Na exchange in patch-clamped murine myocytes using veratridine to induce a large Na influx (35°C). (A) Na exchange estimated from Na/K pump currents. 120 mM NMG-Aspartate on both sides. The extracellular solution contains additionally 7 mM Na or 7 mM K. NMG-Aspartate is exchanged for Na-Aspartate with 3 μM veratridine on the outside. The ∼1 nA inward Na current decays partially, followed by generation of transient outward current upon removal of Na and veratridine. Thereafter, large Na/K pump currents are activated by exchanging 7 mM Na for 7 mM K, and peak currents decay with a time constant of 22 s. Red curves show simulations of Na exchange to a 12 pl cytoplasmic volume. Veratridine dissociates at 0.2 s⁻¹ and pump current is proportional to a Hill equation with a slope of 1.6 (Zong et al., 1992). Peak Na pump current magnitudes determined experimentally are plotted with the simulated pump availability. (B) Na exchange estimated from veratridine-activated Na current. In the presence of veratridine (3 μM), inward Na currents decay partially and outward (reverse) Na currents decay completely with a time constant of 22 s after Na removal. The right panel plots the outward Na current magnitude (y axis) that occurred just after removing extracellular Na in dependence on the time that extracellular Na had been applied (x axis). This time constant, also 22 s, with strong probability reflects the time course over which cytoplasmic Na exchanges with the pipette tip. Simulations assume that Na turnover is determined solely by the pipette access resistance.