Localization of somatostatin receptors in secretion vesicles in anterior pituitary cells and pancreatic islets.

Secretion vesicles in anterior pituitary cells and pancreatic islets appear to translocate somatostatin receptors from the cell interior to the plasma membrane. In this study we attempted to localize somatostatin receptors on either the cytoplasmic or the intraluminal surface of the secretion vesicles. 125I-somatostatin binding was determined in intact secretion vesicles and vesicles disrupted either by sonication or solubilization. The binding of 125I-somatostatin was identical in intact and disrupted vesicles, indicating cytoplasmic orientation of somatostatin receptors. Pronase treatment of intact secretion vesicles removed approximately 90% of specific somatostatin binding. Sonication of pronase treated secretion vesicles did not reveal latent somatostatin binding sites. Gold-conjugated somatostatin binds to isolated secretion vesicles confirming the presence of somatostatin binding sites on the outer surface of these vesicles. We conclude that somatostatin binding sites are located on the cytoplasmic surface of secretion vesicles isolated from anterior pituitary cells and pancreatic islets.