Figure 1. Depletion of the Ca²⁺ store in mouse EDL muscle fibres and Ca²⁺ entry concomitant with refilling of the SR.

Initially, fibres were incubated in 2Ca-5K solution and treated with 2Ca-100K solution for 30 s to observe high-K⁺ contracture (lower left-hand trace). The incubation solution was then changed to 0Ca-5K solution and stimulation with 0Ca-100K solution (30 s) was repeated every 10 min. The time-integrated high-K⁺ contracture gradually decreased with each stimulation, reaching steady state after the 7th-10th treatment, at which time it had diminished to an average of about 12 % of the initial magnitude (partial depletion). Then 10 μm CPA was added to the solution (prior to the 9th treatment in this experiment), and high-K⁺ stimuli were further repeated every 10 min until fibres did not develop contractures (18th, severe depletion). The fibres were also challenged with 25 mm caffeine (25Caf), resulting in only slight tension development. After washout of CPA with 0Ca-5K solution, the fibres were incubated in 2Ca-5K solution for 2 min, followed by washing with 0Ca-5K solution, and were subsequently challenged with 0Ca-100K solution and 25 mm caffeine to determine the extent of refilling of the SR with Ca²⁺. Ordinal numbers indicate the total number of high-K⁺ treatments in Ca²⁺-free solution. The horizontal line indicates the extracellular Ca²⁺ concentration. CPA was present in the media during the period indicated by the thick line. The time scale can be applied to the tension traces but not to the intervals between. The fluorescence ratio signals corresponding to the tension responses below are also shown (upper traces); note that the Ca²⁺ signal at the 9th high-K⁺ stimulation suffered from large movement artifact, when the fibres moved out of the field, distorting the ratio signal.