Specificity of action of the novel antihypertensive agent, BRL 34915, as a potassium channel activator. Comparison with nicorandil.

Experiments have been performed to investigate the specificity of the mechanism of action of the novel antihypertensive agent, BRL 34915. BRL 34915 (0.5-100 microM) and nicorandil (10-500 microM) stimulated the efflux of rubidium from preloaded rabbit isolated mesenteric arteries. BRL 34915 also caused an increase in the rubidium efflux rate constant in other vascular smooth muscles. Tetraethylammonium (0.1-30 mM) inhibited BRL 34915 (10 microM), nicorandil (100 microM) and potassium (30 mM) induced stimulations of rubidium efflux, but had no effect on noradrenaline (30 microM) induced efflux. Only noradrenaline induced efflux was inhibited by apamin (3-100 nM). Examination of other second messenger systems demonstrated that BRL 34915 (at concentrations up to 100 microM) did not have any appreciable effect on cGMP accumulation in rabbit mesenteric artery, cAMP or cGMP phosphodiesterase in rat heart, or cAMP and inositol phosphate accumulation in rat brain slices. Nicorandil (100 microM) caused a small increase in cGMP accumulation in rabbit mesenteric artery. Radioligand binding studies showed that BRL 34915 did not interact with dihydropyridine, 5-hydroxytryptamine, dopamine, alpha 1, alpha 2 or beta adrenoceptor binding sites. [3H]-BRL 34915 did not bind specifically to any site in any tissue studied, either in vitro or ex vivo. Thus we have been unable to demonstrate an effect of BRL 34915 other than of increasing potassium efflux in rabbit vascular smooth muscle. This lends support to other evidence suggesting that BRL 34915 relaxes vascular smooth muscle (and hence lowers blood pressure) by a novel, and specific, mechanism involving hyperpolarisation of the smooth muscle cell membrane.