Inhibition of Myometrial Activity in the Pregnant Rabbit: Evidence for a “New” Factor

When non-pregnant rabbits were linked to 20- or 29-day pregnant rabbits by a cross-circulation technique, the frequency of intra-uterine pressure cycles in the former fell from pre-treatment values of around 8 cycles/10 min to 0.4 cycles/10 min at the end of cross-circulation (P < 0.001). Maximum amplitude fell less abruptly from 29 mm Hg to 19 mm Hg (P < 0.05). Following the cross-circulation which lasted 1 h, activity recovered so that in approximately 2 h maximum amplitude had returned to pretreatment levels and frequency to 50% of pretreatment levels. Similar experiments using progesterone-treated non-pregnant animals in the place of the pregnant does had no effect on the uterine activity of their partners during 1 h of cross-circulation despite the fact that the blood progesterone level of the treated animals was 93 ng/ml. Intravenous injections of large doses of progesterone were also ineffective within a similar time period. Since the latency of the progesterone effect on the myometrium is 8–12 h, the existence of a second myometrial inhibitory substance which acts in conjunction with progesterone in maintaining pregnancy is postulated.