title: Phosphate, nitrendipine and valinomycin increase the Ca2+/ATP coupling ratio of rat skeletal muscle sarcoplasmic reticulum Ca(2+)-ATPase.
abstract: 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 Ca(2+)-ATPase or by inhibition of the sarcoplasmic reticulum Ca2+ channel, but is due to an increased efficiency of transport by Ca(2+)-loaded vesicles. At low Ca2+ concentrations, nitrendipine+valinomycin inhibits Ca2+ uptake by increasing the Ca2+ KM but does not effect equilibrium Ca2+ binding to the Ca(2+)-ATPase (Kd = 0.75 microM). 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 Ca(2+)-ATPase-mediated Ca2+ release are reduced by nitrendipine+valinomycin. The data indicate that nitrendipine and valinomycin act directly on the Ca(2+)-ATPase to decrease the ATP hydrolysis rate, increase the Ca2+ KM, decrease Ca2+ efflux, and increase the Ca2+/ATP coupling ratio of Ca(2+)-loaded vesicles.
authors.affiliation: Department of Biochemistry, Uniformed Services University of the Health Sciences, Bethesda, MD 20814.
authors.forename: T J
authors.initials: TJ
authors.lastname: Beeler
authors.forename: K S
authors.initials: KS
authors.lastname: Gable
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004357:Drug Synergism; D009132:Muscles / Q000187:drug effects* / Q000201:enzymology; D009568:Nitrendipine / Q000494:pharmacology*; D010710:Phosphates / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D014634:Valinomycin / Q000494:pharmacology*
chemical_list: D010710:Phosphates; D014634:Valinomycin; D000255:Adenosine Triphosphate; D009568:Nitrendipine; D000252:Calcium-Transporting ATPases; D002118:Calcium