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Oxytocin (OT), progesterone and prostaglandin F2α (PGF2α) concentrations were measured in the utero-ovarian vein (UOV) of ewes which displayed persistence of the corpus luteum (CL). During the period of expected luteolysis, the frequency of OT and PGF2α pulses in the UOV was significantly (P < 0.005 for both) lower in ewes with persistent CLs, compared with ewes that underwent normal luteal regression. In contrast, the amplitude of both OT and PGF2α pulses was similar in both groups of animals. It is suggested that persistence of the CL resulted from a decreased PGF2α pulse frequency, which may have arisen from a decreased frequency of stimulation by OT. In two persistent CL ewes, however, it appeared that a failure at the level of the uterus may have contributed to the observed decrease in PGF2α release. Although a PGF2α analogue (Lutalyse) infusion into the uterine vein of two ewes with persistent CLs failed to induced luteolysis, it did stimulate a large release of OT into the UOV. This suggests that persistent CLs maybe more resistant to PGF2α and, that at day 22 post-oestrus, these CLs are capable of releasing large quantities of OT into the UOV.

A technique for implanting and maintaining catheters in both utero-ovarian veins (UOV) of intact, conscious sheep has been developed. Both UOV catheters remained patent for more than 2 weeks in 72.7% of animals. In non-pregnant ewes, large pulses of PGF2α were observed in UOV plasma collected on days 13 to 16 of the cycle (frequency 1.3 ± 0.2/12 h; pulse height range 1.0–26.9 ng/ml). In contrast, on days 13 to 16 of gestation, significantly (P < 0.001) fewer pulses of PGF2α (frequency 0.42 ± 0.2/12 h) were observed in both UOVs of pregnant ewes, which were significantly (P < 0.001) smaller in amplitude (pulse height range 1.4–4.9 ng/ml). The maintenance of the corpus luteum (CL) during early pregnancy may therefore directly result from a decreased uterine PGF2α release. During the luteal phase (day 4–12), the PO2 of UOV blood draining an ovary containing a CL (UOV/CL) was significantly higher than blood in the contralateral UOV (P 2 of UOV/CL blood had decreased and was not significantly different to that observed in the contralateral UOV (P >0.1). On day 17, the PO2 of blood in both UOVs was elevated significantly compared to levels observed on the two preceding and the two following days (P < 0.001). We consider that the higher PO2 in UOV/CL blood resulted from a very high luteal blood flow, and that the decrease on days 15 and 16 of the cycle resulted from a decline in luteal blood flow during luteolysis. The subsequent increase in PO2 of blood from both UOVs on day 17 was considered to result from an increased uterine blood flow.