Guanine nucleotide mediated desensitazation of adenylate cyclase in cell free preparations from a Leydig cell tumour

Cell free desensitization of a tumour Leydig cell plasma membrane adenylate cyclase has been demonstrated in the presence of guanine nucleotides. In experiments in which the membranes were pre-incubated with various nucleotides and LH, it was shown that this decreased adenylate cyclase activity was dependent on the presence of GTP and occurred both in the presence and absence of ATP. While pre-treatment with LH alone appeared to enhance subsequent adenylate cyclase activity, this hormone was able to potentiate the desensitizing effect of GTP. The desensitizing effect of GTP was not inhibited by sodium fluoride. In contrast, the GTP analogue p(NH)ppG (guanosine 5′β, λ-imido triphosphate) caused a persistent activation of the adenylate cyclase. GMP and guanosine also initially inhibited the adenylate cyclase activity, but this was entirely reversed by p(NH)ppG plus LH. GDP in addition to GTP caused desensitization but this was only partially reversed by p(NH)ppG plus LH. It is proposed that in similarity with the ovary (Bockaert et al. 1976; Ezra & Salomon 1982a) desensitization of Leydig tumour cell plasma membrane adenylate cyclase may involve a GTP-mediated phosphorylation step.