On the control of prostaglandin release during the bovine estrous cycle. Effects of cloprostenol injections.

The study comprise seven cows which were injected i.m. with 500 microgram of a synthetic prostaglandin analogue (cloprostenol) at different stages of their estrous cycles. Peripheral blood plasma levels of progesterone and the main blood plasma metabolite of prostaglandin F2 alpha, 15-keto-13,14-dihydroprostaglandin F2 alpha were determined. The injection of cloprostenol initiated luteolysis in all cases as evidenced by decreasing progesterone levels and occurrence of heat. In two animals injected on days 6 and 12, respectively, of the estrous cycle no release of endogenous prostaglandin F2 alpha was found. In animals injected on day 13 and 15 of their estrous cycles, respectively, a minor but sustained release of prostaglandin F2 alpha was found. In three animals injected on days 17, 17 and 19 of their estrous cycles, respectively, the endogenous release of prostaglandin F2 alpha had already started and no specific effect attributed to the injection of cloprostenol could be observed. It is concluded that the injection of cloprostenol initiates luteolysis with subsequent heat and ovulation. The release of endogenous prostaglandin F2 alpha occasionally seen in conjunction with the induced luteolysis is different and less pronounced as compared to the release seen during normal luteolysis in the cow.