Action of D-600 on spontaneous and electrically stimulated activity of the parturient rat uterus.

The effects of D-600, the methoxy derivative of verapamil, on the parturient rat uterus have been studied. At concentrations of 10(-8) to 10(-7) M, D-600 abolishes spontaneous contractions and reduces the force of electrically evoked contractions. The first action is associated with a reduction in amplitude of the slow waves (oscillations of membrane potential which underlie rhythmic bursts of spikes initiating contractions). The second is associated with a reduction in frequency of spikes within the bursts accompanying the electrically stimulated contractions. This reduction results from a decrease in the slope of the prepotential in cells in pacemaker areas of the muscle. D-600 also decreased the height and rising velocity of the conducted action potentials, but these actions cannot account entirely for the reduction in contractile force.