Luminal pH regulates calcium release kinetics in sarcoplasmic reticulum vesicles

Abstract

Calcium binding to triads isolated from rabbit skeletal muscle followed a single hyperbolic function in the pH range 5.5-8.0. Maximal binding was obtained at pH 8.0; decreasing the pH decreased the binding capacity and, at pH ≤6.0, increased K(d) 2-fold. These results indicate that lowering the pH diminished calcium binding to calsequestrin, since this protein is the primary source of calcium binding sites in triads. Luminal pH had a marked effect on calcium release induced by 2 mM ATP, at pCa 5.0, pH 6.8. At a constant lutninal [Ca2+] of 0.1 mM, release rate constants (k) and initial rates of release increased steadily as a function of decreasing luminal pH; at luminal pH 7.5, values of k < 0.4 s-1 were found, whereas at pH 5.5 values of k ≃ 10 s-1 were obtained. Increasing luminal [Ca2+] from 0.05 mM to 0.7 mM had no effect on the k values measured at luminal pH 5.5. In contrast, at pH 6.8, increasing luminal [Ca2+] produced a marked increase in k values, that reached maximal values