Ca²⁺ pumping ATPase of cardiac sarcolemma is insensitive to membrane potential produced by K⁺ and Cl⁻ gradients but requires a source of counter-transportable H⁺

Summary

The sensitivity of the Ca²⁺ pumping ATPase of bovine cardiac sarcolemma (SL) to changes in membrane potential was studied in a preparation of sealed SL vesicles. Membrane potential was imposed by preincubating the vesicles in media of defined ion composition (K⁺, Cl⁻, choline⁺ and gluconate⁻) and diluting into media of differing ion composition. The durations of the ion gradients and relative ion permeabilities were determined in separate experiments by the dependence of the half time for net K⁺ (or choline⁺) movement coupled with these anions (Cl⁻ or gluconate⁻), registered by the fluorescence of 1-anilino-8-naphthalene sulfonate (Chiu, V.C.K., Jaumes. D.H. 1980.J. Membrane Biol. 56:203–218). Relative permeabilities were: 1.0, K⁺, ≥10.0, 1 μm valinomycin-K⁺; 4.0, Cl⁻, 0.66, choline⁺; 0.38, gluconate⁻. Durations of the gradients ranged between 17 sec (KCl, valinomycin) to 195 sec (K⁺-gluconate⁻). In separate experiments. active Ca²⁺ uptake was monitored using chlorotetracycline (CTC) fluorescence, a technique validated by 45-Ca²⁺ measureaments (Dixon, D., Brandt, N., Haynes, D.H. 1984.J. Biol. Chem. 259:13737–13741). Active Ca²⁺ uptake was initiated in the presence of monovalent ion gradients. The values of the membrane potentials (E _m ) imposed by the monovalent ion gradients were calculated using the ion concentrations, their relative permeabilities and the Goldman-Hodgkin-Katz equation. No effect of membrane potential on transport rate was observed (≤4%, for 5–7%sd) for imposed potentials as extreme as ≥+71 and ≤−67 mV. Formal analysis shows that the above observations are not compatible with models in which the Ca²⁺ pumping ATPase functions in an electrogenic or charge-uncompensated fashion. Further experimentation showed that the pump rate is slowed when uptake is measured at less-than-adequate concentrations of buffer (5vs. 25mm HEPES/Tris). This, together with further control experiments using nigericin and FCCP, gave evidence that the pump requires a source of counter-transportable H⁺ in the vesicle lumen. The above experimentation also underlines the need for control of internal pH to obviate erroneous interpretation of ion perturbation experiments. The results are compared with results obtained with the Ca²⁺ ATPase pump of skeletal sarcoplasmic reticulum.