Cloprostenol inouced luteal regression in the beef cow. I. Ovarian response and endocrine changes

Forty-two cycling, multiparous beef cows (percentage-Brahman) were given two injections of 500 ug Cloprostenol (CLP) 11 days apart. Cows were randomly allocated to be ovariectomized at 0, 12, 24, 36, 48, 60 or 72 hr after the second CLP injection. Mean CL weight declined within 36 hr after CLP. Mean concentration of P4 in luteal tissue increased between o and 60 hr, while mean P4 content per CL declined by 12 hr after CLP. There was a precipitous decrease in mean serum P4 by 12 hr following CLP injection. Serum E2 was elevated until 24 hr and then declined through 72 hr after CLP. Follicular T concentration increased from 0 to 48 hr and then decreased by 60 hr. We conclude that CLP caused rapid diminution of luteal function which was accompanied by a reduction in P4 content but not in P4 concentration. Futhermore, the concentration of E2 in large follicles decreased by 72 hr post-CLP which is consistent with an alteration of the steroidogenic pathway in the periovulatory follicle.