Recent progress in potassium channel opener pharmacology

Potassium (K) channel openers comprise a diverse group of molecules capable of opening K channels in excitable cells. These agents exhibit their greatest potency in the smooth muscle system but K channels in cardiac muscle, neurones and in secretory cells are also affected. The development of tissue selectivity is currently one major focus of research and evidence is starting to emerge that this can be achieved. The profound effects of the K channel openers in vivo has led to the suggestion that an endogenous K channel opener might exist and exert an important role in blood pressure homeostasis. The discovery of such a substance--endothelium-derived hyperpolarizing factor--has many implications and its role in cardiovascular regulation is currently under investigation. In vivo, initial studies with the K channel openers emphasized their antihypertensive properties. However, later studies have concentrated on the improvement to coronary blood flow produced by these substances together with their protective effect on the ischaemic myocardium, the basis of which is not fully understood. In spite of great efforts, the K channel which forms the target of these agents in smooth muscle is a matter of controversy. The ability of glibenclamide to antagonize the actions of the K channel openers initially led to the suggestion that an ATP-dependent K channel was their site of action in smooth muscle although the most recent data have implicated a smaller conductance K channel.

Cyclosporin A does not possess K+ channel opening properties in smooth muscle

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The contribution of Rb-permeable potassium channels to the relaxant and membrane hyperpolarizing actions of cromakalim, RP49356 and diazoxide in bovine tracheal smooth muscle

1. Cromakalim (1 and 10 microM), RP49356 (5 and 50 microM) and diazoxide (100 and 300 microM) produced full relaxation of smooth muscle strips pre-contracted with 25 mM KCl. These agents caused membrane hyperpolarization and increased 42K and 86Rb efflux. The time taken to achieve the maximum change in each of these parameters (tmax) was less for the higher concentration levels of cromakalim, RP49356 and diazoxide than for the lower concentration levels. 2. Calculation of permeability (P) changes showed that cormakalim (1 and 10 microM) produced a greater rise in PK than PRb, although the PRb:PK ratio was similar at both concentration levels. Similarly RP49356 produced a greater change in PK than PRb. However, in contrast to cromakalim, this difference was more marked at the higher concentration (50 microM) and was reflected by a differential effect of the two concentrations of RP49356 on the PRb:PK ratio. Diazoxide (100 and 300 microM) produced similar changes in PK and PRb. 3. For cromakalim (1 and 10 microM) the tmax for the electrical and mechanical effects and also the profile of change in these parameters corresponded to changes in both PK and PRb. For RP49356 (5 microM), changes in tension and membrane potential were related to both changes in PK and PRb, whereas at 50 microM these responses more closely corresponded to changes in PK. For diazoxide (100 and 300 microM) the electrical and mechanical effects corresponded to changes in both PK and PRb. 4. The results show that changes in 42K and 86Rb efflux induced by cromakalim, RP49356 and diazoxide are good indicators of changes in membrane PK and PRb evoked by these agents. Furthermore, it is concluded that the K channels involved in the mechanical and electrical effects of cromakalim are represented by the opening of a single population through which Rb can pass less easily than K, whilst the K channels associated with actions of diazoxide are equally permeable to both K and Rb. In contrast, the relaxant and membrane hyperpolarizing actions of RP49356 may involve the opening of more than one group of K channels which differ in their permeability to Rb.

Differences between the effects of cromakalim and nifedipine on agonist-induced responses in rabbit aorta

1. The effects of cromakalim on endothelium-denuded rabbit aortic strips were compared with those of the calcium (Ca2+) entry blocking agent, nifedipine. 2. Pre-incubation with cromakalim or nifedipine had no significant effect on the initial phasic component of noradrenaline (NA)-induced responses. 3. Cromakalim (0.3-10 microM), but not nifedipine, inhibited the maintained tonic contractions produced by NA. The effects of cromakalim were antagonized by raising extracellular [K+] or by glibenclamide. 4. Nifedipine inhibited contractions produced by KCl (40 mM) whereas cromakalim had no effect. 5. In Ca2(+)-free physiological salt solution (PSS), cromakalim produced a significant inhibition of both the refilling of and the release of Ca2+ from NA-releasable Ca2+ stores, whereas nifedipine was ineffective. 6. In tissues preloaded with 42K+ cromakalim (0.3-10 microM) produced a concentration-dependent increase in the 42K+ efflux rate coefficient. NA (0.3 microM) also produced an increase in the rate of efflux of 42K+, an effect which was not antagonized by nifedipine (0.3 microM). 7. When microelectrodes were used, cromakalim (1-10 microM) produced a maintained concentration-dependent membrane hyperpolarization. However, low concentrations of cromakalim (less than 1 microM) which relaxed the aorta had no effect on membrane potential. NA had no significant effect on membrane potential. 9. It is concluded that the ability of cromakalim to relax NA-induced contractions in rabbit aorta is not exerted by the indirect closure of nifedipine-sensitive Ca2+ channels. Instead, cromakalim may exert a direct inhibitory action on Ca2+ uptake into and release from Ca2+ stores and additionally inhibit the pathway through which Ca2+ passes from the extracellular fluid to intracellular Ca2+ stores.

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 vein. The effects of glibenclamide on the relaxant responses and rises in 86Rb efflux evoked by cromakalim, RP49356 and diazoxide were studied. The effects of galanin and depletion of intracellular ATP concentrations [( ATP]i) were also examined. Galanin increased mechanical activity and 86Rb efflux, effects most likely mediated via galanin receptors rather than a direct action on a K channel. Glibenclamide inhibited the relaxant responses and rises in 86Rb efflux evoked by cromakalim, RP49356 and diazoxide. Reduction of [ATP]i 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.

Structure-activity relationships of K+ channel openers

Seven groups of synthetic agent, distinguished by a combination of their chemical and pharmacological characteristics exert some or all of their effects by opening plasmalemmal K+ channels primarily in smooth muscle. Progress over the past two years now allows broad structure-activity relationships to be formulated within many of the individual groups of agent. Gillian Edwards and Arthur Weston review the historical basis of these discoveries and comment on the significance of new developments. They focus on the search for tissue and channel selectivity, two factors likely to be important for the successful clinical deployment of these substances as antihypertensive and bronchodilator agents.

A comparison of the actions of cromakalim and nifedipine on rabbit isolated mesenteric artery

This study shows that cromakalim (0.4-10 microM) can concentration-dependently hyperpolarize the smooth muscle cell membrane and increase 86Rb efflux from rabbit mesenteric small arteries at concentrations which inhibit noradrenaline-induced increases in perfusion pressure in this preparation. Hyperpolarisation of the cell membrane by cromakalim was inhibited by prior exposure of the tissue to glibenclamide (1 microM). Noradrenaline (greater than 1 microM) depolarized the smooth muscle cell membrane and this effect was reduced in the presence of cromakalim. Experiments involving repetitive stimulation of the perivascular nerves in this tissue showed that cromakalim (2-10 microM) reduced excitatory junction potential amplitude and fall time without affecting the facilitation process. The results of this study suggest that in rabbit small mesenteric arteries the vasodilator action of cromakalim is a consequence of the opening of 86Rb-permeable potassium channels. It is unlikely that any component of the vasorelaxant effects of cromakalim is due to a direct effect on voltage-operated calcium channels or a prejunctional effect on neuroeffector transmission.

A comparison of the haemodynamic profiles of Ro 31-6930, cromakalim and nifedipine in anaesthetised normotensive rats

The regional haemodynamic profiles of Ro 31-6930, cromakalim and nifedipine were compared using pulsed Doppler flowmetry in the anaesthetised rat. In order of potency, Ro 31-6930 (0.1-300 micrograms/kg), cromakalim (1-300 micrograms/kg) and nifedipine (1-1000 micrograms/kg) produced dose related falls in mean arterial pressure. The hypotensive effects of Ro 31-6930 and cromakalim were accompanied by reflex tachycardia. All three agents reduced renal vascular resistance by 30-50%. Cromakalim exerted a selective action on this vascular bed. Similar maximal reductions in mesenteric vascular resistance (37-50%) were observed; however, cromakalim was the least potent on this vascular bed. Maximal reductions in iliac vascular resistance (65-78%) were observed, with an order of potency as observed on mean arterial pressure. Qualitative differences in the regional haemodynamic profiles of Ro 31-6930, cromakalim and nifedipine are evident from this study. The different profiles of Ro 31-6930 and cromakalim may reflect structural differences between the pharmacophores of these compounds.

The action of diazoxide and minoxidil sulphate on rat blood vessels: a comparison with cromakalim

1. The actions of diazoxide and minoxidil sulphate have been compared with those of cromakalim in rat aorta and portal vein. 2. Diazoxide and minoxidil sulphate hyperpolarized the rat portal vein in a similar manner to cromakalim. 3. Cromakalim, diazoxide and minoxidil sulphate increased 42K and 86Rb efflux from rat portal vein, although minoxidil sulphate had only a small effect on 86Rb efflux. 4. Cromakalim, diazoxide and minoxidil sulphate increased 42K efflux from rat aorta but only cromakalim and diazoxide increased 86Rb efflux from this tissue. 5. Glibenclamide inhibited the relaxant actions of cromakalim, diazoxide and minoxidil sulphate on rat aorta and the increase in 42K efflux produced by these agents in this tissue. 6. Diazoxide relaxed an 80 mM KCl-induced contraction of rat aorta, whilst cromakalim and minoxidil sulphate were without effect. 7. Cromakalim, diazoxide and minoxidil sulphate had no effect on cyclic AMP or cyclic GMP concentrations in rat aorta. 8. It is concluded that diazoxide and minoxidil sulphate like cromakalim exhibit K+ channel opening properties in vascular smooth muscle. Diazoxide exerts an additional inhibitory action not related to the production of cyclic AMP or cyclic GMP. The action of minoxidil sulphate may be primarily located at a K+ channel which is relatively impermeable to 86Rb.

Actions of cromakalim on ionic currents recorded from single smooth muscle cells of the rat portal vein

The effects of cromakalim (BRL34915), a novel K channel opener, on ionic currents in dispersed smooth muscle cells of the rat portal vein were investigated. Application of cromakalim (greater than 3 microM) generated an outward current, the reversal potential of which (-79 mV) was almost the same as the theoretical K equilibrium potential (-80 mV) under the experimental conditions used. When the Ca in the physiological salt solution (PSS) was replaced with Mn, the cromakalim-induced outward current was inhibited markedly. When 4 mM ethylene glycol bis(beta-aminoethyl ether(N,N'-N,N'-tetraacetic acid (EGTA) was added to the pipette solution (bath solution, Ca-free PSS + Mn), cromakalim inhibited the outward current evoked by command pulses. Using a pipette solution containing high Cs with 4 mM EGTA and a bath PSS containing 2.5 mM Ca with 1 microM tetrodotoxin, the Ca inward current was isolated. Cromakalim (greater than 3 microM) inhibited the Ca inward current in a voltage- dependent manner. Cromakalim (30 microM) inhibited the Ca inward current to 0.6 times the control. The decay of the inward Ca current comprised fast and slow components. Cromakalim inhibited the latter component and shifted the voltage-dependent inactivation curve of this current to the left (more hyperpolarized direction) in a parallel manner and delayed recovery from inactivation as estimated using a double pulse protocol. By using a pipette solution containing high Cs with 4 mM EGTA and Ca-free PSS containing 2.5 mM Mn in the bath, Na inward currents, blocked by tetrodotoxin (IC50 = 10 nM), were evoked by depolarizing pulses. Cromakalim (up to 30 microM) had no effect on the Na inward current.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium channel openers and vascular smooth muscle relaxation

Potassium channel openers comprise a diverse group of chemical agents which open plasma-lemmal K-channels. They show selectivity for smooth muscle, although K-channels in cardiac and skeletal muscle, neurones and the pancreatic beta-cell are also affected at relatively high concentrations. In addition, at least one endogenous K-channel opener of vascular origin--endothelium-derived hyperpolarizing factor--exists and in man plays a role in modulating blood vessel tone. The type of K-channel involved in the actions of both exogenous and endogenous K-channel openers is still uncertain, although a prime candidate in smooth muscle seems similar to the [ATPi]-modulated K-channel in the pancreatic beta-cell. This review focuses attention on the action of these agents in vascular smooth muscle and on the possible clinical exploitation of their powerful vasorelaxant properties.

The potassium channel openers: a new class of vasorelaxants

Cromakalim, pinacidil, nicorandil, diazoxide and RP-49356 belong to the class of drugs termed potassium channel openers. In rat portal vein diazoxide, like cromakalim, abolished spontaneous mechanical and electrical activity and in rat aorta caused an increase in 86Rb efflux and inhibited KCl(20 mM)-induced contractions. However, in contrast to cromakalim, diazoxide (greater than 100 microM) also inhibited mechanical responses evoked by 80 mM KCl in rat aorta suggesting that it possesses pharmacological properties in addition to K channel opening. Since glibenclamide can attenuate the effects of cromakalim and diazoxide in vascular tissues, it is possible that a channel resembling the ATP-sensitive K channel found in pancreatic beta-cells may be involved in the vasorelaxant effects of these agents. However, differences exist in the order of potency of cromakalim and diazoxide for producing smooth muscle relaxation and for decreasing insulin secretion in pancreatic beta-cells. Furthermore galanin (which opens ATP-sensitive K channels in beta-cells) increases mechanical activity in rat portal vein. It is anticipated that new chemical developments will produce K channel opening molecules with greater potency and tissue selectivity.

Acetylcholine releases endothelium-derived hyperpolarizing factor and EDRF from rat blood vessels

1. The effects of haemoglobin and methylene blue on the acetylcholine (ACh)-induced electrical and mechanical responses of smooth muscle cells were investigated in rat aorta and rat main pulmonary artery. 2. When the endothelium was intact, ACh induced a transient hyperpolarization and sustained relaxation of tissues precontracted with noradrenaline. Both hyperpolarization and relaxation were absent in preparations without endothelium. 3. Haemoglobin and methylene blue inhibited the ACh-induced relaxation, but not the transient hyperpolarization. 4. In aorta with an intact endothelium, ACh produced an increase in both the rate of 86Rb efflux and tissue cyclic GMP levels. The changes in ion flux were unaffected by either haemoglobin or methylene blue in concentrations which almost abolished the increase in cyclic GMP concentrations. 5. In arteries with an intact endothelium, indomethacin had no effect on the ACh-induced electrical and mechanical responses or on the increase in 86Rb efflux and tissue cyclic GMP levels. 6. It is concluded that in the rat aorta and rat main pulmonary artery, ACh releases two different substances, an endothelium-derived relaxing factor (EDRF) and a hyperpolarizing factor (EDHF), from the endothelial cells. Neither substance appears to be derived from a pathway dependent on cyclo-oxygenase. EDHF seems to play a minor role in the relaxation of noradrenaline-induced contractions.

Characteristics of cromakalim-induced relaxations in the smooth muscle cells of guinea-pig mesenteric artery and vein

1. The effects of cromakalim (BRL 34915) on the smooth muscle cells of guinea-pig mesenteric artery and vein were investigated with microelectrode and tension recording methods. 2. Cromakalim (greater than 10 microM) produced membrane hyperpolarization with an increase in ionic conductance. The hyperpolarization occurred to a greater extent and lasted longer in the vein than in the artery. 3. The hyperpolarization induced by cromakalim in mesenteric vein comprised two components, one of which was Mn sensitive. In mesenteric artery, the hyperpolarization was relatively insensitive to Mn. 4. From the current-voltage relationship measured from arterial smooth muscle membranes, the reversal potential of cromakalim was estimated to be -80 mV. The cromakalim-induced hyperpolarization was not modified in Na- or Cl-deficient solution. 5. In both mesenteric artery and vein, cromakalim relaxed tissues precontracted with high K with (below 40 mM) or without (above 40 mM) hyperpolarization of the membrane. 6. In the mesenteric artery, action potentials evoked by electrical stimulation ceased before the generation of hyperpolarization. 7. Cromakalim produced a cross-desensitization with nicorandil on the evoked membrane hyperpolarization in mesenteric artery. 8. It is concluded that the relaxing actions of cromakalim result from the hyperpolarization which follows the opening of Ca-dependent K channels. The inhibition of a voltage-dependent Ca current may also be involved in this inhibitory effect.

The potassium channel opening action of pinacidil; studies using biochemical, ion flux and microelectrode techniques

In rat aorta and rat portal vein, (-)- and (+)-pinacidil each produced a concentration-dependent inhibition of tension development. Although the (-) isomer was the more potent, concentration effect curves for each isomer were steep with similar slopes. In rat portal vein, tetraethylammonium and procaine antagonised the relaxant effect of (+/-)-pinacidil, whereas 3,4-diamino-pyridine was without effect. Intracellular microelectrode recording in rat portal vein showed that low concentrations of (+/-)-pinacidil reduced the duration of multispike electrical complexes. In both rat aorta and rat portal vein, higher concentrations of (+/-)-pinacidil hyperpolarised the membrane towards the potassium equilibrium potential. (+/-)-Pinacidil increased 86Rb efflux from rat aorta and rat portal vein in a concentration dependent manner. In a separate study, (+/-)-pinacidil increased 42K efflux from rat portal vein. (+/-)-Pinacidil had no effect on cyclic GMP or cyclic AMP levels in rat aorta. It is concluded that pinacidil opens 86Rb-permeable potassium channels in rat aorta and rat portal vein. This mechanism is independent of cyclic nucleotide changes and may be responsible for the antihypertensive effect of pinacidil.

Endothelium-dependent effects of acetylcholine in rat aorta: a comparison with sodium nitroprusside and cromakalim

1. The mechanisms involved in the mechano-inhibitory effects of acetylcholine (ACh) have been compared with those of sodium nitroprusside (SNP) and cromakalim on the rat isolated thoracic aorta. 2. Relaxations produced by ACh were endothelium-dependent, whereas those produced by SNP or cromakalim were endothelium-independent. 3. ACh, cromakalim and SNP relaxed established contractions produced by noradrenaline (NA) and KCl (20 mM) and these relaxations were well-maintained. 4. SNP was a relatively effective inhibitor of contractions produced by KCl (80 mM). ACh was relatively ineffective and cromakalim was without effect against such contractions. 5. Membrane potential and cyclic GMP concentrations were higher in tissues with an intact endothelium whereas rubbed tissues had a higher 86Rb efflux rate coefficient. 6. ACh and cromakalim produced a transient and long-lasting hyperpolarization, respectively. These changes were accompanied by increases in the 86Rb efflux rate coefficient with a time course comparable to that of the electrical changes. 7. Tissue cyclic GMP concentrations were significantly increased in the presence of ACh or SNP, whereas cromakalim had no effect. 8. Transmission electron microscopy showed the presence of endothelial cells on intact tissues. On rubbed preparations, such cells were absent and some damage to the underlying smooth muscle cells was detected. 9. It is concluded that at least two inhibitory substances are released from the endothelial cells by ACh. One of these increases tissue cyclic GMP concentrations and produces an electrically-silent relaxation. The other produces a transient hyperpolarization associated with the opening of 86Rb-permeable K-channels. This event may serve to initiate relaxation processes and to close any open voltage-dependent Ca-channels.

A comparison of the lanthanum and EGTA methods for measuring calcium influx into vascular smooth muscle cells

A comparison has been made of methods utilizing LaCl3 or EGTA for the determination of changes in Ca2+ influx into vascular smooth muscle. Stimulation of 45Ca2+ influx through voltage-operated channels using KCl and blockade of this effect with verapamil were used as test stimuli. Satisfactory results were obtained using ice-cold, Ca2+-free wash solutions containing LaCl3 (10-80.8 mmole.l-1). To achieve a rate of 45Ca2+ exchange of approximately 1% min-1, a wash duration of 60-90 min, depending on the tissue, was necessary. Using an EGTA wash less 45Ca2+ was retained by the tissue and the rate of 45Ca2+ exchange was greater than when LaCl3 was used. However, the changes in 45Ca2+ influx produced by the test stimuli and detected following an EGTA wash were qualitatively similar to those obtained when LaCl3 was used.