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Effects of Equimate (ICI-81008)¹ on Levels of Luteinizing Hormone, Follicle-Stimulating Hormone and Progesterone during the Estrous Cycle of the Mare

Colorado State University, Fort Collins 80523²

Abstract

Jugular blood was collected at 6-hr intervals during estrus and at daily intervals during diestrus from 12 cycling mares. On day 8 of diestrus four mares were given saline (im) while the remaining eight mares were given Equimate (250 µg), a synthetic analog (ICI-81008) of PGF₂. Following treatment, jugular blood was collected at 15-min intervals for 6 hr and then at 6-hr intervals until the end of the subsequent estrus. Blood was again collected at daily intervals until the end of the diestrus following treatment. Day of ovulation was estimated by palpation per rectum. Sera were analyzed for LH, FSH and progesterone by radioimmunoassay.

Injection of Equimate on day 8 of diestrus resulted in an immediate increase in levels of LH and FSH in serum. Concentrations of LH increased from 35.6 ± 8.2 ng/ml to 89,8 ± 25.2 ng/ml (P<.05) within 45 min after injection but had returned to baseline within 3 hours. Concentrations of FSH increased from 178 ± 34 ng/ml to 382 ±0 86 ng/ml (P<.05) within 45 min after the injection and then returned to baseline over the next several hours. Concentrations of progesterone decreased to baseline (< 1 ng/ml) within 24 hr after Equimate and remained low until the end of the next estrus. Concentrations of LH and FSH during the estrus preceding Equimate and the estrus subsequent to Equimate did not differ (P>.05). Likewise, there was no difference (P>.05) between levels of progesterone during the diestrous periods of the cycles preceding, and subsequent to, Equimate. Similarly, concentrations of LH, FSH and progesterone were not different (P>.05) between mares treated with saline and those treated with Equimate during subsequent estrous cycles. Equimate appears to be an effective agent for reducing concentrations of progesterone in mares without interferring with the normal secretion of hormones during the subsequent estrous cycle.

¹ Supported in part by a contract from ICI United States, Inc., Wilmington, DE.

² Animal Reproduction Laboratory, Department of Physiology and Biophysics.