Growth hormone in mares and stallions: pulsatile secretion, response to growth hormone-releasing hormone, and effects of exercise, sexual stimulation, and pharmacological agents.

This research article explores how various factors such as exercise, sexual stimulation, and certain drugs can affect the secretion of growth hormone (GH) in mares and stallions, and sees no significant influence of sex or seasonality on GH concentrations. The researchers also examined the animals’ response to the administration of growth hormone-releasing hormone (GHRH) and found varying, pulsatile secretion patterns of GH.

Study Design and Methods

• The research was conducted through a series of experiments. In Experiment 1, the researchers took hourly blood samples from three mares and three stallions in both summer and winter seasons. Measurments of GH concentration in the blood were recorded.
• In Experiment 2, blood samples were drawn every 10 minutes for a duration of 8 hours from 12 mares. After 6 hours, the mares were administered varying doses of porcine GH-releasing hormone (GHRH) intravenously.
• In Experiment 3, the investigators studied the effects of short bouts of physical exercise or exposure to an estrus mare on plasma GH concentrations in stallions. They measured the fluctuations in GH concentrations after physical restraint and administration of epinephrine.
• Experiment 4 and 5 were designed to study the effects of thyrotropin-releasing hormone (TRH) and sulpiride, a dopamine receptor antagonist, on plasma prolactin and GH concentrations.

Key Findings

• GH concentrations varied in a pulsatile manner in all horses.
• There was no significant effect on GH concentrations based on sex or season.
• Pulsatile secretion of GH occurred in all mares following administration of GHRH, showing varying amplitude range.
• Physical exercise and sexual stimulation caused significant increases in plasma GH concentrations in stallions.
• Restraint using a twitch and administration of epinephrine also triggered a significant increase in plasma GH concentrations.
• Administration of TRH or sulpiride boosted plasma prolactin concentrations. However, they had no significant effect on GH concentrations.

Conclusions

• The research establishes that exercise, sexual stimulation, and certain pharmacological agents can significantly affect the secretion of GH in mares and stallions.
• However, sex or season seems to have no substantial effect on GH secretion amongst the sample group.
• The response of mares to GHRH injection was variable but had no significant relationship with the dosage of GHRH administered.
• The two tested pharmacological agents notably affected plasma prolactin levels while leaving GH concentrations unchanged.