Concurrent uterine venous and ovarian arterial prostaglandin F concentrations in heifers treated with oxytocin.

In preliminary studies 5 of 6 heifers given oxytocin subcutaneously on Day 2--6 of the oestrous cycle had depressed progesterone levels by Day 5, and each had a precocious oestrus and a normal preovulatory LH surge. Oxytocin treatments given once daily on Days 5 and 6, or twice daily on Day 5 and 6 or 4, 5 and 6 resulted in shortened cycles and lower jugular progesterone concentrations in 1 of 5, 2 of 4 and 4 of 5 heifers, respectively. On the basis of these data, uterine venous and ovarian arterial catheters were inserted into 17 animals on Day 3 of the oestrous cycle and oxytocin treatments were given twice daily on Days 4, 5 and 6. Oxytocin treatment significantly depressed jugular progesterone levels by Day 8 and increased uterine venous prostaglandin (PG) F concentrations between 30 and 240 min after each injection. No changes of PGF concentrations occurred in uterine venous blood in control heifers, or in ovarian arterial blood from oxytocin-treated or control animals. It therefore appears that some factor other than PGF or a mechanism other than local veno-arterial transfer of PGF is involved in mediating the inhibitory effects of oxytocin on bovine corpus luteum function.