PHOSPHATE, NITRENDIPINE AND VALINOMYCIN INCREASE THE CA2+/ATP COUPLING RATIO OF RAT SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM CA2+-ATPASE

Nitrendipine and valinomycin act synergistically to stimulate ATP-dependent Ca2+ accumulation by rat skeletal muscle sarcoplasmic reticulum vesicles 3-fold. The stimulation is not caused by activation of the Ca2+-ATPase or by inhibition of the sarcoplasmic reticulum Ca2+ channel, but is due to an increased efficiency of transport by Ca2+-loaded vesicles. At low Ca2+ concentrations, nitrendipine + valinomycin inhibits Ca2+ uptake by increasing the Ca2+ K-M but does not effect equilibrium Ca2+ binding to the Ca2+-ATPase (K-d = 0.75 mu M). In the presence of 50 mM phosphate, nitrendipine + valinomycin increases the steady-state coupling ratio (Ca2+ accumulated per ATP hydrolyzed) from 0.6 to 1.9 by decreasing the rate of ATP hydrolysis by 72%, while reducing the Ca2+ accumulation rate by only 13%. The rates of both passive and Ca2+-ATPase-mediated Ca2+ release are reduced by nitrendipine + valinomycin. The data indicate that nitrendipine and valinomycin act directly on the Ca2+-ATPase to decrease the ATP hydrolysis rate, increase the Ca2+ K-M, decrease Ca2+ efflux, and increase the Ca2+/ATP coupling ratio of Ca2+-loaded vesicles.