Calcium Activation of Heart Mitochondrial Oxidative Phosphorylation

Abstract

Parallel activation of heart mitochondria NADH and ATP production by Ca2+ has been shown to involve the Ca2+-sensitive dehydrogenases and the F0F1-ATPase. In the current study we hypothesize that the response time of Ca2+-activated ATP production is rapid enough to support step changes in myocardial workload (∼100 ms). To test this hypothesis, the rapid kinetics of Ca2+ activation of mV̇O2, [NADH], and light scattering were evaluated in isolated porcine heart mitochondria at 37 °C using a variety of optical techniques. The addition of Ca2+ was associated with an initial response time (IRT) of mV̇O2 that was dose-dependent with a minimum IRT of 0.27 ± 0.02 s (n = 41) at 535 nM Ca2+. The IRTs for NADH fluorescence and light scattering in response to Ca2+ additions were similar to mV̇O2. The Ca2+ IRT for mV̇O2 was significantly shorter than 1.6 mM ADP (2.36 ± 0.47 s; p ≤ 0.001, n = 13), 2.2 mM Pi (2.32 ± 0.29, p ≤ 0.001, n = 13), or 10 mM creatine (15.6.±1.18 s, p ≤ 0.001, n = 18) under similar experimental conditions. Calcium effects were inhibited with 8 μM ruthenium red (2.4 ± 0.31 s;p ≤ 0.001, n = 16) and reversed with EGTA (1.6 ± 0.44; p ≤ 0.01, n = 6). Estimates of Ca2+ uptake into mitochondria using optical Ca2+ indicators trapped in the matrix revealed a sufficiently rapid uptake to cause the metabolic effects observed. These data are consistent with the notion that extramitochondrial Ca2+ can modify ATP production, via an increase in matrix Ca 2+ content, rapidly enough to support cardiac work transitions in vivo.