PROLACTIN ADMINISTRATION TO SEASONALLY ANESTROUS MARES - REPRODUCTIVE, METABOLIC, AND HAIR-SHEDDING RESPONSES

Eight pony mares received 4 mg of recombinant porcine prolactin (rpPRL ) daily for 45 d beginning on January 15; eight control mares received vehicle. Reproductive end points and various indicators of metabolism , hair shedding, and thyroid activity were monitored. Prolactin concen trations peaked in mares treated with rpPRL at 94 +/- 19 ng/mL 2 h aft er injection and were 5.1 +/- 1.7 ng/mL 24 h after injection. Treatmen t with rpPRL increased (P < .01) hair shedding within 14 d, which peak ed at 28 d and then dropped precipitously. Binding of I-125-equine pro lactin confirmed that antibodies were present in rpPRL-treated mares b y 28 d, thus only data up to that time are included herein. The percen tage of mares in estrus after 17 d of treatment was greater (P < .05) for mares treated with rpPRL than for controls, although the average d ay of estrus onset did not differ (P > .1). The date of first ovulatio n, determined from plasma progesterone concentrations, was hastened (P < .005) in mares treated with rpPRL (February 6 +/- 3 d) relative to control mares (March 14 +/- 6 d). Concentrations of insulin-like growt h factor I and nonesterified fatty acids were not affected (P > .1) by treatment with rpPRL but did vary (P < .05) over time. There were min or differences between groups in thyroid hormone secretion. Various as sessments of glucose and insulin indicated no perturbation (P > .1) du e to treatment with rpPRL. We conclude that prolactin mediates the ons et of vernal hair shedding in mares and is in some way involved with o vulation.