A selective binding site for 3H-NECA that is not an adenosine A2 receptor.
In homogenates of NG108-15 cells, adenosine analogues activate adenylate cyclase with the following order of potency: N-ethylcarboxamidoadenosine (NECA) greater than 2-chloroadenosine greater than N6-(L-phenylisopropyl)-adenosine (PIA) = cyclohexyladenosine = 2-phenylaminoadenosine. Adenosine receptor antagonists inhibit NECA-stimulated adenylate cyclase activity with the order of potency 3-isobutyl-1-methyl-xanthine (IBMX) greater than theophylline greater than caffeine. These data suggest that these ligands act at an adenosine A2 receptor. There is an apparently homogenous population of saturable 3H-NECA binding sites in homogenates of NG108-15 cells. These sites have an affinity for 3H-NECA of approximately 1 microM, and are present at a density of approximately 10 pmol/mg protein. Unlabelled NECA, 2-chloroadenosine, IBMX and theophylline displace 3H-NECA binding, with an order of potency that suggests that the 3H-NECA binding site may represent an adenosine A2 receptor. However, PIA, cyclohexyladenosine and 2-phenylaminoadenosine produce no detectable displacement of 3H-NECA binding at concentrations that produce a maximal stimulation of adenylate cyclase activity. Pretreatment of NG108-15 cells with either NECA or PIA produces a homologous desensitization of subsequent responses to all the adenosine analogues, with no effect on subsequent responses to a prostacyclin receptor agonist or NaF. This suggests that all the adenosine analogues examined activate an adenosine A2 receptor. Therefore, the 3H-NECA site at which PIA is inactive cannot represent this receptor.
Keen
M
M
Department of Pharmacology, Medical School, University of Birmingham, U.K.
Kelly
E
E
Nobbs
P
P
MacDermot
J
J
D000241:Adenosine / Q000031:analogs & derivatives* / Q000145:classification / Q000378:metabolism / Q000494:pharmacology*; D019830:Adenosine-5'-(N-ethylcarboxamide); D000067956:Adenylyl Cyclase Inhibitors; D001665:Binding Sites / Q000187:drug effects; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D006822:Hybrid Cells; D010660:Phenylisopropyladenosine / Q000145:classification / Q000378:metabolism / Q000494:pharmacology; D011983:Receptors, Purinergic / Q000145:classification* / Q000187:drug effects / Q000502:physiology; D014407:Tumor Cells, Cultured / Q000187:drug effects
D000067956:Adenylyl Cyclase Inhibitors; D011983:Receptors, Purinergic; D010660:Phenylisopropyladenosine; D019830:Adenosine-5'-(N-ethylcarboxamide); D000241:Adenosine