From cromakalim to different structural classes of K(ATP) channel openers

ATP-Sensitive potassium channel openers (K(ATP)COs) are a group of compounds with a broad spectrum of potential therapeutic applications, as they constitute efficient tools for dampening cell excitability. Interest in the K(ATP)COs was triggered in the early 1980s by the discovery of the benzopyran-based structure cromakalim (CRK), which is a powerful smooth muscle relaxant. CRK can be considered the archetype of K(ATP)COs and is by far the most mimicked structure. In many structure-activity studies various substitutions have been made at the different positions of the benzopyran ring permitting the optimal activity to be correlated with a specific set of structural characteristics and stereochemical features of the molecule. Thus, many potent benzopyran derivatives have been identified. The benzopyran nucleus itself has also been modified in both the aromatic ring and in the pyran moiety. The intention of this review is to bring together all the different structural classes of K(ATP)COs arising from the replacement of CRK benzopyran-based structure with various ring systems; design, structure-activity relationship, and synthesis will be given.     

Comparative effects of K+ channel blockade on the vasorelaxant activity of cromakalim, pinacidil and nicorandil

Three agents with K+ channel blocking activity, procaine, 4-aminopyridine (4-AP) and tetraethylammonium (TEA), were tested for inhibition of vasorelaxation and 86Rb+ efflux induced by cromakalim (BRL 34915), pinacidil and nicorandil in rabbit isolated mesenteric artery. The potency order for inhibition of vasorelaxation was procaine greater than 4-AP greater than TEA and for inhibition of efflux was procaine = 4-AP greater than TEA. The K+ channel blockers did not discriminate between cromakalim, pinacidil or nicorandil on efflux but demonstrated preferential inhibition of vasorelaxation to cromakalim greater than pinacidil greater than nicorandil. In addition, the maximum response to cromakalim was depressed but that to pinacidil and nicorandil was not. The results confirm the role of K+ channel activation in vasorelaxation to cromakalim, pinacidil and nicorandil, but suggest that additional mechanisms may be involved for pinacidil and, in particular, for nicorandil.     

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.     

Effect of three novel K+ channel openers, cromakalim, pinacidil and nicorandil on allergic reaction and experimental asthma.

The anti-allergic and anti-asthmatic activities of three potassium (K+) channel openers, cromakalim, pinacidil, and nicorandil, were investigated. 1) Forty-eight-hour homologous passive cutaneous anaphylaxis (PCA) in mice was not affected by cromakalim, pinacidil, or nicorandil. Ketotifen significantly inhibited the reaction. 2) Antigen-induced histamine release from sensitized guinea pig lung tissue was not affected by cromakalim, pinacidil or nicorandil (except for 10(-4) M nicorandil). Salbutamol inhibited the release of histamine. 3) Histamine, serotonin and LTC4-induced vasculitis in rat back skin was not affected by any of these three K+ channel openers. 4) Antigen-induced constriction of isolated sensitized guinea pig tracheal muscle was relaxed by each of the K+ channel openers. 5) Constrictions of isolated guinea pig tracheal muscle caused by high potassium, histamine, LTC4, or U-46619 were clearly relaxed by each of the three K+ channel openers. 6) Increases of airway resistance caused by histamine, LTD4, or U-46619 in guinea pigs in vivo were inhibited by administration of each of the three K+ channel openers. 7) Experimental asthma caused by the IgE antibody and antigen system in guinea pigs was inhibited by each of the three K+ channel openers.     

K+ channel openers,cromakalim and Ki4032, inhibit agonist-induced Ca2+ release in canine coronary artery

Summary The effects of K⁺ channel openers, cromakalim and an acetoxyl derivative of KRN 2391 (Ki 4032), were studied on force of contraction, increases in intracellular calcium concentration ([Ca²⁺]_i) measured by fura-2 and inositol 1,4,5-trisphosphate (IP₃) production induced by the thromboxane A₂ analogue, U46619, in canine coronary arteries. Upon single dose applications of U46619 at 300 nmol/l, phasic and tonic increases in [Ca²⁺]_i and force were seen, which were almost abolished by cromakalim (10 mol/l) and Ki4032 (100 mol/l).In the absence of extracellular Ca²⁺, U46619 induced a transient increase in [Ca²⁺]_i with a contraction. Cromakalim (0.01–10 mol/l) and Ki4032 (0.1–100 mol/l) concentration-dependently inhibited the increases in [Ca²⁺]_i and contraction. The inhibitory effects of cromakalim and Ki4032 were blocked by the K⁺ channel blocker tetrabutylammonium (TBA) and counteracted by 20 mmol/l KCl-induced depolarization. Cromakalim and Ki4032 did not affect caffeine-induced Ca²⁺ release. Cromakalim reduced U46619-induced IP₃ production significantly and TBA blocked this inhibitory effect. These results suggest that the hyperpolarization of the plasma membrane by K⁺ channel openers inhibits the production of IP₃ and Ca ²⁺ release from intracellular stores related to stimulation of the thromboxane A₂ receptor.Correspondence to T. Yanagisawa at the above address     

Binding and effect of K ATP channel openers in the absence of Mg2+

1 Openers of ATP-sensitive K(+) channels (K(ATP) channels) are thought to act by enhancing the ATPase activity of sulphonylurea receptors (SURs), the regulatory channel subunits. At higher concentrations, some openers activate K(ATP) channels also in the absence of MgATP. Here, we describe binding and effect of structurally diverse openers in the absence of Mg(2+) and presence of EDTA. 2 Binding of openers to SUR2B was measured using a mutant with high affinity for [(3)H]glibenclamide ([(3)H]GBC). In the absence of Mg(2+), 'typical' openers (benzopyrans, cyanoguanidines and aprikalim) inhibited [(3)H]GBC binding with K(i) values approximately 200 x higher than in the presence of MgATP. Minoxidil sulphate and nicorandil were inactive, whereas binding of diazoxide was unaffected by MgATP. 3 In the absence/presence of MgATP, N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine (P1075) activated the Kir6.2/SUR2B channel in inside-out patches with EC(50)=2000/67nM and E(max)=32/134%. In the absence of Mg(2+), responses were variable with only a small part of the variability being explained by a decrease in channel responsiveness with time after patch excision and to differences in the ATP sensitivity between patches. 4 The rank order of efficacy of the openers was P1075>rilmakalim approximately nicorandil>diazoxide>minoxidil sulphate. 5 The data show that structurally diverse openers are able to bind to, and to activate the Kir6.2/SUR2B channel by a pathway independent of ATP hydrolysis. These effects are observed at concentrations used to define the biochemical mechanism of the openers in the presence of MgATP and allow the openers to be classified into 'typical' and 'atypical' KCOs with diazoxide standing apart.     

Inhibition by glibenclamide of the vasorelaxant action of cromakalim in the rat.

1. In rat isolated thoracic aortic rings pre-contracted with noradrenaline (10(-6) M), cromakalim (3 x 10(-7)-3 x 10(-5) M) produced concentration-related relaxation. This effect was progressively inhibited by increasing concentrations of the anti-diabetic sulphonylurea drug, glibenclamide (10(-6)-10(-5) M). 2. In rat isolated portal veins, cromakalim (3 x 10(-8)-10(-6) M) produced concentration-related inhibition of the spontaneous contractive activity and glibenclamide (3 x 10(-7)-3 x 10(-6) M) prevented this inhibitory action in a concentration-dependent manner. 3. In both rat aortic rings and portal veins, cromakalim (10(-5) M) stimulated 86Rb efflux. Prior exposure to glibenclamide (10(-7)-10(-6) M) produced a concentration-related inhibition of this response. 4. In conscious rats, cromakalim, 0.075 mg kg-1 i.v., produced a rapid and sustained fall in arterial blood pressure which was not influenced by pretreatment (2 h) with a large oral dose of glibenclamide (100 mg kg-1). 5. In conscious rats, the hypotensive action of cromakalim, 0.075 mg kg-1 i.v., was abolished by pretreatment (30 min) with glibenclamide, 20 mg kg-1, given by the intravenous route. 6. The results suggest that the vasorelaxant and hypotensive actions of cromakalim involve a K+ channel which can be inhibited by glibenclamide, but which may be distinct from the ATP-sensitive K+ channel of the pancreatic beta-cell.     

Effects of K+ channel blockers on the relaxant action of dihydralazine, cromakalim and nitroprusside in isolated rabbit femoral arteries.

The relaxant responses to dihydralazine and the influence of different K+ channel blockers were studied in isolated rabbit femoral arteries. The prototype K+ channel opener, cromakalim, and nitroprusside, which does not produce relaxation by K+ channel activation were used for comparison. Dihydralazine was most effective on contractions induced by noradrenaline (EC50 = 1.1 microM; Emax = 95%) and relaxed the contractions elicited by 20 mM K+ (EC50 = 2.0 microM; Emax = 81% in preference to 124 mM K(+)-induced contractions (EC50 = 30.1 microM; Emax = 54%). Cromakalim, but not nitroprusside, also selectively relaxed 20 mM K(+)-induced contractions. In noradrenaline-contracted arteries, glibenclamide (10 microM) completely suppressed the relaxant response to cromakalim but did not influence the vasorelaxation produced by dihydralazine or nitroprusside. Tetraethylammonium (8 mM) and Cs+ (4 mM) shifted the concentration-relaxation curve for dihydralazine 2-fold to the right, whereas Ba2+ (0.1 mM), 4-aminopyridine (5 mM) and procaine (0.1 mM) failed to influence dihydralazine-induced responses. Tetraethylammonium (8 mM) shifted the concentration-relaxation curve for cromakalim and nitroprusside 6-fold to the right and suppressed the maximal relaxant effects by about 30%. It is concluded that dihydralazine produces vascular smooth muscle relaxation by a mechanism different from the opening of glibenclamide- and ATP-sensitive K+ channels.     

Modulation of vasorelaxant responses to potassium channel openers by basal nitric oxide in the rat isolated superior mesenteric arterial bed.

1. We have used the isolated buffer-perfused mesenteric arterial bed of the rat to assess the modulation of vasorelaxation to potassium channel openers (KCOs) by basal nitric oxide. 2. The dose-response curves to the KCOs, levcromakalim and pinacidil, in preconstricted preparations were significantly shifted to the left in the presence of the nitric oxide synthase inhibitor (100 microM) NG-nitro-L-arginine methyl ester (levcromakalim, ED50 = 4.47 +/- 0.70 nmol vs. 1.73 +/- 0.26 nmol, P < 0.001; pinacidil, ED50 = 16.1 +/- 4.8 nmol vs. 5.43 +/- 1.10 nmol, P < 0.001). The vasorelaxant responses to papaverine, a vasodilator which acts independently of potassium channels was unaffected by NG-nitro-L-arginine methyl ester (L-NAME). 3. Removal of the endothelium, by perfusion with the detergent CHAPS (0.3%), significantly (P < 0.001) increased the potency of levcromakalim as a vasodilator (ED50 4.47 +/- 0.70 nmol vs. 2.59 +/- 0.31 nmol). The subsequent administration of L-NAME following perfusion with CHAPS did not lead to any additional enhancement of responses to levcromakalim. 4. The presence of the non-selective adenosine antagonist, 8-phenyltheophylline (8-PT, 10 microM) significantly (P < 0.001) shifted the dose-response curve to levcromakalim to the left (ED50 4.47 +/- 0.70 nmol vs. 1.11 +/- 0.32 nmol). In the presence of both L-NAME and 8-PT, the dose-response curve to levcromakalim was also significantly (P < 0.01) shifted to the left compared with control (ED50 in the presence of both L-NAME and 8-PT was 0.42 +/- 0.08 nmol). 5. The presence of 8-bromo cyclic GMP (10 microM) reversed the increase potency of levcromakalim, observed following inhibition of nitric oxide synthase (ED50 in the presence of L-NAME was 0.59 +/- 0.01 nmol and in the presence of 8-bromo cyclic GMP plus L-NAME the ED50 was 3.17 +/- 0.80 nmol). However in the absence of L-NAME, the cell permeable analogue of cyclic GMP, 8-bromo cyclic GMP, did not affect the dose-response curve to levcromakalim compared with control (control ED50 value was 4.16 +/- 0.52 nmol vs. 3.85 +/- 1.13 nmol in the presence of 8-bromo cyclic GMP). 6. The present investigation demonstrates that both basal nitric oxide and adenosine modulate vasorelaxation to the KCOs levcromakalim and pinacidil. The modulatory effect of nitric oxide may be mediated via cyclic GMP.     

Influence of depolarization on vasorelaxant potency and efficacy of Ca2+ entry blockers, K+ channel openers, nitrate derivatives, salbutamol and papaverine in rat aortic rings

We examined the effects of various KCl concentrations on the actions of some vasodilators belonging to different pharmacological classes in rat aortic rings. In some experiments, tissues were precontracted with noradrenaline after blocking voltage-dependent channels to assess the effects of depolarisation unaccompanied by the entry of extracellular Ca²⁺ into the cytosol. Concentration/response curves for the vasorelaxant effect of calcium entry blockers (e.g. diltiazem), K⁺ channel openers (e.g. aprikalim), nitrate derivatives (e.g. nitroglycerin), a β₂-adrenergic agonist (salbutamol) and papaverine were obtained by using endothelium-denuded rat aortic rings precontracted with KCl (20–60 mM) to determine the potencies and efficacies of the drugs. The efficacies and potencies of calcium entry inhibitors were virtually independent of the [KCl]. A reduction in the potency (up to 18-fold) of papaverine occurred without changes in efficacy when the [KCl] was raised from 20 to 60 mM. The decline in potency was even greater for nitrate-like compounds. The potency of K⁺ channel openers in aortic rings precontracted with 30 mM KCl decreased by three- to sixfold compared with those precontracted with 20 mM KCl. With the exception of pinacidil, the efficacy of these agents already started to decline in preparations precontracted with 25 mM KCl and was virtually zero in preparations precontracted with 60 mM KCl. In contrast to other K⁺ channel openers, the vasorelaxant action of pinacidil was relatively resistant to glibenclamide. Salbutamol produced only a slight relaxation even in preparations precontracted with 20 mM KCl. In nitrendipine-pretreated, noradrenaline-precontracted aortic rings, the vasorelaxant effects of aprikalim, but not those of linsidomine or papaverine, declined when the [KCl] of the bathing medium was increased. In conclusion, the vasorelaxant potency and efficacy of calcium entry blockers is independent of the [KCl] used to precontract rat aortic rings, and thus, of the degree of membrane depolarisation. In contrast, increasing the [KCl] strongly reduces the potency and the efficacy of K⁺ channel openers not only in this preparation but also in noradrenaline-precontracted rings in which the entry of extracellular Ca²⁺ was prevented with nitrendipine. This indicates that, with the exception of pinacidil, the vasorelaxant activity of K⁺ channel openers depends on the degree of membrane depolarisation. Finally, the vasorelaxant potency and efficacy of nitrate-like compounds and papaverine are independent of depolarisation per se but they are markedly affected by the influx of Ca²⁺ accompanying elevated [KCl]. Thus, the degree of vessel depolarisation should be taken into consideration when attempting to compare potencies and efficacies among vasorelaxant agents. Received: 4 May 1998 / Accepted: 6 July 1998     

The vasorelaxant effect of evodiamine in rat isolated mesenteric arteries: mode of action.

The roles of the endothelium, Ca2+ and K+ fluxes in the evodiamine-induced attenuation of vascular contractile responses to vasoactive agents were examined. The results showed that: (1) in rat mesenteric artery rings, evodiamine elicited a concentration-dependent attenuation in the contractile response generated by phenylephrine. The inhibitory potency was greater for intact than for endothelium-denuded preparations. Thus, the vasodilator action of evodiamine appeared to be partially endothelium-interactive (dependent). (2) Evodiamine pretreatment had a greater inhibitory effect on the phenylephrine-induced tonic contraction (via Ca2+ influx) than on the phasic contraction (via Ca2+ release). In addition, evodiamine was more potent to inhibit the restoration by CaCl2 of contractile responses to phenylephrine than a potassium depolarizing solution in media that had been kept calcium-free. These results suggest that block of the Ca2+ influx through receptor-mediated Ca2+ channels may be the major mechanism underlying the vasodilator effect of evodiamine. (3) A K+ channel blocker, tetraethylammonium, almost completely abolished the vasodilatation induced by minoxidil (a known K+ channel opener) but not evodiamine. The possible involvement of K+ channel activation of the vasodilator effect produced by evodiamine was therefore excluded.     

Differential effects of endothelin-1 on the vasorelaxant properties of benzopyran and non-benzopyran potassium channel openers.

1. The effects of endothelin-1 (ET-1) on the vasorelaxant properties of structurally different potassium channel openers (PCOs), BRL-38227, Ro 31-6930, SDZ PCO 400, EMD-52692, RP-49356 and pinacidil, were studied. 2. All PCOs evoked concentration-related relaxations of ET-1 (10 nM) or KCl (20 mM) contracted rat isolated aortic rings denuded of endothelium. BRL-38227, EMD 52692, SDZ PCO 400 and Ro 31-6930 were 11-42 times less potent in relaxing contractions to ET-1 than KCl. In contrast, this differential potency was not observed with RP-49356 or pinacidil. 3. BRL-38227 (0.06-3.0 microM), RP-49356 (0.3-3.0 microM) and pinacidil (0.3-3.0 microM) displaced KCl concentration-response curves to the right of controls, without modifying the maximum response. A subcontractile concentration of ET-1 (0.1 nM) prevented the inhibitory effects of low concentrations of BRL-38227 (0.06-0.1 microM) on KCl responses, but failed to modify those to RP-49356, pinacidil or high concentrations of BRL-38227 (0.3-3.0 microM). The inhibitory effects of BRL-38227 (0.1 microM) were also not changed by ET-3 (1.0 nM) or angiotensin II (0.1 nM). 4. In anaesthetized spontaneously hypertensive rats (SHR), cumulative bolus intravenous administrations of BRL-38227 (1-1000 micrograms kg-1, i.v.), Ro 31-6930 (1-1000 micrograms kg-1, i.v.), RP-49356 (10-1000 micrograms kg-1, i.v.) or nitrendipine (0.1-30 micrograms kg-1, i.v.) produced dose-dependent falls in diastolic blood pressure (DBP).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that acetylcholine-mediated hyperpolarization of the rat small mesenteric artery does not involve the K+ channel opened by cromakalim.

1. Acetylcholine causes a concentration-dependent hyperpolarization of the rat small mesenteric artery (diameter at 100 mmHg, 200-400 microns). In the absence of tone the average potential change was from approximately -60 to -75 mV. In the presence of tone induced by endothelin-1 (20 nM), acetylcholine caused vasorelaxation in association with a marked hyperpolarization; from approximately -32 to -71 mV. 2. A number of compounds known to antagonize the actions of cromakalim were tested for their ability to block responses to acetylcholine. Glibenclamide (0.1-3 microM), phentolamine (10-100 microM) and alinidine (1-30 microM) caused a concentration-dependent depolarization of the rat small mesenteric artery which was not dependent on an intact endothelium. Glibenclamide was approximately 10 times more potent than either phentolamine or alinidine, a similar ratio to their potency as antagonists of cromakalim. 3. In the presence of concentrations of the cromakalim antagonists which functionally inhibited responses to cromakalim, only phentolamine and alinidine had a significant effect on the hyperpolarization and functional responses to acetylcholine. Glibenclamide was without effect at the concentrations used. 4. Experiments on pig coronary artery, where acetylcholine causes vasoconstrictor responses, showed that phentolamine and alinidine have some anti-muscarinic activity which could account for their ability to affect vasorelaxant/hyperpolarization responses to acetylcholine in the rat small mesenteric artery. 5. The results suggest that the acetylcholine-mediated hyperpolarization observed in the rat small mesenteric artery does not involve K+ channels opened by cromakalim. This finding differs from other studies performed on the rabbit middle cerebral artery which show hyperpolarizing responses to acetycholine to be glibenclamide-sensitive.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms underlying the vasorelaxant effect induced by proanthocyanidin-rich fraction from Croton celtidifolius in rat small resistance arteries

Proanthocyanidins are condensed tannins present in fruits, vegetables, and flowers, consumed in the human diet. These compounds are believed to decrease coronary heart disease. The present study was designed to investigate the relaxing effects of a proanthocyanidin-rich fraction (PRF) obtained from Croton celtidifolius BAILL (Euphorbiaceae) barks in rat mesenteric arterial bed (MAB) and isolated mesenteric artery (MA). In the MAB pre-contracted with phenylephrine (Phe), PRF (0.1 - 100 microg) induced a concentration-dependent relaxation of 73% (compared to the control). This effect was significantly reduced by the nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine (L-NOARG) or high K(+) solution and completely abolished in vessels perfused with KCl plus L-NOARG. However, the vasorelaxant effect was not altered by indomethacin, atropine, yohimbine, pyrilamine, or K(+)-channel blockers: BaCl(2), glibenclamide, ouabain, and 4-aminopyridine. In isolated MA pre-contracted with Phe, PRF also induced a concentration-dependent relaxation (0.1 - 30 microg/mL), which was in turn inhibited by endothelial removal, guanylyl cyclase inhibitor 1H[1,2,3]oxadiazolo[4,3-alpha]quinoxalin, charybdotoxin (ChTx), and ChTx plus apamin. Moreover, the relaxant effect was not altered by HOE140 and apamin given alone. The present study demonstrates that the vasorelaxing effect of PRF is dependent upon the NO-cGMP pathway in combination with hyperpolarization due to activation of Ca(2+)-dependent K(+) channels.     

K channel openers and suppression of airway hyperreactivity

Openers of ATP-sensitive K⁺ channels (K_ATP channels) can reduce obstruction to airflow by suppressing hyperreactivity of intact airways. This property can be observed in hyperreactive animals with doses that are insufficient to relax airway smooth muscle in situ in normal animals. Hence, the potency of openers of K_ATP channels as inhibitors of bronchospasm is greater in hyperreactive than in normal animals. A closely analogous property has been described in clinical and laboratory studies of established anti-asthma drugs. Such findings raise the possibility that the therapeutic benefit of these drugs may depend upon an opening of K⁺ channels, either directly or indirectly. In this review, John Morley suggests that compounds that open K⁺ channels and impair expression of airway hyperreactivity in the absence of direct smooth muscle spasmolysis will provide a novel approach to symptomatic therapy in asthma.     

Hyperpolarisation of rat mesenteric endothelial cells by ATP-sensitive K(+) channel openers

Membrane potential responses of rat mesenteric endothelial cells were investigated in intact arteries using sharp electrodes. Levcromakalim, an activator of ATP-sensitive K(+) channels (K(ATP)) induced concentration-dependent hyperpolarisation of the endothelial cells, which was reversible by glibenclamide and ciclazindol, inhibitors of K(ATP). Another K(ATP) activator, diazoxide, also hyperpolarised the endothelial cells. Carbachol induced endothelial hyperpolarisation that was inhibited by combinations of apamin and charybdotoxin, but not Ba(2+) and ouabain. Prior stimulation with levcromakalim inhibited carbachol-induced responses, and this inhibitory effect was also sensitive to glibenclamide. 1, 3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl) -2H-benzimidazol-2-one (NS 1619), an activator of large conductance, Ca(2+)-activated K(+) channels (BK(Ca)), induced only small hyperpolarisations of the endothelial cells. Preincubation of tissues with 18 alpha- or 18 beta-glycyrrhetinic acid, inhibitors of gap junction communication, increased the input resistance and depolarised the endothelial cells, and inhibited the hyperpolarising effect of levcromakalim. It is concluded that activation of K(ATP) causes hyperpolarisation of rat mesenteric endothelial cells, probably through gap junctional transfer of smooth muscle hyperpolarisation, and that this may represent an important modulator of endothelial function.     

K+ channel openers suppress myotonic activity of human skeletal muscle in vitro

Isolated fibre bundles from myotonic human skeletal muscle showed after-contractions and spontaneous mechanical activity. The K+ channel openers cromakalim (10-100 mumols/l) and EMD 52962 (1-10 mumols/l) completely suppressed these abnormalities in mechanical activity. Voltage-clamp experiments revealed that cromakalim (100 mumols/l) increased the membrane K+ conductance of isolated, non-myotonic human skeletal muscle fibres 4-fold; Cl- conductance was not altered. The data show that myotonia is suppressed by an increase in in membrane K+ conductance.     

Comparison of the effects of the K(+)-channel openers cromakalim and minoxidil sulphate on vascular smooth muscle.

1 The actions of the potassium channel openers, cromakalim and minoxidil sulphate, were compared in a range of isolated blood vessel preparations. 2 Cromakalim and minoxidil sulphate inhibited spontaneous mechanical activity of the guinea-pig portal vein and relaxed the noradrenaline precontracted rat aorta with similar potency. In contrast, minoxidil sulphate was less potent than cromakalim in inhibiting spontaneous activity in the rat portal vein and was essentially inactive in the noradrenaline precontracted rat mesenteric artery and rabbit aorta. 3 Minoxidil sulphate did not antagonize the effects of cromakalim in the rabbit aorta indicating it was not acting as a partial 'agonist'. 4 Charybdotoxin, noxiustoxin and rubidium failed to discriminate between cromakalim and minoxidil sulphate indicating that the apparently selective effects of minoxidil sulphate were not mediated by either Ca(2+)-activated potassium channels, delayed rectifiers or rubidium impermeable potassium channels. 5 Glibenclamide antagonized the effects of cromakalim in an apparently competitive manner whereas the effects of minoxidil sulphate were antagonized in a non-competitive manner. The involvement of subtypes of ATP-sensitive potassium channels is discussed.