Binding of the K+ channel opener [3H]P1075 in rat isolated aorta: relationship to functional effects of openers and blockers

P1075 [N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine], an analogue of the K+ channel opener pinacidil, was shown to be a K+ channel opener in rat aorta, based on (i) its ability to stimulate 86Rb+ efflux, (ii) its ability to relax contractions in response to noradrenaline under normal conditions (5 mM KCl) but not under depolarized conditions (55 mM KCl), and (iii) the sensitivity of these effects to inhibition by the sulfonylurea glibenclamide. In these assays, P1075 was approximately 20 times more potent than cromakalim. Using a tritiated derivative, [3H] P1075, specific binding could not be detected in microsomal preparations from various tissues. However, in rat aortic strips specific binding of [3H]P1075 has been observed and was reduced by lowering the temperature or by decreasing intracellular ATP levels via metabolic inhibition. Specific [3H]P1075 binding was influenced neither by depolarization (55 mM KCl) nor by lowering the pH from 7.4 to 6.0. Binding was inhibited by representatives from all major families of K+ channel openers, with potencies that correlated well with the potencies obtained in 86Rb+ efflux and relaxation studies. However, stimulation of 86Rb+ efflux occurred at 40 times higher concentrations than did binding (and vasorelaxation). Of the various inhibitors of the K+ channel openers tested, only the sulfonylureas inhibited [3H] P1075 binding with the same rank order of potencies as that required for inhibition of P1075-induced 86Rb+ efflux, although at higher concentrations. The results show that binding of [3H] P1075 is independent of membrane potential but decreases concomitantly with the intracellular ATP level. The excellent correlation between the potencies of the openers and sulfonylurea blockers in binding assays and functional studies suggests that the 'drug receptor' labeled by [3H]P1075 in rat isolated aorta is of functional relevance. However, the fact that binding of the openers occurred at concentrations considerably lower than those required for K+ channel opening and that binding of the sulfonylureas was only reflected at concentrations higher than those needed to block the channel requires complex models to link binding and effect, possibly involving two agonist binding sites coupled by negative cooperativity.