this a general mechanism of gating for all potassium channels? The answer must wait until more channel proteins are purified and shown to be functional potassium channels. Determination of the molecular architecture of these proteins and their manipulation by techniques similar to those used for studying the shaker channel would shed more light on the molecular mechanism of ion selectivity and channel gating.

IX. FUTURE DIRECTIONS Potassium channels have dominated the field of ion channel research for the past several years. The initial euphoria over the discovery of potassium channel opening as a mechanism for the vasodilator/antihypertensive activity of several existing agents is slowly settling down. Many potent analogs of the purported KATp openers (e.g., cromakalim, pinacidil, RP 52,691) have been synthesized over the past few years. However, no clear advantages of KATp openers over the existing antihypertensive drugs (for example, ACE inhibitor, calcium channel blockers, etc.) could be demonstrated. New uses are being explored and they will determine the future of this class of agents. Since there are several types of potassium channels, often in the same tissue, channel and tissue selectivity will play a major role in advancing these agents to clinical stage. Although most of the pharmacological effects of the KATp openers (e.g., cromakalim) are reversed by sulfonyl urea type K,, blockers (e.g., glyburide), cromakalim and other KATP openers do not affect glyburide binding at therapeutically relevant concentrations.ls6 Definitive experiments need to be performed to understand at what level the effects of potassium channel openers are reversed by sulfonyl ureas. Additional complication comes from the controversy over the identity of the potassium channel(s) affected by cromakalim and other prototype compounds. Although I have discussed these agents under the title KAT, openers, it is already clear that these compounds may affect, depending on the tissue, other potassium channels. The controversy over the identity of the channel(s) that mediates their pharmacological effects in various tissues needs to be resolved. Our understanding of the functioning of potassium channels, although still primitive, is increasing at a fast rate. The diversity of potassium channels in various tissues and their roles in controlling basic cell functions are being unraveled by interdisciplinary research. The availability of organic molecules

ATWAL

586

that modulate these channels in a tissue-specific manner would be useful to explore the therapeutic potential of potassium channel modulation. Medicinal chemistry research has so far concentrated on modification of the existing agents, many of which are rather promiscuous. The design of channel- and tissue-specific molecules relies heavily on the progress in understanding the structural biology and the protein chemistry of potassium channels. At the present time, very little is known about the three-dimensional structure of channel protein(s) or the second messenger systems that regulate their activity. Our understanding of the basic mechanism(s) of channel functioning is slowly progressing, but further work is needed to unravel how various channels differ in structure and in their gating properties. This is crucial for understanding the pharmacological effects of existing potassium channel modulators and for the design of tissue- and channel-specific compounds.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19.

20. 21. 22. 23.

24. 25. 26.

27. 28.

N. S. Cook, Trends Pharmacol. Sci. 9, 21 (1988). U. Quast and N. S. Cook, Trends Pharmacol. Sci. 10, 431 (1989). D. W. Robertson and M. I. Steinberg, J. Med. Chem. 33, 1529 (1990). (a) G. Edwards and A. H. Weston, Trends Phurmacol. Sci. 11, 417 (1990); (b) G. Edwards and A. H. Weston, Pharmacol. Ther. 48, 237 (1990). J. M. Evans and S. D. Longman, Annu. Rep. Med. Chem. 26, 73 (1991). J. M. Evans and G. Stemp, Chem. Br. 27, 439 (1991). A. H. Weston and A. Abbott, Trends Pharmacol. Sci. 8, 283 (1987). A. Noma, Nature 305, 147 (1983). A. E. Spruce, N. 8. Standen, and P. R. Stanfield, J. Physiol. 382, 213 (1987). D. L. Cook and C. N. Hales, Nature 311, 271 (19M). Y. Ben-Ari, K. KrnjeviC, and V. Crepel, Neurosci. 37, 55 (1990). H. Bernardi, M. Fosset, and M. Lazdunski, Proc. Natl. Acad. Sci. U S A 85, 9816 (1988). N. B. Standen, J. M. Quayle, N. W. Davies, J. E. Brayden, Y. Huang, and M. T. Nelson, Science 245, 177 (1989). R. J. Kovacs and M. T. Nelson, A m . 1. Physiol. 261, H604 (1991). I. Inoue, H. Nagase, K. I s h i , and T. Higuti, Nature 352, 244 (1991). F. M. Ashcroft, Annu. Rev. Neurosci. 11, 97 (1988). J. R. deWeille, M. Fosset, C. Mourre, H. Schmid-Antomarchi, H. Bernardi, and M. Lazdunski, PfZugers Archiv. Eur. 1. Physiol. 414 (Suppl. l), S80 (1989). G. E. Kirsch, J. Codina, L. Birnbaumer, and A. M. Brown, Am. J. Physiol. 259, H820 (1990). (a) J. R. de Weille, H. Schmid-Antomarchi, M. Fosset, and M. Lazdunski, Proc. Natl. Acud. Sci. U S A 86, 2971 (1988); (b) W. Wang and G. Giebisch, ibid. 88, 9722 (1991). T. Karashima, T. Itoh, and H. Kuriyama, J. Pharmacol. Exp. Ther. 221, 472 (1982). S. Mondot, M. Mestre, C. G. Caillard, and I. Cavero, Br. J. Pharmacol. 95, 813P (1988). G. Burrell, F. Cassidy, J. M. Evans, D. Lightowler, and G. Stemp, J. Med. Chem. 33, 3023 (1990). (a) D. R. Buckle, J. R. S. Arch, C. Edge, K. A. Foster, C. S. V. Houge-Frydrych, I. L. Pinto, D. G. Smith, J. F. Taylor, S. G. Taylor, J. M. Tedder, and R. A. B. Webster, J. Med. Chem. 34, 919 (1991); (b) D. R. Buckle, J. R. S. Arch, A. E. Fenwick, C. S. V. Houge-Frydrych, I. L. Pinto, D. G. Smith, S. G. Taylor, and J. M. Tedder, ibid. 33,3028 (1990). (c) V. A. Ashwood, F. Cassidy, J. M. Evans, S. Gagliardi, and G. Stemp, ibid. 34, 3261 (1991). R. Bergmann and R. Gericke, J. Med. Chem. 33, 492 (1990). R. Bergmann, V. Eiermann, and R. Gericke, J. Med. Chem. 33, 2759 (1990). For 7: (a) M. R. Attwood, P. S. Jones, P. B. Kay, P. M. Paciorek, and S. Redshaw, Life Sci. 48, 803 (1991); (b) P. M. Paciorek, D. T. Burden, Y. M. Burke, I. S. Cowlrick, R. S. Perkins, J. C. Taylor, and J. F. Waterfall, J. Cardiovasc. Pharmacol. 15, 188 (1990). For 8: G. S. Stemp and G. Burrell, EP 0399 834 (1990). For 9: R. Gericke, M. Baumgarth, I. Lues, J. De Peyer, and R. Bergmann, EP 400430 A2 (1990), and Ref. 25.

MODULATION OF POTASSIUM CHANNELS

587

29. For 10: G. S. Stemp and G. Burrell, EP 0426 379 (1991). 30. For 11: J. M. Tedder, I. L. Pinto, and C. M. Edge, EP 0321175 A1 (1989). 31. For 12: S. Matsuhisa, M. Asano, Y. Matsumoto, K. Takayama, T. Yoden, R. Tsuzuki, W. Uchida, and I. Yanagisawa, EP 0432 893 A2 (1991). 32. (a) J. M. Evans, G. Stemp, and F. Cassidy, EP 0205292 A2, 1986; CA, 106, 213921~(1987);(b) D. Binder, EP 0405298 A1 (1990); (c) J. 8 . Press, P. Sanfilippo, J. J. McNally, and R. Falotico, EP 0360 621 (1990). 33. (a) R. P. Hof, U. Quast, N. S. Cook, and S. Blarer, Circ. Res. 62, 679 (1988); (b) G. J. Grover, J. Newburger, P. S. Sleph, S. Dzwonczyk, S. C. Taylor, S. Z. Ahmed, and K. S. Atwal, J. Phurmacol. Exp. Ther. 257, 156 (1991). 34. H. J. Petersen, J. Med. Chem. 21, 773 (1978). 35. (a) K. S. Atwal, S. Moreland, J. R. McCullough, 8 . C. OReilly, S. Z. Ahmed, and D. E. Normandin, Bioorg. Med. Chem. Lett. 2, 83 (1992); (b) K. S. Atwal, J. R. McCullough, and G. J. Grover, U.S. Patent No. 5011834 (1991). 36. H. J. Petersen, German Patent Offenleg No. 2557438 (1976). 37. (a) M. I. Steinberg, 5. A. Wiest, K. M. Zimmerman, P. J. Ertel, K. G. Bemis, and D. W. Robertson, J. Pharmacol. Exp. Ther. 256, 222 (1991); (b) P. W. Manley and U. Quast, Chemia 45, 88 (1991). 38. J. C. Aloup, D. Farge, C. James, 5. Mondot, and 1. Cavero, Drugs Future 15, 1097 (1990). 39. M. N. Palfreyman, N. Vicker, and R. J. A. Walsh, EP 0377532 A1 (1990). 40. D. C. Cook, T. W. Hart, I. M. Mclay, M. N. Palfreyman, and R. J. A. Walsh, EP 0321273 A2 (1989). 41. D. C. Cook, T. W. Hart, I. M. Mclay, M. N. Palfreyman, and R. J. A. Wash, EP 0321274 A1 (1989). 42. M. T. Nelson, Y. Huang, J. E. Brayden, J. Hescheler, and N. B. Standen, Nature 344, 770 (1990). 43. M. Fosset, H . Schmid-Antomarchi, J. R. de Weille, and M. Lazdunski, FEBS Lett. 242, 94 (1988). 44. J. R. de Weille, H. Schmid-Antomarchi, M. Fosset, and M. Lazdunski, Proc. Natl. Acad. Sci. U S A 85, 1312 (1988). 45. J. Daut, W. Mailer-Rudolph, N. von Beckerath, G. Mehrke, K. Gunther, and L. GoedelMeinen, Science 247, 1341 (1990). 46. K. S. Atwal, S. Moreland, J. R. McCullough, S. Z. Ahmed, and D. E. Normandin, Bioorg. Med. Chem. Lett. 2, 87 (1992). 47. G. Haeusler, Clin. Physiol. Biochem. 8 (Suppl. 2), 46 (1990). For a comprehensive review on the role of potassium channels in vascular relaxation, see J. Cardiovasc. Pharmacol. 12 (Suppl. 2), (1988). 48. I. Ahnfelt-Rsnne and H. J. Jurgensen, Drugs Today 25, 65 (1989). 49. M. R. Goldberg and W. W. Offen, Drugs 36, (Suppl. 7), 83 (1988). 50. M. J. VandenBurg, S. R. Woodward, M. Hossain, P. Stewart-Long, and T. C. G. Tasker, J. Hypertens. 4 (Suppl. 6),S166 (1986). 51. K. M. Eckl and W. H. Greb, Clin. Exp. Hypertens. A Ther. Pract. 9, 160 (1987). 52. Scrip, No. 1428, 22 (1989). 53. J. C. Clapham, T. C. Hamilton, S. D. Longman, R. E. Buckingham, C. A. Campbell, and G. L. Ilsley, Arzneim. Forsch. 41 (I), 385 (1991). 54. S. D. Longman, J. C. Clapham, C. Wilson, and T. C. Hamilton, J. Cardiovasc. Pharmacol. 12, 535 (1988). 55. R. Z. Kozlowski, C . N. Hales, and M. L. J. Ashford, Br. J. Pharmacol. 97, 1039 (1989). 56. M. J. Dunne, R. J. Aspinall, and 0. H. Petersen, Br. J. Pharmacol. 99, 169 (1990). 57. (a) P. Lebrun, V. Devreux, M. Hermann, and A. Herchuelz, Eur. J. Pharmacol. 156, 283 (1988); (b) P. Lebrun, V. Devreux, M. Hermann, and A. Herchuelz, I. Pharmacol. Exp. Ther. 250, 1011 (1989); (c) P. Lebrun, M. H. Antoine, M. Hermann, and A. Herchuelz, Eur. J. Pharmacol. 200, 159 (1991). 58. J. Pratz, S. Mondot, F. Montier, and I. Cavero, 1. Pharrnacol. Exp. Ther. 258, 216 (1991). 59. J. K. Smallwood and M. I. Steinberg, 1. Cardiovasc. Pharmacol. 12, 102 (1988). 60. W. Osterrieder, Naunyn-Schmiedeberg's Arch. Pharmacol. 337, 93 (1988). 61. M. C. Sanguinetti, A. L. Scott, G. J. Zingaro, and P. K. S. Siegl, Proc. Natl. Acad. Sci. U S A 85, 8360 (1988).

588

ATWAL

62. T. J. Colatsky and C. H. Follmer, Cardiovasc. Drug Rev. 7, 199 (1989). 63. D. Lathrop, P. P. NAnAsi, and A. Varrb, Br. J. Phurmucol. 99, 119 (1990). 64. F. A. Fish, C. Prakash, and D. M. Roden, Circulation 82, 1362 (1990). 65. N. El-Sherif, R. Mehra, W. B. Gough, and R. H. Zeiler, Circ. Res. 51, 152 (1982). 66. H. Le Marec, K. H. Dangman, P. Danilo, and M. R. Rosen, Circulation 71, 1224 (1985). 67. C. D. Wolleben, M. C. Sanguinetti, and P. K. S. Siegl, J. Mol. Cell. Cardiol. 21, 783 (1989). 68. G. J. Grover, J. R. McCullough, D. E. Henry, M. L. Condor, and P. G. Sleph, J. Phurmacol. Exp. Ther. 251, 98 (1989). 69. G. J. Grover, P. G. Sleph, and S. Dzwonczyk, J. Cardiovasc. Phurmacol. 16, 853 (1990). 70. G. J. Grover, S. Dzwonczyk, and P. G. Sleph, Eur. J. Phurmucol. 191, 11 (1990). 71. (a) G. J. Grover, S. Dzwonczyk, C. S. Parham, and P. G. Sleph, Cardiovasc. Drugs Ther. 4,465 (1990); (b) G. J. Grover, P. G. Sleph, S. Dzwonczyk, and C. S. Parham, Circulation 80,II-499 (1989). 72. (a) M. Maruyama and G. J. Gross, FASEB J. 4, A320 (1990); (b) J. A. Auchampach, M. Maruyama, I. Cavero, and G. J. Gross, Circulation 82,111-571 (1990);(c) G. J. Gross, I. Cavero, and J. A. Auchampach, FASEB J. 5, A1219 (1991). 73. (a) M. A. Murray, J. P. Boyle, and R. C. Small, Br. 1. Phurmacol. 98, 865 (1989); (b) J. R. S. Arch, D. R. Buckle, J. Bumstead, G. D. Clarke, J. F. Taylor, and S. G. Taylor, ibid. 95, 763 (1988). 74. J. E. Nielsen-Kudsk, S. Mellemkjaer, C. Siggaard, and C. B. Nielsen, Eur. 1. Phurmacol. 157, 221 (1988). 75. (a) S. G. Taylor, J. Bumstead, J. E. J. Morris, D. J. Shaw, and J. F. Taylor, Br. J. Pharmacol. 95 (Suppl.), 795P (1988);(b) J. R. S. Arch, D. R. Buckle, J. Bumstead, G. D. Clarke, J. F. Taylor, and S. G. Taylor, ibid. 95, 763 (1988). 76. A. Baird, T. C. Hamilton, D. H. Richards, T. Tasker, A. J. Williams, Br. J. Clin. Pharmucol. 25, 114P (1988). 77. A. J. Williams, A. Hopkirk, E. Lavender, T. Vyse, V. F. Chiew, and T. H. Lee, Am. Rev. Respir. Dis. 139 (4, Part 2), A140 (1989). 78. A. Malmgren, K. E. Anderson, C. Sjogren, and P. 0. Anderson, 1. Urol. USA 142, 1134 (1989). 79. K. E. Anderson, P. 0. Anderson, M. Fovaeus, H. Hedlund, A. Malmgren, and C. Sjogren, Drugs 36 (Suppl. 7), 41 (1988). 80. Scrip, No. 1620121, 31 (1991). 81. J. Restoric and D. Nurse, Neurol. Urodyn. 7, 207 (1988). 82. R. Hatton, C. Heys, M. H. Todd, 0. A. Downing, and K. A. Wilson, Br. J. Pharmacol. 102, 280P (1991). 83. (a) A. Spuler, F. Lehmann-Horn, and P. Grafe. Naunyn-Schmeideberg's Arch. Phurmacol. 339, 327 (1989); (b) P. Grafe, S. Quasthoff, M. Strupp, and F. Lehmann-Horn, Muscle Nerve 13, 451 (1990); (c) S. Quasthoff, A. Spuler, W. Spittelmeister, F. Lehmann-Horn, and P. Grafe, Eur. J. Phurmucol. 186, 125 (1990). 84. K. Krnjevit, Stroke 21 (Suppl. 111), 111-190 (1990). 85. K. P. S. J. Murphy and S. A. Greenfield, Exp. Brain Res. 84, 355 (1991). 86. (a) G. Gandolfo, C. Gottesmann, J.-N. Bidard, and M. Lazdunski, Eur. 1. Phurmucol. 159,329 (1989); (b) G. Gandolfo, S. Romettino, C. Gottesmann, G. Van Luijtelaar, A. Coenen, J.-N. Bidard, and M. Lazdunski, ibid. 167, 181 (1989). 87. M. D. Tricklebank, G. Flockhart, and S. G. Freedman, Eur. J. Pharmacol. 151, 349 (1988). 88. R. S. Perkins, P. M. Paciorek, and J. F. Waterfall, Br. 1. Pharmacol. 98, 808P (1989). 89. I. Piper, E. Minshall, S. J. Downing, M. Hollingsworth, and H. Sadraei, Br. 1. Phurmacol. 101, 901 (1990). 90. (a) A. E. Boyd 111, Diabetes 37, 847 (1988); (b) E. Gylfe, B. Hellman, J. Sehlin, and I.-B. Taljedal, Experentiu 40, 1126 (1984). 91. (a) 8. J. Zunkler, S. Lenzen, K. Manner, U. Panten, and G. Trube, Naunyn-Schmeideberg's Arch. Phurmacol. 337, 225 (1988); (b) P. Rorsman, and G. Trube, Pfliigers Archiv. 405, 305 (1985); (c) J. C. Henquin, Biochem. Biophys. Res. Commun. 156, 769 (1988). 92. M. Fosset, J. R. de Weille, R. D. Green, H. Schmid-Antomarchi, and M. Lazdunski, J. Bid. Chem. 263, 7933 (1988). 93. H. Bemardi, M. Fosset, and M. Lazdunski, Proc. Natl. Acad. Sci. USA 85, 9816 (1988). 94. H. Schmid-Antomarchi, J. R. de Weille, M. Fosset, and M. Lazdunski, J. Biol. Chem. 262, 15840 (1987).

MODULATION OF POTASSIUM CHANNELS 95. S. Amoroso, H. Schmid-Antomarchi, M. Fosset, and M. Lazdunski, Science 247,852 (1990). 96. M. Gopalakrishnan, D. E. Johnson, R. A. Janis, and D. J. Triggle, J. Phrmacol. Exp. Ther. 257, 1162 (1991), and references therein. 97. W. J. Lederer and C. G. Nichols, J. Physiol 419, 193 (1989). 98. R. Weik and B. Neumcke, 1. Membr. Bid. 110, 217 (1989). 99. K. Geisen, V. Hitzel, R. Okomonopoulos, J. Punter, R. Weyer, and H.-D. Summ, Arzneim. Forsch. 35, 707 (1985). 100. P. Ronner, T. J. Higgins, and G. A. Kimmich, Diabetes 40, 885 (1991). 101. T. Niho, T. Notsu, H. Ishikawa, H. Funato, M. Yamazaki, H. Takahashi, I. Tanaka, M. Kayamoto, T. Dabasaki, F. Yamasaki, T. Shinkawa, A. Uemura, and M. Mizota, Folk P h r macol. Jpn. 89, 155 (1987). 102. (a) J. R. McCullough, D. E. Normandin, M. L. Conder, P. G. Sleph, S. Dzwonczyk, and G. J. Grover, Circ. Res. 69,949 (1991); (b) C. A. Sargent, M. A. Smith, S. Dzwonczyk, P. G. Sleph, and G. J. Grover, J. Pharmacol. Exp. Ther. 259, 97 (1991). 103. G. A. McPherson and J. A. Angus, Br. J. Pharmacol. 97, 941 (1989). 104. M. J. Dunne, Br. 1. Pharmacol. 103, 1847 (1991). 105. G. A. McPherson and J. A. Angus, Br. 1. Pharmacol. 100, 201 (1990). 106. K. Bray and U. Quast, Eur. J. Pharmacol. 200, 163 (1991). 107. (a) S. G. Taylor, K. A. Foster, D. J. Shaw, and J. F. Taylor, Br. J. P h a m c o l . 98,881P (1989). (b) J. R. S. Arch, D. R. Buckle, C. Carey, H. Parr-Dobrzanski, A. Faller, K. A. Foster, C. S. V. Houge-Frydrych, I. L. Pinto, D. G. Smith, and S. G. Taylor, J. Med. Chem. 34, 2588 (1991). 108. S. C. Stinson, Chem. Eng. News, September 30, 35 (1991). 109. A. G. Klkber, J. Mol. Cell. Cardiol. 16, 389 (1984). 110. J. N. Weiss, S. T. Lamp, and K. I. Shine, Am. J. Physiol. 256, H1165 (1989). 111. A. A. M. Wilde, D. Escande, C. A. Schumacher, D. Thuringer, M. Mestre, J. W. T. Fiolet, and M. J. Janse, Circ. Res. 67, 835 (1990). 112. A. S. Harris, A. Bisteni, R. A. Russell, J. C. Brigham, and J. E. Firestone, Science 119, 200 (1954). 113. S. Bekheit, M. Restivo, R. Henkin, M. Boutjdir, K. Gooyandeh, and R. Jean-Bart, J. A m Coll. Cardiol. 13, 142A (1989). 114. J. A. Auchampach and G. J. Gross, FASEB J. 5, A1103 (1991). 115. P. F. Kantor, W. A. Coetzee, S. C. Dennis, and L. H. Opie, Circulafion 76, IV-17 (1987). 116. C. D. Wolleben, M. C. Sanguinetti, and P. K. S. Siegl, J. Mol. Cell. Cardiol. 21, 783 (1989). 117. L. Chi, A. C. G. Uprichard, and B. R. Lucchesi, J. Mol. Cell. Cardiol. 21 (Suppl. 11), S89 (1989). 118. A. L. Blatz and K. L. Magleby, Trends Neurosci. 10, 463 (1987). 119. J. J. Singer and J. V. Walsh, Jr., Pflugers Archiv. 408, 98 (1987). 120. I. Bar0 and D. Escande, Pflugers Archiv. 414 (Suppl. l), S168 (1989). 121. F. Ahmed, R. W. Foster, R. C. Small, and A. H. Weston, Br. J. Phamcol. 83, 227 (1984). 122. 0. P. Hamil, A. Marty, E. Neher, 8. Sakmann, and F. J. Sigworth, Pflugers Archiv. 391, 85 (1981). 123. R. Inoue, K. Okabe, K. Kitamura, and H. Kuriyama, Pfliigers Archiv. 406, 138 (1986). 124. (a) R. Inoue, K. Kitamura, and H. Kuriyama, Pflugers Archiv. 405, 173 (1985); (b) G. Isenberg and V. Klockner, ibid. 405, R62 (1986); (c) N. S. Cook and D. G. Haylett, J. Physiol. 358, 373 (1985). 125. S. Chung, P. H. Reinhart, 8 . L. Martin, D. Brautigan, and I. 8. Levitan, Science 253, 560 (1991), and references therein. 126. H. Kume, A. Takai, H. Tokuno, and T. Tomita, Nature 341, 152 (1989). 127. J. Sadoshima, N. Akaike, H. Kanaide, and M. Nakamura, Am. J. Physiol. 255, H754 (1988). 128. D. A. Ewald, A. Williams, and I. B. Levitan, Nature 315, 503 (1985). 129. D. L. Williams, Jr., G. M. Katz, L. Roy-Contancin, and J. Reuben, Proc. Natl. Acad. Sci. U S A 85, 9360 (1988). 130. A. P. Moms, D. V. Gallacher, R. F. Irvine, and 0. H. Petersen, Nature 330, 653 (1987). 131. C. H. Gelband, N. J. Lodge, and C. Van Breeman, Eur. J. P h m c o l . 167, 201 (1989). 132. C. H. Gelband, J. R. McCullough, and C. van Breeman, Biophys. J. 57, 509a (1990). 133. U. Klockner, U. Trieschmann, and G. Isenberg, Anneim. Forsch. 39, 120 (1989). 134. K. Hermsmeyer, Drugs 36, (Suppl. 7), 29 (1988). 135. S. Hu, H. S. Kim, P. Okolie, and G. 8. Weiss, J. Pharmacol. Exp. Ther. 253, 771 (1990). 136. A. C. Wickenden, S. Grimwood, T. L. Grant, and M. H. Todd, Br. J. Phrmacol. 103, 1148 (1991).

590

ATWAL

U. Klockner and G. Isenberg, Br. 1. Pharmacol. 97, 957 (1989). R. Boer, A. Grassegger, C. Schudt, and H. Glossmann, Eur. J. Pharrnacol. 172, 131 (1989). C. E. Bear and 0. H. Petersen, Pfliigers Archiv. 410, 342 (1987). B. Marqueze, M. J. Seagar, and F. Couraud, Eur. J. Biochem. 169, 295 (1987). G. Gimenez-Gallego, M. A. Navia, J. P. Reuben, G. M. Katz, G. J. Kaczorowski, and M. L. Garcia, Proc. Natl. Acad. Sci. U S A 85, 3329 (1988). 142. E. Moczydlowski, K. Luchhesi, and .A. Ravindran, J. Membr. B i d . 105, 95 (1988). 143. R. MacKinnon and C. Miller, Science 245, 1382 (1989). 144. F. Bontems, C. Roumestand, P. Boyot, 8. Gilquin, Y. Doljansky, A. Menez, and F. Toma, Eur. 1. Biochem. 196, 19 (1991). 145. W. Massefski, Jr., A. G. Redfield, D. R. Hare, and C. Miller, Science 249, 521 (1990). 146. T. Tominaga, H. Katagi, and S. T. Ohnishi, Brain Res. 460, 376 (1988). 147. T. Ohnishi, U.S. Patent No. 5,006,512 (1991). 148. (a) H. Matsuura, T. Ehara, and Y. Imoto, Pfliigers Archiv. 410, 596 (1987); (b) T. Shibasaki, J. Physiol. 387, 227 (1987). 149. (a) F. Conti and E. Neher, Nature 285, 140 (1980); (b) C. D. Benham and T. B. Bolton, J. Physiol. 340, 469 (1983). 150. D. Noble and R. W. Tsien, J. Physiol. 200, 205 (1969). 151. M. C. Sanguinetti and N. K. Jurkiewicz, J. Gen. Physiol. 96, 195 (1990). 152. E. M. Vaughan Williams, Eur. Heart 1. 6 (Suppl. D), 145 (1985). 153. J. P. Arena and R. S. Kass, MoZ. Pharmacol. 34, 60 (1988). 154. C. H. Follmer and T. J. Colatsky, Circulation 82, 289 (1990). 155. (A) T. K. Knilans, D. A. Lathrop, P. P. Nanasi, A. Schwartz, and A. Varr6, Br. J. Pharmacol. 103,1568 (1991); (b) S. C. Black, L. Chi, D-X Mu, B. R. Lucchesi, J. Pharmacol. Exp. Ther. 258, 416 (1991). 156. M. Gwilt, H. W. Dalrymple, K. J. Blackburn, R. A. Burges, and A. J. Higgins, Circulation 78 (No. 4, Pt. 2), 150 (1988). 157. S. P. Connors, P. D. Denis, E. W. Gill, and D. A. Terrar, J. Med. Chem. 34, 1570 (1991). 158. (a) D. Hafner, F. Berger, U. Borchard, A. Kullmann, and A. Scherlitz. Arzneim. Forsch 38,231 (1988); (b) E. Carmeliet, J. Pharmacol. Exp. Ther. 232, 817 (1985); (c) A. W. Gomoll and M. J. Bartek, Eur. J. Pharmacol. 132, 123 (1986). 159. E. P. Baskin, C. M. Serik, A. A. Wallace, L. M. Brookes, H. G. Selnick, D. A. Claremon, and J. J. Lynch, Jr., J. Cardiovasc. Pharmacol. 18, 406 (1991). 160. R. Lis, D. D. Davey, T. K. Morgan, Jr., W. C. Lumma, Jr., R. A. Wohl, V. K. Jain, C.-N. Wan, T. M. Argentieri, M. E. Sullivan, and E. H. Cantor, J. Med Chem. 30, 2303 (1987), and references therein. 161. For 4 2 T. K. Morgan, Jr., R. Lis, W. C. Lumma, Jr., R. A. Wohl, K. Nickisch, G . B. Phillips, J. M. Lind, J. W. Lampe, S. V. Di Meo, H. J. Reiser, T. M. Argentieri, M. E. Sullivan, and E. H. Cantor, J. Med. Chem. 33, 1087 (1990). 162. For 43: R. Lis, D. D. Davey, T. K. Morgan, Jr., W. C. Lumma, Jr., R. A. Wohl, V. K. Jain, C.-N. Wan, T. M. Argentieri, M. E. Sullivan, and E. H. Cantor, J. Med. Chem. 30,2303 (1987). 163. For 44: P. E. Cross and J. E. Arrowsmith, EP 0286277 A1 (1988). 164. For 45: W. C. Lumma, Jr., R. A. Wohl, D. D. Davey, T. M. Argentieri, R. J. DeVita, R. P. Gomez, V. K. Jain, A. J. Marisca, T. K. Morgan, Jr., H. J. Reiser, M. E. Sullivan, J. Wiggins, and S. S. Wong, 1. Med. Chem. 30, 755 (1987). 165. D. S. Grega, M. A. Werz, and R. L. Macdonald, Science 235, 345 (1987). 166. T. Hoshi, S. S. Garber, and R. W. Aldrich, bcience 240, 1652 (1988). 167. D. Krause, S. C. Lee, and C. Deutsch, Pfliigers Archiv. 412, 133 (1988). 168. K. Dunlap, Pfliigers Archiv. 403, 170 (1985). 169. D. Choquet, P. Sarthou, D. Primi, P. A. Cazenave, and H. Korn, Science 235, 1211 (1987). 170. K. Watanabe and M. Gola, Neurosci. Lett. 78, 211 (1987). 171. D. Kim and D. E. Clapham, Science 244, 1174 (1989). 172. K. W. Ordway, J. V. Walsh, Jr., and J. J. Singer, Science 244, 1176 (1989). 173. Y. Kurachi, H. Ito, T. Sugimoto, T. Shimizu, I. Miki, and M. Ui, Nature 337, 555 (1989). 174. D. Kim, D. L. Lewis, L. Graziadei, E. J. Neer, D. Bar-Sagi, and D. E. Clapham, Nature 337, 557 (1989). 175. N. P. Franks and W. R. Lieb, Nature 333, 662 (1988). 176. N. P. Franks and W. R. Lieb, Science 254, 427 (1991). 177. D. M. Papazian, T. L. Schwarz, B. L. Tempel, Y. N. Jan, and L. Y. Jan, Science 237,749 (1987). 137. 138. 139. 140. 141.

MODULATION OF POTASSIUM CHANNELS

591

178. (a) B. L. Tempel, D. M. Papazian, T. L. Schwartz, Y. N. Jan, and L. Y. Jan, Science 237, 770 (1987); (b) T. Takumi, H. Ohkubo, and S. Nakanishi, ibid. 242, 1042 (1988); (c) M. J. Christie, J. P. Adelman, J. Douglass, and R. A. North, ibid. 244, 221 (1989); (d) A. Wei, M. Covarrubias, A. Butler, K. Baker, M. Pak, and L. Salkoff, ibid. 248, 599 (1990). 179. R. MacKinnon, Nature 350, 232 (1991). 180. C. Miller, Science 252, 1092 (1991). 181. T. Hoshi, W. N. Zagotta, and R. W. Aldrich, Science 250, 533 (1990). 182. W. A. Catterall, Science 242, 50 (1988). 183. W. N. Zagotta, T. Hoshi, and R. W. Aldrich, Science 250,568 (1990). 184. C. M. Armstrong and F. Bezanilla, J. Gen. Physiol. 70, 567 (1977). 185. F. Bezanilla, E. Perozo, D. M. Papazian, and E. Stefani, Science 254, 679 (1991). 186. I. Angel and S. Bidet, Fund. Clin. Pharmacol. 5, 107 (1991).