The central hypothesis to be examined in the proposed studies is that regulation of cardiac cellular response to catecholamine occurs not only at the level of the beta receptor and adenylate cyclase but also more distally in the stimulus-response sequence at the level of the Ca channel. We will determine whether the Ca channel can be regulated by cardiac development, by catecholamine-mediated mechanisms, and by other hormonal and ionic mechanisms. Regulation of Ca channel ligand binding sites and of Ca channel function by adrenergic and other hormonal mechanisms may be noncoordinate so regulation of Ca channel ligand binding sites will be examined and related to independently determined functional properties of the Ca channel. Three related model systems will be used: chick embryo ventricular strips, ventricular membranes, and intact cultured chick ventricular cells. The primary experimental approaches to studying beta receptor and Ca channel regulation include contractility measurements of ventricular strips and cultured cells, ligand binding studies of the beta receptor and Ca channel in membranes and intact cultured cells, 45Ca flux measurements in cultured cells, and cAMP and adenylate cyclase assays. These techniques will be employed to examine three hypotheses a) sympathetic innervation alters beta receptor properties and coupling to adenylate cyclase. As a consequence of altered cellular cAMP content, expression of Ca channel ligand binding sites in the sarcolemma is altered. Ca channel function evolves during cardiac development and alterations in channel function can be accounted for only in part by concomitant alterations in beta adrenergic receptor-effector coupling. Beta adrenergic modulation of Ca channel function will be assessed by contractile and 45Ca flux response of cultured heart cells to isoproterenol and intrinsic Ca channel properties will be assessed by response to the Ca channel agonist BAY k8644. b) Rapid agonist-induced beta receptor desensitization does not alter intrinsic Ca channel properties but beta receptor down-regulation produces upregulation of number and function of Ca channels. c) Thyroid hormone alters cellular Ca kinetics which are related to altered Ca channel properties. Cell growth in medium with decreased Ca concentration can directly modulate Ca channel properties. Taken together these studies will offer new insights into regulation of cardiac contractile function and mechanisms of how function might be altered during development and disease states.
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