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

• Published in: Endocrinology (Philadelphia) Vol. 127; no. 3; p. 1136
• Main Authors: Gorodeski, G I, Sheean, L A, Utian, W H
• Format: Journal Article
• Language: English
• Published: United States 01.09.1990
• Subjects: Animals; Estradiol - pharmacology; Female; Mifepristone - pharmacology; Myometrium - drug effects; Myometrium - physiology; Oxytocin - metabolism; Oxytocin - pharmacology; Pregnancy; Pregnancy, Animal - metabolism; Progesterone - blood; Progesterone - pharmacology; Rats; Rats, Inbred Strains; Receptors, Angiotensin - metabolism; Receptors, Oxytocin; Uterine Contraction - drug effects
• ISSN: 0013-7227
• DOI: 10.1210/endo-127-3-1136

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.