Myometrial oxytocin receptors levels in the pregnant rat are higher in distal than in proximal portions of the horn and correlate with disparate oxytocin responsive myometrial contractility in these segments.

Distal (D) segments of the pregnant rat uterine horn express myometrial oxytocin receptors (MORs) earlier than proximal (P) segments (day 18 vs. 20, respectively); the levels in D segments remain higher than in P segments throughout days 21-22 and correlate with the segment-specific myometrial sensitivity to oxytocin. While progesterone (P4) had no effect on MOR levels, RU486 increased (t1/2 6-12 h) MOR levels both in D and P segments, particularly in days 12-17, and the levels in the P segment equaled those in the D segment. Estradiol had no effect on MOR levels in days 20-22; in days 16-19 estradiol increased MOR levels particularly in the P segment, and the levels in the latter were higher than in the D segment. Capillary plasma P4 levels were higher in P vs. in D myometrial segments. These results indicate, in the pregnant rat, a local uterine control of D greater than P MOR expression by P4 withdrawal beginning in day 18. We hypothesize that the D greater than P MOR expression determines a role for oxytocin in initiating myometrial contractions in the D segment, while in active labor another class of agent(s) assume that function in more proximal segments.