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The influence of somatostatin was studied on cyclic AMP levels and adenylate cyclase activity in cyc- variants of S49 lymphoma cells. These cells are deficient in the guanine nucleotide site that mediates hormone-induced adenylate cyclase stimulation, but their cyclase can be stimulated by forskolin. Somatostatin maximally decreased the 30 microM forskolin-stimulated cyclic AMP levels by 35%. Half-maximal suppression occurred at about 0.1 nM somatostatin. Somatostatin (up to 1 microM) had no effect on the 100 microM forskolin-stimulated adenylate cyclase activity in cyc- membrane preparations when guanine nucleotides were not present. In the presence of GTP, however, which by itself caused a small decrease in activity, somatostatin maximally inhibited the enzyme by 20-25%. GTP was half-maximally effective at 0.1 microM, and half-maximal inhibition by somatostatin was observed at 0.1- 1 nM. In the presence of the stable GTP analog guanosine 5'-O-(3-thiotriphosphate) (1 microM), which decreased the stimulated activity by about 40% after a short lag period, somatostatin (1 microM) did not cause a further decrease in final activity but reduced the lag period by about 50%. The data indicate that membranes of cyc- variants contain a regulatory site that mediates both guanine nucleotide and hormone-induced inhibition of the adenylate cyclase and suggest that the mechanisms of activation and inactivation of this inhibitory site are similar to those of the stimulatory component missing in cyc-membranes.

Occurrence of a hormone-sensitive inhibitory coupling component of the adenylate cyclase in S49 lymphoma cyc- variants.