Electron Microscopic Cytochemistry of 5-Hydroxytryptamine (5-HT) in the Beta Cells of Guinea Pig Endocrine Pancreas

In order to localize 5-hydroxytryptamine (5-HT) stores in the islet of Langerhans of guinea pig pancreas, a cytochemical procedure for the ultrastructural differentiation between catecholamines and indoleamines was used. Islet tissue of animals receiving reserpine was similarly analyzed. The results obtained with the cytochemical reactions were correlated with those of pancreatic amine assays performed before and after reserpine administration. Dense reactive granules were observed in β cells of normal guinea pigs with the cytochemical procedures. These granules correspond to those normally observed in β cells and known to store insulin. After reserpine administration, the reactivity of the granules disappeared and simultaneously a marked decrease in pancreatic 5-HT content was observed. The possible participation of insulin was discarded in view of results drawn from in vitro experiments. Light and electron microscopic observations also suggested that the granular population of β cells, reflecting their insulin content, was not markedly affected by reserpine administration. The evidence obtained indicates that 5-HT present in β cells of guinea pig endocrine pancreas is localized within the same cytoplasmic granule in which insulin is stored. (Endocrinology83: 917, 1968)