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1. Cromakalim (1 and 10 microM), RP49356 (5 and 50 microM) and diazoxide (100 and 300 microM) produced full relaxation of smooth muscle strips pre-contracted with 25 mM KCl. These agents caused membrane hyperpolarization and increased 42K and 86Rb efflux. The time taken to achieve the maximum change in each of these parameters (tmax) was less for the higher concentration levels of cromakalim, RP49356 and diazoxide than for the lower concentration levels. 2. Calculation of permeability (P) changes showed that cormakalim (1 and 10 microM) produced a greater rise in PK than PRb, although the PRb:PK ratio was similar at both concentration levels. Similarly RP49356 produced a greater change in PK than PRb. However, in contrast to cromakalim, this difference was more marked at the higher concentration (50 microM) and was reflected by a differential effect of the two concentrations of RP49356 on the PRb:PK ratio. Diazoxide (100 and 300 microM) produced similar changes in PK and PRb. 3. For cromakalim (1 and 10 microM) the tmax for the electrical and mechanical effects and also the profile of change in these parameters corresponded to changes in both PK and PRb. For RP49356 (5 microM), changes in tension and membrane potential were related to both changes in PK and PRb, whereas at 50 microM these responses more closely corresponded to changes in PK. For diazoxide (100 and 300 microM) the electrical and mechanical effects corresponded to changes in both PK and PRb. 4. The results show that changes in 42K and 86Rb efflux induced by cromakalim, RP49356 and diazoxide are good indicators of changes in membrane PK and PRb evoked by these agents. Furthermore, it is concluded that the K channels involved in the mechanical and electrical effects of cromakalim are represented by the opening of a single population through which Rb can pass less easily than K, whilst the K channels associated with actions of diazoxide are equally permeable to both K and Rb. In contrast, the relaxant and membrane hyperpolarizing actions of RP49356 may involve the opening of more than one group of K channels which differ in their permeability to Rb.

The effects of pinacidil have been compared with those of glyceryl trinitrate (GTN) using the aorta and portal vein of the rat and the trachealis and taenia caeci of the guinea-pig. In aorta, both pinacidil and GTN inhibited responses to noradrenaline and showed some selective inhibition of contractions to 20 mM K+. Responses to 80 mM K+ were little affected. In trachealis, both pinacidil and GTN inhibited spontaneous tone and selectively relaxed spasms to 20 mM K+. Responses to 80 mM K+ were unaffected. In portal vein, pinacidil completely inhibited spontaneous electrical and mechanical activity. GTN reduced the amplitude of tension waves and extracellularly-recorded discharges, but increased the frequency of spontaneous electrical and mechanical activity. In portal vein, pinacidil inhibited contractions to noradrenaline and selectively inhibited responses to 20 mM K+. GTN had little inhibitory effect on responses to either noradrenaline or K+. In portal veins loaded with 86Rb as a K+-marker, pinacidil significantly increased the 86Rb efflux rate coefficient whilst GTN had no effect on 86Rb exchange. In taenia caeci, both pinacidil and GTN inhibited the spontaneous tone of the preparation. These inhibitory effects were not antagonized by apamin. It is concluded that pinacidil and GTN do not share a common relaxant mechanism. Evidence has been obtained that pinacidil exerts its inhibitory effects by the opening of apamin-insensitive, 86Rb-permeable K+ channels.

(+/-) 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.