Neural regulation of pancreatic polypeptide release

To clarify the nervous system's role in the regulation of pancreatic polypeptide (PP) release, extrinsic innervation to the in situ pancreas was eliminated in five dogs. Before and 2 weeks after denervation, PP was measured during insulin hypoglycemia and ingestion of a protein meal. Exogenous insulin caused a similar marked hypoglycemia in both control and denervated dogs. Hypoglycemia caused a significant increase in plasma PP in control dogs from a baseline of 42 ± 8 pg/ml to 86 ± 18 pg/ml at 20 minutes (P < 0.01). In denervated dogs plasma PP did not increase with hypoglycemia, and levels were significantly less than in the control animals at 30 and 60 minutes (P < 0.05). With a protein meal, PP increased in the control animals from 53 ± 12 to 116 ± 16 pg/ml at 10 minutes (P < 0.05), 164 ± 22 pg/ml at 20 minutes (P < 0.05), and 193 ± 20 pg/ml at 60 minutes (P < 0.01). Denervation markedly blunted this response, and PP increased only from 53 ± 6 to 64 ± 4 pg/ml at 10 minutes, to 83 ± 72 pg/ml at 20 minutes, and to 91 ± 8 pg/ml at 60 minutes. PP became significantly elevated above baseline in denervated dogs only at 60 minutes (P < 0.05), and PP was significantly lower than in the control group at 10, 20, and 60 minutes after the meal (P < 0.05). Immunostaining for insulin, glucagon, somatostatin, and PP showed no difference in the number and distribution of these endocrine cells in predenervation and postdenervation specimens. Adrenergic and cholinergic nerves were seen in all control specimens but, except for a few adrenergic fibers, were not seen in denervated animals. Peptidergic nerves that contained vasoactive intestinal polypeptide (VIP) were seen in all areas of the pancreas before and after denervation. This study confirms that the initial rise in PP with a protein meal is governed by vagal cholinergic pathways. Later postprandial PP secretion is controlled by an interplay between these cholinergic pathways and other uncertain influences such as hormonal or substrate changes. The pancreas has a rich intrinsic peptidergic system of VIP-containing nerves.