Changes in plasma melanocyte-stimulating hormone, ACTH, prolactin, GH, LH, FSH, and thyroid-stimulating hormone in response to injection of sulpiride, thyrotropin-releasing hormone, or vehicle in insulin-sensitive and -insensitive mares.

This research examines how two hormones, sulpiride and thyrotropin-releasing hormone (TRH), affect the pituitary responses in horses that are either insulin-sensitive or insulin-insensitive. The study found that the hormone responses were generally higher in insulin-insensitive horses compared to insulin-sensitive ones.

Objective of the study

• The study was designed to investigate the hormonal responses of the pituitary gland in horses to the administration of sulpiride and thyrotropin-releasing hormone (TRH), both commonly used in the diagnosis of pituitary pars intermedia dysfunction (PPID), a common endocrine disorder in horses.

Research methodology

• The researchers used 12 mares, six of which were insulin-sensitive and six were insulin-insensitive. The insulin sensitivity of the mares was previously determined by their glucose responses to direct injections of human recombinant insulin.
• The study was conducted using a replicated 3 by 3 Latin square design, administering treatments of sulpiride, TRH, or saline (as the control vehicle) intravenously.
• Blood samples were collected from the mares at specific intervals following injection (from 10 minutes before to 120 minutes after), to determine concentrations of various hormones in the plasma.

Findings

• The study found that the plasma concentrations of ACTH did not vary with the treatment or the insulin sensitivity of the mares.
• However, the concentrations of melanocyte-stimulating hormone (MSH) and prolactin increased in response to both sulpiride and TRH, and the response was greater in insulin-insensitive horses.
• The thyroid-stimulating hormone (TSH) levels increased only in response to TRH, with a greater increase observed in the insulin-sensitive mares.
• The concentrations of LH and FSH and GH hormones varied with time, but were not affected by either the treatment or the insulin sensitivity of the horses.

Conclusion

• The findings suggest that the hormonal responses of the pituitary gland to sulpiride and TRH may be influenced by the insulin sensitivity of the horse. This could potentially have implications for diagnosing and understanding PPID in horses.
• The elevated MSH responses to sulpiride and TRH in insulin-insensitive horses were similar to those observed in horses with PPID, suggesting possible links between insulin resistance and PPID.