| dc.description |
The cycâ variants of S49 lymphoma cells have served as powerful tools for studying the components and mechanisms of hormone-induced adenylate cyclase stimulation, as these cells are deficient in the guanine nucleotide regulatory site (Ns) mediating hormone, guanine nucleotide, cholera toxin and fluoride-induced stimulations of the enzyme<sup>1</sup>. Because of this deficiency, membranes of these cells have been used for reconstitution of the system by inserting the coupling component derived from other cell types<sup>2â 4</sup>. The hormone-sensitive adenylate cyclase is not only stimulated by hormones but can also be inhibited by a wide variety of hormones and neurotransmitters<sup>5â 8</sup>, and there is some evidence that hormonal inhibition may be mediated by a distinct guanine nucleotide regulatory site<sup>5â 8</sup>. Studies in cyc<sup>â </sup> cells lacking a functional Ns may therefore answer this unresolved, important question. We have recently observed<sup>9</sup> that stable GTP analogues can inhibit cyc<sup>â </sup> adenylate cyclase stimulated by purified, preactivated Ns or forskolin, which can activate adenylate cyclase even in the absence of a functional Ns (ref. 10). The data indicated that these Ns-deficient cells contain an inhibitory guanine nucleotide site, Ni. To strengthen this concept, we investigated whether the cyc<sup>â </sup> adenylate cyclase can be inhibited by a hormone. We report here that somatostatin decreases cyclic AMP levels in cyc<sup>â </sup> cells, inhibits the forskolin-stimulated adenylate cyclase and causes a concomitant increase in a high affinity GTPase activity in cyc<sup>â </sup> membranes. The data strongly suggest that both the hormone- and guanine nucleotide-induced adenylate cyclase inhibitions in cyc<sup>â </sup> cells are mediated by Ni and that the mechanisms of activation and inactivation of Ni are similar to those established for Ns. |
