Vasoactive intestinal polypeptide (VIP) has been found in pancreatic nerves in several species. Studies were conducted to determine if VIP could be a parasympathetic neurotransmitter in the canine endocrine pancreas. To verify that VIP is localized in pancreatic parasympathetic nerves, sections of canine pancreas were immunostained for VIP. VIP staining was identified in the majority of neuronal cell bodies in intrapancreatic parasympathetic ganglia. In addition, VIP was localized in nerve fibers innervating pancreatic islets in the proximity of alpha cells. Next, to determine if VIP is released during electrical stimulation of parasympathetic nerves, pancreatic spillover of VIP was measured during vagal nerve stimulation (VNS) in anesthetized dogs. VIP spillover increased from a baseline of 630±540 pg/min to 2580±540 pg/min (δ = + 1950±490 pg/min, p <0.01). Pancreatic VIP release during VNS was not affected by atropine, whereas ganglionic blockade with hexamethonium nearly abolished the VIP response to VNS (p < 0.005 vs control), suggesting that VIP is a postganglionic neurotransmitter in the dog pancreas. To examine the effects of VIP on pancreatic hormone secretion, synthetic VIP was infused locally into the pancreatic artery. VIP, at a low dose (5 pmol/min), increased glucagon secretion from 1750±599 to 3800±990 pg/min (Δ = + 2060±870 pg/min, p < 0.05), but did not affect insulin secretion (Δ = -1030±760 μU/min, NS). Thus, VIP is contained in and released from pancreatic parasympathetic nerves in proximity to islet alpha cells and exogenous VIP, at a dose which approximates the increase of VIP spillover during VNS, preferentially stimulates glucagon vs insulin secretion. Therefore, VIP is likely to function as a parasympathetic neurotransmitter in the endocrine pancreas in dogs. We hypothesize that VIP could mediate the glucagon response to parasympathetic activation which has been shown to resistant to cholinergic blockade with atropine in several species.
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