Melanocyte-Stimulating Hormone Response to Exercise, Twitching, Epinephrine Injection, Substance P Injection, and Prostaglandin-F2α Administration in Mares.

Five experiments were performed to test the hypothesis that α-melanocyte-stimulating hormone (MSH) is secreted in response to various stressors in horses similar to prolactin, growth hormone, and adrenocorticotropin (ACTH). There was considerable variation in resting concentrations of MSH and in the degree of stimulation in responders; thus all data sets were tested for heterogeneity of variance and corrected for as needed before analysis. In experiment 1, 12 mares were used in a switchback design to test the effect of a 2-minute exercise bout on MSH secretion. Plasma MSH concentrations were constant when mares were not exercised but increased (P < .05) immediately (2 minutes) after exercise and were still elevated 5 minutes later. In experiment 2, six mares were twitched for 2 minutes and six mares were not twitched. Twitching stimulated (P < .05) both MSH and ACTH relative to controls. Experiments 3, 4, and 5 tested the acute effects of intravenous injection of epinephrine at 5 μg/kg of body weight, intravenous injection of 100 μg substance P, and intramuscular injection of 10 mg prostaglandin-F2α in mares compared to controls (6, 5, and 6 mares per treatment group, respectively). Concentrations of MSH increased (P ≤ .05) after treatment in all three experiments. Plasma concentrations of ACTH also increased (P < .01) after administration of epinephrine and prostaglandin-F2α in experiments 3 and 5; plasma ACTH was not measured in experiment 1 or 4 because we have previously reported that exercise and substance P stimulate plasma ACTH concentrations. As hypothesized, MSH is secreted in response to various stimuli similar to that observed previously for prolactin, growth hormone, and ACTH.