PARENT SESSION
Follicular Development

IN VIVO EVIDENCE FOR AN OXYTOCIN RECEPTOR-MEDIATED RESPONSE IN THE OVINE PREOVULATORY FOLLICLE.

Custer, Edward¹, McCracken, John², ^{1 2}

ABSTRACT- Oxytocin (OT) and its mRNA have been detected in bovine granulosa cells. In vitro, OT exerts a specific dose dependent stimulatory effect on progesterone (P) secretion by bovine granulosa cells obtained before, but not after the LH surge (Voss, A.K. and Fortune, J.E. (1991) Molec. Cell. Endo. 78:17-24). To determine if this phenomenon also occurred in vivo, we used ovarian autotransplant sheep which allow direct infusion of OT into the arterial supply of the ovary and the periodic collection of ovarian venous blood for P measurement. The cycles of transplant sheep (n=3) were synchronized with two i.m. injections of Lutalyse (5mg) given 4 hours apart. 24 hrs later, when a preovulatory follicle is known to be present, ovarian venous blood was collected at -20 and -10 mins (controls) before giving a 10 min intra-ovarian infusion of OT (625ng/min). Samples were taken at +2, 5, 10, 15, 30, 40, and 50mins. At +60mins (25hr post Lutalyse) an i.v. injection of GnRH (5g) was given to produce an LH surge. A 2^nd intra-ovarian infusion of OT (625ng/min) was given 15hrs post GnRH (i.e. after the preovulatory follicle was exposed to endogenous LH but before follicular rupture). Ovarian venous blood was collected at the same times as for the 1^st OT challenge. P conc. peaked sharply (150% of controls) during the 1^st OT infusion (pre-LH surge) but there was no change in P after the 2^nd OT challenge (post-LH surge). These in vivo findings indicate that OT stimulates P from the ovary with a preovulatory follicle before the LH surge but not after the LH surge and are consistent with results obtained with bovine granulosa cells in vitro. Taken together, these studies suggest that OT receptors may be present in granulosa cells before, but not after the LH surge. Such a transient expression of putative OT receptors may indicate a functional role for follicular OT in the granulosa/lutein cell shift. (Supported by USDA grant # 98-35203-6635)

KEY WORDS: follicle, oxytocin, oxytocin receptor, granulosa cell