Effects of oxytocin on ionic currents underlying rhythmic activity and contraction in uterine smooth muscle.

The influence of oxytocin on the electrical and mechanical activity of uterine smooth muscle strips was studied under voltage-clamp conditions. 1. At a concentration of 0.1 mU/ml, oxytocin caused a slight depolarization of the resting potential and also increased the amplitude of the action potential. The maximal frequency of the rhythmic activity, which can be produced by depolarizing current pulse, is increased by about 20%. 2. Oxytocin increased the peak of the inward current without modification of the reversal potential. This effect is enhanced in a sodium-free solution. With oxytocin the steady-state inactivation of the inward current is not modified and the increase in the current intensity can be related to an increase in the maximal conductance. The amplitude of the outward current is not affected. 3. The first component (phasic-like) of the contractile response obtained for brief depolarizations is increased by oxytocin. This effect may be explained by the increase in the intensity of the inward current. The second component (tonic-like) of the contraction associated with long-lasting depolarizations and obtained in manganese-containing solution is not modified. The increased frequency of the rhythmic activity after oxytocin administration may also result in increased contractility by summation.