A role of H+ flux in active Ca2+ transport into sarcoplasmic reticulum vesicles. I. Effect of an artificially imposed H+ gradient on Ca2+ uptake.

To ascertain the function of H+ flux in active Ca2+ transport into sarcoplasmic reticulum vesicles, the effect of pH gradient on Ca2+ transport was examined. A transient H+ gradient (inside-acidic) was imposed on K+-loaded sarcoplasmic reticulum vesicles with the aid of K+-H+ exchange driven by nigericin. This proton gradient was dissipated rapidly and concomitantly with ATP-driven Ca2+ transport. Under these conditions, the initial rate of the Ca2+ uptake was increased about 1.5-fold. The stimulation of Ca2+ uptake was completely lost when the pH gradient was cancelled with an uncoupler plus membrane permeable cation before Ca2+ uptake. These results are interpreted in terms of H+ efflux coupled with Ca2+ transport.