Acidosis and Ca²⁺ content of SR. (A) Specimen I_NCX signals recorded from a rabbit ventricular myocyte in response to rapid caffeine (20 mM) application during control and combined acidosis (2 min). The caffeine pulse was preceded by at least 15 conditioning AP-clamp pulses (CL = 2 s). The control and acidosis AP templates for the clamp pulses were the same as those used for I_Ca,L measurements (Fig. 8). (B) Summary of integrated I_NCX demonstrating that extracellular acidosis (n = 7) decreased Ca²⁺ content, whereas intracellular (n = 7) and combined acidosis (n = 6) increased content. **, P < 0.01; *, P < 0.05, paired; control compared with 2 min of acidosis.