Abstract
The contracture produced by removal of the external Na was studied in the pregnant rat myometrium and this was compared with that of the non-pregnant myometrium. The Na-free contracture was small compared with spontaneous phasic contractions in the circular muscle of non-pregnant rat, but the contracture increased during pregnancy. On the other hand, in the longitudinal muscle, the contracture was large in both non-pregnant and pregnant myometria. The change in circular muscle did not seem to result from an increased sensitivity to the external Ca. Since the contracture induced by 40mM K had properties similar to those of the Na-free contracture, it is likely that weak tension development in non-pregnant circular muscle is either due to a limited Ca influx or due to a low efficiency of the excitation-contraction coupling for the sustained contraction compared with the phasic contraction. A small contraction which was independent of the external Ca was observed in Na-free solution in the both layers of non-pregnant myometrium, but this was not observed in the longitudinal muscle of pregnant myometrium.
