Immunoelectron microscopic study of the luminal release of chromogranin A from rat enterochromaffin cells

Recently we found that raising the intraluminal pressure caused an increase in the luminal release of serotonin from enterochromaffin (EC) cells and s erotonin immunoreactivity normally restricted within the secretory granules was diffusely scattered over the extragranular matrix. In the present stud y we investigated the intracellular localization of chromogranin A, a prote in co-stored with serotonin in the EC cells, after stimulating the luminal release of serotonin. In situ vascularly and luminally perfused rat duodenu m was exposed to intraluminal pressure and fixed for immunoelectron microsc opic study. For immunoelectron microscopy, the pre-em bedding DAB reaction for serotonin combined with the postembedding immunogold reaction for chrom ogranin A was used. Results showed that a number of secretory granules labe led with immunogold chromogranin A immunoreactivity located close to the ap ical plasma membrane. Some EC cells showed that one part of the apical cyto plasm was protruded into the lumen and a number of secretory granules with immunogold labeling were included in the protruded cytoplasm. These results suggest that EC cells may release chromogranin A into the intestinal lumen together with serotonin, by means of a different manner of secretion from that in serotonin.