Possible relationship between the release of calcium ions and inactivation of the potassium conductance induced by noradrenaline in smooth muscle of the guinea-pig vas deferens.

Abstract

In order to examine whether any change in cytoplasmic Ca2+ concentration occurs during the noradrenaline (NA)-induced change in the membrane potential, the isometric tension of smooth muscle of the guinea-pig vas deferens was recorded together with membrane potentials using the sucrose-gap method. NA induced a depolarization and an increase in cytoplasmic Ca2+ comprising two (initial and second) phases. In time courses, the depolarization and the second phase of the increase in cytoplasmic Ca2+ were similar. Only at the phase around the onset of the depolarization, the membrane conductance was increased but then, it was decreased. Lowering the external Ca2+ concentration and application of D600 suppressed all of the NA-induced responses. In addition, D600 diminished the size of the NA-induced decrement in the membrane conductance. The present results suggest that the initial phase of the increase in the cytoplasmic Ca2+ is due to the influx of Ca2+ and the second phase, probably to the release of Ca2+ from some intracellular stores. It was discussed that the release of Ca2+ might occur at the internal surface of the membrane, and the membrane might be left less permeable to K+ as a result.