The isolated ventricular cell must retain the physiologic characteristics of its native tissue if it is to be useful in studying excitation-contraction coupling. A wide variability in isolated cell size, rest sarcomere length, amount and velocity of sarcomere shortening and action potential has been observed. This proposal seeks to understand this variability by correlating the function of individual cells, as examined by sarcomere motion and rest and action potentials, with the structure of the same cell. Cells will be isolated from the right and left ventricle of the rabbit and rat using enzymatic and mild mechanical dispersion. Rabbit myocardiuim will be studied because it has physiologic properties much more like those of other mammals than those of the rat. The rat will be studied because it has such strikingly different physiologic properties and has been used in many isolated cell investigations. The experiments will make use of the prominent effects induced by altering the rate and pattern of stimulation on contractility and the action potential and the characteristic changes in these effects induced by inotropic agents. Timed changes in extracellular ionic concentrations during rest and contraction will take advantage of the short diffusion distance. The proposal will provide a detailed examination of the physiological and ultrastructural properties of the same cell. This will enable us to assess whether intrinsic in situ differences in cells or the effects of the isolation procedure are the basis of cell-to-cell variability. Sarcomere shortening and action potential characteristics will be related to cell shape, size and ventricular site of origin. Species differences will allow structrue-function correlations to be made. The changes in myocardial performance produced by altering the rate and pattern of stimulation have been used in multicellular preparations to study contractility and to characterize the effects of disease. By producing a detailed quantitative description of the changes in sarcomere shortening velocity induced by altering the pattern of stimulation, this investigation will provide a basis at the cellular level for the force-interval relationship in the intact heart. By assessing the importance of extracellular calcium concentration at different times during activation and rest, this proposal will provide new data for testing models of excitation-contraction coupling. This proposal will provide a basis for future isolated cell investigations of disease-induced derangements in excitation-contraction coupling.