Differential A(1)-adenosine receptor reserve for inhibition of cyclic AMP accumulation and G-protein activation in DDT(1) MF-2 cells.
1. The A(1)-adenosine receptor (A(1)AdoR) reserve for N(6)-cyclopentyladenosine (CPA) mediated inhibition of (-)isoprenaline stimulated cyclic AMP accumulation and stimulation of [(35)S]-guanosine-5'-O-(thiotriphosphate) (GTPgammaS) binding, a measure of guanine nucleotide binding protein (G-protein) activation, was determined in DDT(1) MF-2 cells. 2. Inactivation of the A(1)AdoRs with the chemoreactive ligand 8-cyclopentyl-3-[3-[[4-(fluorosulphonyl)benzoyl]oxy]propyl]-1-p ropylx anthine (FSCPX) caused a progressive rightward shift of the concentration-response curves for CPA to inhibit cyclic AMP accumulation, with a maximum of 10 fold increase in the EC(50) value. In contrast, inactivation of A(1)AdoR's caused only a 1.7 fold rightward shift in the CPA concentration-response for stimulation of [(35)S]-GTPgammaS binding. 3. The A(1)AdoR occupancy-response relationship for CPA inhibition of cyclic AMP accumulation was hyperbolic with 43% receptor occupancy required to elicit the maximal response, i.e. a 57% A(1)AdoR reserve. In contrast, the A(1)AdoR occupancy-response relationship for CPA mediated stimulation of [(35)S]-GTPgammaS binding was linear indicating little or no receptor reserve for G-protein activation. The relationship between CPA stimulation of [(35)S]-GTPgammaS binding and cyclic AMP inhibition was also hyperbolic with 44% G-protein activation sufficient to cause maximal inhibition. 4. The data suggest that the A(1)AdoR reserve for CPA mediated inhibition of cyclic AMP accumulation occurs at the level of G-protein interaction with adenylyl cyclase. However, each A(1)AdoR appears to activate a constant fraction of the total G-protein population suggesting signal amplification at the receptor-G-protein level which may also contribute to the receptor reserve for CPA.
Baker
S P
SP
Department of Pharmacology and Therapeutics, University of Florida College of Medicine, Box 100267, Gainesville, Florida, FL 32610, USA.
Scammells
P J
PJ
Belardinelli
L
L
D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000037:antagonists & inhibitors*; D004305:Dose-Response Relationship, Drug; D019204:GTP-Binding Proteins / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D009130:Muscle, Smooth / Q000378:metabolism; D018047:Receptors, Purinergic P1 / Q000502:physiology*; D014970:Xanthines / Q000494:pharmacology
D018047:Receptors, Purinergic P1; D014970:Xanthines; C100640:8-cyclopentyl-3-(3-((4-(fluorosulfonyl)benzoyl)oxy)propyl)-1-propylxanthine; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C048599:N(6)-cyclopentyladenosine; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D000241:Adenosine