The potassium channel opening action of pinacidil; studies using biochemical, ion flux and microelectrode techniques

In rat aorta and rat portal vein, (-)- and (+)-pinacidil each produced a concentration-dependent inhibition of tension development. Although the (-) isomer was the more potent, concentration effect curves for each isomer were steep with similar slopes. In rat portal vein, tetraethylammonium and procaine antagonised the relaxant effect of (+/-)-pinacidil, whereas 3,4-diamino-pyridine was without effect. Intracellular microelectrode recording in rat portal vein showed that low concentrations of (+/-)-pinacidil reduced the duration of multispike electrical complexes. In both rat aorta and rat portal vein, higher concentrations of (+/-)-pinacidil hyperpolarised the membrane towards the potassium equilibrium potential. (+/-)-Pinacidil increased 86Rb efflux from rat aorta and rat portal vein in a concentration dependent manner. In a separate study, (+/-)-pinacidil increased 42K efflux from rat portal vein. (+/-)-Pinacidil had no effect on cyclic GMP or cyclic AMP levels in rat aorta. It is concluded that pinacidil opens 86Rb-permeable potassium channels in rat aorta and rat portal vein. This mechanism is independent of cyclic nucleotide changes and may be responsible for the antihypertensive effect of pinacidil.