Pertussis toxin blocks both cyclic AMP-mediated and cyclic AMP-independent actions of somatostatin. Evidence for coupling of Ni to decreases in intracellular free calcium

Abstract

The neuropeptide somatostatin inhibits hormone release from GH4C1 pituitary cells via two mechanisms: inhibition of stimulated adenylate cyclase and a cAMP-independent process. To determine whether both mechanisms involve the guanyl nucleotide-binding protein Ni, we used pertussis toxin, which ADP-ribosylates Ni and thereby blocks its function. Pertussis toxin treatment of GH4C1 cells blocked somatostatin inhibition of both vasoactive intestinal peptide (VIP)-stimulated cAMP accumulation and prolactin secretion. In membranes prepared from toxin-treated cells, somatostatin inhibition of VIP-stimulated adenylate cyclase activity was reduced and 125I-Tyr1-somatostatin binding was decreased more than 95%. In contrast, pertussis toxin did not affect the biological actions or the membrane binding of thyrotropin-releasing hormone. These results indicate that ADP-ribosylated Ni cannot interact with occupied somatostatin receptors and that somatostatin inhibits VIP-stimulated adenylate cyclase via Ni. To investigate somatostatin's cAMP-independent mechanism, we used depolarizing concentrations of K+ to stimulate prolactin release without altering intracellular cAMP levels. Measurement of Quin-2 fluorescence showed that 11 mM K+ increased intracellular [Ca2+] within 5 s. Somatostatin caused an immediate, but transient, decrease in both basal and K+-elevated [Ca2+]. Consistent with these findings, somatostatin inhibited K+-stimulated prolactin release, also without affecting intracellular cAMP concentrations. Pertussis toxin blocked the somatostatin-induced reduction of [Ca2+]. Furthermore, the toxin antagonized somatostatin inhibition of K+-stimulated and VIP-stimulated secretion with the same potency (ED50 = 0.3 ng/ml). These results indicate that pertussis toxin acts at a common site to prevent somatostatin inhibition of both Ca2+- and cAMP-stimulated hormone release. Thus, Ni appears to be required for somatostatin to decrease both cAMP production and [Ca2+] and to inhibit the actions of secretagogues using either of these intracellular messengers.