Europeun Journal of Pharmacofogy, 190 (1990) 75-84 Elsevier

75

WP 51540

Jeanette

Longmore,

Donald T. Newgreen

and Arthur H. Weston

Smooth Muscle Research Group, Department of Physiological Sciences, Unioersiry of Manchester. Oxford Road, Manchester Ml3 9PT. U.K. Received 5 March 1990. revised MS received 29 .rune 1990, accepted 31 July 1990

The study investigated the possible involvement of an ATP-sensitive potassium (K) channel in the relaxant actions of K channel openers in rat portal vein. The effects of glibenclamide on the relaxant responses and r&es in 86Rb efflux evoked by cromakalim, RP49356 and diazoxide were studied. The effects of galanin and depletion of intracellular ATP concentrations ([ATP],) were also examined. Galanin increased mechanical activity and s6Rb efflur, effects most likely mediated via galanin receptors rather than a direct action on a K channel. Glibenclamide inhibited the relaxant respondes and rises in &Rb efflux evoked by cromakalim, RP49356 and diazoxide. Reduction of ]ATPli caused relaxation and this effect was partially reversed by glibenclamide. The restored activity was abolished by cromakalim. These results suggest that an ATP-sensitive K channel is present on rat portal vein and that it may be involved in the relaxant actions of cromakalim, RP49356 and diazoxide.

ATP-sensitive

K’ channels; Cromakalim: RP49356; Diazoxide:

1. Introduction

Potassium (K) channels which are regulated by changes in intracellular levels of adenosine triphosphate ([ATP]i) have been identified in pancreatic &cells (including rat insulinoma cells such as the RINMSF cell line; Schmid-Antomarchi et al., 1987; Ziinkler et al., 1988) cardiac cells (Noma, 1983) and more recently in vascular smooth muscle (Standen et al., 1989). In pancreatic P-cells the channels are open under resting physiological conditions (Findlay et al., 1988) and can be closed by sulphonylurea-type drugs such as glibenclamide. glipizide and tolbutamide (S&mid-Antomarchi et

Correspondence to: A.H. Weston. Smooth Muscle Research Group, Department of Physiological Sciences, University of Manchester, Oxford Road. Manchester Ml3 9pT. U.K. 0014-2999/90/$03.50

Glibenclamide;Galanin;

Portal vein (rat)

ler et al., al., 1987; Sturgess et al., 1988; Z 1988). This effect of the sulphonylureas is associated with a decrease in E6Rb efflux (a marker for K) and membrane depofdrisation (De Weille et al.. 1987; Schmid-Antoma chi et al., 1987). Furthermore, in rat insulinon.. cells a good correlation exists between the aftnity of these drugs for specific sulphonylurea b z&rag sites and their ability to block ATP-sensit ;tt3 K channels (SchmidAntomarchi et al., 1987). ATP-sensitive K channels in pancreatic &cells can also be opened by galanin, a polypeptide hormone. This effect is associated with an increase in s6Rb efflux and membrane hyperpolarisation, changes which are susceptible to blockade by glibenclamide (De Weille et al., 1988). In addition, recent electrophysiological studies have demonstrated that in the presence of physiological concentrations of ATP,, d&oxide and cromakalim (at

c 1990 Elsevier Science Publishers B.V. (Biomedical Division)

36

entrations e.g. ) 100 ~~) can also open nnels (Sturgess et al., 1988; Ziinkler et unne et al., 1989; 1990; but see Ashford et al., 1988). In contrast to pancreatic /3-cells, the ATP-sensitive K ~ha~~~el in cardiac and vascular sm~tb muscle is closed under resting physiological conditions and the channels open in response to a fall ATP], (Noma, 1983; Weiss and Lamp, 19P7; Standen et at.. 19g9). In cardiac muscle this response is associated with the shortening of action potential duration (Noma, 1983; Fosset et al., 1988). It has been suggested that cromakalim and RP49356 can open the cardiac ATP-sensitive K channel since both these agents shor!en action ~teRtia1 duration (Escande et al., 1988b: Mondot et al., 1988: S~guinetti et al., 1988). an effect which is susceptible to bfockade by glibenclamide (Esrande et al.. 198Sa: Sanguinetti et al., 1988; Fosset et al., 1988). In vascular smooth muscle the ATP-sensitive K channel is opened by cromakalim and this effect is inhibited by gli~ncla~de (Standen et al., 1989). In the present study we used glibenclamide to investigate the possible involvement of an ATPsensitive # charmel in the relaxant action of a number of K channel opening drugs in rat portal vein. The K channel openers used were cromakalim (BRL 34915. a benzopyran derivative), diazoxide (a benzothiadiazine) and RP49356 (thioformamide derivative), agents which belong to different chemical groups and show little structural si~l~ty (Edwards and Weston, 1989). In addition. to further clarify the role of ATP-sensitive M channels the effects of galanin and of reduction in [ATP]i were also studied. Some of these results have been presented to the British Pharmacolo~~al Society (Longmore et al., 1989; 19%).

2.1. Tissue bath studies The veins were mounted for isometric tension recording in a 20 ml organ bath containing oxygenated MOPS physiological salt solution (PSS) maintained at 37OC and pH 7.4 (Jetley and Weston, 1980). The veins were suspended under 0.5 g tension and allowed to equilibrate for 60 min after which time tension was adjusted to 0.5 g if necessary. Spontaneous mechanical activity was quantified using integrators (Hamilton et al., 1986). The integrated value obtained during a 5 min period of exposure to each drug concentration was expressed as a percentage of the value obtained in the 5 ruin period immediately prior to administration of the drug. In each experiment vehicle- and tim~match~ control tissues were used.

2.1.1. Experiment A Following the initial equilibration period, cumulative additions of ~romaka~m, RP49356, diazoxide and galanin were made at 5 min intervals. Concentration-effect experiments using cromakalim, RP49356 and d&oxide were repeated following a 30 min incubation with one of the following concentrations of glibencla~de: 0.1, 0.3 or 1.0 pM. Each portal vein was exposed to only one of the relaxants under test and to one concentration of glibenclamide. 2.1.2. Experiment B Following the initial equi~bration period, tissues were incubated in a modified MOPS PSS buffer (DG buffer) containing 0 mM glucose, 1 mM 2-deoxyglucose and 0.24 pg/ml oligomycin for 2-3 h. During this incubation period the DG buffer was changed every 20 min. Tissues were then exposed to glibenclamide (1 FM) for 1 h. Cromalcalim was then added in a cumulative fashion at 5 min intervals.

aterlals and methods 2.2. %b e&&u studies Portal veins each appro~mately 2 cm in length were obtained from male Sprague-Dawley rats (300-450 g) which were killed by stunning and bleeding. The animals were supplied by the University of Nanchester Animal Unit.

Each portal vein was suspended in a plastic vial containing 3 ml MOPS PSS at 37OC, pH 7.4 and aerated with 100% oxygen. Tissues were then transferred to vials containing 3 ml MOPS PSS

and 86Rb (5 pCi/mI) for a 90 min loading period. For experiments using DG buffer the tissues were incubated in this buffer throughout the loading period and the efflux collection periods. 86Rb was allowed to efflux from the tissues by transferring them, at 2 min intervals, to a series of vials containing MOPS PSS but no radioactivity {for details see Hamilton et al., 1986). The following drugs were present during 18-28 min of the efflwr; cromakalim (10 FM), RP49356 (10 PM), d&oxide (500 CM), galanin (10 nM), conclude (1 FM), cromakalim (10 FM) and ~bencl~de (1 FM), RP49356 (10 1rM) and glibenclamide (1 PM), diazoxide (500 b&M)and glibenclamide (1 FM) or vehicle control (ethanol or DMSO). The efflux data were expressed in terms of the efflux rate efficient (fractions loss of %Rb from the tissue) standardized for a 1 min period, expressed as a percentage of that in the tissue (Hamilton et al., 1986).

Portal veins were incubated in 80 ml of either normal MOPS PSS or DG buffer at 37OC pH 7.4 and aerated with 100% oxygen for 2.5 h. Tissues were then plunged into liquid nitrogen for IO s and allowed to thaw in 1 ml ice cold 20% perchloric acid. Each tissue was homogenized separately with a Potter glass/glass hcmogenizer. The homogenate was centrifuged at 3000 X g for 15 min at 4OC. The protein pellet was solubilized in 1 ml 1 M NaOH and 100 ~1 of the resulting solution was used for protein determination (Pierce Protein Assay Reagent, with bovine serum albumin as the standard). Of the supematant 250 ~1 was retained and the pH adjusted to pH 7.8 (using BDH Universal Indicator) with IO M KOH ~ntai~ng 0.5 M t~eth~ol~ne. The resulting potassium perchlorate precipitant was removed by centrifugation at 3000 X g for 15 min at 4OC. Of the neutralized supematant 200 ~1 was assayed for ATP using a biolu~n~~nce technique (Sigma). 2.4. Drugs and sofurions The following substances were used: ( f )cromakalim (Beecham); ( rf:)-RP49356 (Rhane-

Poulenc); d&oxide (Glaxo); galanin (Ba~hem); oligomycin (Sigma); 2-deoxyglucose (Sigma); MOPS (3-(N-morpholino)-propane sulphonic acid (Crdbiochem)); 86RbCl (Amersham). Stock solutions were prepared by dissolving cromakalim (10 mM) and RP49356 (IO mM) in 70% v/v ethanol:distilled water, d&oxide (10 mM) in DMSO (dimethyl sulphoxide; Sigma), galanin (0.1 mM) in double distilled water, glibenclamide (1 mM) in absolute ethanol, oligomycin (0.24 pg/ml) in 50% ethanol : distilled water, 2-deoxyglucose was added directly to glucose free MOPS PSS. The composition of MOPS PSS is listed in Hamilton et al. (1986). 2.5. Analysis of data The effects of cromakalim, RP49356, d&oxide and galanin on 86Rb efflux were analyzed separately using two-factor analysis of vari 0.05). Incubation in DG buffer reduced [ATP]; from 0.54 + 0.009 pg/mg (contro1) to G-024+ 0.~ jtgjmg prokin (n = 4; P < 0.05).

1

225

li)

.

.

10

100

I

,1

1 [Galanln],

nY

Fig. 3. Rat portal vein: effects of gala&t ( on spontaneous mechanical activity. Ordinate: % initial control value. Abscissa: concentration of galanin on a logarithmic scale. Points show mean values (n = 6) and vertical bars signify f S.E.M. vahres.

de

Ol__ 0

.Ol

.l

[Cromakalimf,

10

1

100

p

Fig. 4. Rat portal vein: effects of cromakalim on mechanical activity in the presence of glibenclamide (1 phi) in DG buffer ) and normal MOPS PSS (0). For details see fig. I legend.

3.2.

drugs on *‘Rb The effects of t e m~ifyin~ ef~~~ are s~owu in figs. 5 and 6. ANWA reveaiied sig~~f~~~Rt overall- main effects of drug using cromakalim t in experiments treat = 17.4). ~~493~6 (F(3.2lO~ = 5.35). diW(3 ~ox~de ~~(3,210~ = 21.2) and galanin (F(1,140) = 4.~~~. ~rom~ai~m, ~~493~~ arId diazoxide sig~~f~cant~y increased %b effhtx and this effect was antagonist by ~~beucla~de. In normal MOPS g~~~~c~~~de alone had no significant effect on e loss of %b efflux from rat postal vein. For 5 and 6. In t‘ es incubated in caused a small, nc~am~de (I $ but st~t~st~~a~~y ~o~-s~gn~f~~atlt decrease in the al level 0.73 It 0.0% com(n = 4) following 10 min ~~~~bation with g~be~c~a~de; Student’s t-test paired ~om~a~so~ (P = 0.084).

225

t.25

s E

0.25

. RP49356

~~ibe~c~a~de (0.1-W $¶w) alone produced a concentration-dependent increase in spontaneous rn~ba~c~ activity of rat portal vein. This effect was cbara~te~zed by increases in the amplitude and duration of contractions with little change in baseline tension. This observation is not entirely consistent with those of previous studies in rat

S

;z

12

36

20

24

28

32

36

Tlmr Into ettlux imln) Fig. 5. Effects of K channel openers and ~~ncla~de on the toss of %b from rat portal vein. (A) Cromakalim (IO $H) . ~~nc~~rn~d~ {I $&) alone (a). cromakal~ (10 gl.ibenclamide (1 PM) (0); (B) RP49356 (10 PM) ~~~cl~~e (1 PM) alone (0). RP49356 (10 NM) and glibenclamide (1 PM) (01; (Cl d&oxide (500 CM) alone )1~~ncla~de (1 FM) alone (0). diazoxide (500 CM) and ~~~ncla~de (XFM) (ok (A) shows vahtes for vehicle (ethanol) control. Ordinate: efflux rate coefficient (% per min); absicissa: time into efflux (min). Points show mean values (n = 6) and vertical bars signify + S.E.M. For reasons of clarity not all fS.EM. values are shown. The horizontal bar indicates the timfe period over which the rn~if~n~ drugs were present. Asterisks denote significant differences from control values (P < 0.05, ANOVA, Student’s t-test with Dunnett’s correction for multipPe comparisons). + Denotes significant differences from d&oxide (100 PM) atone (Studentized Rangetest).

ii

s

0.50

12

16

20 24 26 Tlmc Inlo rfflux (ml@

32

36

ai

i

c E "0 E 8 a E

125

1.00

0.75

-_

is

Further channels lack of (present

0.50

14

12

16

20 Tlma

Fig. 6. Effect of galanin

24

26

32

36

Into oiflux(mln)

) and vehicle control (0) on the loss

of 86Rb from rat portal vein. For details see fig. 5 legend.

portal vein. Winquist et al. (1989) showed that whilst glibenclamide (0.3 PM) increased spontaneous activity no such effect was observed at concentrations of l-10 PM. Furthermore, Buckingham et al. (1989) reported that glibenclamide was ineffective over a 0.3-3 PM concentration range. There is no immediate explanation for this discrepancy, although it may reflect differences in the manner in which spontaneous activity was assessed and also the different solvents for glibenclamide: in the present study ethanol was used and intregrated mechanical activity (which incorporates duration, frequency and amplitude of contraction) was employed to quantify spontaneous activity. In previous studies, DMSO (which has a marked relaxant effect, see fig. 1) was used (Winq ;t et al., 1989; Buckingham et al., 1989) and the contractile amplitude alone was the indicator of spontaneous mechanical activity (Winquist et al., 1989). The characteristic contractile effect of glibenclamide observed in the present study (increased wave duration and amplitude but not of baseline tension) may reflect a preferential action of this drug on pacemaker cells (see also below ‘efflux studies*). Since baseline tension is closely related to membrane potential (Bolton, 1979) the lack of effect of glibenclamide on this parameter suggests that glibenclamide did not cause significant membrane depolarisation in this tissue consistent with preliminary electrophysiological studies in our laboratory (McHarg, personal communication).

support for a preferential blockade of K on pacemaker cells is provided by the effect of glibenclamide on @‘Rb efflux study) or 42K efflux (Edwards and Weston, personal communication) at conantrations which produce marked changes in mechanical activity. Since the number of pacemaker cells is small in comparison to the total number of smooth muscle cells in rat portal vein, changes in *6Rb efflux from these cells make only a small contribution to the overall loss of 86Rb from the tissue. The three K channel openers tested each produced complete inhibition of spontaneous activity. The order of potency was cromakalim > RP49356 > d&oxide with an approximate ratio of 1, 2.9 and 141, when IC,, values were compared. These potency ratios are comparable to those previously reported for cromakalim and diazoxide in rat portal vein (Wir quist et al., 1989) and rat aorta (Newgreen et al., 1989) and for cromakalim and RP49356 in guinea-pig pulmonary artery (Eltze, 1989). In pancreatic /3-cells, glibenclamide is an effective inhibitor of ATP-sensitive K channels in the nanomolar concentration range (De Weille et al., 1988). If a similar/identical K channel is involved in the relaxant actions of cromakalim, d&oxide and RP49356 in rat portal vein, the concentrations of glibenclamide used in the present study should produce quite large changes in the positions of the concentration-effect curves for these drugs. However, the observed shifts were relatively small and this was reflected by small changes in the respective I&, values. These changes in the IC, values were comparable to those previously reported for rat portal vein (Winquist et al., 1989; Buckingham et al., 1989) and other vascular tissues (Newgreen et al., 1989; Wilson, 1989; Eltze, 1989). In spite of the structural diversity between cromakalim, RP49356 and d&oxide (Edwards and Weston, 1989), the present study showed that glibenclamide (0.1 and 0.3 PM) had similar inhibitory effects on the relaxant actions of all three agents. Thk could suggest that the three agents i*%eract with the same site(s) and that this interaction is inhibited by glibenclamide. Alternatively it is passib]e that the three agents act at different sites on

the same K.channel and that glibenclamide acts at a distal location. However. glibenclamide (1 PM) pr~uced a comparatively larger shift in the IC,, value for cromakalim (16.8-fold shift) than for RP49356 and d&oxide (6.1-fold and 4.6-fold shifts respectively: see table 1). Therefore it is possible that cromakalim, particularly at high concentrations may interact with an additional site which is more sensitive to the effects of glibenclamide. The conce: trations of glibenclamide used in the present study were more comparable to those required to block ATP-sensitive K channels in cardiac muscle (Mondot et al., 1988; Sanguinetti et al., 1989; Escande et al.. 1988a). Based on the order and absolute affinity of sulphonylureas for specific binding sites. it has been suggested that the cardiac K channel is similar but not identical to that in the pancreas (Fosset et al., 1988). A similar argument may apply to vascular smooth muscle. In rat aorta Quast and Webster (1989) showed that the ability of sulphonylureas to inhibit cromakalim-stimulated increases in “Rb efflux correlated with the ability of these drugs to inhibit [ ‘H]glibenclamide binding in pancreatic /%-cells.However, Standen et al. (1989) have shown that the ATP-dependent K channel in vascular smooth muscle differs from that found in some other tissues in that it has a higher conductance. Thus, it is possible that any similarities between the effects of ghbenclamide in the pancreas and in cardiac and vascular tissues reflect siinilarities in the ‘sulphonylurea recognition site’ rather than the channel itself. The results of the efflux studies show that cromakalim (10 CM). RP49356 (10 FM) and diazoxide (500 PM) can open a group of K channels which are permeable to Rb and susceptible to blockade by glibenclamide. The relevance of this channel in the relaxant actions of cromakalim and RP49356 is unclear. Glibenclamide (1 FM) markedly inhibited the rises in 86Rb efflux evoked by cromakalim (10 PM) and RP49356 (10 PM). However, these concentrations of the K channel openers were still capable of causing almost complete relaxation in the presence of 1 PM ghbenclamide (see fig. 1). K channels have been shown to differ in their permeabilitites to K and

Rb (Quast and Baumlin, 1988; Longmore et al., 1990) and it is possible that in the presence of glibenclamide (1 PM) the relaxant effects were associated with the opening of a K channel which was relatively impermeable to Rb. In pancreatic j%cells, galanin causes the opening of the ATP-sensitive K channel (De Weille et al., 1988). If a similar ATP-dependent K channel exists in rat portal vein then galanin should cause relaxation and generally mimic the effects of K channel openers. However, galanin produced increases in contractile frequency and amplitude and also baseline tension. Previous studies in mammalian smooth muscle have shown the effects of galanin are diverse. In rabbit femoral artery, femoral vein or basilar artery galanin has no effect, whereas it produces contraction of guinea-pig trachea, rat aorta and rat intestine and causes inhibition of spontaneous activity in smooth muscle strips isolated from canine small intestine (Rijkaeus, 1987; Longmore et al., 1989). These effects are mediated via a specific galanin receptor (Muramatsu and Yanihara, 1988). In the present study galanin also produced a small increase in *6Rb efflux which may reflect the direct opening of a K channel. However, it more likely reflects the indirect opening of K channels which are activated by membrane depolatisation and/or calcium entry into the cells. In conditions of lowered [ATP], the spontaneous activity of portal vein was diminished but it could be partially restored by glibenclamide. This observation is analogous to that reported by Fosset et al. (1988) who showed that in guinea-pig cardiac muscle cells glibenclamide (50 nM) could partially reverse the shortening of action potential duration induced by depletion of [ATP]i. These authors proposed that this finding was consistent with the opening of ATP-sensitive K channels which were susceptible to blockade by glibenclamide. A similar argument could be used to interpret the observations made in the present study. Indeed, in rat and rabbit mesenteric arteries an ATP-sensitive K channel has been described (Standen et al., 1989) although a similar channel has yet to identified in rat portal vein. However, it is possible that restoration of spontaneous mechanical activity by glibenclamide represents an effect

83

on pacemaker cells, since under normal conditions glibenclamide caused a 50% increase in mechanicat activity (see fig. 2). This seems unlikely since under normal conditions the mechanoexcitatory effects of glibenclamide are not assoc;iated with any detectable change in 86Rb efflux, whereas under ATP-depleted conditions a 10 min exposure to ‘benclamide caused a reduction, albeit small, lFh in “Rb effhuc indicating K channel blockade. It is ossible that a greater effect of glibenclamide on E Rb efflux would have been seen follo~g a 1 h incubation period, as was used in the tissue bath experiments. IIowever, in the present study it was not possible to monitor flux changes over such long time periods. In the present study “ghbenclamide-restored spontaneous activity was abolished by cromakalim. This effect was accompanied by an approximate 2-fold decrease in the IC,, value for cromakalim in ATP-depleted tissues. Such ~tentiation of the action of cronl~~rn might be relevant ‘in viva’ in conditions of hypoxia. Indeed studies by Harder et al. (1987) and Angersbach and Nicholson (1988) have indicated that the effects of K channel opening drugs are marked during or following hypoxia. In conclusion, the present results suggest that ATP-sensitive K ch,annels may exist in rat portal vein and these channels are involved in the relaxant actions of crom~a~~n, RP49356 and diazoxide. In consideration of the relatively high concentrations of glibenclamide required to exert inhibitory effects, the channel may possess a ‘sulphonylurea recognition site’ which is similar, although not identical, to that associated with the ATP-sensitive K channel in pancreatic &ds.

and other compounds enhancing membrane K+ umductance, but not by Ca2+ antagonists or hydrahtzine, in an Attica] model of o%uhtsive arterial disease, NaunynSchmiedeb.Arch Pharmacot. 337,341. Ashford. M.L.J., C.N. Hales and R.Z. Kozlowski, 1988, Di-tide but not BRL 34915, activates ATP-sensitive potap sium channels in a rat insulinoma cell fine, J. Physiol. 409, 53P. Bucki@am. R.E.. T.C. Hamilton, D.R. Howlett, S. Mootoo and C. Wilson, 1989, inhibition by giibenclamide of the vasorelaxant action of cromakalim in the rat, Br. J. Pharnlau3L 97, 57. De Weihe, J., H. ~~d-~torn~c~, M. Fosset and M. Ladunski. 1988, ATP-sensitive K+ channels that are blocked by hypogyi~mia-indu~in8 sulfonylureas in insulin-secreting cells are activated by &min, a byperglycemia-inducing hormone, Proc. Nat;. Acad. Sci. U.S.A. 85. 1312. Dunne. M.J.. R.J. Aspinall and O.H. Peterson. 1990, The effects of cromakahm on ATP-sensitive potassium channels in insulin secretin ceBs, Br, J. Pharmacol. 99, 169. Dunne, M-J.. D.I. Yule, D.U. Gatlacher and 0-H. Pe)lerson, 1989. Cromakahm (BRL 34915) and d&oxide activate ATP-regulated potassium channels in insulin secreting ceBs, Pfhigers Arch. 414 (Suppl. I), S154. Edwards, G. and A.H. Weston, 1989, Potassium c&nnef openers: their development and prospects, Curr. Ctrdiovast. Patents 1, 1810. Eltze, M.. 1989, Glibenclamide is a competitive antagonist of cromakalim, pinacidil and RP49356 in guine.tl-pig pulmonary artery, European J. Pharmacol. 165. 231. Escande, D., D, Thuringer, M. Laville, J. Courteix and I. Cavero, 1988a. RP49356 is a potent opener of ATP-modulated potassium channels in cardiac myocytes. Br. J. Rafael. 95,814P. E&xnde. D.. D. ~M~ger, S. Leguern and 1. Cavero, 1988h. The potassium channel opener cromakalim (BRL 34915) activates ATP-dependent K+ channels in isolated wdiac myocytes, Biochem. Biophys. Res. Commun. 154.620. Findlay, J., M.J. Dunne and O.H. Peterson, 1985, ATP-sensitive iuward rectifier and voltage- and calcium-activated K+ channels in cultured pancreatic islet celts, J. Membr. Biol. 88, 15% Fosset, M., J.R. De Weille. R.D. Green, H. S&mid-Antomarchi

and iti, La&m&i, 1988. Antidiabetic sutphonylureas control action potential properties in heart tells via h&h affm-

J.L. was supported by a grant from Rh&ne Poulenc and D.T.N. was supported by a SERC CASE award.

References

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CD. Nicholson, 1988, Enhancement of muscle blood cell fhtx and pOt by crorn~~~ (BRL 34915)

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~&tlar mechanism for cerebral vasospasm after experimentd subarachnoid hemorrhage in the do.g+ European J. Pharmacol. 80.875. Jetley, M. and A.H. Weston, 1980, Some effects of sodium

) and nifedipine on H. Weston. 1990. The relalionromakalim and diazoxide on K srnvutb muscle, Br. J. Pharmacol. 99. 3P. benclamide

in rat portal

and I. Cavero. 1988, activating properties. Br. J. Pharmacol. 95. d13P. ~4~ramats~. 1. and N. Yanaihara. 1988, Contribution of galanin to non-cholinergic. non-adrenergic transmission in rat ileum, Br. J. Pharmacol. 94. 1241. J. Longmore and A.H. Weston. 1988. The nchnide on the action of cromakalim, diminoxidil sulphate on rat aorta. Br. J. armacol. 96. 116P. Noma. A.. 1983. ATP-regulated K + channels in cardiac muscle, ure 305, 14’7. W. and Y. daubs. 1988, Compatison of the effluxes of + and %Rbf elicited by cromakalim (BRL 34915) in tome and phasic vascular tissue, Naunyn-Schmiedeb. Arch. Pharmacol. 338, 319. Quast. U. and C. Webster. 1989. Sulfonylureas and tetraethylmodem ions as inhibitors of cromakahm-stimulated efflux. Naunyn-Schmiedeb. Arch. Pharmacol. 339.

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1988. BRL 34915 (cromakalim) activates ATP-sensitive K+ current in cardiac muscle. Proc. Natl. Acad. Sci. U.S.A. 85, 8360. Schmid-Antomarchi, H.. J. De Weille. M. Fosset and M. Lazdunski, 1987. The receptor for antidiabetic sulfonylureas controls tfx activity of the ATP-modulated K+ channel in insulin-secreting cells, J. Biol. Chem. 262. 15840. Standen. N.B.. J.M. Quayle. N.W. Davies, J.E. Brayden. Y. Huan; and M.T. Nelson, 1989. Hyperpolarising vasodilators activate ATP-sensitive K+ channels in arterial smooth muscle. Science 245. 177. Sturgess. N.C.. R.Z. Kozlowski, C.A. Carington. C.N. Hales and M.L.F. Ashford. 1988. Effects of sulphonylureas and d&oxide on insulin secretion and nucleotide-sensitive channels in an insulin-secreting cell line. Br. J. Pharmacol. 95. 83. Weiss. J.N. and S.T. Lamp, 1987, Glycolysis preferentially inhibits ATP-sensitive K+ channels in isolated guinea-pig cardiac myocytes. Science 238, 67. Wilson. C.. 1989, Inhibition by sulphonylureas of vasorelaxation-induced by potassium channel activators in vitro, J. Autonom. Pharmacot. 9, 71. Winquist, R.J.. L.A. &aney, A.A. Wallace, E.P. Baskin, R.B. Stein, M.L. Garcia and G.J. Kaczorowski, 1989, Glyburide blocks the relaxation response to BRL 34915 (cromakahm), minoxidil sulfate and diazoxide in vascular smooth muscle, J. Pharmacol. Exp Ther. 248, 149. Zunfder, B.J., S. Lenzen, K. Manner, U. Pauten and G. Trube, 1988. Concentration-dependent effects of tolbutamide. meglitimide. glipizide, glibenclarnide and d&oxide on ATP-regulated K+ currents in pancreatic ~-cells. NaunyttSchmiedeb. Arch. Pharmacol. 337, 225.

Effects of cromakalim, RP49356, diazoxide, glibenclamide and galanin in rat portal vein.

The study investigated the possible involvement of an ATP-sensitive potassium (K) channel in the relaxant actions of K channel openers in rat portal v...
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