Ultrastructural simultaneous localization of vasoactive intestinal polypeptide immunoreactivity and the acetylcholine-like cation in rat endocrine pancreas.

The ultrastructural co-localization of vasoactive intestinal polypeptide (VIP) and acetylcholine (ACh)-like cation was demonstrated in the rat endocrine pancreas. Immunoperoxidase cytochemical procedure for the detection of VIP was followed by ionic fixation of ACh by silicotungstic acid. All the VIP-immunoreactive nerve endings displayed punctiform precipitates revealing ACh-like cations in the small clear vesicles. Almost all the VIP-immunoreactive nerve terminals were in contact with endocrine A, B or D cells, but not in the close vicinity of blood vessels. Nerve endings devoid of VIP immunoreactivity but containing ACh-like cations were seen either in contact with endocrine cells or in close vicinity to blood vessels. These data suggest that VIP might modulate, concomitantly with ACh, the hormonal secretion of the endocrine pancreas, although the possibility of the neurosecretion of ACh and VIP into the blood vessels should not be excluded. ACh might also control some function of the endothelial cells of blood vessels.