The neurotransmitters norepinephrine and acetylcholine regulate cardiac function through specific receptors on the sarcolemma. Interaction of ligands with these receptors results in alterations in the intracellular levels of cyclic nucleotides in the heart. Direct binding assays for beta-adrenergic receptors and muscarinic cholinergic receptors now make it possible to directly assess the number and properties of these receptors in a variety of tissues including the heart. In addition, radioimmunoassay techniques make it possible to determine levels of cyclic-AMP and cyclic-GMP with small amounts of tissue. A culture system for collagenase dissociated heart cells will be established and used to investigate questions regarding the relationships between ligand interaction with cell surface receptors, changes in intracellular cyclic nucleotide levels, and alterations of the rate at which heart cells beat. Initial experiments will involve the study of heart cell proliferation and differentiation in culture. The effects of catecholamines and cyclic-AMP on cell growth and differentiation will be studied. Another group of experiments will be designed to investigate the temporal and concentration-effect relationships between agonist interaction at beta-adrenergic and muscarinic cholinergic receptors and changes in intracellular cyclic nucleotide levels. These results will be compared to those of similar experiments in which the effects of these agents on cardiac function will be assessed. The possible regulation of the postsynaptic response to catecholamines and acetylcholine will be investigated in several experimental paradigms. In one series of experiments the molecular basis for catecholamine-induced desensitization will be examined. Another group of experiments will be carried out to investigate the possible role of muscarinic cholinergic receptors in the regulation of the cardiac response to acetylcholine. Evidence exists in the literature which suggests that thyroxine influences the responsiveness of the myocardial cell to catecholamines. The effects of this hormone on the beta-adrenergic receptor/adenylate cyclase system will be investigated in the cultured heart cell system. Using such a system that is free from external or secondary influences, it should be posssible to directly assess the effects of thyroid hormone on cardiac responsiveness to catecholamines, and to determine the molecular basis for any change that might occur.