LH secretion during the periovulatory period and corpus luteum development, LHRH regulation of FSH secretion and progesterone metabolism, in cattle

The objectives of experiments 1 and 2 described in this dissertation were to determine the effects of gonadotropin release around the periovulatory period of the estrous cycle on subsequent ovarian function in cattle. When heifers were treated with daily injections of a LHRH antagonist starting 48 h before, during or 48 h after the preovulatory surge of LH, development of the corpus luteum (CL) following ovulation was decreased according to time of initiation of treatment. Initiation of treatment 48 h before the preovulatory surge of LH resulted in development of a smaller CL. Additionally, function of the resulting corpora lutea in treated heifers was also impaired. Secretion of progesterone in treated heifers represented about 25% of controls. Furthermore, treatment of heifers with an LHRH antagonist before or during the preovulatory surge of LH resulted in inhibition of the secondary rise in FSH which is thought to be responsible for initiation of growth and development of ovarian follicles. Number and size of ovarian follicles developing in heifers treated with LHRH antagonist were reduced as a consequence and no waves of follicular development were observed. In the third study discussed herein, the objective was to estimate the amounts of progesterone metabolized by the brain and liver of cattle during the luteal phase of the estrous cycle. Heifers with indwelling cannulae in the carotid artery, and jugular, hepatic, mesenteric and portal veins were studied to determine concentrations of progesterone in circulation and to estimate blood flow through the liver and portal drained viscera. Progesterone uptake by the brain during the luteal phase of the estrous cycle was about 15%. Progesterone uptake by the liver and portal drained viscera of heifers was about 75% of total progesterone in circulation. Moreover, differences were found in amounts of progesterone taken up by the liver among heifers