DESCRIPTION (Investigator's Abstract): Activation of adenosine receptors exerts important effects in the heart. The presence of a functional adenosine A3 receptor on the heart cell suggests a novel cardiac function for this receptor. Using cultured cardiac atrial and ventricular cells as models, the overall objective of the present proposal is to investigate the signaling mechanism, regulation, and function of the adenosine A3 receptor and to investigate the role of A1 and A3 receptors and the underlying mechanisms in preconditioning of cardiac myocytes. Specifically, the proposal will 1) carry out pharmacological characterization of the cardiac A3 receptor, 2) investigate the potential effectors, such as phospholipases, adenylyl cyclase and KATP channel, to which the A3 receptor is coupled, and 3) characterize desensitization of A3 receptor-coupled effectors. Because A1 and A3 receptors may have different signaling mechanisms and functions and may exhibit differential desensitization by adenosine agonist, parallel studies on the A1 receptor will be performed for direct comparison. Using a novel cardiac ventricular myocyte model of preconditioning, the proposal will further 4) investigate the role of A1 and A3 receptors in initiating and mediating the protective effect of preconditioning, and 5) study the mechanism by which activation of each receptor induces preconditioning and determine the role of protein kinase C and KATP channel in mediating this protective phenomenon. Moreover, atrial myocytes express only the A1 receptor and exhibit characteristics of preconditioning similar to the characteristics of A1 receptor-mediated preconditioning in the ventricular myocytes. Thus, atrial myocytes represent a useful "null" myocyte model for the A3 receptor and atrial myocytes transfected with A3 receptor cDNA will be used to further test the function and signaling mechanism of A3 receptor during preconditioning. The proposal should yield important and new information on the cardiac actions and signaling mechanisms of adenosine and its receptor subtypes.