ABSTRACT
The present study was attempted to investigate the effect of vasoactive intestinal polypeptide (VIP) on secretion of catecholamines (CA) and to establish whether there is the existence of a noncholinergic mechanism in adrenomedullary CA secretion from the isolated perfused rat adrenal gland. The perfusion into an adrenal vein of VIP (3 X 10-6 M) for 5 min or the injection of acetylcholine (ACh, 5.32 X 10-3 M) resulted in great increases in CA secretion. Tachyphylaxis to releasing effect of CA evoked by VIP was not observed by the repeated perfusion. The net increase in adrenal CA secretion evoked by VIP still remained unaffected in the presence of atropine or chlorisondamine. However, the CA release in response to ACh was greatly inhibited by the pretreatment with atropine or chlorisondamine. The releasing effects of CA evoked by either VIP or ACh were depressed by pretreatment with nicardipine, TMB-8, and the perfusion of Ca2+-free medium. Moreover, VIP- as well as ACh-evoked CA secretory responses were markedly inhibited under the presence of (Lys1, Pro2.5, Arg3.4, Tyr6)-VIP or naloxone. CA secretory responses induced by ACh and high K+ (5.6 X 10-2 M) were potentiated by infusion of VIP (3 X 10-6 M for 5 min). Taken together, these experimental results indicate that VIP causes CA release in a fashion of calcium ion-dependence, suggesting strongly that there exists a noncholinergic mechanism that may be involved in the regulation of adrenomedullary CA secretion through VIP receptors in the rat adrenal gland, and that VIP may be the noncholinergic excitatory secretagogue present in the chromaffin cells.