The dependence on internal pH of Ca2+-fluxes across sarcoplasmic reticulum vesicular membranes

The interdependence of the competition between Ca2+ and hydrogen ions for the internally located low-affinity Ca2+ binding sites of sarcoplasmic reticulum vesicles and the pH-dependent splitting rate of phosphoenzyme was investigated. Sarcoplasmic reticulum vesicles were preincubated at a selected pH and passive Ca2+ loading, active Ca2+ uptake at the same pH as well as active Ca2+ uptake at a distinct pH (pH-jump method) were observed. In addition, Cai-Cao exchange in the absence and presence of ADP and ATP-ADP exchange were measured. The overall ATP splitting rate was assayed with leaky vesicles in the presence of varied Ca2+ concentration and four different pH. All experiments were carried out at Ca2+ concentrations sufficient to saturate the externally located activating high-affinity binding sites at all pH and in the absence of affecting concentrations of monovalent cations. Active Ca2+ transport (particularly evident applying the pH-jump method) is facilitated at low intravesicular pH, reflecting the favoured Ca2+ release to the intravesicular space, in contrast to the reverse pH-dependence of passive Ca2+ accumulation and the initial rate of Cai-Cao exchange, both favoured by elevated internal Ca2+ binding capacity. The rates of ATP splitting, the continuing slow rate of Cai-Cao exchange, and the ATP-ADP exchange are optimal at an intermediate proton concentration, reflecting the influence of protons on partial reaction steps occurring later in the reaction cycle and the accelerated exchange of Ca2+ at the internal low-affinity sites as well as the establishment of a new pseudo equilibrium between the possible reaction intermediates. The pool of rapidly exchangeable Ca2+ is enlarged whereas the rate of slow exchange is unaltered or diminished (pH 7.8) by ADP.