Thyrotropin releasing hormone interactions with growth hormone secretion in horses.

The research explores how thyrotropin-releasing hormone (TRH) affects the secretion of growth hormone in horses, finding that TRH inhibits growth hormone release in response to certain stimuli, an effect that disappears after multiple injections of TRH.

Study Design and Participants

• The study used a variety of light horse mares, stallions, and geldings.
• The research goal involved extending observations regarding TRH’s inhibitive impact on growth hormone (GH) secretion, and the evaluation of the hypothesis that stimulation of endogenous TRH would decrease the normal rate of GH secretion.

Research Findings

• In Experiments 1 and 2, the researchers found that administering TRH to mares decreased the GH response to stimuli like exercise and aspartate infusion.
• Time analysis indicated a TRH inhibitory effect that lasted approximately 60 minutes, which was no longer apparent by 120 minutes.
• When stallions were given a single injection of TRH, there was no evident impact on GH concentrations, while prolactin concentrations did increase.
• Experiment 5 involved administering 11 hourly injections of TRH to geldings, which had no discernible effect on GH concentrations.
• Experiment 6 revealed that geldings, who were pretreated with five hourly injections of TRH, had exercise-induced GH responses similar to those of control geldings, suggesting the absence of the inhibitory effect after five TRH injections.
• Analyses of pooled data showed lowered endogenous GH concentrations 45 to 75 minutes after a TRH injection, but not thereafter.

Additional Experiments and Findings

• Experiment 7 involved feeding 6-n-propyl-2-thiouracil to stallions to decrease thyroid activity, potentially increasing endogenous TRH secretion.
• Treated stallions exhibited lowered concentrations of thyroxine and heightened thyroid-stimulating hormone levels. However, plasma concentrations of GH and prolactin remained unaffected.
• Notably, the GH response to aspartate and the prolactin response to sulpiride were significantly higher in treated stallions compared to controls.

Conclusions

• TRH was found to inhibit GH secretion triggered by exercise and aspartate, with the inhibitory effect lasting at least an hour but less than two hours.
• With multiple injections, this inhibitory effect of TRH seemed to wane.
• Reducing thyroid feedback did not result in changes to basal GH and prolactin secretion.