Figure 3. Reduction of tonicity, regardless of its absolute value, increases Na⁺ pump density.

A, the number of Na⁺ pumps per gram of muscle (determined by [³H]ouabain binding) increases significantly when muscles pre-equilibrated in π= 1 medium are exposed to π= 0.5 (a) and in those pre-equilibrated in π= 2 medium and then transferred to π= 1 (b; n = 6 muscle pairs for each change of π). B, in the continuous (100 min) presence of a saturating concentration of ouabain ([oua]_o= 50 μm), muscles that were exposed to π= 1 medium for 50 min and then transferred to π= 0.5 for another 50 min exhibited a significantly greater (43 %, P < 0.04) ouabain binding than their paired control muscles, which had been kept in π= 1 medium. Likewise, a similar increase of ouabain binding was produced by the π= 2 →π= 1 transfer (46 %, P < 0.02). These data indicate that the stimulation of the active Na⁺ transport promoted by fibre swelling is related to an actual increase of the pump density, although an additional effect on the Na⁺–K⁺ exchange cannot be discarded (n = 4 muscle pairs for each transfer).