Ca(2+) activation of heart mitochondrial oxidative phosphorylation: role of the F(0)/F(1)-ATPase

Ca²⁺ has been postulated as a cytosolicsecond messenger in the regulation of cardiac oxidativephosphorylation. This hypothesis draws support from the well-knowneffects of Ca²⁺ on muscle activity, which is stimulated inparallel with the Ca²⁺-sensitive dehydrogenases (CaDH). Theeffects of Ca²⁺ on oxidative phosphorylation were furtherinvestigated in isolated porcine heart mitochondria at the level ofmetabolic driving force (NADH or ) and ATPproduction rates (flow). The resulting force-flow (F-F) relationshipspermitted the analysis of Ca²⁺ effects on several putativecontrol points within oxidative phosphorylation, simultaneously. TheF-F relationships resulting from additions of carbon substrates aloneprovided a model of pure CaDH activation. Comparing this curve withvariable Ca²⁺ concentration(Ca²⁺]) effects revealed an approximatetwofold higher ATP production rate than could be explained by a simpleincrease in NADH or via CaDH activation. The half-maximal effectof Ca²⁺ at state 3 was 157 nM and was completely inhibitedby ruthenium red (1 µM), indicating matrix dependence of theCa²⁺ effect. Arsenate was used as a probe to differentiatebetween F₀/F₁-ATPase and adenylate translocaseactivity by a futile recycling of ADP-arsenate within the matrix,catalyzed by the F₀/F₁-ATPase. Ca²⁺increased the ADP arsenylation rate more than twofold, suggesting adirect effect on the F₀/F₁-ATPase. Theseresults suggest that Ca²⁺ activates cardiac aerobicrespiration at the level of both the CaDH andF₀/F₁-ATPase. This type of parallel control ofboth intermediary metabolism and ATP synthesis may provide a mechanismof altering ATP production rates with minimal changes in thehigh-energy intermediates as observed in vivo.