Figure 3. The influence of successive step changes in extracellular tonicity upon muscle fibre volume.

Filled squares denote mean normalized volumes, V_c (±s.e.m., n= 4 fibres except between 75 and 135 min, when n= 3 as cell swelling reduced the number of fibres that could be visualized in each frame). The horizontal bars denote predicted volumes, V_p, after each step change in osmolarity, taking into account the actual normalized volumes, V_c, recorded immediately before each such change (see Methods). Initial extracellular osmolarity was 227 mosmol l⁻¹, and this was reduced in steps to 216 mosmol l⁻¹ (a), 200 mosmol l⁻¹ (b) and 171 mosmol l⁻¹ (c). Finally, the bathing solution was replaced with standard 227 mosmol l⁻¹ Ringer solution once more (d). Note that fibres remained residually swollen on return to standard Ringer solution (from d), but then showed gradual volume recovery, in contrast to the ΔV_S+ seen in similarly swollen fibres (from b) in hypotonic solutions.