[The coexistence of neuropeptides and catecholamines in the adrenal gland. Research on paracrine effects on adrenal cortex cells].
The aim of the present review was to compare in mammals and amphibians the data concerning the presence of neuropeptides in the chromaffin cells and the possible action of these substances on adrenocortical cell function. Major homologies are to be found concerning the coexistence in chromaffin granules of catecholamines, Met-and Leu-enkephalins, and their precursor, proenkephalin A. However, the inhibitory action that might be exerted by enkephalins in vitro on corticosteroid production in mammalian adrenal gland, does not occur in amphibia. Dynorphin has been isolated in bovine adrenal medulla extracts; the presence of this opioid peptide has not been reported yet in amphibian interrenal tissue. All chromaffin cells of the frog interrenal gland contain VIP-like immunoreactivity whereas this neuropeptide is not contained in the adrenal medulla of mammals, exept in certain pheochromocytomas. In the frog, VIP, Metenkephalin and catecholamines are co-sequestered in the same chromaffin granules. In addition, synthetic porcine or chicken VIP stimulate in vitro the secretion of corticosteroids by frog interrenal fragments. In mammals, the steroidogenic action of VIP has been observed exclusively in tumor cell lines. The existence of somatostatin has been demonstrated in the human adrenal medulla and in pheochromocytomas, but not in amphibia. Somatostatin has been found to inhibit the response of adrenocortical cells to angiotensin II in mammals. A similar effect of somatostatin was not observed in amphibia. The coexistence of VIP and catecholamines in frog chromaffin granules and the stimulatory effect exerted by VIP on corticosteroidogenesis suggest that, in these animals, VIP may be co-liberated with noradrenaline during stress conditions, and thus may act locally on adrenocortical cells to stimulate corticosteroid secretion.