Metabolism of L-DOPA and 5-HTP in the exocrine pancreas studied with autoradiography in the electron microscope

Abstract

By application of autoradiographic technique in the electron microscope, the cellular and subcellular distribution of radioactivity in the exocrine pancreas of the mouse was investigated after intravenous administration of L-DOPA-3H or dl -5-HTP-3H. Autoradiographic silver grains were recorded over acinar cells, whereas very few grains were found over other cells in the exocrine pancreas. Twenty minutes after administration of 5-HTP-3H autoradiographic grains appeared over all compartments of the acinar cells, but quantitative analyses of autoradiographic sections revealed a concentration of the label to the zymogen granules. This granular preponderance of the labeling was still more marked at 1 hour. Two and 3 hours after injection of 5-HTP-3H, decreasing levels of radioactivity were recorded over zymogen granules. A very similar labeling pattern was obtained when DOPA-3H was injected, but this amino acid had a more rapid turnover. Injection of a decarboxylase inhibitor before the administration of the labeled amino acids abolished the granular concentration of autoradiographic grains, which were now uniformly distributed over all parts of the acinar cells. This observation, together with earlier data, was taken as evidence that most of the silver grains over the zymogen granules represent monoamines formed by decarboxylation of the administered precursors. Pretreatment with an MAO inhibitor before the administration of dl -5-HTP-3H resulted in an increased level of radioactivity compared with animals given only the labeled amino acid.