269 scorned, but it has proved very successful in this particular instance where no nonpeptide tachykinin antagonists have been produced as a result of rational drug-design strategies. In future, increasing automation of radioligand binding assay techniques, permitting screening of thousands of compounds per week, coupled with computerized data capture/analysis and eventual commercial availability of cloned human receptors transfected into stable cell lines, will ensure that targeted screening of compound libraries will remain at the forefront of many drug discovery programmes. A nonpeptide antagonist for the NK, receptor cannot, therefore, be far away. K. J. WATLING
Parke-Davis Neuroscience Resiarcir Centre, Addenbrooke’s Hospital Site, Hills Road, Cambridge, UK CB2 2QB.
References
1 Guard, S. and Watson, S. P. (1991) Neurochem. ht. 18, 149-165 2 Nakanishi, S. (1991) Annu. Rev. Neurosci. 14, 123-136 3 Pemow, B. (1983) Phannacol. Rev. 35, 85-141 4 Maggio, J. E. (1988) Annu. Rev. Neurosci. 11,13-28 5 Snider, R. M. et al. (1991) Science 251, 435-437 6 McLean, S. et al. (1991) Science 251, 437-439 7 Snider, R. M., Longo, K. P., Drozda. S. E., Lowe, J. A. III-and Lee&m, S. E: (1991) Proc. Nut! Acud. Sci. USA 88, 10042-10044 8 Lecci, A., Giuliani, S., Patacchini, R., Viti, G. and Maggi, C. A. (1991) Neurosci. Lett. 129, 299-302 9 Birch, I’. J., Harrison, S. M., Hayes, A. G., Rogers, H. and Tyers, M. B. (1992) Br. j. Phannacof. 105,508-510 10 Guard. S. and Watline;. K. I. Br. 1. Phannbcol. (in press) y. _ ’ 11 Lembeck, F., Donnerer, J., Tsuchiya, M. and Nagahisa, A. (1992) Br. 1. Pharmncol. 105,527-530 12 Radhakrishnan, V. and Henry, J. L. (1991) Neurosci. Lett. 132, 39-43 13 De Koninck, Y. and Henry, J. L.
Kf channel openers and ‘natural’cardioprotection Kf channel openers are principally characterized by their ability to relax vascular smooth muscle via the opening of plasmalemma Kf channels (for review see Ref. 1). Because low doses of such agents increase coronary blood flow in experimental models, much research has been directed towards exploiting them for the treatment of angina*. More recently, pharmacologists have provided substantial evidence on their ability actively to protect the jeopardized myocardium during acute ischaemia3. Cardioprotection from ischaemia has been observed for various K+ channel openers having diverse chemical strucincluding nicorandi15, tures4, cromakalim6, pinacidi17 and aprikalims (formerly RP52891), and thus appears to be characteristic of this class of drugs, even though they are not all equally efficacious as cardioprotective agents. In parallel with this finding, cellular physiologists have recently accumulated compelling evidence that plasmalemma K+ those particularly channels, regulated by intracellular ATP concentration, come into play to prevent irreversible cell damage
in the early stages of an ischaemic insult7,9. It is the strengthening of this natural mechanism that appears to be the basis for the cardioprotective activity of K+ channel openers. Activation of ATP-sensitive K+ channels in ischaemia In 1983, NomalO identified a novel type of Kf channel in cardiac cells, characterized by its sensitivity to intracellular ATP, which acts as a natural blocker. In the normally oxygenated heart, these ATP-sensitive K+ channels remain closed because the intracellular ATP content is high. Rapid activation of these channels during experimental hypoxia generates a large repolarizing current that, by shortening the action potential, leads to early contractile failure”. These events are readily prevented by the sulfonylurea glibenclamide11*12,a specific blocker of ATPsensitive K’ channels in the heart”. Ischaemia, however, is a more complex pathological situation than experimental hypoxia, because during ischaemia cardiac myocytes not only undergo oxygen deprivation but also suffer from the interruption of the perfusion.
(1991) Proc. Nut! Acud. Sci. USA 88, 11344-11348 14 Lei, Y-H., Barnes, I’. J. and Rogers, D. F. (i992j Irr. 1. Pharmucol. 105.261-262 15 Delay-Goyet, P. and Lundberg, J. M. (1991) Eur. J. Phartnacol. 203,157-158 16 Garret, C. et al. (1991) Proc. Nat1 Acud. Sci. USA 88,10208-10212 17 Gitter, B. D. et al. (1991) Eur. J_ Pharmucol. 197,237~238 18 Beresford, I. J. M., Birch, P. J_, Hagan, R. M. and Ireland, S. J. (1991) Br. J. Phanrzacol. 104,292-293 19 Watlmg, K. J.,.Guard, S., Howson, W. and Walton, L. (1991) Br. 1. Pharmacol. 104,27P 20 Fardin, V., Foucault, F., Bock, M. D. and Garret, C. (1992) Br. J. Pharmucol. 105, 8OP 21 Venepalli, B. R. et al. (1992) 1. Med. Chem. 35.374-378 22 Appell, k. C., Fragile, B. J., Loscig, J., Singh, S. and Tomczuk, B. E. (1992) Mol. Ph&nacol. 41,77%778. 23 Aimone, L. D., Appell, K. C., Chippari, S. C.. Harris, A. L. and Ward, S. 1.11991) Sot. NeurosCi. Abstr. 17,320.g . . ’ 24 Advenier; C. et al. (1992) Br. 1. Phurmucol. 105, 77TJ 25 Emonds-Ah, X. et al. (1992) Life Sci. 50, PLl&PL106
Thus, not only does the action potential shorten, but K’ ions accumulate in the interstitial spaces14. The exact sequence of the events comprising early ischaemia is still a matter of conjecture7, but one scheme is outlined in Fig. 1, whereby ischaemia causes opening of ATP-sensitive K+ channels, the ultimate consequence being that the electrical and mechanical activity in the ischaemic zone is rapidly arrested. If, as seems likely, the mechanisms described in Fig. 1 operate during ischaemia in. vivo, they would prolong celI survival by conserving energy for the active extrusion of intracellular Ca” and Na+, a process upon which cell integrity and viability depend. Glibenclamide, which reduces both action potential shortening’s9 and extracellular K+ accumulation9*U and which delays tension decline7V9,impairs post-ischaemic recovery of mechanical function9,15 (increased cytosolic free ADP accounts for sulfonylureas being only partially effective in blocking cellular K+ loss during ischaemia?. K+ channel openers afford cardioproteclion At concentrations that do not myocardial function, depress cromakalim, pinacidil, aprikalim O1992,ElsevierSciencePublishenLtd(UK)
TiPS -July 2992 /Vol. 231
270
descending coronary artery” (the model), myocardium stunned nicorandil and aprikalim also improved the recovery of depressed myocardial contractility within the ischaemic segment during reperfusion whereas, by contrast, glibenclamide or tolbutamide aggravated left ventricular dysfunction” (Fig. 2). That cardioprotection is directly related to the activation of K+ channels is supported by the finding that the cardioprotective effects of K+ channel openers are prevented by pretreatment with glibenclamide, irrespective of the model used3. Finally, these drugs can be presumed to affect the myocardium directly because cardioprotection can occur in the absence of significant haemodynamic changes8. More recently”, the antiischaemic effects of nicorandil confirmed clinically in were patients undergoing percutaneous angiotransluminal coronary plasty. During this procedure, the time that elapsed before signs of ischaemia were observed in the ECG was prolonged by 63% after administration of intravenous nicorandil. Under the same experimental conditions nitrate was ineffective. glyceryhri-
“I
arrest of
myocardial cell
rapidanestof etecbfcat autvtty
AK+ UlU
$+
’
Fi@. 1. Bcheme of natural cellular mechanisms affording myocardialprotection at the earty stage of iechaemia Uppef traces in each section represent action potential mon&ngs in the ischaemic region. The arrest of coronary flow and resultant critical ted&ton of oxygen de&red to the ischaemic zone leads to a moderate decrease in rntracelhrlarATP, an increase in intracellular ADP and a fall in intracellularp/f. Because tntrac&lular ACP and cytosolicprvtons counteract the channel closure evoked by ATP, theiiknxease durtngischaemia means thatintracellular ATP has to fall less significantly before ATP-sensittve K’ chainels open [Lederer, W. J. and Nichols, C. G. (1999) J. Physiil. (Lond.) 419. 1X3-212]. Eztracellular adenostne concentration also increases duringischaemia and this, in turn, can open ATP-sensitive K+ channels via activation of a G protein, if intracellular ATP is reduced (Kirsch, G. E. et al. (1999) Am. J. Physiol. 259 H82O-H826]. ActkWon of ATP-sensittve K+ channels leads to a large repolanzing current that shortens the duration of the action potential. As a consequence of reduced tissue perfusion, K+ ions leaving the cells via the opened channels accumulate in the extracellular space, thereby reducing the force driving them to leave the celland resulttngin cell depolarization: the ultimate consequence of action potential shortening and cell depolarization is a rapid arrest of electrical and mechanics1 acthrity in the ischaemic zone.
and nicorandil significantly reduce the damage caused by transient global ischaemia in perfused rat heart in vitro, as shown by an improvement in the post-ischaemic mechanical function, an inhibition of contra&me formation, a reduction in lactate dehydrogenase release and a preservation of intracellular stocks of ATP.
In dogs submitted to prolonged coronary occlusion followed by reperfusion, nicorandil or aprikalim given systemically at doses producing minor haemodynamic effects reduced infarct size by 30 to 56%8*17.By contrast, glibenclamide increased infarct size by 38%~~. In anaesthetized dogs undergoing transient ligation of the left anterior
Strengthening the natural cardioprotective mechanism The molecular target for K+ channel openers in cardiac cells is the ATP-sensitive K+ channelz0,21. The effects of the drugs on these channels is to shift to the right the channel-opening versus ATP concentration curve22-24 (Fig. 3). Thus, the channels will assume the operative state at a higher ATP concentration in the presence of the drug than in its absence, an effect that is also produced by low pH or elevated intracellular ADP16, as described in Fig. 1. Therefore, a tentative explanation for the anti-ischaemic activity of low concentrations of K+ channel openers in the heart is as follows: in the presence of the drug, the number of activated AT&sensitive K” channels at the onset of ischaemia is sufficient to accelerate the shortening of the action potential and to speed up the loss of contractile activity within the ischaemic zone’ even though, in the absence of ischaemia, these drugs at the same concentrations fail to activate appreciably K+
TiPS -July
1992 [Vol. 131
271
anism was recently shown to be susceptible to @er&mide blockad@ and may be mediated by the release of adenosine from the ischaemic myocardium~. Irr this regard, one can suggest that therapy with K4 channel openera may afford a permanent ‘chemical preconditioning’ which confers on the heart the extraordinary ability to better withswtd transient oxygen deprivation and co~~u~~~s~~~~ during acute myocardial Mar&m. Acknowledpnenls
We are gratefttfto Dr 1. Findfay for helpful discusson and to Karen Pepper for help with the manuscript.
channelsz* and thus do not modify baseline ektrical and mechanicat fl3l-l~Ol.l.
Finally, there are striking analogies between the cardioprotection conferred by K’” channel
openers
and
ischaemic
pre-
conditioning, e-g. increased tolerante of cardiac myocytes to an ordinarily lethal ischaemic insult, achieved by an initial brief exposure to ischaemia. This mech-
Laborafoire de Physiolagie Cell&ire, Blit. 443. Univtife’ Paris-Xl, 91405 Otsay and
*IWne-Poulenc Rarer Central Research, CRVA, 13 pui &ks Gzwsde,BP 24,94403 t&plr-seine,
RP49356
References 1 Weston, A.
ATP t -~~__-_~____~_--~--~I_c
+
f
0
-
10s
I
10 pA
1.0
0.0 -rri
1
VP
10+
1o-4
10-3
ATP concentration
Fw.
10-2
10-l
(mM)
Fe. 3. the K* ch~neio~r RP4S356(a raivwmtcform of whtchapntrat&nis the active ~~ decreases tha A~-~ of ATP-sensifive K* &am&s. IWwez effecloflWlrnnATPonfKfAT~~chamtcikfrwnan~ottpatcfireorrrdad~~ and duringtheir stimulatiun try 30 @ fttWW.56. Bath and pipette sotuttonsixwtained 140 m&f KCt, Hotding potential: -40 mV, B&w: dose-rasponse relationa for ATP block@ kthw, channels in the absence (0) or p~?san# (s) of RP49356 (3tJphf). The open pmbabit& In each ATP so&ton w&snormatkd by nsfarenceto its maxtmum vaksztn #~~ so&t&k N&ias 8ra maans f SEM. Data wwa abtabd kam 81 &s&out @a&k. Fmm F&f.22, w&hpermWm.
H. and Ed&, G. (1992) Bitlcketa. PltIz?wracot. 43.47-54 2 Gross. G. (1991) in Carrent Dnrgs Paf~~~C~~~M~~~~~J_~~), pp. 82-92, Current Patent Ltd 3 Grover, G. J. (1991) in Currcnf Drugs: Potassium ChannelModulatorsParr, I. J., ea.), pp. 29-38, Current Patent Ltd 4 Edwards* G. and Weston, A. W. (1%) m&5 Pkarwca$. Sci_ llz 4X7-422 5 Gross,G. J., pieper, G. M.. Farber,N. E, W&tier, D. C. and Hard, H. (1989) Am. J. Car&l, 63, llJ-17J 6 Gmver, G. J. et nl. (1991) J. Pharmacol. Exp. 2&r. 257,136-162 7 Cole, W. C,, McPherson, C. D. and Sontag, D. (19QX)C&c. Res. 69, S7r-58x 8 Aucharnpach, J. A., Manryama, M.6, Cavem, I and Gross, G. J. (1991) J. PhamlacoLEaQ.I-her. us, Q61-967 9 Gasser, R. N. A. and Vaughan-Jones, R. D. (1990) J. PkysioL &?a&.) &I, 7X3-741 X0 Noma, A. (1983) Nafzrre 3@S, 147-l@ 11 Deutsch, N., Klitzner, T. S., Lamp. S. T. and Weiss, J, N. (1991) Am. 1. Physiot. 261, H671-H676 X2 z. A. M. ef aI. (1990) Circ. Rcs. 67, 13 Fosset, M., de We&z, J- R, Green, R D., Schkd-Antomar&. H. and Lazduns?& M. (1988) J. Biol. Chem. 263,7%X+-7936 14 KlBber, A. C. (1984) 1. Mol. Cell. Cardiot. 16,389-394 15 Gross, G. J., Auchampach, J- A- 4 EAanr)ramrt,I# (1%) CirculrrfiDn 82 (m)z @ (Abstr.) 16 Venkatesh, N., Lamp, S+?: and Weiss, J. N. (1991) Circ. Rcs. 69,623-h37 17 Endo, T. et al. (1988) J. Csrdiovasc. Pharmacol. X2,587-592 18 Gmss, G. J., pieper, G. M. andWarlti‘e+,
lx c. f1887)J. Ca&iawsc. Pkunn~. IQ (Suppl. a), S76-84
2X5 -Jr&f 1992 fVol. 131
272 19 Saih, S. et al. (1991) J. Am. Coil. Cardiof. 17,377A 20 Escande, D. et al. (1989) PJliigen Arch. 414,669-675 21 Qua&, U. and Cook, N. S. (1989) Trends Pkarmaco?. Sci. 10.431-43S
22 Thuringer,
D. and Escande, D. (1989) Mol. Pharmncol. 36, 897-902 23 Nakayama, K., Fan, Z., Marumo, F. and Hiraoka, M. (1990) Circ. Res. 67, 1124-1133 24 Ripoll, C., Lederer, W. J. and Nichols,
C. G. (199O)j. Phannacoi. Exp. Ther. 255, 429-435 25 Gross, G. J. and Auchampach, J. A. (1992) Circ. Res. 70,223-233 26 Liu, G. S. et aI. (1991) C~rc~~at~ont34, 350-356
This and That: hair pigments, the hypoxic basis of life and the Virgilia~ journey of the spermatozoon WISDOM is that we are an oxygen-dependent species, requiring an oxygen-rich environment for survival. However, as is usually the case, received wisdom is guilty of an over-simplification. The relationship between 0, and life can be viewed from another perspective, one rooted in evolutionary historv. Life evolved under anaerobic, reductive conditions. Since then, -the world has become fatal unless the sufferer is rapidly oxygen-rich, and reductive enbrought to a lower altitude. At vkmments are now limited to 30000 feet, an unadapted person such locations as the sulfureta will die in 2 minutes from oxygen found in bogs, mud flats, voldeficiency.The effectsof 4 depricanoes, and oceanic depths,
THE FUXXIVED
and, on a smaller but more frequently repeated scale, in the rumen of sheep and the guts of insectsf. Life in such environmentsrevolvesaround a sulfatesulfide redox system. Higher forms of life are based on 02 and the controlled oxidation of energy-yielding substrates. Yet, as the composition of plasma carries an atavistic biochemical memory of our evolution from a marine environment, so an examination of celluiar biochemistry yields evidence of the anoxic origins of life. Dry air at sea level (760 mmHg pressure) contains a partial pressure of 4 (Pod of around 16OmmHg. As we ascend from sea level, the PO* drops and we experience more and more physiological difficulties. At 10000 feet, POZis 110 mmHg. Without acclim-
ation, the climber is giddy and light-headed. At 20000 feet, the PO:!is 70 mmHg, and the climber
relies on tanked oxygen. Otherwise, he will suffer acute mountain sickness, characterized by headache, nausea, sickness, loss of
appetite, sleeping difficulty and periodic breathing. Pulmonary edema may occur, which can be
vation have been well described by the old balloonists*. The highest site of habitation by adapted people on the globe is Aucanquilcha, a mine in Peru at 20000 feet. This is the twilight zone; humans cannot adapt to permanent life below half of atmospheric pressure, equivalent to a PO2 of 80 mmHg. It appears indubitable from this that humans require an O2 tension of 110 mmHg or more for a reasonably comfortable existence. The reason, however, that such a high 0~ tension is required is that mammals have evolved exceptionally efficient barriers to protect the cellular milieu from the high ambient O2 levels existing a few centimeters away. Between the air that is inhaled and the mitochondrion - the site of oxidative phosphorylation where the O2 is consumed - 02 tension drops from 160 mmHg to around 2 mmHg or lower (Fig. 1). This is equivalent to the O2 tension in dry air at 100000 feet. The 4 tension existing on top of Mount Everest, the highest point on the globe, is equivalent to that found in blood on the venous side of the capillaries. Without protection from external 4, therefore, the
pressure of oxygen, even at an
altitude far above the top of Everest, is high enough to be cytotoxic. Cardiac myoglobin has an OZ affinity of 1.3 mmHg (Ref. Cardiomyocytes function 3). normally down to 02 tensions of around 0.2 mmHg. The ultimate site of cellular oxidation is a protected, rarified milieu, with an O2 tension equivalent to that at 19 miles (30 km) above sea level. These calculations involve a simplification in that the external environment is assumed to contain dry air, whereas the partial pressure of water at sea level is
47 mmHg. sturation of inhaled air in the airways is largely responsible for the fall in Po2 in the
trachea and bronchi. The basic fact remains, however, that to duplicate in the physical world the biological 02 gradient between the body surface and the interior of a mitochondrion a few millimeters away requires an ascent of many miles. Even within the attenuated environment of the cell, the mitochondrion is under intense oxidative stress. Each mitochondrion generates about 10’ superoxide radicals per day. Mitochondria protect themselves from the oxidative damage these radicals cause by rapid turnover. Per cell, about 100 mitochondria per day undergo lysosomal digestion. As mito~ondria have a half-life considerably shorter than that of their host cells, there is a functional advantage to them having their own DNA. Oxygen and its partially reduced radicals, superoxide, peroxide and hydroxyl, are so cytotoxic that in the course of evolution effective barriers have developed to protect
Parke-Davis Neuroscience Resiarcir Centre, Addenbrooke’s Hospital Site, Hills Road, Cambridge, UK CB2 2QB.
References
1 Guard, S. and Watson, S. P. (1991) Neurochem. ht. 18, 149-165 2 Nakanishi, S. (1991) Annu. Rev. Neurosci. 14, 123-136 3 Pemow, B. (1983) Phannacol. Rev. 35, 85-141 4 Maggio, J. E. (1988) Annu. Rev. Neurosci. 11,13-28 5 Snider, R. M. et al. (1991) Science 251, 435-437 6 McLean, S. et al. (1991) Science 251, 437-439 7 Snider, R. M., Longo, K. P., Drozda. S. E., Lowe, J. A. III-and Lee&m, S. E: (1991) Proc. Nut! Acud. Sci. USA 88, 10042-10044 8 Lecci, A., Giuliani, S., Patacchini, R., Viti, G. and Maggi, C. A. (1991) Neurosci. Lett. 129, 299-302 9 Birch, I’. J., Harrison, S. M., Hayes, A. G., Rogers, H. and Tyers, M. B. (1992) Br. j. Phannacof. 105,508-510 10 Guard. S. and Watline;. K. I. Br. 1. Phannbcol. (in press) y. _ ’ 11 Lembeck, F., Donnerer, J., Tsuchiya, M. and Nagahisa, A. (1992) Br. 1. Pharmncol. 105,527-530 12 Radhakrishnan, V. and Henry, J. L. (1991) Neurosci. Lett. 132, 39-43 13 De Koninck, Y. and Henry, J. L.
Kf channel openers and ‘natural’cardioprotection Kf channel openers are principally characterized by their ability to relax vascular smooth muscle via the opening of plasmalemma Kf channels (for review see Ref. 1). Because low doses of such agents increase coronary blood flow in experimental models, much research has been directed towards exploiting them for the treatment of angina*. More recently, pharmacologists have provided substantial evidence on their ability actively to protect the jeopardized myocardium during acute ischaemia3. Cardioprotection from ischaemia has been observed for various K+ channel openers having diverse chemical strucincluding nicorandi15, tures4, cromakalim6, pinacidi17 and aprikalims (formerly RP52891), and thus appears to be characteristic of this class of drugs, even though they are not all equally efficacious as cardioprotective agents. In parallel with this finding, cellular physiologists have recently accumulated compelling evidence that plasmalemma K+ those particularly channels, regulated by intracellular ATP concentration, come into play to prevent irreversible cell damage
in the early stages of an ischaemic insult7,9. It is the strengthening of this natural mechanism that appears to be the basis for the cardioprotective activity of K+ channel openers. Activation of ATP-sensitive K+ channels in ischaemia In 1983, NomalO identified a novel type of Kf channel in cardiac cells, characterized by its sensitivity to intracellular ATP, which acts as a natural blocker. In the normally oxygenated heart, these ATP-sensitive K+ channels remain closed because the intracellular ATP content is high. Rapid activation of these channels during experimental hypoxia generates a large repolarizing current that, by shortening the action potential, leads to early contractile failure”. These events are readily prevented by the sulfonylurea glibenclamide11*12,a specific blocker of ATPsensitive K’ channels in the heart”. Ischaemia, however, is a more complex pathological situation than experimental hypoxia, because during ischaemia cardiac myocytes not only undergo oxygen deprivation but also suffer from the interruption of the perfusion.
(1991) Proc. Nut! Acud. Sci. USA 88, 11344-11348 14 Lei, Y-H., Barnes, I’. J. and Rogers, D. F. (i992j Irr. 1. Pharmucol. 105.261-262 15 Delay-Goyet, P. and Lundberg, J. M. (1991) Eur. J. Phartnacol. 203,157-158 16 Garret, C. et al. (1991) Proc. Nat1 Acud. Sci. USA 88,10208-10212 17 Gitter, B. D. et al. (1991) Eur. J_ Pharmucol. 197,237~238 18 Beresford, I. J. M., Birch, P. J_, Hagan, R. M. and Ireland, S. J. (1991) Br. J. Phanrzacol. 104,292-293 19 Watlmg, K. J.,.Guard, S., Howson, W. and Walton, L. (1991) Br. 1. Pharmacol. 104,27P 20 Fardin, V., Foucault, F., Bock, M. D. and Garret, C. (1992) Br. J. Pharmucol. 105, 8OP 21 Venepalli, B. R. et al. (1992) 1. Med. Chem. 35.374-378 22 Appell, k. C., Fragile, B. J., Loscig, J., Singh, S. and Tomczuk, B. E. (1992) Mol. Ph&nacol. 41,77%778. 23 Aimone, L. D., Appell, K. C., Chippari, S. C.. Harris, A. L. and Ward, S. 1.11991) Sot. NeurosCi. Abstr. 17,320.g . . ’ 24 Advenier; C. et al. (1992) Br. 1. Phurmucol. 105, 77TJ 25 Emonds-Ah, X. et al. (1992) Life Sci. 50, PLl&PL106
Thus, not only does the action potential shorten, but K’ ions accumulate in the interstitial spaces14. The exact sequence of the events comprising early ischaemia is still a matter of conjecture7, but one scheme is outlined in Fig. 1, whereby ischaemia causes opening of ATP-sensitive K+ channels, the ultimate consequence being that the electrical and mechanical activity in the ischaemic zone is rapidly arrested. If, as seems likely, the mechanisms described in Fig. 1 operate during ischaemia in. vivo, they would prolong celI survival by conserving energy for the active extrusion of intracellular Ca” and Na+, a process upon which cell integrity and viability depend. Glibenclamide, which reduces both action potential shortening’s9 and extracellular K+ accumulation9*U and which delays tension decline7V9,impairs post-ischaemic recovery of mechanical function9,15 (increased cytosolic free ADP accounts for sulfonylureas being only partially effective in blocking cellular K+ loss during ischaemia?. K+ channel openers afford cardioproteclion At concentrations that do not myocardial function, depress cromakalim, pinacidil, aprikalim O1992,ElsevierSciencePublishenLtd(UK)
TiPS -July 2992 /Vol. 231
270
descending coronary artery” (the model), myocardium stunned nicorandil and aprikalim also improved the recovery of depressed myocardial contractility within the ischaemic segment during reperfusion whereas, by contrast, glibenclamide or tolbutamide aggravated left ventricular dysfunction” (Fig. 2). That cardioprotection is directly related to the activation of K+ channels is supported by the finding that the cardioprotective effects of K+ channel openers are prevented by pretreatment with glibenclamide, irrespective of the model used3. Finally, these drugs can be presumed to affect the myocardium directly because cardioprotection can occur in the absence of significant haemodynamic changes8. More recently”, the antiischaemic effects of nicorandil confirmed clinically in were patients undergoing percutaneous angiotransluminal coronary plasty. During this procedure, the time that elapsed before signs of ischaemia were observed in the ECG was prolonged by 63% after administration of intravenous nicorandil. Under the same experimental conditions nitrate was ineffective. glyceryhri-
“I
arrest of
myocardial cell
rapidanestof etecbfcat autvtty
AK+ UlU
$+
’
Fi@. 1. Bcheme of natural cellular mechanisms affording myocardialprotection at the earty stage of iechaemia Uppef traces in each section represent action potential mon&ngs in the ischaemic region. The arrest of coronary flow and resultant critical ted&ton of oxygen de&red to the ischaemic zone leads to a moderate decrease in rntracelhrlarATP, an increase in intracellular ADP and a fall in intracellularp/f. Because tntrac&lular ACP and cytosolicprvtons counteract the channel closure evoked by ATP, theiiknxease durtngischaemia means thatintracellular ATP has to fall less significantly before ATP-sensittve K’ chainels open [Lederer, W. J. and Nichols, C. G. (1999) J. Physiil. (Lond.) 419. 1X3-212]. Eztracellular adenostne concentration also increases duringischaemia and this, in turn, can open ATP-sensitive K+ channels via activation of a G protein, if intracellular ATP is reduced (Kirsch, G. E. et al. (1999) Am. J. Physiol. 259 H82O-H826]. ActkWon of ATP-sensittve K+ channels leads to a large repolanzing current that shortens the duration of the action potential. As a consequence of reduced tissue perfusion, K+ ions leaving the cells via the opened channels accumulate in the extracellular space, thereby reducing the force driving them to leave the celland resulttngin cell depolarization: the ultimate consequence of action potential shortening and cell depolarization is a rapid arrest of electrical and mechanics1 acthrity in the ischaemic zone.
and nicorandil significantly reduce the damage caused by transient global ischaemia in perfused rat heart in vitro, as shown by an improvement in the post-ischaemic mechanical function, an inhibition of contra&me formation, a reduction in lactate dehydrogenase release and a preservation of intracellular stocks of ATP.
In dogs submitted to prolonged coronary occlusion followed by reperfusion, nicorandil or aprikalim given systemically at doses producing minor haemodynamic effects reduced infarct size by 30 to 56%8*17.By contrast, glibenclamide increased infarct size by 38%~~. In anaesthetized dogs undergoing transient ligation of the left anterior
Strengthening the natural cardioprotective mechanism The molecular target for K+ channel openers in cardiac cells is the ATP-sensitive K+ channelz0,21. The effects of the drugs on these channels is to shift to the right the channel-opening versus ATP concentration curve22-24 (Fig. 3). Thus, the channels will assume the operative state at a higher ATP concentration in the presence of the drug than in its absence, an effect that is also produced by low pH or elevated intracellular ADP16, as described in Fig. 1. Therefore, a tentative explanation for the anti-ischaemic activity of low concentrations of K+ channel openers in the heart is as follows: in the presence of the drug, the number of activated AT&sensitive K” channels at the onset of ischaemia is sufficient to accelerate the shortening of the action potential and to speed up the loss of contractile activity within the ischaemic zone’ even though, in the absence of ischaemia, these drugs at the same concentrations fail to activate appreciably K+
TiPS -July
1992 [Vol. 131
271
anism was recently shown to be susceptible to @er&mide blockad@ and may be mediated by the release of adenosine from the ischaemic myocardium~. Irr this regard, one can suggest that therapy with K4 channel openera may afford a permanent ‘chemical preconditioning’ which confers on the heart the extraordinary ability to better withswtd transient oxygen deprivation and co~~u~~~s~~~~ during acute myocardial Mar&m. Acknowledpnenls
We are gratefttfto Dr 1. Findfay for helpful discusson and to Karen Pepper for help with the manuscript.
channelsz* and thus do not modify baseline ektrical and mechanicat fl3l-l~Ol.l.
Finally, there are striking analogies between the cardioprotection conferred by K’” channel
openers
and
ischaemic
pre-
conditioning, e-g. increased tolerante of cardiac myocytes to an ordinarily lethal ischaemic insult, achieved by an initial brief exposure to ischaemia. This mech-
Laborafoire de Physiolagie Cell&ire, Blit. 443. Univtife’ Paris-Xl, 91405 Otsay and
*IWne-Poulenc Rarer Central Research, CRVA, 13 pui &ks Gzwsde,BP 24,94403 t&plr-seine,
RP49356
References 1 Weston, A.
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+
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0
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I
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1
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Fe. 3. the K* ch~neio~r RP4S356(a raivwmtcform of whtchapntrat&nis the active ~~ decreases tha A~-~ of ATP-sensifive K* &am&s. IWwez effecloflWlrnnATPonfKfAT~~chamtcikfrwnan~ottpatcfireorrrdad~~ and duringtheir stimulatiun try 30 @ fttWW.56. Bath and pipette sotuttonsixwtained 140 m&f KCt, Hotding potential: -40 mV, B&w: dose-rasponse relationa for ATP block@ kthw, channels in the absence (0) or p~?san# (s) of RP49356 (3tJphf). The open pmbabit& In each ATP so&ton w&snormatkd by nsfarenceto its maxtmum vaksztn #~~ so&t&k N&ias 8ra maans f SEM. Data wwa abtabd kam 81 &s&out @a&k. Fmm F&f.22, w&hpermWm.
H. and Ed&, G. (1992) Bitlcketa. PltIz?wracot. 43.47-54 2 Gross. G. (1991) in Carrent Dnrgs Paf~~~C~~~M~~~~~J_~~), pp. 82-92, Current Patent Ltd 3 Grover, G. J. (1991) in Currcnf Drugs: Potassium ChannelModulatorsParr, I. J., ea.), pp. 29-38, Current Patent Ltd 4 Edwards* G. and Weston, A. W. (1%) m&5 Pkarwca$. Sci_ llz 4X7-422 5 Gross,G. J., pieper, G. M.. Farber,N. E, W&tier, D. C. and Hard, H. (1989) Am. J. Car&l, 63, llJ-17J 6 Gmver, G. J. et nl. (1991) J. Pharmacol. Exp. 2&r. 257,136-162 7 Cole, W. C,, McPherson, C. D. and Sontag, D. (19QX)C&c. Res. 69, S7r-58x 8 Aucharnpach, J. A., Manryama, M.6, Cavem, I and Gross, G. J. (1991) J. PhamlacoLEaQ.I-her. us, Q61-967 9 Gasser, R. N. A. and Vaughan-Jones, R. D. (1990) J. PkysioL &?a&.) &I, 7X3-741 X0 Noma, A. (1983) Nafzrre 3@S, 147-l@ 11 Deutsch, N., Klitzner, T. S., Lamp. S. T. and Weiss, J, N. (1991) Am. 1. Physiot. 261, H671-H676 X2 z. A. M. ef aI. (1990) Circ. Rcs. 67, 13 Fosset, M., de We&z, J- R, Green, R D., Schkd-Antomar&. H. and Lazduns?& M. (1988) J. Biol. Chem. 263,7%X+-7936 14 KlBber, A. C. (1984) 1. Mol. Cell. Cardiot. 16,389-394 15 Gross, G. J., Auchampach, J- A- 4 EAanr)ramrt,I# (1%) CirculrrfiDn 82 (m)z @ (Abstr.) 16 Venkatesh, N., Lamp, S+?: and Weiss, J. N. (1991) Circ. Rcs. 69,623-h37 17 Endo, T. et al. (1988) J. Csrdiovasc. Pharmacol. X2,587-592 18 Gmss, G. J., pieper, G. M. andWarlti‘e+,
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272 19 Saih, S. et al. (1991) J. Am. Coil. Cardiof. 17,377A 20 Escande, D. et al. (1989) PJliigen Arch. 414,669-675 21 Qua&, U. and Cook, N. S. (1989) Trends Pkarmaco?. Sci. 10.431-43S
22 Thuringer,
D. and Escande, D. (1989) Mol. Pharmncol. 36, 897-902 23 Nakayama, K., Fan, Z., Marumo, F. and Hiraoka, M. (1990) Circ. Res. 67, 1124-1133 24 Ripoll, C., Lederer, W. J. and Nichols,
C. G. (199O)j. Phannacoi. Exp. Ther. 255, 429-435 25 Gross, G. J. and Auchampach, J. A. (1992) Circ. Res. 70,223-233 26 Liu, G. S. et aI. (1991) C~rc~~at~ont34, 350-356
This and That: hair pigments, the hypoxic basis of life and the Virgilia~ journey of the spermatozoon WISDOM is that we are an oxygen-dependent species, requiring an oxygen-rich environment for survival. However, as is usually the case, received wisdom is guilty of an over-simplification. The relationship between 0, and life can be viewed from another perspective, one rooted in evolutionary historv. Life evolved under anaerobic, reductive conditions. Since then, -the world has become fatal unless the sufferer is rapidly oxygen-rich, and reductive enbrought to a lower altitude. At vkmments are now limited to 30000 feet, an unadapted person such locations as the sulfureta will die in 2 minutes from oxygen found in bogs, mud flats, voldeficiency.The effectsof 4 depricanoes, and oceanic depths,
THE FUXXIVED
and, on a smaller but more frequently repeated scale, in the rumen of sheep and the guts of insectsf. Life in such environmentsrevolvesaround a sulfatesulfide redox system. Higher forms of life are based on 02 and the controlled oxidation of energy-yielding substrates. Yet, as the composition of plasma carries an atavistic biochemical memory of our evolution from a marine environment, so an examination of celluiar biochemistry yields evidence of the anoxic origins of life. Dry air at sea level (760 mmHg pressure) contains a partial pressure of 4 (Pod of around 16OmmHg. As we ascend from sea level, the PO* drops and we experience more and more physiological difficulties. At 10000 feet, POZis 110 mmHg. Without acclim-
ation, the climber is giddy and light-headed. At 20000 feet, the PO:!is 70 mmHg, and the climber
relies on tanked oxygen. Otherwise, he will suffer acute mountain sickness, characterized by headache, nausea, sickness, loss of
appetite, sleeping difficulty and periodic breathing. Pulmonary edema may occur, which can be
vation have been well described by the old balloonists*. The highest site of habitation by adapted people on the globe is Aucanquilcha, a mine in Peru at 20000 feet. This is the twilight zone; humans cannot adapt to permanent life below half of atmospheric pressure, equivalent to a PO2 of 80 mmHg. It appears indubitable from this that humans require an O2 tension of 110 mmHg or more for a reasonably comfortable existence. The reason, however, that such a high 0~ tension is required is that mammals have evolved exceptionally efficient barriers to protect the cellular milieu from the high ambient O2 levels existing a few centimeters away. Between the air that is inhaled and the mitochondrion - the site of oxidative phosphorylation where the O2 is consumed - 02 tension drops from 160 mmHg to around 2 mmHg or lower (Fig. 1). This is equivalent to the O2 tension in dry air at 100000 feet. The 4 tension existing on top of Mount Everest, the highest point on the globe, is equivalent to that found in blood on the venous side of the capillaries. Without protection from external 4, therefore, the
pressure of oxygen, even at an
altitude far above the top of Everest, is high enough to be cytotoxic. Cardiac myoglobin has an OZ affinity of 1.3 mmHg (Ref. Cardiomyocytes function 3). normally down to 02 tensions of around 0.2 mmHg. The ultimate site of cellular oxidation is a protected, rarified milieu, with an O2 tension equivalent to that at 19 miles (30 km) above sea level. These calculations involve a simplification in that the external environment is assumed to contain dry air, whereas the partial pressure of water at sea level is
47 mmHg. sturation of inhaled air in the airways is largely responsible for the fall in Po2 in the
trachea and bronchi. The basic fact remains, however, that to duplicate in the physical world the biological 02 gradient between the body surface and the interior of a mitochondrion a few millimeters away requires an ascent of many miles. Even within the attenuated environment of the cell, the mitochondrion is under intense oxidative stress. Each mitochondrion generates about 10’ superoxide radicals per day. Mitochondria protect themselves from the oxidative damage these radicals cause by rapid turnover. Per cell, about 100 mitochondria per day undergo lysosomal digestion. As mito~ondria have a half-life considerably shorter than that of their host cells, there is a functional advantage to them having their own DNA. Oxygen and its partially reduced radicals, superoxide, peroxide and hydroxyl, are so cytotoxic that in the course of evolution effective barriers have developed to protect
