Figure 2. Basic properties of Na/K pump currents in murine cardiac myocytes.
Solutions contain 20 mM TEA on both sides to minimize K currents, 4 mM Mg on the outside to promote seal formation without Ca, and NMG as the major extracellular cation. Pump currents are initiated by replacing 7 mM Na for 7 mM K on the extracellular side. (A) Outward Na/K pump currents in the presence of 110 mM cytoplasmic Na and no cytoplasmic K are large (>6 pA/pF) and stable. Capacitance decreases and increases by ~1% within solution switch times when K is applied and removed, respectively. (B) Outward Na/K pump currents in the presence of 25 mM cytoplasmic Na and 90 mM cytoplasmic K decay over several seconds by more than 50%. Membrane capacitance decreases immediately during K application, as just described, and can then continued to decrease slowly but to a lesser extent during current decay; capacitance recovers slowly over 20 s after removing extracellular K. Typical for rodent Na/K pumps, currents are quite resistant to heart glycosides but can be rapidly and effectively blocked by a high concentration of ouabain (0.3 mM). (C and D) Cytoplsmic Na dependence of peak and steady state (10 s) pump currents at 37°C with and without 90 mM cytoplasmic K. (C) In the presence of 90 mM K, the K50 for peak current is 19 mM Na and the Hill coefficient is 2.4, while the 10 s current shows a K50 of 30 mM and Hill slope of 2.6. (D) In the absence of cytoplasmic K, peak currents are best described by a Hill equation with a K50 of 6 mM cytoplasmic Na and a Hill slope of 1.6. The 10 s current is shifted to a K50 of 37 mM with a Hill slope of 1.1 Although currents are smaller in the presence of cytoplasmic K, the fractional decay of current is greater at all Na concentrations. (E) Cytoplasmic Na dependence of peak and quasi steady state (15 s) pump currents at 23°C without cytoplasmic K. Data points are the average of values from at least 5 separate myocytes. (F) The Na dependence of the fractional decay of pump current (1–15 s/peak) is well described by the occupancy of a single Na binding site with an apparent Kd of 12 mM. The solid and dotted lines in panels E and F are predictions for overtly simplified Na depletion and inactivation models, described in Materials and methods.