Fig. 7. Comparison of SR Ca²⁺ uptake and sarcolemmal Ca²⁺ extrusion. (A) Total amount of Ca²⁺ taken up by the SR, sarcolemmal Ca²⁺ extrusion (SL) and the sum of the two (TOT) in the absence (black bars) and the presence of 1 µmol l^–1 isoproterenol (gray bars). (B) The relative contribution of the SR Ca²⁺ uptake (SR/TOT) and sarcolemmal Ca²⁺ extrusion (SL/TOT) to the total Ca²⁺ elimination from the cytosol during a contraction elicited by a 200 ms depolarization. SR Ca²⁺ uptake was calculated using individual Ca²⁺ transients from the 7 experiments in Fig. 4B and the relationship between [Ca²⁺]_i and SR Ca²⁺ uptake in Fig. 6C. Sarcolemmal Ca²⁺ extrusion was obtained from the time integral of the tail currents in the corresponding 7 cells in Fig. 4A. Significant differences: ** (P<0.01, N=7) and *** (P<0.001, N=7).