The adenosine Ri agonist, phenylisopropyladenosine, reduces high affinity isoproterenol binding to the beta-adrenergic receptor of rat myocardial membranes.
Adenosine attenuates beta-adrenergic receptor mediated activation of adenylate cyclase in myocardial membranes via adenosine Ri receptors. The effects of adenosine analogs on the binding characteristics of beta-adrenergic receptors were examined in the present study utilizing rat ventricular membranes treated with adenosine deaminase. In 125I-cyanopindolol/isoproterenol competitive binding experiments phenylisopropyladenosine (PIA) significantly increased the IC50 for isoproterenol from 48 +/- 6 nM to 140 +/- 48 nM and steepened the slope of the competition curves from -0.56 +/- 0.03 to -0.90 +/- 0.21. Computer analysis of these curves indicated that binding of isoproterenol to the high affinity state of the beta-adrenergic receptor was eliminated in the presence of PIA. PIA had no effects in the presence of GPP(NH)P. 2-chloroadenosine, a less specific Ri agonist, caused smaller increases in IC50 and slope, without significantly affecting high affinity binding. 2',5'-dideoxyadenosine, a P-site agonist, had no significant effects on isoproterenol binding. During the time course of the competitive binding experiments the membranes displayed isoproterenol-sensitive adenylate cyclase activity in the absence of added GTP. These data suggest that adenosine attenuates catecholamine-induced activation of adenylate cyclase via Ri receptors by decreasing the ability of beta-adrenergic agonists to promote the formation of a high affinity complex composed of the agonist, receptor and stimulatory guanine nucleotide binding protein.
Romano
F D
FD
Department of Physiology, University of Massachusetts Medical School, Worcester 01655.
Fenton
R A
RA
Dobson
J G
JG
D000241:Adenosine / Q000031:analogs & derivatives* / Q000032:analysis; D000262:Adenylyl Cyclases / Q000032:analysis; D000818:Animals; D003627:Data Interpretation, Statistical; D006321:Heart / Q000187:drug effects*; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000378:metabolism*; D008297:Male; D008566:Membranes / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D010660:Phenylisopropyladenosine / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*
D011943:Receptors, Adrenergic, beta; D010660:Phenylisopropyladenosine; D000262:Adenylyl Cyclases; D000241:Adenosine; D007545:Isoproterenol