Effects of cromakalim (BRL 34915) on mechanical responses of rat vas deferens to noradrenaline and naphazoline.

The contractile responses produced by noradrenaline were compared to those produced by naphazoline in the rat isolated vas deferens, a preparation which is usually quiescent. The responses to noradrenaline were biphasic: prominent spontaneous contractile activity was superimposed on contractions produced by naphazoline. After removal of naphazoline the rat vas deferens displayed periodic spontaneous activity. The effects of cromakalim and verapamil were compared on contractions induced by noradrenaline and naphazoline and on spontaneous activity induced by exposure to naphazoline. Cromakalim (1-10 x 10(-7) M) shifted to the right the cumulative concentration-response curve of naphazoline but not that of noradrenaline; at the same range of concentrations, cromakalim inhibited naphazoline-induced periodic spontaneous activity. Verapamil (3-30 x 10(-8) M) flattened in a dose-dependent manner the concentration-response curves of noradrenaline and naphazoline; higher concentrations of verapamil (3-30 x 10(-6) M) were required to modify periodic spontaneous activity. It was concluded that naphazoline but not noradrenaline can block K+ channel(s) and that this property can represent an important difference between imidazolines and phenethylamines.