UNUSUAL EFFECT OF PROLONGED VASOACTIVE-INTESTINAL-PEPTIDE (VIP) ADMINISTRATION ON THE ADRENAL GROWTH AND CORTICOSTERONE SECRETION IN THE RAT

VIP is widely distributed in the body, and among others is present wit hin the hypothalamo-pituitary-adrenal axis. The aim of this study was to investigate the effect of prolonged VIP administration on the struc ture and function of the rat adrenal cortex. Adult female rats were tr eated for 7 or 14 days with VIP (1.5 mug/100 g/day). This treatment di d not change adrenal weight and evoked only slight changes in the ster eologic parameters of adrenal cortex (volume of adrenocortical zones, average cell volume and number of cells in the gland). On the contrary , basal plasma corticosterone levels were markedly lowered (44% and 26 % of control values in 7- and 14-day experiments, respectively), an ef fect accompanied by a significant decrease in corticosterone output by adrenal slices. After standardized ether stress, blood corticosterone concentration was similar in control and 14-day VIP-treated rats, whi le corticosterone secretion by adrenal quarters was higher in VIP-admi nistered animals. Thus, prolonged VIP administration results in an unu sual inhibition of corticosterone secretion in the rat, which is not c oupled with a compensatory hypertrophy of the rat adrenal cortex. Such changes may depend on a modulatory action of VIP on the hypothalamo-p ituitary-adrenal axis, and its possible mechanism is discussed.