Preovulatory surges of LH were experimentally induced in all animals by the intravenous administration of purified bovine LH (bLH; preovulatory LH surge = Day 0) starting 48 h after the second injection of PGF2a. Purified bLH, at 200 ^g/ml, had activity equivalent to that of standard available preparations (NIH-LH-S1 and LH-G3-226B). Administration of LH according to the protocol started about 24 h before the typical time of the endogenous preovulatory surge of gonadotropins. LH was administered at this time so that the LH surge to induce ovulation and stimulate the initial stages of luteinization would be similar in timing and amount among all animals. Doses of LH were calculated to achieve an initial concentration of 100 ng/ml of plasma (6.17 X 10 5 mg/kg BW) and to maintain a concentration of 50 ng/ml of plasma (3.09 X 10-5 mg/kg BW) with subsequent injections at 20-min intervals for 3 h.
Measurements and Sample Collections
To verify accuracy of exogenous bLH treatment to induce ovulation and initial stages of luteinization, blood samples were collected every 20 min from 2 h before the first bLH treatment to 2 h after the last injection, and samples were subsequently analyzed for concentrations of LH. Ovulation and development of the CL (diameter in millimeters) were monitored by ultrasonography using a 7.5-MHz linear array probe and monitor (Aloka 500V Coro-metrics) every day until Day 12 of the estrous cycle and every other day until the end of the experiment on Day 28.