Arginin-vasopressin regulates proliferative activity of the regenerating rat adrenal cortex

Enucleation-induced adrenal regeneration is a classic model to investigate adrenocortical proliferation in vivo, which is dependent not only on pituitary ACTH release, but also on various other neural and endocrine signals. Arginin-vasopressin (AVP), mainly acting via V1 receptors, regulates hypothalamic-hypophyseal-adrenal axis function, acting on both its central and peripheral branches. Here, we studied whether endogenous AVP system modulates rat adrenal regeneration. Reverse transcription-polymerase chain reaction (PCR) detected only the mRNAs of V1a and V1b receptors in normal and regenerating adrenals. The expression was very low, and semi-quantitative conventional and real-time PCR showed that it was down-regulated in regenerating adrenals in relation to the time elapsed from enucleation. AVP (three subcutaneous injections 28, 16 and 4 h before sacrifice) raised metaphase index at day 5, but not at day 8 of regeneration. Unexpectedly, both V1-receptor and V2-receptor antagonists increased metaphase index at days 5 and 8 of regeneration. Neither AVP nor AVP-receptor antagonists affected plasma levels of corticosterone in rats bearing regenerating adrenals. It is concluded that AVP, acting via V1 receptors located in adrenals, exerts a stimulating effects on adrenal regeneration. Due to the down-regulation of V1-receptor expression in regenerating adrenals, this effect is very weak and is easily overcome by a tonic inhibitory action of endogenous AVP systems probably involving extra-adrenal indirect mechanisms.