Interestrous interval of cyclic gilts is decreased by systemic but not intrauterine administration of exogenous oxytocin

Oxytocin (OT) stimulates pulsatile secretion of uterine prostaglandinF(2 al pha) in ruminants, but the role of OT in the estrous cycle regulation of pi gs is not clear. These studies were performed to determine the effect of ex ogenous OT on interestrous interval of intact cyclic gilts. Intrauterine in fusion of 80 USP units three times daily on d 10 to 16 after estrus did not decrease interestrous interval (24.5 +/- 1.3 d) compared with control gilt s (22.5 +/- 1.3 d). In contrast, i.m. injections of 20 USP units of OT twic e daily or 80 USP units of OT three times daily on d 10 to 16 after estrus decreased (P < 0.05) interestrous interval (20.0 +/- 0.3 or 19.5 +/- 0.4 d, respectively) compared with control gilts (20.5 +/- 0.3 d). When gilts rec eived a single i.m. injection of 0 or 1 mg of estradiol valerate on d 11 an d twice daily i.m. injections of 0 or 20 USP units OT on d 10 to 16 after e strus, an interaction (P = 0.05) between OT and estradiol valerate was dete cted. In the absence of exogenous estradiol valerate, injection of OT decre ased interestrous interval (19.0 +/- 0.5 d) compared with injection of vehi cle (20.4 +/- 0.5 d). However, when gilts were injected with 1 mg of estrad iol valerate to inhibit luteolysis, OT did not prevent the increase in inte restrous interval (25.4 +/- 0.5 d) compared with injections of vehicle (24. 7 +/- 0.5 d). These results indicate that 1) exogenous OT administered by i ntrauterine infusion on d 10 to 16 did not decrease interestrous interval o f intact cyclic gilts, 2) exogenous OT administered i.m. on d 10 to 16 shor tened interestrous interval, and 3) exogenous OT did not prevent the increa se in interestrous interval induced by estradiol valerate.