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Laboratory of Cardiac Energetics, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1061
Ca2+ has been postulated as a cytosolic
second messenger in the regulation of cardiac oxidative
phosphorylation. This hypothesis draws support from the well-known
effects of Ca2+ on muscle activity, which is stimulated in
parallel with the Ca2+-sensitive dehydrogenases (CaDH). The
effects of Ca2+ on oxidative phosphorylation were further
investigated in isolated porcine heart mitochondria at the level of
metabolic driving force (NADH or 
) and ATP
production rates (flow). The resulting force-flow (F-F) relationships
permitted the analysis of Ca2+ effects on several putative
control points within oxidative phosphorylation, simultaneously. The
F-F relationships resulting from additions of carbon substrates alone
provided a model of pure CaDH activation. Comparing this curve with
variable Ca2+ concentration
([Ca2+]) effects revealed an approximate
twofold higher ATP production rate than could be explained by a simple
increase in NADH or 
via CaDH activation. The half-maximal effect
of Ca2+ at state 3 was 157 nM and was completely inhibited
by ruthenium red (1 µM), indicating matrix dependence of the
Ca2+ effect. Arsenate was used as a probe to differentiate
between F0/F1-ATPase and adenylate translocase
activity 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 a
direct effect on the F0/F1-ATPase. These
results suggest that Ca2+ activates cardiac aerobic
respiration at the level of both the CaDH and
F0/F1-ATPase. This type of parallel control of
both intermediary metabolism and ATP synthesis may provide a mechanism
of altering ATP production rates with minimal changes in the
high-energy intermediates as observed in vivo.
metabolism; ATP synthesis; dehydrogenase; force-flow analysis
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