Modulation of cardiac cyclic AMP metabolism by adenosine receptor agonists and antagonists.
The mechanism(s) underlying adenosine receptor-mediated modulation of cardiac cAMP levels has been investigated using detergent-permeabilized embryonic chick ventricular myocytes. The beta-adrenergic receptor agonist isoproterenol (ISO) stimulated adenylyl cyclase activity in detergent-permeabilized cells by 5-10-fold, with an EC50 value of 0.3 microM. Three adenosine receptor agonists, (R)-N6-phenylisopropyladenosine, N6-(3-iodo-4-aminobenzyl)adenosine, and 5'-N-ethylcarboxamidoadenosine, inhibited ISO (10 microM)-stimulated adenylyl cyclase activity in a concentration-dependent manner. The maximum inhibition of the ISO-stimulated adenylyl cyclase activity by (R)-N6-phenylisopropyladenosine (10 microM) was 30-40%. This inhibition was antagonized by the adenosine receptor antagonists xanthine amine congener and 8-cyclopentyl-1,3-dipropylxanthine and was abolished by pertussis toxin treatment, suggesting that the inhibition of adenylyl cyclase activity is mediated by A1 adenosine receptors acting via a pertussis toxin-sensitive guanine nucleotide-binding protein (G protein). Because the adenosine receptor agonists had no detectable effect on phosphodiesterase activity, the adenosine receptor-mediated inhibition of adenylyl cyclase activity appears to account for the cAMP-lowering effect of adenosine receptor agonists seen in intact cardiac myocytes. Moreover, two A1 adenosine receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine and 3-(4-amino)phenethyl-1-propyl-8-cyclopentylxanthine, stimulated basal adenylyl cyclase activity in the absence of an adenosine receptor agonist; this stimulation was abolished by pretreatment of the cells with pertussis toxin. We postulate that "precoupled" A1 adenosine receptor-G protein complexes, present in the cardiac myocytes, exert a tonic inhibitory influence on adenylyl cyclase activity and that some adenosine receptor antagonists remove this tonic inhibition by destabilizing these precoupled receptor-G protein complexes.
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Department of Pharmacology, University of Illinois, Chicago 60612.
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D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D002478:Cells, Cultured; D002642:Chick Embryo; D000242:Cyclic AMP / Q000378:metabolism*; D004789:Enzyme Activation; D009132:Muscles / Q000166:cytology / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D011983:Receptors, Purinergic / Q000187:drug effects*
D011983:Receptors, Purinergic; D000242:Cyclic AMP; D010727:Phosphoric Diester Hydrolases; D000262:Adenylyl Cyclases