Prolonged activation of inhibitory somatostatin receptors increases adenylate cyclase activity in wild-type and G sα-deficient (cyc −) S49 mouse lymphoma cells

Abstract

Many cells develop enhanced adenylate cylcase activity after prolonged exposure to drugs that acutely inhibit the enzyme and it has been suggested that this adaptation may be due to an increase in G sα. We have treated wild-type and G sα-deficient cyc − S49 mouse lymphoma cells with a stable analogue (SMS 201–995) of the inhibitory agonist somatostatin. After incubation with SMS for 24 h, the forskolin-stimulated cAMP synthetic rate in intact cyc − cells was increased by 76%, similar to the increase found in the wild-type cells. Forskolin-stimulated adenylate cyclase activity in the presence of Mn 2+ was also increase in membranes prepared from SMS-treated cyc −cells; however, guanine nucleotide-mediated inhibition of adenylate cyclase activity was not changed despite a small increase in inhibitory G iα subunits detected by immunoblotting. Pretreatment of cyc − cells with pertussis toxin prevented SMS from inducing the enhancement of forskolin-stimulated cAMP accumulation in intact cells. After chronic incubation of cyc − cells with SMS, exposure to N-ethylmaleimide, which abolished receptor-mediated inhibition of cAMP accumulation, did not attenuate the enhanced rate of forskolin-stimulated cAMP synthesis compared to N-ethymaleimide-treated controls. These results with cyc − cells demonstrate that an adaptive increase in adenylate cyclase activity induced by chronic treatment with an inhibitory drug can occur in the absence of expression of G sα.