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.