The Electrogenic Sodium/Potassium Pump and Passive Sodium Influx of Isolated Guinea Pig Ventricular Myocytes

Electrogenic Sodium/Potassium Pump in Ventricular Myocytes. Introduction: The membrane current generated by the sodium/potassium pump was measured in ventricular myocytes isolated from guinea pig hearts. Methods and Results: Cells were impaled with microelectrodes to minimize dialysis of the intracellular milieu and thus maintain normal physiological conditions. To selectively record pump current, it was necessary to suppress potassium currents with external barium and to buffer intracellular calcium with a calcium chelator. Exposure to potassium‐free solution was used to inhibit the sodium/potassium (Na/K) pump and load the cell with intracellular sodium (Na i ). When the pump was subsequently reactivated by replacing extracellular potassium, there was a sharp increase of a transient outward membrane current, which declined back to control with a time constant of 50.7 ±12.9 seconds (mean ± SD, n = 11). The increase of outward membrane current became larger after longer periods of potassium removal and was abolished by the pump inhibitor strophanthidin. These features show that the transient outward current was generated by the Na/K pump. The pump current of quiescent myocytes was assessed by two different methods. First, it was evaluated directly by measuring the loss of outward pump current upon exposure to a maximal blocking dose of strophanthidin. The loss of outward current was 20.3 ± 5.5 pA (n = 11), which is equivalent to a pump current density of 0.164 /μA/cm 2 . The second method was to estimate passive Na influx into quiescent myocytes, by measuring the charge extruded by the pump after a period of potassium removal. The charge extruded during recovery reflects the magnitude of Na i accumulation during potassium removal. Passive Na influx was found to be 4.58 ± 2.4 pmol/cm 2 per second. If passive Na influx is normally balanced fully by Na extrusion on the Na/K pump, this will result in an average pump current of 18.0 pA (equivalent to a pump current density of 0.147 μA/cm 2 ). Thus, although the two methods for estimating the pump current of quiescent myocytes were different, their results agreed well. A pump current of 20 pA will contribute only about 0.4 mV to the resting potential of ventricular myocytes. This conclusion was confirmed experimentally by showing that pump inhibition depolarized the resting potential of myocytes by only 0.45 ± 0.05mV(n = 10). Conclusion: An important finding of this study is that the electrogenic Na/K pump makes a much lower contribution to the resting potential of ventricular muscle than was previously supposed. ( J Cardiovasc Electrophysiol, Vol. 3, pp. 225–238, June 1992 )