Capillary orientation of rat pancreatic D-cell processes: evidence for endocrine release of somatostatin.

We examined the ultrastructure of somatostatin-containing pancreatic D-cells in the rat in order to shed light on the function and mode of action of somatostatin in the pancreas. D-cells were first identified by indirect immunocytochemistry with the peroxidase-antiperoxidase technique on semithin (1-micron) sections of 2% glutaraldehyde-1.7% paraformaldehyde-fixed tissue from the tail of the pancreas. Fine ultrastructure of the positively identified D-cells was examined in adjacent sections (0.08 micron) by electron microscopy. D-cells characteristically exhibited long cytoplasmic projections that extended to capillaries. Each cell was divided arbitrarily into three zones of roughly equal size, nuclear, central, and capillary, and distribution of secretory granules into each zone was quantified. In unstimulated cells, secretory granules were dispersed throughout the D-cell. In sections obtained from rats stimulated to secrete somatostatin by infusion of 20 mM glucose-5 mM theophylline-20 mM L-arginine HCl, 75 +/- 4% of the D-cell granules was polarized to the capillary end of the cell, while only 54 +/- 2% was is this region in unstimulated rats (P less than 0.05). These studies suggest that pancreatic somatostatin is released into islet capillaries.