Investigations on calmodulin antagonistic effects of bepridil in intact and skinned fibres of smooth muscle.

Potassium-induced contractions of rat aortic strips are completely inhibited by N-benzyl-beta-(isobutoxymethyl)-N-phenyl-1-pyrrolidine-ethylamine (bepridil, Cordium) 10(-5) mol/l. In contrast, contractions induced by norepinephrine are only partially inhibited (by 40%) by identical doses of bepridil. Since the latter effect could be due to an inhibition of calcium release from intracellular compartments it was investigated how bepridil influences the effect of calcium on functionally isolated contractile structures. Smooth muscle preparations (guinea pig taenia coli and pig coronary artery) were skinned with octoxinol and suspended in adenosine triphosphate salt solution. During long-term incubation (greater than 40 min) of skinned fibres in low-calcium solution (pCa greater than 8) the calmodulin (CaM) is extracted. A calcium-induced contraction occurs only by adding CaM, showing the necessity of CaM for contraction. In the presence of bepridil 10(-4) mol/l the following effects were observed: submaximal contractions induced by calcium (1.1-1.6 mumol/l) were inhibited completely, the effects of calcium (4 mumol/l and 30 mumol/l) were inhibited by 83 +/- 4.5% and 53 +/- 10.5% respectively (mean +/- S.E.M.; n = 5). These effects could be completely antagonized by addition of calmodulin, final concentration 5 mumol/l. Verapamil did not show any inhibitory effects even in high concentrations (up to 10(-3) mol/l). Thus the effects of bepridil on the contractile mechanism in vascular smooth muscle are at least partly mediated through an intracellular mechanism - most probably inhibition of calmodulin.