Activation of Thyroid Membrane Adenylate Cyclase by Purine Nucleotides

Abstract

Abstract The effect of purine and pyrimidine nucleotides on the activation of adenylate cyclase by thyrotropin and prostaglandin E1 in beef thyroid plasma membranes was investigated. At 1 mm ATP concentration, GTP increases basal and thyrotropin-stimulated activity 2- to 4-fold at concentrations of 1 to 30 µm. Significant stimulation occurs at 0.1 µm. Greater maximal stimulation occurs with ITP (5- to 7-fold) over the concentration range of 20 to 50 µm. In order of decreasing maximal effect the nucleoside triphosphates are ITP g dGTP g GTP g XTP. At low concentrations GTP g ITP. Pyrimidine nucleotides had low activity. The enhancement occurs also with IDP and GDP and at 1 mm with 5'-IMP and 5'-GMP. The ITP and IDP effects have a 1 to 2 min lag period before they become manifest. F--activated cyclase is not stimulated and is partially inhibited at high nucleotide concentrations; thus, thyrotropin-stimulated cyclase may exceed F--stimulated activity in some conditions. Prostaglandin E1-stimulated activity is also enhanced by these nucleotides but, in contrast to thyrotropin activity, maximal stimulation by GTP = dGTP = ITP g XTP. Potassium ion increases thyrotropin-stimulated adenylate cyclase in the presence of 20 µm ITP, whereas there is no K+ effect on stimulation evoked by prostaglandin E1 even in the presence of ITP. Furthermore, the prostaglandin and thyrotropin stimulations are additive in the absence and in the presence of ITP and GTP. It is concluded that the nucleotides interact with the cyclase system not at the catalytic site, that the nucleotides can interact at more than one site, and that the thyrotropin and prostaglandin receptors are probably separate.