University of Florida

Daniel C. Sharp, Ph.D.

The laboratory addresses two areas of reproductive biology: 1) mechanisms of pregnancy establishment, and 2) mechanisms of seasonal reproductive cycle control. We have demonstrated that maternal production of prostaglandin F2-alpha (PGF) on or about 14 days after ovulation is instrumental in regressing the corpus luteum. In the presence of a conceptus, however, maternal endometrial PGF production is markedly reduced, indicating a conceptus mediated PGF inhibitory mechanism. Cervical/uterine stimulation results in oxytocin and prostaglandin F2-alpha metabolite (PGFM) release in nonpregnant mares, but in pregnant mares, the release of PGFM is blocked, further supporting the concept of an equine conceptus PGF-inhibitory substance. Current studies include efforts to identify the equine PGF-inhibitory factor.

We have also demonstrated that the pineal gland is involved in timing of the annual reproductive cycle. Current studies indicate that exposure to increased photoperiod creates sensitivity to the positive feedback effects of estrogen on pituitary Luteinizing Hormone (LH) synthesis and secretion, possibly through pineal-mediated events which are yet to be defined. "Downstream" of the pineal, our studies are focused on regulation of pituitary gonadotropin synthesis and regulation. Using pituitary stalk-transected mares, we have shown that administration of Gonadotropin-Releasing Hormone (GnRH), but not estrogen, results in re-establishment of messenger ribonucleic acid (mRNA) encoding for LH subunit (alpha or beta) synthesis. These studies have led to the hypothesis that hypothalamic-pituitary signal transduction in horses may be different from established dogma in other domestic species. We have also shown that disparate gonadotropin secretory pattern (high Follicle Stimulating Hormone, FHS: low LH) during the vernal transition from anestrus into the breeding season is associated with development of three to four successive anovulatory follicles displaying abnormal steroidogenic patterns. Analysis of mRNA encoding steroidogenic enzymes demonstrated that the presumptive site of "steroidogenic defect" may lie in the 17-alpha-hydroxylase enzyme. Current studies are also examining the potential role of cholesterol side-chain cleavage enzyme and abundance of gonadotropin receptors in these systems.

Representative Publications:
Vernon, M.W., M.T. Zavy, R.L. Asquith and D.C. Sharp. Prostaglandin F2 in the Equine Endometrium: Steroid modulation and production capacities during the estrous cycle and early pregnancy. Biol. Reprod. 25:581-589, 1981.

Zavy, M.T., M.W. Vernon, D.C. Sharp and F.W. Bazer. Endocrine aspects of early pregnancy in pony mares: A comparison of uterine luminal and peripheral plasma levels of steroids during the estrous cycle and early pregnancy. Endocrinology 115:214-219, 1984.

Zavy, M.T., D.C. Sharp, F.W. Bazer, A. Fazleabas, F. Sessions and R.M. Roberts. Uterine protein secretions of the mare during the oestrous cycle and pregnancy. Identification of stage specific and hormonally induced polypeptides using two dimensional gel electrophoresis. J. Reprod. Fertil. 64:199-207, 1982.
Sharp, DC, Thatcher, MJ, Salute, ME, and Fuchs, AR. Relationship between endometrial oxytocin receptors and oxytocin-induced prostaglandin F2-alpha release during the oestrous cycle and early pregnancy in pony mares. Jour Reprod Fertil 109:137-144, 1997.

Porter, MB, Cleaver, BD, Robinson, G, and Sharp, DC. The effect of pulsatile gonadotrophin-releasing hormone and estradiol administration on luteinizing hormone and follicle stimulating hormone concentrations in pituitary stalk-sectioned ovariectomized pony mares. Domestic Animal Endocrinology 14:275-285, 1997.

Porter, MB, Cleaver, BD, Peltier, MR, Robinson, G, and Sharp, DC. The effect of pulsatile gonadotropin-releasing hormone and estradiol administration on luteinizing hormone and follicle stimulating hormone concentrations in pituitary stalk-sectioned ovariectomized pony mares. Domestic Animal Endocrinology 14:275-285, 1997. 

Return to top


sharp

Daniel C. Sharp, Ph.D.
Professor Emeritus
Department of Animal Sciences
Ph.D., University of Massachusetts
Postdoc., University of Wisconsin