Effect of insulin-induced hypoglycaemia on secretion patterns and rates of corticotrophin-releasing hormone, arginine vasopressin and adrenocorticotrophin in horses.

The research is about understanding how insulin-induced hypoglycemia affects the secretion rates of certain hormones in horses. It found that insulin administration led to increased secretion rates of corticotrophin-releasing hormone (CRH), arginine vasopressin (AVP), and adrenocorticotropic hormone (ACTH).

Method and Experimentation

• The researchers conducted a non-surgical method to collect blood from the pituitary veins of seven horses every 0.5 to 1 minute before and after the administration of insulin.
• Another group of horses were not exposed to the pituitary vein cannulation process, to compare the hormonal secretions. These were used as the control group.
• The given insulin dosage was 0.4 U/kg, administered intravenously. This dosage successfully lowered the plasma glucose levels in all horses studied.

Results

• The researchers detected a significant increase in cortisol concentrations after insulin administration in both groups of horses.
• Most horses showed discomfort as glucose levels fell.
• The degree of cortisol increase and the severity of hypoglycaemic symptoms were found to be inversely ranked with the glucose nadir (the point at which glucose levels were the lowest).
• The administration of insulin led to increased secretion rates of ACTH, AVP, and CRH in horses with PitVen cannulae. The hormonal increments were inversely ranked with the glucose nadir.

Interpretation

• In individual horses, minute-to-minute AVP and ACTH concentrations in PitVen blood were always correlated. However, minute-to-minute CRH and ACTH concentrations were correlated only when glucose dropped below 3.4 mmol/l.
• In less hypoglycaemic horses, ACTH secretion rose despite little or no change in CRH.
• The researchers suggest that in horses, AVP is the primary acute signal for ACTH release both before and during hypoglycaemia. However, the increasing magnitude of ACTH increments induced by greater degrees of hypoglycaemia is determined largely by selective CRH release, which then augments corticotroph responses to AVP.

Conclusion

• These findings deepen the understanding of the role of hypoglycemia in the secretion rates of ACTH, AVP, and CRH in horses, and can potentially assist in the study of similar phenomena in other non-ruminant species.