Effect of galanin on arginine-stimulated pancreatic hormone release from isolated perifused rat islets

Evidence that galanin is a parasympathetic, rather than a sympathetic, neurotransmitter in the baboon pancreas

To determine whether galanin is a pancreatic sympathetic neurotransmitter regulating insulin secretion in the baboon, as it is in the dog, we evaluated galanin for inhibitory effects on insulin secretion in conscious baboons, determined if baboon pancreatic islets are innervated by galaninergic fibers using immunohistochemistry, and measured galanin content in the major sympathetic ganglion supplying the pancreas. Surprisingly, infusion of galanin (1 microgram/kg per min) had no effect on arginine-stimulated secretion of either insulin (71 +/- 14 vs. 88 +/- 17 microU/ml; P = NS) or glucagon (104 +/- 12 vs. 94 +/- 9 pg/ml; P = NS). By contrast, growth hormone secretion was markedly increased during galanin infusion. In the baboon celiac ganglion, no galanin immunoreactivity was detectable in sympathetic neuronal cell bodies by immunostaining and their content of galanin-like immunoreactivity, determined by radioimmunoassay, was only 3% of that in dog celiac ganglion (5.2 +/- 0.8 vs. 158 +/- 13 pmol/g; P < 0.001). By contrast, galanin immunoreactivity was observed in many nerve fibers in the baboon exocrine pancreas and occasionally in baboon pancreatic islets. Moreover, galanin content of the baboon pancreas was similar to that of dog (8.7 +/- 1.5 vs. 5.5 +/- 1.2 pmol/g; P = NS). The finding of galanin immunoreactivity in many neuronal cell bodies in baboon intrapancreatic ganglia suggests a parasympathetic source for these galaninergic fibers in the baboon. Together these data demonstrate that galanin is likely to be a parasympathetic neurotransmitter in the baboon pancreas, without major effects on insulin or glucagon secretion.

Interactions of galanin and arginine on growth hormone, prolactin, and insulin secretion in man

Galanin (GAL), a 29 amino acid neuropeptide, is known to increase both basal and growth hormone-releasing hormone (GHRH)-induced growth hormone (GH) secretion while not significantly increasing prolactin (PRL) secretion in man. GAL is also endowed with an inhibiting effect on glucose-stimulated insulin release in animals, but not in man. We studied the effect of GAL (80 pmol/kg/min infused over 60 minutes) on the arginine- (ARG, 30 g infused over 30 minutes) stimulated GH, PRL, insulin, and C-peptide secretion in eight healthy volunteers (age, 20 to 30 years). GAL induced an increase of GH (GAL v saline, area under curve [AUC], mean +/- SEM: 316.5 +/- 73.9 v 93.2 +/- 20.9 micrograms/L/h, P less than .05), but failed to modify both PRL and insulin secretion. GAL enhanced the ARG-induced stimulation of both GH (1,634.1 +/- 293.1 v 566.9 +/- 144.0 micrograms/L/h, P less than .02) and PRL secretion (1,541.9 +/- 248.8 v 1,023.8 +/- 158.7 micrograms/L/h, P less than .02). On the contrary, GAL blunted the ARG-stimulated insulin (816.3 +/- 87.7 v 1,322.7 +/- 240.9 mU/L/h, P less than .05), as well as C-peptide secretion (105.1 +/- 9.8 v 132.8 +/- 17.3 micrograms/L/h, P less than .02). ARG administration induced a transient increase of glucose levels (P less than .01 v baseline) followed by a significant decrease (P less than .05 v baseline). This latter effect was prevented by the coadministration of GAL. In conclusion, these results show that in man GAL potentiates the GH response to ARG, suggesting that these drugs act at the hypothalamic level, at least in part, via different mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)