A CELLULAR AND ENDOCRINE CHARACTERIZATION OF THE ORIGINAL AND INDUCEDCORPUS-LUTEUM AFTER ADMINISTRATION OF A GONADOTROPIN-RELEASING-HORMONE AGONIST OR HUMAN CHORIONIC-GONADOTROPIN ON DAY 5 OF THE ESTROUS-CYCLE

To determine whether injection of hCG or GnRH-agonist on d 5 after est rus (d 0) has a differential functional effect on an induced and the o riginal corpus luteum (CL), two experiments were conducted. In Exp. 1, nonlactating Holstein cows were injected on d 5 with saline (n = 4; T 1), a GnRH-agonist (Buserelin(R), 8 mu g i.m.; n = 4; T2), or hCG (1,0 00 IU, i.v., and 2,000 IU, i.m.; n = 4; T3). Induced CL were removed o n d 13 and weights were different (GnRH-agonist < hCG). In vitro produ ction of progesterone by CL tissue (mu g/g; mu g/CL) was affected by t reatment (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 w as removed on d 17 for in vitro culture. Day-17 CL weights and in vitr o production of progesterone by original CL were not affected by treat ment. 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, th e rate of plasma progesterone decrease was not different between treat ments. Electron micrographic study of the original and induced CL indi cates 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.