The Localization and Release of Immunoreactive Vasoactive Intestinal Polypeptide (VIP) in Bovine Adrenal Medulla

Abstract

Vasoactive intestinal polypeptide (VIP), originally isolated from the porcine small intestine, is known to be widely distributed throughout the body including the central and peripheral nervous systems in various species. In the present study, we demonstrated the existence, subcellular distribution and mode of release of VIP-like immunoreactivity (VIP-LI) in the bovine adrenal medulla by radioimmunoassay.
In tissue extracts from fresh bovine adrenal medulla, a considerable amount of VIP-LI (101.1 ± 24.3ng/g wet weight) was detected, and its concentration was about 100 times and 30 times higher than those of neurotensin-LI and somatostatin-LI, respectively, on a molar basis. On chromatographic analysis, the majority of adrenal VIP-LI was comfirmed to have the same molecular size as synthetic VIP (1-28), and a small peak of macromolecular VIP-LI corresponding to pro-VIP was also found.
In studies using a retrograde venous perfusion system of the bovine adrenal gland, marked releases of both VIP-LI and catecholamine (CA) were observed immediately after the infusion of potassium solution of a concentration of 56mM in a Ca²⁺-dependent manner. Ba²⁺ (2mM) also stimulated the releases of VIP-LI and CA from the adrenal gland. Carbachol (10^-4M) stimulated CA secretion as much as high potassium and Ba²⁺, but the magnitude of VIP-LI release was lower.
The subcellular distribution of VIP in the adrenal medulla was investigated by a method of differential centrifugation and discontinuous density gradient. VIP-LI was mainly found in mitochondrial fraction, which contain mitochondria and synaptosome, while little was found in highly purified chromaffine granule fraction.
These results suggest that VIP in the adrenal gland is mainly localized in the splanchnic nerve endings and may play a role as neurotransmitter/neuromodulator in the adrenal medulla.