The relationship between somatostatin binding and cyclic AMP-stimulated protein kinase inhibition

Somatostatin binding and the ability to inhibit cyclic AMP stimulated protein kinase were investigated utilizing isolated pancreatic islets, anterior pituitary plasma membranes, adipocytes, erythrocyte ghosts, hepatic plasma membranes, and anterior pituitary secretion vesicles. Three types of response were observed. With type I response, somatostatin bound specifically to pancreatic islets and anterior pituitary secretion vesicles and inhibited cyclic AMP stimulated protein kinase. In type II response, adipocytes and anterior pituitary plasma membranes exhibited somatostatin binding but no effect of the ligand on the kinase. In erythrocyte membrane ghosts and hepatic plasma membranes, there was neither specific somatostatin binding nor protein kinase inhibition (type III response). The absence of somatostatin binding in erythrocytes or hepatic plasma membranes cannot be explained by degradation of the ligand per se. Secretory vesicles isolated from the anterior pituitary gland bind somatostatin with an average affinity which exceeds that observed in plasma membrane (for pituitary secretory vesicles Kd1 = 8.5 X 10(-8)M, Kd2 = 5.2 X 10(-7)M; for pituitary membranes Kd1 = 1.9 X 10(-8)M, Kd2 = 8.1 X 10(-7)M). The molar concentration of high affinity binding sites (Ro) for plasma membranes was 6.9 X 10(-10)M; for secretory vesicles 3.6 X 10(-9)M. Calculated in terms of somatostatin binding per U 5'nucleotidase activity, the binding for plasma membranes becomes 8.4 X 10(-14) mole/U 5'nucleotidase; secretory vesicles 4.4 X 10(-13) mole/U 5'nucleotidase. Thus, secretory vesicles are fivefold richer in high affinity receptor sites than plasma membranes. It is suggested that in order for somatostatin to act, both a receptor and an effector unit must be present. In the case of tissues secreting polypeptide hormones by granule extrusion, the secretory vesicle may possess both the receptor and the effector units. It is postulated that during the process of fusion of the plasma and secretory vesicle membranes, a high affinity binding site for somatostatin is incorporated into the plasma membrane, thereby allowing somatostatin to act at a specific locus in the cell in inhibiting hormone release.