A Cellular and Endocrine Characterization of the Original and Induced Corpus Luteum After Administration of a Gonadotropin-Releasing Hormone Agonist or Human Chorionic Gonadotropin on Day Five of the Estrous Cycle1,2,3

To determine whether injection of hCG or GnRH-agonist on d 5 after estrus (d 0) has a differential functional effect on an induced and the original corpus luteum (CL), two experiments were conducted. In Exp. 1, nonlactating Holstein cows were injected on d 5 with saline (n = 4; T1), a GnRHagonist (Buserelin, 8 mg i.m.; n = 4; T2), or hCG (1,000 IU, i.v., and 2,000 IU, i.m.; n = 4; T3). Induced CL were removed on d 13 and weights were different (GnRH-agonist < hCG). In vitro production of progesterone by CL tissue ( mg/g; mg/CL) was affected by treatment (GnRH-agonist < hCG) and dose of LH (ng/mL) in culture media. Experiment 2 was a replicate of Exp. 1, except that the original CL was removed on d 17 for in vitro culture. Day-17 CL weights and in vitro production of progesterone by original CL were not affected by treatment. The daily rate of increase of plasma progesterone from d 6 to d 13 differed: saline < GnRH-agonist < hCG ( P < .01). From d 14 to 17, the rate of plasma progesterone decrease was not different between treatments. Electron micrographic study of the original and induced CL indicates that LH-like exposure delays involution of steroidogenic luteal cells. In summary, the higher levels of progesterone from d 6 to d 13 of the estrous cycle following an injection of hCG vs GnRH-agonist on d 5 is due to a greater response of hCG-induced CL.