In vitro studies on the mode of action of pinacidil.

(+/-) Pinacidil inhibited noradrenaline-induced contractions in rat aorta and portal vein. The spontaneous tone of guinea-pig bronchial and taenia caeci muscles was relaxed and the spontaneous mechanical activity of rat portal vein was abolished. (+/-) Pinacidil abolished contractions produced by low concentrations of KCl in rat aorta and portal vein, but had relatively little effect on responses to high KCl concentrations. The mechano-inhibitory effects of (+/-) pinacidil were antagonised by tetraethylammonium or procaine. In studies with the purified enantiomers of pinacidil, (-) pinacidil was approximately 20 times more potent that (+) pinacidil. Measurements of electrical activity showed that low concentrations of (+/-) pinacidil selectively inhibited spontaneous electrical discharges in rat portal vein. The duration of multispike electrical complexes was shortened, but spike frequency within a complex and the rate of spike rise and fall were unaffected. At higher concentrations, a dose-dependent hyperpolarisation was observed in both rat aorta and portal vein and the membrane potential approached EK. Using both 86Rb and 42K, (+/-) pinacidil produced a concentration-dependent increase in isotope exchange which correlated with those concentrations at which electrical and mechanical inhibitory effects were observed. Using radioimmunoassay, no pinacidil-induced changes in cyclic AMP or cyclic GMP concentrations were detected in rat aorta. These electrical, ion flux and biochemical measurements suggest that the in vitro mechano-inhibitory effects of pinacidil are associated with the opening of 86Rb-permeable K+ channels in smooth muscle. These effects were observed at concentrations of pinacidil similar to those found in vivo in the plasma of experimental animals and man. It is thus concluded that the hypotensive and antihypertensive effects of pinacidil are the consequence of the cessation of ongoing electrical activity and hyperpolarisation which follows the opening of K+ channels in vascular smooth muscle.