Effects of changing Ca2+-to-H+ ratio on Ca2+ uptake by cardiac sarcoplasmic reticulum

Ca uptake by fragments of cardiac sarcoplasmic reticulum (SR) in the absence of oxalate depends on the ratios between concentrations of free Ca2+ and H+ and on the level of Ca loading. Using a rapid quenching technique, we showed that at [Ca2+]f less than 1 microM or in short time intervals after ATP addition (less than or equal to 1 s), pH decrease led to a significant inhibition of Ca accumulation. If Ca transport was quenched in 1 s, [Ca2+]f for half-maximal stimulation of the process would be 0.3, 1.0, and 4.0 microM at pH 7.2, 6.8, and 6.2, respectively. At higher [Ca2+]f and at longer intervals, low pH increases the amount of Ca taken up by SR. These phenomena could not be explained by an interaction of ethyleneglycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA) with the Ca2+-pump system since they were also demonstrated without application of Ca-EGTA buffers. Scatchard plot analysis of ATP-independent Ca binding to SR shows that apparent Kd for Ca increases from 0.3 microM at pH 7.2 to 7.4 microM at pH 6.2. pH shifts do not change the number of high-affinity Ca-binding sites. The results suggest a competition between Ca2+ and H+ for high-affinity binding sites on Ca2+-ATPase molecules. This may cause a delay of myocardial relaxation during ischemia when a decrease in intracellular pH takes place.