Fig. 1. Experimental protocol. SR Ca²⁺ uptake was measured by clearance of the SR Ca²⁺ content by a brief exposure to 10 mmol l^–1 caffeine (Caf) at –80 mV (Clear). The SR was then loaded during a 3 or 10 s depolarization (Load). The SR Ca²⁺ uptake during the depolarization to different membrane potentials (V_m) was quantified as the Caf releasable Ca²⁺ content and obtained by integration of the I_NCX elicited by Caf (Release).