Coexistence of vasoactive intestinal peptide and enkephalins in the adrenal chromaffin granules of the frog

The chromaffin cells of the frog adrenal gland were studied at the electron microscopic level by means of the immunoperoxidase technique. Using specific antibodies against Met-enkephalin, Leu-enkephalin and vasoactive intestinal peptide (VIP), the coexistence of the three neuropeptides in the chromaffin cells was demonstrated. Furthermore, all three peptides were co-located in the 200-400 nm dense-core vesicles which also stained for dopamine-beta-hydroxylase. These results, which suggest concomitant release of catecholamines and neuropeptides by the chromaffin cells, support the concept that VIP is involved in the stimulation of adrenal steroid secretion during stress conditions.