It has been postulated that the positive inotropic effect of catecholamines is mediated, at least in part, by the cyclic AMP-dependent protein kinase catalyzed phosphorylation of membrane proteins that control calcium transport. The object of this proposal is to determine if the phosphorylation reactions involved in the mediation of the positive inotropic effect of catecholamines are also involved in the regulation of the sensitivity of the myocardium to calcium. The proposed experiments are prompted by the previous observations that young embryonic chick hearts which display and increased sensitivity to the positive inotropic effect of calcium also contain higher levels of cyclic AMP and activated cyclic AMP-dependent protein kinase than older hearts. The hypothesis to be tested is that the increased sensitivity of the heart to calcium during development is due to an increased state of phosphorylation of membrane proteins controlling calcium transport. The effect of isoproterenol and calcium on the phosphorylation of membrane proteins will be examined using intact embryonic and newborn chick hearts equilibrated with 32Pi. Sarcolemma will be purified and analyzed to identify the membrane proteins phosphorylated in control and in treated hearts. Quantitative correlations will be made between the state of phosphorylation of isolated membranes and their ability to accumulate calcium. It is expected that the data obtained will further our understanding of the mechanisms mediating the positive inotropic effect of catecholamines and those regulating the sensitivity of the myocardium to calcium.