Effects of anions, pH and magnesium on calcium accumulation and release by sarcoplasmic reticulum vesicles

In the absence of oxalate, Ca2+ accumulation by isolated sarcoplasmic reticulum vesicles may show a transient behavior in which the vesicles accumulate during the first 2 min of incubation as much as twice the amount of Ca2+ which is retained after 5-7 min, when Ca2+ accumulation approaches a steady state. Before Ca2+ release begins, the Ca2+ accumulation can reach 200-250 nmol/mg protein. The spontaneous release of the "extra" Ca2+ initially accumulated appears to be triggered by the attainment of a sufficiently high concentration of free Ca2+ inside the vesicles. The amplitude of the transient phase of Ca2+ accumulation reaches a high value near pH 6.0 and is increased by free Mg2+. At optimal concentrations of H+ and Mg2+, the amount of Ca2+ accumulated during the transient is augmented by various anions, in the order maleate > or = propionate > or = succinate > chloride > sulfate > acetylglycine. The divalent anions have their maximum effects at 20-40 mM and the monovalent anions, at 40-200 mM. At 200 mM, all of the carboxylic anions tested significantly reduce the amount of Ca2+ retained in the steady state.