The mechanism by which adenosine and cholinergic agents reduce contractility in rat myocardium. Correlation with cyclic adenosine monophosphate and receptor densities.

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Abstract
The adenosine analogue phenylisopropyladenosine decreased the basal and isoproterenol-stimulated contractile state of isolated rat left atria. The ED50 levels for both responses were similar, suggesting that direct and antiadrenergic effects may be mediated by the same receptor. Phenylisopropyladenosine decreased the cyclic adenosine monophosphate content of isolated atria and inhibited isoproterenol-stimulated adenylate cyclase activity in membranes prepared from atria and ventricles, but not as much as did methacholine. A maximally effective concentration of phenylisopropyladenosine or methacholine greatly reduced atrial contractility measured in the presence of either isoproterenol (1 microM) or Ro7-2956 (a phosphodiesterase inhibitor, 1 mM); however, in the presence of isoproterenol plus Ro7-2956, the contractile effects of phenylisopropyladenosine and methacholine were greatly attenuated. From the contractile data and cyclic adenosine monophosphate analyses, we conclude that direct and antiadrenergic contractile effects of both phenylisopropyladenosine and methacholine result primarily from their effects on cyclic adenosine monophosphate metabolism. The densities of adenosine, muscarinic, and beta-adrenergic receptors in rat atrial membranes were found to be 30, 551, and 24 fmol/mg protein, respectively, based on equilibrium-binding assays conducted with 125I-aminobenzyl-adenosine, [3H]quinuclidinyl benzilate, and 125I-labeled pindolol. The greater effectiveness of methacholine than phenylisopropyladenosine as a negative inotropic agent and an inhibitor of adenylate cyclase in atria may be related to the relative densities of muscarinic and adenosine receptors.