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

摘要

25 normally cyclic Holstein heifers were used to examine the effects of oxytocin on cloprostenol-induced luteolysis, subsequent ovulation, and early luteal and follicular development. The heifers were randomly assigned to 1 of 4 treatment groups: (1) SC-SC (n = 6); (2) SC-OT (n = 6); (3) OT-SC (n = 6); and (4) OT-OT (n = 7). The SC-SC and SC-OT received continuous saline infusion, while OT-SC and OT-OT received continuous oxytocin infusion (1:9 mg/day) on days 14-26 after oestrus. All animals received i.m. injection of 500μg cloprostenol (Estrumate) 2 days after initiation of infusion (day 16) to induce luteolysis. SC-OT and OT-OT received oxytocin twice daily (12 h apart) (0.33 USP units/kg body weight, sc) on days 3-6 of the oestrous cycle following cloprostenol-induced luteolysis, while SC-SC and OT-SC received an equivalent volume of saline. Daily plasma progesterone (P4) concentrations prior to cloprostenol-induced luteolysis and rates of decline in P4 following the induced luteolysis did not differ between oxytocin-infused (OT-OT and OT-SC) and saline-infused (SC-SC and SC-OT) groups. Duration of the oestrous cycle was shortened in saline-infused heifers receiving oxytocin daily during the first week of the oestrous cycle. In contrast, oxytocin injections did not result in premature inhibition of luteal function and return to oestrus in heifers that received oxytocin infusion (OT-OT). Day of ovulation, size of ovulating follicle and time of peak LH after cloprostenol administration for oxytocin and saline-treated control heifers did not differ. During the first 3 days of the oestrous cycle following luteal regression, fewer (P<0.01) follicles of all classes were observed in the oxytocin-infused animals. Day of emergence of the first follicular wave in heifers treated with oxytocin was delayed (P<0.05). It is concluded that continuous infusion of oxytocin during the mid-luteal stage of the oestrous cycle has no effect on cloprostenol-induced luteal regression, timing of preovulatory LHpeak or ovulation. An episodic rather than continuous administration of oxytocin during the first week of the oestrous cycle results in premature loss of luteal function. There are minor inhibitory effects of oxytocin on follicular growth during the first 3 days of the oestrous cycle following cloprostenol-induced luteolysis.