Potassium efflux enhancement by cromakalim (BRL 34915) in rabbit mesenteric artery: an indirect effect independent of calcium?

The novel cardioprotective agent BMS-180448 activates a potassium conductance in cardiac and vascular smooth muscle

The goal of the present study was to further characterize the effects of the novel cardioprotective agent BMS-180448 on potassium fluxes in cardiac and vascular smooth muscle. Exposure of voltage-clamped guinea pig ventricular myocytes to BMS-180448 (300 microM) produced an inhibition of IK followed by the delayed (5.5 +/- 0.5 min) activation of a large time-independent potassium current. At 100 microM, BMS-180448 produced only inhibition of IK. The BMS-180448 activated current was refractory to block by 30 microM glyburide but was largely inhibited by 100 microM alinidine (84 +/- 6% inhibition at +40 mV). Cromakalim (100 microM)-activated currents were fully inhibited by 3 microM glyburide and 79 +/- 4% blocked by 100 microM alinidine. The current responses to BMS-180448 were unaffected by the inclusion of 10 mM UDP (100 microM ATP) in the pipette. BMS-180448 also produced a concentration-dependent increase in 86Rb efflux from aortic strips; efflux responses were increased in low calcium medium and fully antagonized by 3 microM glyburide. Thus, BMS-180448 activates a potassium conductance in both cardiac and smooth muscle. The glyburide sensitivity of the BMS-180448-induced increase in 86Rb efflux from the aortic preparations suggests that this drug activates IKATP in vascular smooth muscle. Moreover, the observation that BMS-180448 (100 microM) partially inhibits the effects of cromakalim in ventricular muscle cells suggests that these drugs interact, directly or indirectly, with a common site in cardiac muscle.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Potentiation of P1075-induced K+ channel opening by stimulation of adenylate cyclase in rat isolated aorta

1. The effects of analogues and stimulators of cyclic AMP on the 86Rb+ efflux-stimulating and binding properties of P1075, an opener of ATP-dependent potassium channels, were studied in rat aortic rings. The increase in 86Rb+ efflux stimulated by P1075 was taken as a qualitative measure of K+ channel opening. 2. Forskolin, a direct activator of adenylate cyclase, isobutylmethylxanthine (IBMX), a phosphodiesterase inhibitor, and dibutyryl-cyclic AMP (db-cyclic AMP), a membrane permeant cyclic AMP-analogue, relaxed rat aortic rings contracted by noradrenaline with EC50 values of 0.06, 2 and 10 microM, respectively. 3. Forskolin, IBMX and db-cyclic AMP produced concentration-dependent increases of the 86Rb+ efflux induced by P1075 (50 nM) by up to twofold with EC50 values of about 0.1, 1.7 and 81 microM. At these concentrations the agents had little effect on the basal rate of 86Rb+ efflux. 4. The 86Rb+ efflux produced by P1075 in the presence of the cyclic AMP stimulators was inhibited by glibenclamide, a blocker of ATP-sensitive potassium channels. 5. IBMX (100 microM) induced a leftward shift of the concentration-86Rb+ efflux curve of P1075 without increasing the maximum. The enhancements of P1075-stimulated 86Rb+ efflux produced by combinations of forskolin and IBMX were either additive or less than additive. 6. The protein kinase A inhibitor, H-89, inhibited P1075-stimulated 86Rb+ efflux in the presence of IBMX significantly more than in the absence of IBMX, suggesting that the effect of increased cyclic AMP levels is mediated by protein kinase A. 7. At high concentrations, forskolin and IBMX slightly increased basal 86Rb+ efflux and inhibited the tracer efflux induced by P1075.8. Binding of [3H]-P1075 to rat aortic rings was either unaffected or inhibited by forskolin, IBMX and db-cyclic AMP.9. This study shows that moderate stimulation of the cyclic AMP system potentiates the K+ channel opening effect of P1075 by activation of protein kinase A. The fact that binding of [3H]-P1075 remains unchanged or is diminished favours the hypothesis that the K'channel openers activate ATP-dependent K+ channels by an indirect mechanism.

Characteristics of the contractile response of rabbit aorta produced by cromakalim in calcium-free solution

1 The effect of potassium channel opening compounds has been investigated in the smooth muscle of rabbit aorta under Ca-free conditions. Examination of the characteristics of the response has been performed using cromakalim as the prototype compound. 2 In order of potency, Ro 31-6930, cromakalim, minoxidil sulphate and pinacidil each produced a contraction in rabbit aortic strips bathed in Ca-free MOPS-buffered physiological salt solution (PSS). In contrast, forskolin, glyceryl trinitrate and nifedipine each failed to increase tension under identical conditions. Cromakalim also evoked contraction of bovine trachealis muscle bathed in Ca-free PSS. 3. The contractile response to cromakalim in rabbit aortic strips was of delayed onset (15-20 min) and reached a plateau after approximately 120 min (1.8 g maximum with 1 microM cromakalim). No cromakalim-induced tension changes were observed in either 1 mM or 2.5 mM Ca-containing PSS. 4. Raising the [KCl] of the Ca-free PSS to 65.9 mM fully inhibited the cromakalim-induced contraction in rabbit aortic strips. In addition, pretreatment of aortic strips with the sulphonylurea glibenclamide antagonized the subsequent mechanical response to cromakalim. 5. In Ca-free PSS, cromakalim (1 microM) stimulated 42K-efflux with a time-course corresponding to the contractile event. Glibenclamide (1 microM) inhibited this cromakalim-induced 42K-efflux. 6. In sharp microelectrode studies in bovine trachealis, cromakalim (10 microM) produced a sustained membrane hyperpolarization in normal PSS. In contrast, the cromakalim-induced hyperpolarization in Ca-free PSS was not sustained. The fading of the hyperpolarization was temporally correlated with the increase in tension under these experimental conditions. 7. It is concluded that the K-channel opener-induced smooth muscle contractile response revealed in Ca-free PSS is the consequence of K-channel opening. The nature of the detailed mechanism which underlies this contractile phenomenon remains to be determined.

Cromakalim inhibits contractions of the rat isolated mesenteric bed induced by noradrenaline but not caffeine in Ca(2+)-free medium: evidence for interference with receptor-mediated Ca2+ mobilization

The effects of the K+ channel opener cromakalim on phasic contractions induced by noradrenaline and caffeine were studied in the rat isolated mesenteric bed. In the presence of 1.4 mM Ca2+, 1-s pulses of noradrenaline increased the perfusion pressure of the preparation concentration dependently (midpoint at 92 +/- 10 microM noradrenaline). Cromakalim (0.3 and 1 microM) inhibited these contractions in a non-competitive manner. Contractions elicited by 1-s pulses of noradrenaline (100 microM) were inhibited by the dihydropyridine Ca2+ antagonist isradipine by maximally 24 +/- 1%, indicating that only a minor component of this contraction depended on Ca2+ entry via dihydropyridine-sensitive Ca2+ channels. Cromakalim was a much more effective inhibitor of these contractions (maximum inhibition by 80%, midpoint of the inhibition curve at 171 +/- 15 nM). The effect of cromakalim was stereoselective, inhibited by the sulphonylurea glibenclamide, and abolished in partially depolarizing media (KCl = 35 and 50 mM). In Ca(2+)-free medium, cromakalim inhibited the contraction induced by noradrenaline (100 microM) by maximally 69 +/- 4%, with a midpoint at 58 +/- 14 nM. The effect of cromakalim was again stereoselective, inhibited by glibenclamide, and abolished in the presence of 50 mM KCl. Contractions induced by caffeine (10 and 100 microM) were not affected by cromakalim (1 microM). The results indicate that, in rat mesenteric resistance vessels, cromakalim interferes with the ability of noradrenaline, but not caffeine, to mobilize Ca2+ from intracellular stores. The antivasoconstrictor effect of cromakalim against noradrenaline is inhibited by glibenclamide and appears to be linked to the ability of cromakalim to hyperpolarize the cell membrane.

Effect of depolarizing and hyperpolarizing agents on the membrane potential difference of primary cultures of rabbit aorta vascular smooth muscle cells

Vascular smooth muscle cells of rabbit aorta were enzymatically dispersed, kept in primary culture, and studied between days 1 and 7 in a bath rinsed with Ringer-like solution at 37 degrees C. The electrical membrane potential difference (PD) was measured with microelectrodes. The mean value of PD was -50 +/- 0.4 mV (n = 53). Cromakalim (BRL 34915), 1 mumol/l and 10 mumol/l, hyperpolarized the membrane potential by 9 +/- 1 mV (n = 11) and 15 +/- 1 mV (n = 53) respectively. Glibenclamide (10 mumol/l) abolished the hyperpolarizing effect of chromakalim (n = 6). Simultaneous addition of cromakalim and glibenclamide (both 10 mumol/l, n = 11) and glibenclamide itself (10 mumol/l, n = 7) had no effect on PD. In patch-clamp experiments in outside-out-oriented Ca(2+)-sensitive K+ channels, cromakalim increased the open probability (Po) only slightly and only with a cytosolic Ca2+ activity of 1 mumol/l. In all other series cromakalim had no effect on the Po of these channels. Forskolin (10 mumol/l) hyperpolarized PD by 6 +/- 1 mV (n = 13). The nucleotides UTP, ATP and ITP (10 mumol/l) depolarized PD by 12 +/- 1 mV (n = 7), 8 +/- 1 mV (n = 65) and 5 +/- 1 mV (n = 6) respectively. GTP, [alpha, beta-methylene]ATP and adenosine had no significant effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in the K(+)-channels opened by cromakalim, acetylcholine and substance P in rat aorta and porcine coronary artery

1. The effects of acetylcholine and substance P on the efflux of 86Rb+ and 42K+ from rat aorta and pig coronary artery, respectively, were compared with those of the K+ channel opening agent, cromakalim. 2. In rat aorta preloaded with 86Rb+ and/or 42K+, acetylcholine produced transient, concentration-dependent increases in the efflux rate coefficients of these tracers (maximum approximately 35%). These effects were abolished by endothelial cell removal. 3. Donor/acceptor experiments with rat aorta suggested that at least some of the efflux of 86Rb+ seen in the presence of acetylcholine was not derived from the endothelium, but came from the smooth muscle itself. 4. Acetylcholine (10 microM)-induced 86Rb+ efflux was reduced by tetraethylammonium (TEA, 10 mM) to 33% and ouabain (300 microM) to 54% of control. Preincubation with Ba2+ (100 microM) did not significantly inhibit acetylcholine-induced efflux. 5. Acetylcholine-induced 42K+/86Rb+ efflux was unaffected by preincubation with glibenclamide (10 microM). In contrast, the 42K+/86Rb+ efflux induced by cromakalim was inhibited by glibenclamide (50 nM) by 50%. 6. Acetylcholine (0.3-10 microM)-induced inhibition of phenylephrine (1 microM)-induced tone was abolished by endothelial cell removal but unaffected by glibenclamide. Cromakalim-induced relaxations were endothelium-independent and were inhibited by glibenclamide in a concentration-dependent manner. 7. LG-monomethyl L-arginine (L-NMMA, 250 microM) produced a significant (37 +/- 14%) inhibition of acetylcholine-induced 86Rb+ efflux whereas DG-monomethyl L-arginine was without effect. In the tissue bath L-NMMA inhibited relaxations produced by acetylcholine (0.3-10 microM), but was without effect on responses to cromakalim. 8. In the pig coronary artery, substance P induced an endothelium-dependent efflux of 86Rb+ and 42K+, which was unaffected by preincubation with glibenclamide (10 microM) or L-NMMA (250 microM). 9. The present study shows that acetylcholine and substance P each open K(+)-channels in arterial smooth muscle. However, the insensitivity of the stimulated 86Rb/42K+ efflux to inhibition by glibenclamide suggests that the K(+)-channel opened by these agents is different from the K(+)-channel opened by cromakalim. In addition, the inability of L-NMMA to inhibit fully the acetylcholine- and substance P-stimulated 86Rb+ efflux suggests that in rat aorta and pig coronary artery the endothelium-derived hyperpolarizing factor(s) (EDHF) is different from endothelium-derived relaxing factor (EDRF).

Humoral and cellular effects of the K(+)-channel activator cromakalim in man

The effect of cromakalim, a K(+)-channel activator, on the plasma renin-angiotensin-aldosterone system, catecholamines and alpha-atrial natriuretic peptide, and on the intraerythrocyte concentration and transmembrane fluxes of Na+ and K+ has been investigated in 18 normal male subjects, in a double-blind parallel study. After a run-in period on placebo for 1 week, the subjects were treated either with placebo (n = 6) or cromakalim (n = 12) for 1 week. Plasma renin activity was significantly increased during cromakalim. No effect of cromakalim on plasma angiotensin II, aldosterone, adrenaline, noradrenaline and alpha-atrial natriuretic peptide was demonstrated. The intra-erythrocyte K+ concentration was decreased during cromakalim administration and Ca2(+)-dependent K(+)-channels in red blood cells were increased.

BRL 34915 (cromakalim) enhances voltage-dependent K+ current in cultured rat hippocampal neurons

The whole cell voltage-clamp recording technique was used to study the effects of the benzopyran antihypertensive agent BRL 34915 on voltage-dependent outward currents in cultured rat hippocampal neurons. Extracellular perfusion with BRL 34915 (10-500 microM) produced a dose-dependent increase in the sustained (minimally inactivating) voltage-dependent outward current without altering the transient outward current (IA) or producing a change in the resting current at -60 mV. One-half maximal facilitation of the sustained outward current occurred at 40 microM and maximal facilitation (59%) at 100 microM. The increase in outward current occurred at all potentials where hippocampal neurons exhibited outward rectification (greater than -30 mV). The effect of BRL 34915 was slow to develop (requiring as long as 5-15 min to achieve maximal effect) and persisted for at least 10-15 min after cessation of the drug superfusion. Like the sustained outward current recorded under control conditions, the outward current augmented by BRL 34915 was inhibited by the K+ channel blocker tetraethylammonium (20 mM). These results indicate that BRL 34915 can enhance the activity of sustained voltage-dependent K+ channels in mammalian CNS neurons.