Net Ca2+ accumulation in vesicles of dog cardiac sarcoplasmic reticulum (CSR) was evaluated at three different pHs: 6.0, 6.8 and 7.6. The Ca2+ sequestration by CSR depends on Ca2+ concentration and on pH values. The curves that show the relationship between Ca2+ accumulated by CSR and external Ca2+ 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 microM at pH 7.6 and 6.8, respectively). Opposite results were obtained under pH 6.0 (13.66 +/- 1.29 microM). Net calcium release during active accumulation of Ca2+ and Ca2+ efflux from passively 45Ca2+ loaded CSR microsomes were significantly higher at alkaline pH than at acidic pH. The results suggest that in CSR alkaline pH would promote the increase in the rates of both, Ca2+ release and active Ca2+ accumulation, while opposite effects would be expected under acidic pH. Therefore, pH changes may regulate both, the Ca2+ level upon which the SR Ca2+ pump works (permeability effect) and the sequestration rate of the Ca2+ pump (variation in the affinity for calcium).