5-Hydroxytryptamine synthesis, storage, and secretion from rat pancreatic acinar cells

Pancreatic acinar cells are known to synthesize serotonin and dopamine from extracellular precursors. In this study, we found that small amounts of serotonin, alone, were preferentially stored in zymogen granules of acinar cells. Serotonin was apparently incorporated into mature rather than newly formed granules. This was based on the fact that the amine, rather than the newly synthesized zymogen protein, appeared first in the purified granule fraction; additionally, concentrations of cycloheximide, which markedly inhibited de novo synthesis of zymogen protein, did not affect the incorporation of serotonin into the granule fraction. Serotonin, synthesized by the acinar cells, can be secreted along with amylase in a time- and concentration-dependent manner following stimulation of acinar cells by a variety of pancreatic secretagogues. Whether serotonin secreted into pancreatic juice has a biological function remains unknown.