title: Adenosine-receptor-mediated stimulation of low-Km GTPase in guinea-pig cerebral cortex.
abstract: Inhibition of receptor-coupled adenylate cyclase by hormones is proposed to be associated with GTP hydrolysis. Since adenosine inhibits cerebral-cortical adenylate cyclase via A1 adenosine receptors, the present study attempts to verify this mechanism for A1-selective adenosine derivatives. In guinea-pig cortical membranes N6-(phenylisopropyl)adenosine (PIA) increased the Vmax. of the low-Km GTPase, with an EC50 (concentration causing 50% of maximal stimulation) of about 0.1 microM, and the stimulatory effect was competitively antagonized by 5 microM-8-phenyltheophylline. The rank order of potency of the stereoisomers of PIA and of 5-(N-ethylcarboxamido)adenosine (NECA) to stimulate GTPase correlated with their ability to inhibit adenylate cyclase activity (R-PIA greater than NECA greater than S-PIA). Competition binding studies with (-)-N6- ([125I]iodo-4-hydroxyphenylisopropyl)adenosine suggest that adenylyl imidodiphosphate (p[NH]ppA), an essential component of the GTPase assay system, is a more potent A1-receptor agonist than ATP, with an IC50 (concentration giving half-maximal displacement of radioligand binding) of 7.9 microM. On the basis of the p[NH]ppA concentration used in the GTPase assay (1.25 mM), enzyme stimulation by adenosine seems to be highly underestimated. Nevertheless, adenosine-induced GTP hydrolysis reflects an increased turnover of guanine nucleotides at the Ni regulatory site and appears to be a crucial step in the sequence of events processing the inhibitory signal to adenylate cyclase.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000262:Adenylyl Cyclases / Q000378:metabolism; D000266:Adenylyl Imidodiphosphate / Q000494:pharmacology; D000818:Animals; D002540:Cerebral Cortex / Q000201:enzymology*; D004789:Enzyme Activation / Q000187:drug effects; D020558:GTP Phosphohydrolases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006168:Guinea Pigs; D007700:Kinetics; D010660:Phenylisopropyladenosine / Q000031:analogs & derivatives / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011983:Receptors, Purinergic; D013268:Stimulation, Chemical; D013806:Theophylline / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D000266:Adenylyl Imidodiphosphate; D010660:Phenylisopropyladenosine; D000255:Adenosine Triphosphate; D013806:Theophylline; C028322:8-phenyltheophylline; D010744:Phosphoric Monoester Hydrolases; D020558:GTP Phosphohydrolases; D000262:Adenylyl Cyclases