Cromakalim, nicorandil and pinacidil: Novel drugs which open potassium channels in smooth muscle

Gen. Pharmac. Vol. 20, No. I, pp. 1-9, 1989

0306-3623/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc

Printed in Great Britain. All rights reserved

MINIREVIEW CROMAKALIM, NICORANDIL A N D PINACIDIL: NOVEL DRUGS WHICH OPEN POTASSIUM CHANNELS IN SMOOTH MUSCLE T. C. HAMILTONl and A. H. WESTON 2 IBeecham Pharmaceuticals Research Division, Medicinal Research Centre, Coldharbour Road, Harlow, Essex, CM19 5AD [Tel: (0279) 622101] and 2Smooth Muscle Research Group, Department of Physiological Sciences, Medical School, University of Manchester, Manchester Ml3 9PT, U.K. (Received 30 October 1987; received for publication 17 June 1988)

cidil, as drugs which enhance K+-efflux in smooth muscle, and the evidence for the type(s) of K+-channels affected by these drugs. Additionally, pharmacological studies, both in vitro and in vivo, have suggested a number of potential therapeutic applications of these drugs in disorders of smooth muscle and these are briefly reviewed.

INTRODUCTION

The modulation of K+-channel opening is not a novel physiological mechanism. Inhibition of contraction of the mammalian heart by acetylcholine and relaxation of smooth muscle of the gastro-intestinal tract by noradrenaline are examples of inhibitory effects resulting from K ÷ channel opening (Sakmann et al., 1983; Bulbring and Tomita, 1987). However, the pharmacological and clinical exploitation of this mechanism using synthetic molecules is a recent and novel development and stems from experimental findings with cromakalim, nicorandil and pinacidil; their chemical structures are shown in Fig. 1. The novel amidochromanol, cromakalim (Ashwood et al., 1984, 1986) lowers blood pressure in animal models by relaxing blood vessels (Buckingham et al., 1986b). Mechanistic studies have shown that cromakalim hyperpolarises vascular smooth muscle by opening K+-channels and increasing K+-conductance (Hamilton et al., 1986; Weir and Weston, 1986b). These results extended to vascular smooth muscle the electrophysiological findings in cardiac tissue which suggested enhancement of outward K+-conductance by cromakalim (Cain and Metzler, 1985). The effects of the nicotinamide ester, nicorandil, on the coronary circulation in the dog were described by Uchida et al. (1978). Subsequently, nicorandil was shown to increase membrane K ÷-conductance in vascular smooth muscle cells (Furukawa et al., 1981; Itoh et al., 1981; Karashima et al., 1982; Inoue et al., 1983, 1984; Weir and Weston, 1986b) and also to activate soluble guanylate cyclase (Holzmann, 1983; Schmidt et al., 1985). Thus, nicorandil has mixed actions contributing to its smooth muscle relaxant activity. Recently, a retrospective examination of the vasorelaxant properties of the cyanoguanidine derivative, pinacidil (Petersen et al., 1978; Ahnfelt-Ronne, 1988) has revealed that this antihypertensive agent hyperpolarises vascular smooth muscle by opening K+-channels (Bray et al., 1987; Southerton et al., 1987a). The purpose of this article is to review the mechanism of action of cromakalim, nicorandil and pina-

GENERAL

In vivo experiments The early work of Uchida et al. (1978) in anaesthetised dogs established that nicorandil produced a marked increase in coronary blood flow. Comparison of the effects of nicorandil with those of other nitro-vasodilators, primarily glyceryl trinitrate, showed that a broad similarity exists in vivo between the effects of nicorandil and those of glyceryl trinitrate. Although some reduction in systemic blood pressure is seen, this effect is transient and the predominant action of nicorandil is to increase coronary blood flow under a variety of conditions (Sakai et al., 1981; Shiraki et al., 1981; Abiko et al., 1984; Korb et al., 1985; Kuromaru and Sakai, 1986; Lamping et al., 1984; Sakanashi et al., 1986; Shimshak et al., 1986). In contrast to nicorandil, the in vivo effects of both cromakalim and pinacidil in rats, cats and dogs are characterised by dose-related reductions in systemic blood pressure, cromakalim being up to 10 times more potent by the oral route than pinacidil (Longman et al., 1988). A comparison of the effects of cromakalim with those of the Ca :+ channel blocker, nifedipine, showed that cromakalim was more potent as a blood pressure lowering drug and produced less reflex tachycardia than nifedipine (Buckingham et al., 1986b). In the cat, the changes in arterial blood flow induced by cromakalim included enhanced renal blood flow whereas nifedipine caused slight renal vasoconstriction. Pinacidil has a similar profile to nifedipine in these models (Longman et al., 1988). Cohen (1986) has reviewed the preclinical pharmacology of pinacidil and reported that this drug is up to 3 times more potent than hydralazine as an • anti-hypertensive agent. For both cromakalim (Buck1

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PHARMACOLOGY

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Experiments have been carried out using several types of smooth muscle including various blood vessels, bronchial and gastro-intestinal preparations. In all tissues, cromakalim, nicorandil and pinacidil are capable of inhibiting spontaneous tone and of reducing agonist induced contractions. However, although nicorandil is capable of relaxing contractions produced by both low ( < 2 0 m M ) and high (> 80 mM) concentrations of KCI, cromakalim and pinacidil are only effective against increased tension produced by low (~<30mM) KCI concentrations; contractions generated by KCI />80mM are essentially unaffected by these two agents (Allen et al., 1986a; Clapham and Wilson, 1987; Hamilton et al., 1986; Weir and Weston, 1986a, b). As described for their anti-hypertensive activity, the vasorelaxant properties of cromakalim and pinacidil also reside primarily in the ( - ) enantiomers (Buckingham et al., 1986a; Bray et al., 1987) and strongly suggest that these drugs have a stereospecific mechanism of action. MODE OF ACTION OF CROMAKALIM, NICORANDIL AND PINACIDIL: EVIDENCE FOR M O D U L A T I O N OF K+-CHANNEL OPENING

Electrophysiological

data

Early microelectrode experiments in a variety of isolated smooth muscles with nicorandil (Furukawa et al., 1981), cromakalim (Hamilton et al., 1986) and pinacidil (Southerton et al., 1987a) indicated that these agents were capable of modulating K+-channel opening. Observations were extended to a variety of smooth muscle-containing tissues using microelectrode recordings, often with simultaneous estimation of membrane resistance. Exposure to nicorandil produced a hyperpolarisation in guineapig mesenteric and portal vein, in rat portal vein, in pig and canine mesenteric arteries and in pig and guinea-pig coronary arteries. In several of these tissues, a decrease in membrane resistance was observed and on-going spontaneous electrical discharges are abolished (Furukawa et al., 1981; Itoh

1984; Weir and Weston, 1986b). In comparative experiments, the nitro vasodilators, sodium nitroprusside and glyceryl trinitrate, had little effect on membrane potential or membrane resistance, indicating that the hyperpolarising actions of nicorandil were distinct from those associated with typical nitro-vasodilators (Ito et al., 1978; Karashima, 1980; Taylor et al., 1988). The K+-channel opening action of nicorandil is not restricted to vascular smooth muscle. Similar membrane effects have also been observed in dog and guinea-pig trachea, and in guinea-pig small intestine (Inoue et al., 1983; Yamanaka et al., 1985; Allen et aL, 1986b). In vascular smooth muscle, cromakalim has a hyperpolarising effect, as evidenced in rat portal vein and rabbit mesenteric artery (Fig. 2), rat and rabbit aortae and rabbit pulmonary artery (Hamilton et al., 1986; Kreye et al., 1987a, b; Southerton et al., 1987b; Taylor et al., 1988). In the rat portal vein, the effects of cromakalim were particularly marked. Spontaneous electrical discharges were abolished and the membrane potential increased to a value very close to the potassium equilibrium potential (EK) (Hamilton et al., 1986). In guinea-pig taenia caeci and trachea, and in rat uterus, inhibition of on-going electrical activity and hyperpolarisation were also observed (Allen et al., 1986a; Hollingsworth et al., 1987; McHarg, Weir and Weston, unpublished). Pinacidil has only recently been recognised as a K+-channel opener (Southerton et al., 1987a) and the electrophysiological effects of this drug have been less widely studied than those of cromakalim or nicorandil. However, in rat portal vein, spontaneous electrical activity was abolished and, in this tissue and in rat aorta, a marked hyperpolarisation was produced (Bray et al., 1987; Southerton et al., 1987a, 1988). Some effects of cromakalim, nicorandil and pinacidil on K+-channels in cardiac muscle have also been detected. The changes (negative inotropic) are observed at concentrations higher than those required to produce K+-channel opening in vascular smooth muscle (Grosset and Hicks, 1986; Cohen and Colbert, 1986; Hamilton et al., 1987). They have reinforced the view that K +-channel opening plays a significant part in the actions of these compounds in vivo. The actions of nicorandil on cardiac muscle have been more extensively studied. In canine atrial and Purkinje fibres, the most striking action of nicorandil is a shortening of action potential duration, qualitatively similar to that produced by muscarinic agonists (Yanagisawa and Taira, 1980, 1981; Imanishi et al., 1983). This action has now been studied in single ventricular cells and is associated with the opening of a previously unknown K+-channel (Kakei et al., 1986). An early report (Cain and Metzler, 1985) described the shortening of action potential duration by cromakalim in guinea-pig papillary muscle and this has recently been confirmed (Scholtysik, 1987). Similar effects on cardiac action potentials have been described in another study in which the effects of pinacidil on canine Purkinje and ventricular fibres were compared with those of nicorandil and cromakalim (Steinberg et al., 1988). All three agents

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Fig. 2. Effects of BRL 34915 (cromakalim) on membrane potential in rabbit mesenteric artery (a and b) and on membrane potential and mechanical activity in rat portal vein (c and d). Different cells in the same segment of rabbit mesenteric artery were used with washout of BRL 34915 between a and b (McHarg and Weston, unpublished observations). For rat portal vein, membrane potential was recorded from continuous impalement of a single cell, tension being recorded in the stretched tissue; BRL 34915 was washed out between c and d (from Hamilton et al., 1986, with permission). were found to produce major shortening of action potential duration. Ion f l u x experiments

mechanical inhibition. The possible reasons for this are discussed in a later section. Hamilton et al. (1987) have also shown that cromakalim stimulates 42Kefflux in rabbit mesenteric artery.

The electrophysiological changes produced in smooth muscle by cromakalim, nicorandil and pinacidil are strongly indicative of an action on K+-channels. Such a view is supported by the shortening of the cardiac action potential described above. However, stimulation of smooth muscle Na÷/ K÷-ATPase could theoretically account for some of the observed effects of these substances. For this reason, measurements of K+-efflux have been performed in a variety of tissues to characterise further the actions of cromakalim, pinacidil and nicorandil. Virtually all experiments have been carried out using S6Rb as a K÷-marker. Although Smith et al. (1986) have shown that the use of S6Rb results in an underestimate of K÷-flux, S6Rb is a widely used substitute, especially when absolute flux values are unimportant (Imaizumi and Watanabe, 1981; Bolton and Clapp, 1984; Weir and Weston, 1986b). In rat portal vein, and guinea-pig trachea and taenia caeci, nicorandil produced an increase in 86Rb efflux (Allen et al., 1986b; Weir and Weston, 1986a, b). Similar changes in S6Rb eft]ux were produced by cromakalim and pinacidil in the portal vein (Fig. 3), taenia caeci and trachea of the guinea-pig, in the aorta and portal vein of the rat, and in the aorta, pulmonary, ear and mesenteric arteries of the rabbit (Allen et al., 1986a; Hamilton et aL, 1986; Weir and Weston, 1986a, b; Bray et al., 1987; Coldwell and Howlett, 1986, 1987; Cook et al., 1988; Kreye et aL, 1987a, b; Quast, 1987; Southerton et al., 1987a, b, 1988). In general, these changes in S6Rb efflux are only produced by concentrations of cromakalim, nicorandil and pinacidil which produce substantial

Effects o f K +-channel blockers The K ÷-channel blocking drugs, tetraethylammonium (TEA), procaine, 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP) are relatively non-selective for individual types of K+-channel. Thus, in a whole tissue experiment (as opposed to a single channel investigation), the apparent ability of these agents to modify tissue responses needs cautious interpretation. The blocking agents may themselves cause a contraction, due presumably to the blockade of certain K÷-channels open at rest. Any inhibitory effects of these substances on the responses to putative K+-channel activators could therefore be the result of functional antagonism. In dog mesenteric artery and trachea and in guinea-pig trachea, the hyperpolarising effects of nicorandil were abolished by exposure to TEA (Inoue et al., 1983; Allen et al., 1986b). In guinea-pig trachea, the electrical and mechanical effects of cromakalim were antagonised by TEA, procaine and 4-AP (Allen et al., 1986a). The action of cromakalim on S6Rb efflux in guinea-pig portal vein was antagonised by TEA and by 3,4-DAP (Quast, 1987) and in rabbit mesenteric artery by TEA, but not apamin (Coldwell and Howlett, 1987). In rat portal vein, the inhibitory effects of cromakalim and pinacidil on mechanical activity were blocked by TEA or procaine (Southerton and Weston, 1987). In rabbit mesenteric artery pre-contracted by KCI or noradrenaline, the relaxant effects of cromakalim were inhibited by TEA, procaine and 4-AP (Wilson, 1987). These effects do not in themselves prove that cromakalim,

4

T.C. HAMILTONand A. H. WESTON a

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Fig. 3. Effects of pinacidil 10 #M (Pin) on 86Rb etflux from (a) rat aorta and (b) rat portal vein. Ordinate scale; 86Rbefflux rate co-efficient expressed as % min-~. Abscisca scale--time (min) after start of the efflux period. Tissues were exposed to Pin between the 14th and 22nd min of the efflux period. Broken lines show ~6Rb etflux from control tissues. Each point is the mean derived from 6 experiments: vertical bars show SEM values. From Weston et al. (1988) with permission.

nicorandil and pinacidil interact with K+-channels. However, the data are consistent with this view. Reduction in mechanical responses to KCI

Physiological salt solutions (PSS) containing raised K+-concentrations have been widely used for the assessment of Ca 2÷-entry blocking drugs in vitro. The depolarisation of smooth muscle cells in this simple way allows the opening of voltage-dependent Ca2+-channels in a variety of smooth muscles (Hof and Vuorela, 1983). High K+-solutions can also be used to provide evidence in favour, or otherwise, of a K÷-channel opening action. If a tissue is exposed to a putative K+-channel opening drug in sufficient concentration, the potassium permeability of the membrane will dominate all other ion permeabilities, provided that the K+-channel is able to remain open under conditions of raised membrane potential. The cells will assume a membrane potential at or close to the theoretical EK value, provided no compensatory mechanism is activated. Addition of K ÷ to the PSS will lower the membrane potential to a new EK but a contraction will only be produced when the new E K is less negative than the potential at which the voltagedependent Ca2+-channel opens. Although this potential is not known for certain, it probably lies between - 4 0 and - 5 0 m V (Bolton et al., 1984; Hamilton et al., 1986). Thus, a K+-channel opener should be able to relax a contraction produced by addition of KCI, 20 mM (E K approximately - 5 0 mV), but have no effect on a contraction produced by KCI, 80 mM (E K approx. - 2 0 mV). Such a phenomenon is, in fact, observed with both cromakalim and pinacidil, but not with nicorandil, which produces substantial relaxation of 80 mM KCI contractions (see General Pharmacology, in vitro experiments). These observations are consistent with the view that cromakalim and pinacidil are quite selective K+-channel modulators whilst nicorandil exerts both a K+-channel opening and additional inhibitory effects (see next section).

Effects on adenylate cyclase, guanylate cyclase and Na+/K + ATPase

Although, as detailed in previous sections, nicorandil is capable of opening K÷-channels and hyperpolarising smooth muscle cells, several pieces of evidence strongly suggest that this drug also possesses additional actions. In guinea-pig mesenteric artery, nicorandil produces mechanical inhibition without a detectable change in membrane potential (Itoh et al., 1981) and Weir and Weston (1986b) observed that, in rat aorta, the drug produced mechanical inhibition without an increase in 86Rb efflux. Furthermore, nicorandil is capable of inhibiting contractions produced by KCI ( > 8 0 m M ) (Inoue et al., 1983; Allen et al., 1986b; Weir and Weston, 1986b; Clapham and Wilson, 1987). In guinea-pig trachea, Allen et al. (1986b) observed that, when the hyperpolarising effects of nicorandil were blocked by procaine, mechanical inhibition by nicorandil was little affected. Evidence now exists that the additional inhibitory action possessed by nicorandil is associated with stimulation of guanylate cyclase activity (Holzman, 1983; Schmidt et al., 1985; Sumimoto et al., 1987). Although the mechanism by which guanylate cyclase relaxes smooth muscle is not known for certain, it seems likely that cyclic G M P together with its G-kinase enhances Ca 2÷ pumping at the smooth muscle plasma membrane, reducing intracellular Ca2+-concentrations (Suematsu et al., 1984). In contrast, neither cromakalim nor pinacidil has any effect on guanylate cyclase or adenylate cyclase as determined from measurement of tissue cyclic G M P or cyclic A M P concentrations (Kauffman et al., 1986; Coldwell and Howlett, 1987; Taylor et al., 1988). Furthermore, cromakalim has no effect on the activity of rat purified brain Na+/K+-ATPase (Southerton, unpublished). TYPE OF

OF

K+-CHANNEL

CROMAKALIM,

INVOLVED NICORANDIL

IN AND

THE

ACTIONS

PINACIDIL

No clear picture of the type of K+-channel involved in the action of these agents has yet emerged.

Cromakalim, nicorandil and pinacidil on smooth muscle K ÷ channels On the basis of their observations in rat portal vein, Hamilton et al. (1986) speculated that cromakalim might open at least two types of K+-channel. One of these, a pacemaker channel, seemed to be opened at low concentrations of cromakalim, whilst a second channel required higher concentrations of this agent. This idea has recently been supported by observations in guinea-pig portal vein (Quast, 1987) and in rat uterus (Hollingsworth et al., 1987). Since the effects of nicorandil and pinacidil in rat portal vein are qualitatively very similar to those of cromakalim (Weir and Weston, 1986b; Southerton et al., 1988), a similar argument could also apply to these agents. Some progress has been made towards identifying the different types of K+-channel and K÷-currents which can be recorded from isolated patches or isolated smooth muscle cells, respectively. The picture which emerges is complex but clearest when CaZ+-dependent K+-channels and K+-currents are considered. So far, three different Ca2+-dependent K+-channels have been identified in rabbit portal vein on the basis of their sensitivities to Ca 2+ and to TEA (Inoue et al., 1985, 1986). These channels have been designated KL, KM and Ks (large, medium and small, respectively) according to their unit conductances in symmetrical K+-conditions. A similar KL-type channel has been identified in rabbit jejunum and guinea-pig jejunum and mesenteric artery (Benham et al., 1985, 1986). Using a combination of whole tissue microelectrode recording and single cell voltage clamp methods, two Ca2+-dependent K+-currents have been identified in guinea-pig small intestine (Yamanaka et al., 1985; Nakao et al., 1986). One of these is activated by ATP and by the nonadrenergic, non-cholinergic inhibitory transmitter and is blocked by the bee venom toxin, apamin. Its single channel characteristics in smooth muscle have not yet been identified (Yamanaka et al., 1985). The other is associated with spike after-hyperpolarisation and is apamin-insensitive. It is likely, but not confirmed, that this current is carried by K L channels (Nakao et al., 1986). Several studies have shown that the actions of cromakalim, nicorandil and pinacidil are unaffected by apamin (Yamanaka et al., 1985; Allen, 1986a; Weir and Weston, 1986b; Bray et al., 1987; Coldwell and Howlett, 1987), thus eliminating the apaminsensitive K+-channel. Recently, it has been reported that cromakalim opens a Ca 2+-dependent K+-channel of the K1 type in cultured rabbit aortic cells (Kusano et al., 1987). Preliminary observations with pinacidil suggest that it, too, acts on a large conductance Ca2+-dependent K+-channel (Hermsmeyer, 1988). However, in contrast to the findings of Kusano et al. (1987), Beech and Bolton (1987) have recently shown that in isolated cells of rabbit portal vein, cromakalim evokes a sustained outward K+-current which is essentially calcium and voltage independent. K~--channels are closed at resting membrane potentials and probably function to repolarise cells since they are activated by an increase in intracellular Ca2+-concentration and by depolarising potentials. The opening of such a channel by cromakalim or pinacidil could apparently account for the inhibition of spike firing seen in spontaneously active tissues,

but not for the subsequent hyperpolarisation so often observed. Furthermore, it seems unlikely that a Ca2÷-dependent K+-channel could be involved in hyperpolarisations since intracellular Ca 2+ concentrations are likely to be very low under such circumstances. Studies in rabbit mesenteric artery (Coidweli and Howlett, 1986), and in rat portal vein (Southerton and Weston, 1987), have shown that blockade of Ca2+-influx by nifedipine or La 3+ did not affect the ability of cromakalim to enhance 86Rb efflux. Additionally, in rabbit mesenteric artery bathed in Ca2÷-free solution (containing EGTA), cromakalim still increased S6Rb efflux but the re-addition of Ca 2+ to the buffer returned the rate of efflux to basal levels. Thus, based on these efflux data, the K+-channel activated by cromakalim in rabbit mesenteric artery is not dependent on Ca2÷-influx but Ca 2+ can close the opened K+-channel (Howlett and Coldwell, 1987). Alternatively, if cromakalim of pinacidil were to increase the binding affinity of Ca 2+ for the channel, a shift in the opening probability-potential relationship as described by Singer and Walsh (1987) might occur, allowing channel openings at more negative (hyperpolarising) potentials to take place. Such possibilities can now be experimentally tested. The results of Yamanaka et al. (1985) suggest that nicorandil interacts with a Ca2+-independent K+-channel. If it is possible to draw an analogy with cardiac muscle, the recent work of Kakei et al. (1986) in single ventricular cells suggests that nicorandil opens a K÷-channel of only 5pS unit conductance, smaller than anything so far observed in smooth muscle. CLINICALAPPLICATIONS Cromakalim, nicorandil and pinacidil are currently at various stages of clinical assessment. Nicorandil has been marketed in Japan as an anti-anginal agent for 2 yr. Pinacidil was approved for use as an antihypertensive drug in Denmark in April 1987. Cromakalim is in Phase II/III clinical trials as monotherapy in hypertension. Angina pectoris

Recent studies in patients with effort angina have shown favourable results with nicorandil in comparison with established therapy, e.g. the nitrovasodilators and calcium-entry blocking agents (Belz et al., 1984; Hayata et al., 1986; Araki et al., 1987). Nicorandil decreases preload, like the nitrovasodilators, but also reduces afterload, as would be expected from in vitro experiments with arteries. Some recent studies have suggested that, in vasospastic angina, the primary defect may be a decrease in the K÷-permeability of the smooth muscle cells of coronary arteries (Uchida, 1985; Iwaki et al., 1987). If this is correct, nicorandil, with a combination of guanylate cyclase stimulation and K+-channel opening properties, should be potentially useful in this condition. Hypertension

The primary effect of cromakalim and pinacidil is dilation of peripheral arterioles with a reduction in afterload and these drugs are currently being evalu-

6

T.C. HAMILTONand A. H. WESTON

ated for use in the treatment of hypertension. Clinical experience with pinacidil has recently been reviewed by Goldberg (1988). To date, some 4000 hypertensive patients have been treated with this drug which produces a decrease in blood pressure and increase in heart rate, effects sometimes associated with a typical "vasodilator headache". Oedema is also a prominent side-effect of pinacidil, as a consequence of the reduced blood pressure and stimulation of renal compensatory mechanisms. As with vasodilator drugs such as hydralazine, the incidence of both oedema and headache is reduced by a thiazide diuretic. Therefore, combination of pinacidil with a diuretic may represent the optimum treatment regime; this has been recommended recently for pinacidil by the F D A Cardiorenal Drugs Advisory Committee in the USA. An initial investigation of cromakalim in 70 patients (active and placebo) with mild essential hypertension has found that single oral daily doses, in the range 0.5-1.5mg, lower blood pressure (Vandenberg et al., 1986, 1987). Some incidence of headache was reported, particularly at the highest dose, but oedema was not seen. The differences in haemodynamic profile between cromakalim and pinacidil in animals (see previously) suggest that cromakalim may possess a beneficial effect on renal blood flow in man. O T H E R POSSIBLE CLINICAL INDICATORS

(a) Asthma In vitro experiments have shown that cromakalim, nicorandil and pinacidil relax bronchial smooth muscle (see previous section) and this activity is reflected for oral cromakalim by bronchodilator activity in both the guinea-pig and man (Buckle et al., 1987; Baird et al., 1988). Thus, these drugs may have potential as novel drug treatment of asthma. Such in vivo findings suggest that the opening of K+-channels, and the resultant hyperpolarisation by drugs such as cromakalim, may be an effective means of antagonising the hyperreactivity of airways smooth muscle associated with this condition. (b) Peripheral vascular disease In animal models of chronic ischaemic skeletal muscle disease, cromakalim is able to improve tissue blood flow and oxygen availability (Angersbach and Nicholson, 1985). Such activity is not possessed by other vasorelaxants such as hydralazine and nifedipine and strongly suggests that K+-channel activation is important for this effect of cromakalim. Drugs such as cromakalim (and pinacidil) therefore have potential in the treatment of occlusive vascular diseases, e.g. Raynaud's disease and intermittent claudication, and these useful properties could also be an added benefit in hypertension or congestive heart failure. FUTURE DIRECTIONS OF RESEARCH

Do other K+-channel opening drugs exist? Tenuous links exist between the K÷-channel modulators described in this review and the anti-hypertensive agents, diazoxide and minoxidil.

Diazoxide produces hyperglycaemia as a side-effect of its use in hypertension, a property which seems to be associated with the opening of ATP-dependent K+-channels in pancreatic fl-cells (Henquin and Meissner, 1982; Trube et al., 1986). The antihypertensive activity of minoxidil is believed to reside in its metabolite, minoxidil sulphate, the effects of which may be associated with K÷-channel opening (Meisheri and Cipkus, 1987). Minoxidil exhibits the side-effect of hypertrichosis, a property which it shares with pinacidil and diazoxide. Such observations raise the possibility that minoxidil and diazoxide also exert their anti-hypertensive effects by opening K+-channels in smooth muscle. However, there are several reasons for rejecting this view. In the case of minoxidil the single published report (Meisheri and Cipkus, 1987) is very brief and describes a smooth muscle profile which is dissimilar to that of either cromakalim, nicorandil or pinacidil. Furthermore, cromakalim has no effects on pancreatic fl-cells in vitro and does not interfere with the ability of glibenclamide to promote insulin secretion by closing ATP-dependent K+-channels (Sturgess et al., 1985; Wilson et al., 1988). Although further evidence is required, there are at present few clear published links between the actions of diazoxide and minoxidil on smooth muscle and those of the K÷-channel openers---cromakalim, pinacidil and nicorandil. What are the properties o f the K ÷-channel(s) involved in the actions o f these drugs? Preliminary data concerning the Ca2÷-dependency of the K÷-channel(s) opened by cromakalim and pinacidil are equivocal while nicorandil probably exerts its effects on a Ca2+-independent K-channel. Whole single cell and patch techniques are now available to answer such questions and rapid progress in this area can be expected. Can an increase in K÷-conductance and hyperpolarisation explain all the actions o f these drugs? In the case of nicorandil, the answer is clearly negative, since this drug possesses guanylate cyclase stimulating activity. Indeed, its profile in vivo resembles that of a nitrovasodilator more than a K+-channel activator. Both cromakalim and pinacidil seem quite selective and no significant actions other than K+-channel opening have been found. However, both cromakalim and pinacidil are capable of inhibiting agonist-induced contractions in tissues in which membrane potential changes are believed to be unimportant (although the opening of K+-channels can still be detected). In addition, concentrations approximately five times greater than those required to inhibit mechanical responses are required to show detectable increases in K+-channel opening. The possibility of an additional action is thus still possible, but none has so far been detected. CONCLUSION AND SUMMARY

Cromakalim and pinacidil are novel drugs which lower blood pressure in vivo and open membrane K+-channels in a variety of smooth muscles in vitro. The resulting shift in membrane potential towards the

Cromakalim, nicorandil and pinacidil on smooth muscle K + channels K + - e q u i l i b r i u m potential leads to the closure o f voltage ( d e p o l a r i s a t i o n ) - d e p e n d e n t Ca2+-channels, a n d muscle relaxation. A g o n i s t - i n d u c e d c o n t r a c t i o n which result from the o p e n i n g o f v o l t a g e - d e p e n d e n t Ca2+-channels are also inhibited. In some tissues this m e m b r a n e action is s h a r e d by nicorandii, a drug which also stimulates soluble guanylate cyclase. In vivo the effects o f nicorandil result from a combin a t i o n of K + - c h a n n e l opening a n d a n increase in cellular cyclic G M P concentrations. Clinical experience with c r o m a k a l i m , nicorandil a n d pinacidil is relatively limited. However, initial results in patients with cardiovascular diseases like hypertension a n d a n g i n a pectoris seem promising. T h e i r use in the t r e a t m e n t of o t h e r disorders o f s m o o t h muscle, like asthma, is u n d e r investigation.

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