The oestrous cycle of the ewe after active immunization against prostaglandin F-2 alpha

Six ewes were immunized against a prostaglandin F-2alpha-protein conjugate. Between 24 and 82 days after immunization the regular cyclic occurrence of oestrus was abolished in all six ewes. Further investigations of the immunized animals revealed that the blockade of oestrus was due to a persistence of the CL and the constantly elevated (greater than 2 ng/ml) blood levels of progesterone. Surgical enucleation of the persistent CL was promptly followed by a fall in progesterone concentrations (less than 0-5 ng/ml), normal oestrus and a subsequent return to a state of constantly elevated blood progesterone levels. These results show that neutralization of the biological activity of PGF by active immunization against PGF-2alpha results in a failure of luteal regression and provide evidence that endogenous PGF is involved in normal luteal regression in this species.