The effects of cortisol, vasopressin (AVP), and corticotropin-releasing factor administration on pulsatile adrenocorticotropin, alpha-melanocyte-stimulating hormone, and AVP secretion in the pituitary venous effluent of the horse.

This research paper investigates how cortisol, vasopressin, and corticotropin-releasing factor affect the secretion of adrenocorticotropin, alpha-melanocyte-stimulating hormone, and vasopressin in horses. The results indicate that, despite variations in the concentrations of these hormones, their pulsatile secretion continues under the administration of cortisol, corticotropin-releasing factor, or vasopressin.

Research Methods

• The research was carried out by observing the plasma ACTH, arginine vasopressin (AVP), and alpha MSH levels at 5-minute intervals from five unanesthetized horses during cortisol infusion and after administering AVP or ovine (o) CRF through an intravenous bolus.
• In control experiments where no hormone was administered, the researchers studied the correlation between the timing of concentration changes in ACTH and alpha MSH.

Observations and Findings

• During cortisol infusion, there was an observed 70% increase in jugular cortisol levels. This was associated with a drop in average ACTH, AVP, and alpha MSH secretion rates and ACTH peak secretion rate. However, the pulse frequencies of these hormones remained unchanged.
• Upon administration of AVP, there was a rise in plasma concentrations to levels corresponding to spontaneous peaks in pituitary venous blood. This resulted in increased ACTH and alpha MSH secretion in all horses. Additionally, spontaneous AVP peaks in pituitary venous blood were noticed 90-180 minutes after AVP injection, which indicates exogenous hormone did not stifle AVP secretion.
• Administering oCRF led to a sustained elevation in plasma CRF and an increase in ACTH and alpha MSH secretion, but not AVP secretion, in all horses. Despite plasma CRF levels in excess of 400 pmol/liter, the pulsatile secretion of ACTH and alpha MSH continued. The timing of concentration changes in AVP and ACTH remained highly correlated.

Conclusion

• The experiment concluded that pulsatile ACTH secretion continues during cortisol, oCRF, or AVP administration. Similarly, the secretion of alpha MSH is stimulated by oCRF and AVP administration and suppressed by cortisol.
• While the timing of concentration changes in ACTH and alpha MSH is highly correlated, the correlation of the actual concentrations of these two hormones showed significant variation among different horses.