Fragments of adult rat pancreas were incubated in vitro with tritiated serotonin at concentrations from 10−8 to 10−7 M. The pancreas exhibited an uptake of serotonin which was saturable, with an uptake constant (Km) of 8.75 × 10−7 M, and a Vmax of 873 pmoles per gram. Specificity was determined by the addition of fluoxetine or norepinephrine to the reaction mixture, both at 10−5 M. Fluoxetine significantly reduced the 3H‐5HT uptake, whereas norepinephrine was without effect. Metergoline (10−6 M), a specific 5‐HT postsynaptic receptor blocker, similarly had no effect on the serotonin uptake in the pancreas. Radioautography of the fragments following uptake of tritiated serotonin (5 × 10−8 M) revealed silver‐ grain aggregates dispersed along blood vessels in the interstitial spaces of the exocrine and endocrine pancreas, areas known to be traversed by nerve fibers. There were no silver‐ grain aggregates over the exocrine or islet parenchymal cells. These data support the hypothesis that the pancreas is innervated by serotonergic fibers. Further evidence for this hypothesis was provided by a preliminary study demonstrating the presence of tryptophan hydroxylase in pancreatic homogenates. These serotonergic fibers may be involved in the regulation of pancreatic secretion.
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