Ca ²⁺/H ⁺ exchange via the plasma membrane Ca ²⁺ ATPase in skeletal muscle

The aims of this work were to determine: 1) whether Ca ²⁺ exit via the plasmalemmal Ca ²⁺ ATPase (PMCA) is coupled to H ⁺ entry via a Ca ²⁺/H ⁺ exchange; 2) whether operation of PMCA has an absolute requirement on external H ⁺ (H _o); and 3) the stoichiometry and voltagedependence of the Ca ²⁺/H ⁺ exchange. Barnacle muscle cells were used because of the ease with which they can be internally-perfused (e.g., with ⁴⁵Ca), voltage-clamped and impaled with a pH electrode. Thus, the simultaneous measurement of plasmalemmal Ca ²⁺ and H ⁺ fluxes can be measured. The effects of H _o, intracellular ATP, PMCA blockers, and membrane potential (V _M) were studied on PMCA-mediated Ca ²⁺/H ⁺ exchange. The results indicate that: i) Ca ²⁺ efflux is promoted by external acidification, is accompanied by a membrane depolarization, and by an intracellular acidification greater than the one resulting from H _o "leak" and PMCA-mediated ATP hydrolysis; ii) H _o-dependent Ca ²⁺ efflux is inhibited by PMCA blockers and by ATP depletion and is accelerated by membrane depolarization (∼3 fold by 20 mV depolarization); iii) the coupling ratio of the Ca ²⁺/H ⁺ exchange depends on Ho: at an extracellular pH (pH _o)=6.5, the ratio is 1Ca ²⁺:∼3H ⁺; at pH _o=8.2, Ca ²⁺ efflux rate is 3 times slower and the ratio is 1Ca ²⁺:<1H ⁺.