Pertussis toxin blocks somatostatin inhibition of calcium mobilization and reduces the affinity of somatostatin receptors for agonists.

Both forskolin, the activator of adenylate cyclase, and 8-bromocyclic (cAMP) increase cytosolic calcium levels (measured using Quin 2) and adrenocorticotropin (ACTH) release from a tumor cell line of the mouse anterior pituitary (AtT-20/D16-16). Somatostatin (SRIF) blocks the ACTH release response to each secretagogue but only inhibits forskolin-stimulated calcium mobilization suggesting that SRIF prevents the formation of cAMP rather than blocking the ability of cAMP to raise intracellular calcium concentrations. SRIF itself lowers intracellular calcium levels. The ACTH release response but not the rise in cytosolic calcium levels induced by the membrane-depolarizing agent K+, is blocked by SRIF, indicating that SRIF can interfere with some intracellular event, other than calcium mobilization or cAMP formation, to reduce ACTH secretion. Pertussis toxin uncouples SRIF receptors from adenylate cyclase by catalyzing the ADP-ribosylation of an inhibitory guanine nucleotide binding protein (Ni) in AtT-20 cell membranes. Pretreatment of AtT-20 cells with pertussis toxin abolishes the inhibition by SRIF of the ACTH release response and of the rise in cytosolic calcium induced by forskolin. In addition, the ability of SRIF to inhibit basal calcium levels is prevented by pertussis toxin treatment. Pertussis toxin treatment also reduced the ability of SRIF to inhibit K+-evoked ACTH release. SRIF receptor binding studies using the ligand 125I-CGP-23996 revealed that pertussis toxin treatment greatly diminished the affinity of the SRIF receptor for SRIF and its structural analogs. These results indicate that, in addition to coupling SRIF receptors to adenylate cyclase, Ni is also involved in the lowering by SRIF of resting calcium levels.(ABSTRACT TRUNCATED AT 250 WORDS)