Stimulatory Effects of Calcium on Respiration and NAD(P)H Synthesis in Intact Rat Heart Mitochondria Utilizing Physiological Substrates Cannot Explain Respiratory Control in Vivo

Abstract

Mitochondrial TCA cycle dehydrogenase enzymes have been shown to be stimulated by Ca 2؉ under various substrate and ADP incubation conditions in an attempt to determine and understand the role of Ca 2؉ in maintaining energy homeostasis in working hearts. In this study, we tested the hypothesis that, at physiological temperature and 1 mM extramitochondrial free magnesium, Ca 2؉ can stimulate the overall mitochondrial NAD(P)H generation flux in rat heart mitochondria utilizing pyruvate and malate as substrates at both subsaturating and saturating concentrations. In both cases, we found that, in the physiological regime of mitochondrial oxygen consumption observed in the intact animal and in the physiological range of cytosolic Ca 2؉ concentration averaged per beat, Ca 2؉ had no observable stimulatory effect. A modest apparent stimulatory effect (22-27%) was observable at supraphysiological maximal ADP-stimulated respiration at 2.5 mM initial phosphate. The stimulatory effects observed over the physiological Ca 2؉ range are not sufficient to make a significant contribution to the control of oxidative phosphorylation in the heart in vivo.Mitochondria synthesize ATP by oxidative phosphorylation in response to cellular ATP demand, thereby maintaining the cytosolic phosphorylation potential to drive a variety of processes coupled to ATP hydrolysis. One of the central questions in cardiac physiology is the nature of the mitochondrial response to changes in cellular ATP demand, i.e. whether feedback from products of ATP hydrolysis is sufficient to explain observed phenomena in cardiac phosphoenergetics at various levels of work (1). On the basis of the apparent stability of creatine phosphate/ATP and P i over a wide range of workloads, Balaban et al. (2,3) postulated that this apparent stability is due to a "positive feedback" mechanism enabling mitochondrial response to match the demand. More properly, such a mechanism would be an example of open-loop control, where parallel Ca 2ϩ -dependent pathways would stimulate ATP utilization and synthesis. Cytosolic free Ca 2ϩ , which is a signal coupling cardiac electrical excitation to mechanical contraction, thereby increasing ATP demand, was identified as a putative open-loop controller activating mitochondrial matrix dehydrogenases and F 1 F 0 -ATPase in studies on isolated mitochondria utilizing glutamate/malate (4) or 2-oxoglutarate alone (5, 6) as substrate, whereas pyruvate is a physiological substrate entering the terminal oxidative pathway in the mitochondrial TCA cycle. In this study, we measured the respiration and redox responses of mitochondria to extramitochondrial free Ca 2ϩ when utilizing the physiological substrates pyruvate and malate at saturating, physiological, and subphysiological concentrations at physiological temperature and free magnesium (Mg 2ϩ ) concentration. We tested the hypothesis that an increase in Ca 2ϩ concentration could support higher respiration without a reduction in NAD(P)H concentration over the physiological range for respiration rates...

Mitochondrial performance in heat acclimation—a lesson from ischemia/reperfusion and calcium overload insults in the heart

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Extra-matrix Mg2+ limits Ca2+ uptake and modulates Ca2+ uptake–independent respiration and redox state in cardiac isolated mitochondria

“…However, the importance of [Ca 2+ ] m on mitochondrial dehydrogenase activities is strongly dependent on the substrate used. For example, it was reported that pyruvate dehydrogenase is activated at almost 100%, regardless of the [Ca 2+ ] m , and that α-ketoglutarate dehydrogenase is a more physiologically relevant target of Ca 2+ regulation (Vinnakota et al 2011, Wan et al 1989). This means that the greatest effect of Ca 2+ on bioenergetics would be with α-ketoglutarate as the substrate.…”

Enhanced charge-independent mitochondrial free Ca2+ and attenuated ADP-induced NADH oxidation by isoflurane: Implications for cardioprotection

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