BRL 34915 (cromakalim) activates ATP-sensitive K+ current in cardiac muscle.

Abstract

The mechanism by which the antihypertensive agent BRL 34915 (cromakalim) affects action potential duration (APD) and effective refractory period (ERP) in isolated cardiac muscle was investigated. BRL 34915 (13 pM) shortened ERP of ferret (Mustelaputoriusfuro) and guinea pig (Cavia porcellus) papillary muscles in a concentrationdependent fashion. The reduction in ERP resulted from a decrease in APD. ERP and APD of papillary muscles were also reduced during hypoxia produced by bubbling the physiological bathing solution with N2 instead of 02. Reduction of APD during hypoxia has previously been attributed to activation of ATP-sensitive K+ channels in heart. Glyburide, an inhibitor of ATP-sensitive K+ channels, prevented or reversed the shortening of ERP and APD produced by hypoxia and BRL 34915, respectively. These results suggest that BRL 34915 acts by opening ATP-sensitive K+ channels in heart. The actions of BRL 34915 were temperature-dependent, decreasing ERP 64% at 3TC, but having'no effect at 22C. The effect of BRL 34915 on K+ currents was tested directly in voltage-clamped guinea pig ventricular myocytes. As observed with the papillary muscles, BRL 34915 was without effect at 22rC. At 360C, BRL 34915 (after a delay) increased outward currents positive to, and less so at potentials negative to, the K+ current reversal potential. The normal inwardly rectifying current-voltage relationship for peak K+ currents during 200-msec pulses was changed to one that was nearly ohmic. The current activated by BRL 34915 was blocked by glyburide. The data support the hypothesis that BRL 34915, like hypoxia, activates ATPsensitive K+ channels in the heart. Based upon the profound temperature sensitivity of BRL 34915 action, this activation may be indirect, perhaps by means of modulation of an enzymatic activity that regulates gating of these channels. BRL 34915 and glyburide will be valuable tools for studying the role of ATP-sensitive K ' channels in normal and abnormal cardiac function.Potassium (K+) channels of cardiac muscle cells have many functions, including setting ofthe resting membrane potential and determining refractoriness. Some of these channels are regulated by neurohormones, such as acetylcholine (1) physiological concentrations of K+ (5). Thus, total outward K+ conductance is dramatically increased when IK(ATP) channels are activated. Activation of these channels, such as during experimentally induced internal ATP depletion, hypoxia, or treatment with uncouplers of the oxidative phosphorylation pathway (4-8), markedly shortens action potential duration (APD). These channels are probably activated during transient periods of ischemia in intact hearts, perhaps protecting ischemic cells from Ca2+ overload (9) but also increasing extracellular [K and causing a dispersion in refractoriness, events that can precipitate arrhythmias (10).Study of IK(ATP) channels has been facilitated by the discovery that sulfonylurea antidiabetic drugs, such as glyburide, are specific blockers ofthis channel in pancreati...

Effects of cromakalim on the membrane potassium permeability of frog skeletal muscle in vitro

“…potent in smooth muscle where the EC50 for the increase in 86Rb efflux is around 1 JM (Quast & Cook, 1989). In cardiac muscle the EC50 is in the range 5-30 JM (Osterrieder, 1988;Escande et al, 1988;Sanguinetti et al, 1988) whilst in insulin-secreting cells cromakalim is relatively ineffective (Garrino et al, 1989;Dunne et al, 1990). Our results in frog skeletal muscle, like those of Spuler et al (1989) in human muscle suggest a potency between those in cardiac muscle and insulin-secreting cells.…”

Alkali cation permeability and caesium blockade of cromakalim‐activated current in guinea‐pig ventricular myocytes

“…A relatively small outward K + current is also evident at positive potentials (see below) and inward Na+ and Ca2+ currents inactivate rapidly and make little contribution to whole-cell current measured at the end of 100-200 ms voltage steps. described here, outward current measured at +80 mV in the presence of 100puM cromakalim (a concentration that induces an effect 80% of maximal) was 4.4 + 0.5 nA (range 1.5-12.5 nA, 31 cells) while under control conditions it was 0.65 + 0.03 nA, in good agreement with previous reports (Escande et al, 1988;Sanguinetti et al, 1988 Figure 2b). …”

“…As illustrated in previous reports (Sanguinetti et al, 1988;Liu et al, 1990) ATP in cardiac myocytes can induce a decrease (rather than an increase) in total inward current at negative potentials associated with the appearance of a large outward current at positive potentials (Spruce et al, 1986; and pancreatic fl-cell (Ashcroft et al, 1989). This may reflect differences in the permeability characteristics of K(ATP) channels of various tissues, in addition to their differing sensitivities to cromakalim, sulphonylureas and ATP (for review see Quast & Cook, 1989b).…”

“…Cromakalim is a benzopyran derivative that activates ATPsensitive K+ [K(ATP)] channels in cardiac myocytes (Escande et al, 1988;Sanguinetti et al, 1988), arterial smooth muscle cells (Standen et al, 1989) and an insulin-secreting cell line (Dunne et al, 1989). Since the relaxation of smooth muscle and decrease of blood pressure that it induces are inhibited by sulphonylureas such as glibenclamide (Standen et al, 1989;Quast & Cook, 1989a;Winquist et al, 1989;Cavero et al, 1989;Eltze, 1989) which block K(ATP) channels (SchmidAntomarchi et al, 1987;Sturgess et al, 1988), K(ATP) channel activation may be the primary mechanism of action of cromakalim.…”

Effects of beraprost on the transmembrane potentials of guinea‐pig ventricular muscles during normoxia and hypoxia‐reoxygenation

“…Two hypotheses have been proposed to explain the APD shortening produced by hypoxia or metabolic inhibitors, i.e., a decrease in a slow inward current (McDonald et al, 1973;Schneider et al, 1974) and an increase in a potassium outward current (Vleugels et al, 1980;Isenberg et al, 1983). Since the discovery of ATP-sensitive K+ (K+ATP) channels in cardiac muscle (Noma, 1983), it has been assumed that activation of K+ATP channels plays a major role in the APD shortening during hypoxia or metabolic inhibition (Noma & Shibasaki, 1985;Fosset et al, 1988;Sanguinetti et al, 1988;Friedrich et al, 1990) and probably during myocardial ischaemia in vivo.…”