VASOACTIVE INTESTINAL POLYPEPTIDE (VIP) STIMULATES HORMONAL SECRETION OF THE RAT ADRENAL-CORTEX IN-VITRO - EVIDENCE THAT ADRENAL CHROMAFFIN CELLS ARE INVOLVED IN THE MEDIATION OF THE MINERALOCORTICOID, BUT NOT GLUCOCORTICOID SECRETAGOGUE ACTION OF VIP

The in vitro effect of vasoactive intestinal polypeptide (VIP) on mineralo- and glucocorticoid secretion was investigated in the rat by using adrenal slices, containing zona-medullaris cells, and fragments of adrenocortical autotransplants regenerated from gland capsular-tissue implants, which are completely deprived of chromaffin cells. VIP dose-dependently enhanced basal aldosterone (ALDO) and corticosterone (B) secretions by both kinds of tissue preparations. The maximal effect was observed at a concentration 10(-8) M, but the rise was about 3-fold higher in the case of ALDO secretion by adrenal slices. Corticotropin-inhibiting peptide (CIP), a competitive inhibitor of ACTH, and l-alprenolol (AL), a beta-adrenoceptor antagonist, did not affect basal steroidogenesis of both preparations. CIP (10(-6) M) completely annulled the maximal stimulatory effect of VIP on B production by adrenal slices and ALDO and B secretion by autotransplant quarters, but it did only reduce (by about 50%) that on ALDO production by adrenal slices. AL (10(-7) M) caused a 73% decrease in VIP (10(-8) M)-stimulated ALDO secretion by adrenal slices and when added with CIP (10(-6) M) completely annulled it. In light of these findings, it seems reasonable to suggest that VIP exerts a secretagogue effect on rat adrenal steroidogenesis by a two-fold mechanism: (i) non-specific stimulation of both mineralo- and glucocorticoid secretion by activation of a population of ACTH receptors, and (ii) specific stimulation of mineralocorticoid secretion through the enhancement of catecholamine release by chromaffin cells.