Figure 4. Na⁺ efflux measurement.

A, a rabbit myocyte was first exposed to K⁺-free NT without divalent cations to cause Na⁺ loading of the cell. Then the cell was superfused with Na⁺-free solution (containing 4 mm K⁺) in the presence (outward leak) and absence (pump + leak) of 100 μm strophanthidin (10 mm ouabain was used for rat). The time course of [Na⁺]_i decline was used to calculate d[Na⁺]_i/dt. B, dependence of the rate of [Na⁺]_i decrease on [Na⁺]_i in the presence (leak) and absence (pump + leak) of strophanthidin. The difference between these two curves is the Na⁺ pump-mediated Na⁺ efflux. The outward leak depended linearly on [Na⁺]_i with a slope of 0.036 ± 0.002 min⁻¹ (n = 6) in rabbit and 0.069 ± 0.004 min⁻¹ (n = 6) in rat myocytes. C, [Na⁺]_i dependence of the Na⁺ efflux attributable to the Na⁺−K⁺ pump in rabbit (n = 6) and rat (n = 8) myocytes.