Effects of oxytocin on cloprostenol-induced luteolysis, follicular growth,ovulation and corpus luteum function in heifers

Twenty-five normally cyclic Holstein heifers were used to examine the effec ts of oxytocin on cloprostenol-induced luteolysis, subsequent ovulation, an d early luteal and follicular development. The heifers were randomly assign ed to 1 of 4 treatments: Group SC-SC (n = 6), Group SC-OT (n = 6), Group OT -SC (n = 6) and Group OT-OT (n = 7). The SC-SC and SC-OT groups received co ntinuous saline infusion, while Groups OT-SC and OT-OT received continuous oxytocin infusion (1:9 mg/d) on Days 14 to 26 after estrus. All animals rec eived 500 Gig, im cloprostenol 2 d after initiation of infusion (Day 16) to induce luteolysis. Groups SC-OT and OT-OT received oxytocin twice daily(12 h apart) (0.33 USP units/kg body weight, sc) on Days 3 to 6 of the estrous cycle following cloprostenol-induced luteolysis, while Groups SC-SC and OT -SC received an equivalent volume of saline. Daily plasma progesterone (P-4 ) concentrations prior to cloprostenol-induced luteolysis and rates of decl ine in Pq following the induced luteolysis did not differ between oxytocin- infused (OT-OT and OT-SC) and saline-infused (SC-SC and SC-OT) groups (P > 0.1). Duration of the estrous cycle was shortened in saline-infused heifers receiving oxytocin daily during the first week of the estrous cycle. In co ntrast, oxytocin injections did not result in premature inhibition of lutea l function and return to estrus in heifers that received oxytocin infusion (OT-OT). Day of ovulation, size of ovulating follicle and time of peak LH a fter cloprostenol administration for oxytocin and saline-treated control he ifers did not differ (P > 0.1). During the first 3 d of the estrous cycle f ollowing luteal regression, fewer (P < 0.01) follicles of all classes were observed in the oxytocin-infused animals. Day of emergence of the first fol licular wave in heifers treated with oxytocin was delayed (P < 0.05). The r esults show that continuous infusion of oxytocin during the mid-luteal stag e of the estrous cycle has no effect on cloprostenol-induced luteal regress ion, timing of preovulatory LH peak or ovulation. Further, the finding supp ort that an episodic rather than continuous administration of oxytocin duri ng the first week of the estrous cycle results in premature loss of luteal function. The data suggest minor inhibitory effects of oxytocin on follicul ar growth during the first 3 d of the estrous cycle following cloprostenol- induced luteolysis. (C) 2000 by Elsevier Science Inc.