Two studies were conducted to evaluate the effects of acute hyperprolactinemia on : 1) the timing of ovulation and 2) the termination of the luteal phase in Standardbred mares. Persistently elevated prolactin levels were established in regularly cycling mares by administering 25 mg metoclopramide (MCP; a dopamine antagonist), i.v., every 2 hours for 4 days, beginning either day 17 (follicular phase) or day 11 (luteal phase) of the estrous cycle. Control mares received saline only. In the first experiment (during the follicular phase), the number of days to ovulation from the start of infusion did not significantly differ between the MCP- and vehicle-treated groups (5.4±0.5 versus 6.3±1.3 days, respectively; P<.10). Profiles of serum concentrations of follicle stimulating hormone (FSH) and of luteinizing hormone (LH) aligned relative to day of ovulation were similar between treatment groups. In the second study (during the luteal phase), neither time to termination of the luteal phase (6.0±0.3 versus 5.3±0.3 days from start of treatment) nor time to ovulation (12.8±1.5 versus 9.8±0.5 days) was different in MCP- compared to vehicle-treated mares, respectively (P>0.05). Profiles of serum concentrations of progesterone, FSH and LH aligned relative to days from previous ovulation were not altered by MCP treatment. We conclude that a short-term elevation in serum prolactin, such as that which occurs during the immediate post-partum period, does not alter luteal function or the timing of ovulation in the cycling horse mare.
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