Diminution of the in Vitro Response to Luteinizing Hormone by Corpora Lutea Induced by Gonadotropin Releasing Hormone Treatment of Postpartum Suckled Beef Cows

Abstract

Ovaries were removed from eight beef cows that were exhibiting ovarian cycles and 13 that had been given 250 µg gonadotropin releasing hormone (GnRH) during the postpartum period. The two groups (estrous cycle cows and postpartum GnRH-treated cows) were further divided into subgroups, with corpora lutea (CL) removed on either day 5 or day 7 (day 5 CL and day 7 CL) after estrus or GnRH treatment. Blood was collected and the plasma analyzed for progesterone, and luteal cells were dispersed and incubated with lutenizing hormone (LH) and(or) PGF2α. Eight of the postpartum suckled cows ovulated in response to GnRH treatment, as evidenced by ovulation papillae, and one cow had ovulated before GnRH treatment. Only cows that ovulated in response to GnRH treatment were included in the subgroups. Although corpora lutea weights did not differ between estrous cycle cows and postpartum GnRH-treated cows on day 5, corpora lutea were more than two times larger in estrous cycle cows than in postpartum GnRH-treated cows on day 7. Plasma progesterone concentrations increased with time to day 5 for both estrous cycle cows and postpartum GnRH-treated cows. A divergence between the progesterone concentrations in the two groups was observed between days 5 and 7; as the concentration of progesterone continued to increase to day 7 in estrous cycle cows, concentrations declined in postpartum GnRH-treated cows. Response of luteal tissue from postpartum GnRH-treated cows to LH on day 7 was similar to the response on day 5, but the response of luteal tissue from estrous cycle cows to LH on day 7 was at least 2.5 times greater than that on day 5. It is concluded from these data that corpora lutea in estrous cycle cows became responsive to LH in vitro between days 5 and 7 and continued to develop. Corpora lutea from postpartum GnRH-treated cows, however, did not become responsive to LH in vitro and, therefore, did not continue to develop beyond day 5 post-treatment.