Action of angiotensin II on the electrical and mechanical activity of rat uterine smooth muscle.

The effects of angiotensin II (A II) were studied on membrane potentials, ionic currents and isometric contractions in uterine smooth muscle strips from pregnant rats. In reference solution, A II in a low concentration (5 x 10(-9) M) exerted no effect on the resting potential or the action potential amplitude but significantly increased the rate of repetitive firing. However, in tetraethylammonium chloride (TEA)-containing solution, an increase in the action potential amplitude was always observed. The inward and outward currents were increased simultaneously, so that it was necessary to block the outward current with TEA ions in order to measure the enhancement of the inward current. The reduction of the deactivation time constants of the outward current as well as the increase in the inward current intensity could account for the acceleration of rhythmic activity in the presence of the peptide. The mechanical response to A II consisted of early tetanus which was followed by increased twitch contractions. The calcium contractures were also increased in depolarized strips. The tetanus could be dependent on the increase in and prolongation of repetitive firing while the enhancement of twitch contractions and calcium contractures could be explained by the increase in inward current intensity.