The effect of pH on the calcium dependence of calcium accumulation in dog cardiac muscle sarcoplasmic reticulum

Abstract

Net Ca 2+ accumulation in vesicles of dog cardiac sarcoplasmic reticulum (CSR) was evaluated at three different pHs: 6.0, 6.8 and 7.6. The Ca 2+ sequestration by CSR depends on Ca 2+ concentration and on pH values. The curves that show the relationship between Ca 2+ accumulated by CSR and external Ca 2+ concentrations were shifted with pH changes, both in the absence and in the presence of potassium oxalate. Considering the curve at pH 6.8 as reference, a lower Ca concentration was needed to obtain the half-maximal value in Ca sequestration under pH 7.6 (0.04 ± 0.006 and 0.79 ± 0.09 μ m at pH 7.6 and 6.8, respectively). Opposite results were obtained under pH 6.0 (13.66 ± 1.29 μ m). Net calcium release during active accumulation of Ca 2+ and Ca 2+ efflux from passively 45Ca 2+ loaded CSR microsomes were significantly higher at alkaline pH than at acidic pH. The results suggest than in CSR alkaline pH would promote the increase in the rates of both, Ca 2+ release and active Ca 2+ accumulation, while opposite effects would be expected under acidic pH. Therefore, pH changes may regulate both, the Ca 2+ level upon which the SR Ca 2+ pump works (permeability effect) and the sequestration rate of the Ca 2+ pump (variation in the affinity for calcium).