This proposal will apply and extend recently developed tissue culture techniques to modulate the contribution of the sodium pump to the physiological processes of automaticity, excitation and conduction of cardiac cells. The unique feature of this study will be to exploit those features of cultured embryonic chick heart cells (polystrands) that have made these preparations suitable for electrophysiologic, ion transport and biochemical studies, i.e., minimal diffusion distance, mechanical stability, morphologically simple extracellular space. We plan to define and quantitatively describe the conditions necessary to create a mismatch between the active and passive membrane transport processes of cultured heart cells. The sodium pump density and transport capacity in these cells will be determined from measurements of 3H-ouabain binding, ATPase activity, Na/K and ATP/ADP content and ouabain-sensitive K-influx. The electrophysiological consequences of the adaptive changes in sodium pump function will be evaluated utilizing microelectrode recording techniques to study changes in pacemaker activity, resting membrane potentials, action potential configuration, and conduction velocity as well as to relate the electrogenic pump current to the membrane potential during recovery from Na loading. Correlative studies will include measurements of potassium, sodium, calcium transport and intracellular ionic content. The design of these experiments should provide new insights concerning the physiological implications of active transport on cardiac cell function. The long-term goal is to determine whether an adaptive change in the transport capability of cardiac cells brought about changes in either the number and/or activity of Na pump sites in the membrane can be related to the electrophysiological and electrochemical properties of cardiac muscle.