An upper limit for the electrogenic Na-K pump contribution to maximum diastolic potential in feline cardiac Purkinje fibers in steady state.

We have estimated an upper limit for the electrogenic contribution of the Na-K pump to diastolic transmembrane potential. We simultaneously monitored the maximum diastolic potential and the extracellular space potassium activity during exposure to a very high concentration of ouabain. Exposure to ouabain caused a depolarization of approximately 3 mV (n = 33 experiments) over 34 +/- 3 s (mean +/- standard error) prior to any change in extracellular K activity. In four experiments, we monitored intracellular sodium activity and observed it to rise with approximately the same temporal lag (delay = 26 +/- 7 s). We also measured relative membrane conductance in one series of experiments and observed it to decrease to 91 +/- 2% of its control value by the time extracellular space K began to rise. Following the initial increase in extracellular space K activity the subsequent membrane depolarization is shown to be accurately predicted solely from the measured increase in extracellular space K activity as calculated from the Goldman equation. Limitations of the method and possible interpretations of the data are discussed. We interpret this ouabain-induced depolarization that occurs prior to the rise in external K to be an upper limit to the Na-K pump's electrogenic contribution to steady-state membrane potential.