The integrative control of adrenocorticotrophin secretion: a critical role for corticotrophin-releasing hormone.

This study investigates how the hormone adrenocorticotrophin (ACTH) is regulated, focusing on the relationship between ACTH and substances that either stimulate (corticotrophin-releasing hormone (CRH), and arginine vasopressin (AVP)) or inhibit (cortisol) its secretion, using an in-vitro system with equine anterior pituitary cells.

Research Methodology

• The researchers conducted their study using perifused equine anterior pituitary cells. The experiments were set up to imitate the hormonal conditions found in a horse’s body naturally.
• The hormones cortisol and CRH were continuously perifused at concentrations of either 0 or 100 nmol/l for cortisol and 0 or 0.02 nmol/l for CRH.
• AVP was released in 10 nmol/l pulses, applied for 5 minutes at 30-minute intervals. The researchers then observed the effects on ACTH secretion.

Findings

• The researchers found that in the presence of 0.02 nmol CRH/l, 100 nmol cortisol/l had no significant overall impact on how ACTH reacted to pulses of AVP. ACTH’s total area response to the first and second AVP pulses was significantly decreased in the presence of 100 nmol cortisol/l. However, ACTH’s incremental area was not affected by cortisol at any AVP pulse.
• Contrastingly, in samples perifused with no CRH, the presence of 100 nmol cortisol/l significantly decreased both the total and incremental areas of ACTH in response to all AVP pulses.
• The study also noted a decreased ACTH response over time. This reduction was significantly more pronounced in the presence of 0.02 nmol CRH/l and 0 nmol cortisol/l and when 6 pulses of AVP were given. But no difference was observed in the fall off with time when there was no presence of CRH, regardless of whether cortisol was present or not.

Conclusions

• The results suggest that ACTH secretion is not regulated solely by independent actions of secretagogues like CRH and AVP or inhibitors like cortisol. Instead, a complex interplay between these factors determines the extent of ACTH production.
• The researchers hypothesize that CRH might have a role in protecting the ACTH response to AVP pulses, especially in the presence of cortisol. This could indicate that in living organisms, maintained ‘background’ CRH could promote ACTH response to AVP, even when cortisol levels are high due to stress.