Investigation of the effect of different regulatory peptides on adrenocortical cell proliferation in immature rats: Evidence that endogenous adrenomedullin exerts a stimulating action

Compelling evidence indicates that the active growth of immature rat adrenal glands is sustained not only by an increased release of pituitary ACTH, but also by other ancillary mechanisms. We investigated whether vasoactive intestinal peptide (VIP), atrial natriuretic peptide (ANP), adrenomedullin (ADM) and proadrenomedullin N-terminal 20 peptide (PAMP) play a relevant role in these mechanisms. These four regulatory peptides were chosen because previous studies demonstrated that they are expressed in rat adrenals and are able to modulate the secretory activity and growth of zona glomerulosa (ZG), i.e., the adrenal layer. Groups of immature (20-day old) rats were given three subcutaneous injections (28, 16 and 4 h before sacrifice) of 2 nmol/100 g of the four peptides and/or selective antagonists of their receptors (VIP-A, ANP-A, ADM-A and PAMP-A), and 0.1 mg/100 g vincristin 3 h before autopsy. Adrenal glands were collected, processed for light microscopy, and the mitotic index (MI; percentage of metaphase-arrested cells) was evaluated in the subcapsular ZG. Neither VIP nor VIP-A affected MI. Both ANP and ANP-A decreased MI and their effects displayed additivity. ADM and PAMP raised MI and the effect was abolished by ADM-A and PAMP-A, respectively. When administered alone ADM-A, but not PAMP-A, significantly lowered MI. Collectively, our findings suggest that: i) neither VIP nor PAMP are involved in the regulation of immature rat adrenals; ii) ANP exerts a non-receptor-mediated inhibitory action, whose physiological relevance remains to be investigated; and iii) endogenous ADM system plays a relevant role in the mechanisms underlying the maintenance of high growth rate during adrenal maturation.