Experiments have been planned to characterize long-term effects of two regulators of cardiac contractility, catecholamines and adenosine. Normally the heart is chronically and simultaneously exposed to these endogenously occurring substances which regulate contractility by modifying the activity of the enzyme adenylate cyclase. In heart, this enzyme is activated by beta-adrenergic (B-A) agonists and inhibited by adenosine agonists. Chronic exposure to these agonists leads to some degree of desensitization to the contractile effects of both. The acute contractile and biochemical effects of catecholamines and adenosine on themyocardium have been extensively studied. However, even though chronic regulation of B-A and adenosine receptors is clearly of physiological relevance, B-A down-regulation has only been partially characterized in heart; and control of adenosine receptors has not been examined. We propose to expose cultured intact heart cells to B-A and adenosine agonists and to monitor receptor down-regulation and recovery from the down-regulated state. Based on what is currently known about receptors linked to adenylate cyclase we have hypothesized that adenosine influences down-regulation of B-A receptors. As a novel approach to these problems, we propose to examine long-term interactions between B-A and adenosine agonists with regard to down-regulation of receptor number. Changes in the properties of B-A and adenosine receptors will be correlated with alterations in the physiological responses of the heart. Our long term goal is to identify at a molecular level the factors responsible for down-regulation of receptors. A common strategy used for the treatment of cardiovascular diseases is to reduce cardiac contractility by regulating the activity of adenylate cyclase. Currently this is accomplished with beta blockers; supplemental treatment modalities might depress contractility by inhibiting or down-regulating adenylate cyclase. Studies will be performed using cultured embryonic heart cells which are stable for the requisite time period. Radioiodinated ligands will be synthesized as probes for B-A and adenosine receptors. Nonlinear least squares computational methods will be required for complex data analysis. Production of cyclic AMP by cultured heart cells, and tension developed by isolated muscle strips will be measured as indexes of physiological function. The involvement of guanine nucleotides, endocytosis and protein synthesis in the control of receptors will be evaluated.