Effects of mitochondrial uncoupling on Ca²⁺signaling during excitation-contraction coupling in atrial myocytes

Mitochondria play an important role in intracellular Ca(2+) concentration ([Ca(2+)]i) regulation in the heart. We studied sarcoplasmic reticulum (SR) Ca(2+) release in cat atrial myocytes during depolarization of mitochondrial membrane potential (ΔΨm) induced by the protonophore FCCP. FCCP caused an initial decrease of action potential-induced Ca(2+) transient amplitude and frequency of spontaneous Ca(2+) waves followed by partial recovery despite partially depleted SR Ca(2+) stores. In the presence of oligomycin, FCCP only exerted a stimulatory effect on Ca(2+) transients and Ca(2+) wave frequency, suggesting that the inhibitory effect of FCCP was mediated by ATP consumption through reverse-mode operation of mitochondrial F1F0-ATPase. ΔΨm depolarization was accompanied by cytosolic acidification, increases of diastolic [Ca(2+)]i, intracellular Na(+) concentration ([Na(+)]i), and intracellular Mg(2+) concentration ([Mg(2+)]i), and a decrease of intracellular ATP concentration ([ATP]i); however, glycolytic ATP production partially compensated for the exhaustion of mitochondrial ATP supplies. In conclusion, the initial inhibition of Ca(2+) transients and waves resulted from suppression of ryanodine receptor SR Ca(2+) release channel activity by a decrease in [ATP], an increase of [Mg(2+)]i, and cytoplasmic acidification. The later stimulation resulted from reduced mitochondrial Ca(2+) buffering and cytosolic Na(+) and Ca(2+) accumulation, leading to enhanced Ca(2+)-induced Ca(2+) release and spontaneous Ca(2+) release in the form of Ca(2+) waves. ΔΨm depolarization and the ensuing consequences of mitochondrial uncoupling observed here (intracellular acidification, decrease of [ATP]i, increase of [Na(+)]i and [Mg(2+)]i, and Ca(2+) overload) are hallmarks of ischemia. These findings may therefore provide insight into the consequences of mitochondrial uncoupling for ion homeostasis, SR Ca(2+) release, and excitation-contraction coupling in ischemia at the cellular and subcellular level.