Figure 6. Changes in SR Ca²⁺ content in skinned fibres maintained at pCa 7.3.

A, force responses in a skinned EDL fibre upon emptying SR of Ca²⁺ in repeated load–release cycles as in Fig. 4. The SR was loaded to the same level (approximately endogenous level) on each repetition shown; the reduced area of the force response in the middle trace indicates that the SR lost net Ca²⁺ during the 2 min period in solution at pCa 7.3 (1 mm total EGTA; Cr added to buffer ADP at ∼10 μm; see main text). B, similar responses in a SOL (I) fibre, which displayed a small increase in SR Ca²⁺ content after 2 min in the pCa 7.3 solution. The EDL and SOL (I) fibres shown were typical of type: CSQ1 75 and 23% of EDL average, CSQ2 absent and present, and SERCA1 amounts 135 and ∼1% of EDL average, respectively.