Immohistochemical studies and secretory effects of neuropeptides and regulatory peptides in the isolated rat pancreas

This study investigates the distribution and secretory effects of the neuropeptides vasoactive intestinal polypeptide (VIP) and galanin (GAL) and the pancreatic peptide hormones, glucagon (GLU) and somatostatin (SOMA) in the isolated rat pancreas. The effect of the parasympathetic neurotransmitter, acetylcholine (ACh) was also studied for comparison. Immunohistochemical studies identified GLU and SOMA mainly in the peripheral cells of the islet of Langerhans with the population of GLU was more than double that of SOMA cells. The number of SOMA-immunoreactive cells per islet ranged from 1-6. GAL and VIP were seen mainly in nerve fibres and cell bodies located in the exocrine part of the pancreas. Stimulation of isolated pancreatic segments with different concentrations of either GLU, SOMA, VIP, GAL or ACh resulted in differential effects on amylase secretion. Some secretagogue-evoked secretory responses consisted of an initial inhibition followed by an elevation in amylase output depending upon the concentration of the secretagogue. In Fura - 2 acetomethy lester (AM) loaded acinar cells, VIP, GAL, GLU and SOMA elicited biphasic changes in intracellular free calcium [Ca²⁺]_i concentration with of an initial decrease followed by a gradual increase. In the continuous presence of the neuropeptides and peptide hormones, ACh evoked large increase in [Ca²⁺]_i. The results of this study have indicated that the neuropeptides and peptide hormones are present in neural and endocrine cells of the pancreas and they act to mobilise intracellular Ca²⁺ which in turn mediates enzyme secretion.