Spontaneous and stimulated adrenocorticotropin and vasopressin pulsatile secretion in the pituitary venous effluent of the horse.

This study observes the pulsatile secretion of ACTH and AVP hormones in horses, noting that these secretions are more frequent and brief than expected. The research also emphasizes a strong correlation between changes in these hormones within the pituitary venous effluent and indicates that the horse could serve as a valuable animal model for studying hormone secretion regulation.

Understanding the Research: Key Findings

• The researchers measured the plasma ACTH, arginine vasopressin (AVP), and catecholamines at 5-minute intervals in the pituitary venous effluent of an unanesthetized horse. They found pulses of ACTH and AVP to be frequent (usually between 15-25 minutes) and surprisingly brief (most lasting less than 10 minutes).
• The research showed a significant relationship in the changes of concentration of ACTH and AVP both at rest and following a mild hypoglycemic stimulus. Indeed, the correlation between the hormones was highly significant. This points to a close temporal relationship between those hormones’ fluctuations.
• The study demonstrated substantial variation in the pulse amplitude ratio of AVP to ACTH, even though a significant correlation was observed between simultaneous plasma ACTH and AVP values. This finding suggests that factors other than AVP might be influencing ACTH secretion.
• Notably, cortisol was found to have a greater suppressive effect on the ACTH pulse amplitude than on the AVP pulses. This observation suggests that cortisol might play a bigger role in controlling ACTH secretion than initially believed.
• The researchers identified significant gradients in the concentration of ACTH and AVP between the pituitary effluent and jugular plasma. In some instances, the gradient was over 50-fold for ACTH and 500-fold for AVP. This gradient was also observed for other hormones such as norepinephrine, epinephrine, and dopamine, but the correlation was only significant for norepinephrine.
• Lastly, through their findings, the researchers concluded that the horse may serve as a valuable animal model for studying the regulation of hypothalamic and pituitary hormone secretion. This conclusion is based on the ability to sample frequently for several hormones and to observe a significant gradient in hormonal secretion between the pituitary venous effluent and the jugular plasma.