Figure 2. Effect of hypotonicity on net ouabain-blockable release of Na⁺ into Na⁺-free (Tris) media, determined by atomic absorption spectrophotometry.

A, time course of the increase in active Na⁺ efflux (ΔJ_pump) and internal Na⁺ concentration ([Na⁺]_i) upon replacement of a solution of π= 1 by one of π= 0.5 at time = 0. ΔJ_pump rises, in spite of the reduction in [Na⁺]_i. Each experimental ΔJ_pump value represents data from four muscle pairs (one member of each pair in the presence of 50 μm ouabain) and is expressed per cm² of superficial sarcolemma using the relationship 430 cm² g⁻¹ of muscle (Venosa, 1991). [Na⁺]_i was calculated from the release of Na⁺ into π= 0.5 medium and the muscle Na⁺ content at the end of the washout period of muscles kept in ouabain-free medium. The inset shows the time course of the increment in intracellular water (ΔV_W) in π= 0.5 medium (n = 8). The experimental values were obtained from wet (drained but not blotted) muscle mass determinations. In control experiments it was found that the volume of the extracellular space (Na⁺ space) is not significantly changed by π= 1 →π= 0.5 transfer. Values from this curve were used to calculate [Na⁺]_i. B, ΔJ_pump as a function of [Na⁺]_i. The numbers next to the experimental points indicate time (min) in π= 0.5 medium. The plot emphasises the fact that as fibres swell and [Na⁺]_i falls, the active Na⁺ extrusion increases. Near swelling completion, when the hypotonic effect reaches a maximum, ΔJ_pump as a growing function of [Na⁺]_i starts to become apparent.