In NG108-15 cells inhibition of both N-type calcium channel current and adenylyl cyclase by somatostatin (SRIF) was not sustained but rapidly desensitized in the continued presence of the drug. The degree and rate of desensitization were concentration-dependent, and the desensitization was homologous with respect to the ␦-opioid receptor. We have been unable to obtain evidence for the involvement of G protein-coupled receptor kinases (GRKs) in this desensitization. SRIF-induced desensitization of N-type calcium channel currents was not reduced in cells stably overexpressing a dominant negative mutant of GRK2 or following intracellular dialysis with GRK2-and GRK3-blocking peptides or with heparin. Inhibitors of protein kinase A, protein kinase C, and protein kinase G were also without effect. In contrast, both the rate and degree of SRIF-induced desensitization were reduced by pretreatment with phenylarsine oxide or concanavalin A, both inhibitors of receptor endocytosis. Furthermore, SRIF-induced desensitization was enhanced by monensin, which prevents receptor recycling back to the plasma membrane. Similarly, SRIF-induced desensitization of adenylyl cyclase inhibition was not reduced in cells stably overexpressing dominant negative mutant GRK2 but was reduced in cells pretreated with the receptor endocytosis inhibitor hyperosmotic sucrose or concanavalin A. These data are consistent with the view that SRIF-induced desensitization in NG108-15 cells results from receptor internalization.The cellular effects of somatostatin (SRIF) 1 are mediated by specific cell surface receptors. To date, five somatostatin receptor genes have been cloned (sst 1 to sst 5 ). sst receptors all couple through G i /G o proteins, and some or all of these receptor subtypes have been observed to inhibit adenylyl cyclase (1, 2), to activate tyrosine phosphatase activity (3), to activate phospholipase C (4), to activate mitogen-activated protein kinase (5), to inhibit high voltage activated calcium channel currents (6 -9), and to activate a potassium conductance (10 -11). As with other G protein-coupled receptors, desensitization or waning of the response in the continued presence of an agonist is a salient feature of sst receptor-mediated signaling.There is now good evidence that desensitization of G proteincoupled receptor-mediated responses can arise from a number of different mechanisms depending on both the type of receptor and the type of cell in which it is expressed. At the receptor level, desensitization can be due to phosphorylation by specific G protein-coupled receptor kinases (GRKs) (12-17); phosphorylation by second messenger kinases such as protein kinase A (18 -20), protein kinase C (12,15,21,22), calcium/calmodulindependent protein kinase (23), and casein kinase (24); receptor palmitoylation (25-27); and receptor internalization (28 -30). In addition, modulation of post receptor components of the receptor-effector pathway, such as the level of G protein expression (31), can also underlie desensitization.In the present st...

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