Review article
Mitochondrial calcium transport in the heart: Physiological and pathological roles

https://doi.org/10.1016/j.yjmcc.2009.03.001Get rights and content

Abstract

That intramitochondrial free calcium ([Ca2+]m) plays various critical roles in both normal physiological and pathological conditions in the heart is now well-accepted, and evidenced by the interest and work in this area of the last two decades. However, controversies remain; such as the existence of beat-to-beat mitochondrial Ca2+ transients, role of [Ca2+]m in modulating whole-cell Ca2+ signalling, whether or not [Ca2+]m is critical for increases in ATP supply upon increased demand, and its role in cell death by both necrosis and apoptosis, especially in formation of the mitochondrial permeability transition pore and in ischaemic preconditioning. Neither is there a consensus as to whether inhibiting the Ca2+ influx or efflux pathways – the Ca2+ uniporter (MCU) and Na+/Ca2+-excahnger (mNCX), respectively – is cardioprotective, largely due to lack of specific inhibitors of these transporters. Ruthenium red, Ru360, clonazepam and CGP37157 are all very effective in isolated mitochondria, but reports of their effectiveness in whole cell and heart studies vary considerably, which partly accounts for the lack of a consensus on protective effects. The purification and cloning of the transporters, and development of more specific inhibitors, would produce a step-change in our understanding of the role of these apparently critical but still elusive proteins. However, developments in fluorescent indicators, proteins and imaging technology have meant that [Ca2+]m can now be measured reasonably specifically in intact cells and hearts, and interactions of the mitochondrial Ca2+ transporters with those of the sarcolemma or sarcoplasmic reticulum are being revealed. This has gone a long way to bringing the transporters to the forefront of cardiac research.

Introduction

Intramitochondrial free [Ca2+] ([Ca2+]m) is now recognised as having several important roles in the heart, under both physiological and pathological conditions; for example in the regulation of energy production, in modulating whole-cell Ca2+ signalling, and in the transition from reversible to irreversible injury during ischaemia–reperfusion, especially by formation of the mitochondrial permeability transition pore (MPTP). There has been a resurgence of interest in [Ca2+]m over the last two decades, sparked by development of targeted probes for measuring [Ca2+]m in living cells, and which has continued with the refinement of these probes and of imaging techniques to visualise [Ca2+] in various subcellular compartments, including mitochondria. The major pathways for mitochondrial Ca2+ transport in the heart are the Ca2+-uniporter (MCU) for Ca2+ influx and the mitochondrial Na+/Ca2+ exchanger (mNCX) for Ca2+ efflux. However, a continuing impediment to progression of research in this field is that the mitochondrial Ca2+ transport proteins have not been purified or cloned, and that use of the current inhibitors of these pathways is problematic in living cells, especially cardiomyocytes.

In this review I will summarise the development of theories on the role of [Ca2+]m in ATP production and Ca2+ signalling from historical studies through to present day controversies, including techniques for measuring [Ca2+]m. I will also discuss the role of [Ca2+]m in ischaemia–reperfusion injury in the heart, and whether the mitochondrial Ca2+ transporters are a realistic target for protective strategies; I will not describe the role of the MPTP apoptosis or necrosis in any detail as it is covered by other articles in this issue and in recent reviews [1], [2], [3].

Section snippets

Properties of the transporters

Ca2+ uptake by mitochondria was first described in the early 1960s in isolated kidney mitochondria as an energy dependent process, the energy coming from respiration [4], [5]. Large amounts of Ca2+ can be taken up by mitochondria together with inorganic phosphate (Pi) [6] and at levels above about 2 μM [Ca2+]m form an osmotically inactive calcium phosphate salt [7]. The nature of the calcium phosphate complex, and its low solubility in the mitochondrial matrix compared with that in solutions

Fluorescent and luminescent probes

Research into mitochondrial Ca2+ signalling was given a boost with the development in 1992 of a technique for measuring [Ca2+]m in living cells using the photoprotein aequorin targeted to mitochondria [54]. This, together with the continuing development of fluorescent indicators [55] and more recently genetically encoded fluorescent probes [56] has sparked a great deal of work and interest, not only in the role of [Ca2+]m as a regulator of metabolism, but also its role in controlling whole-cell

Evidence for beat-to-beat mitochondrial Ca2+ transients

In simple terms, excitation–contraction (EC) coupling in the heart occurs by depolarisation of the cell membrane (sarcolemma) causing L-type Ca2+-channels to open that then allow a small amount of Ca2+ into the cell. This triggers a much larger release of Ca2+ from the SR via the RyR – calcium-induced calcium release – that activates contractile proteins in a process that consumes ATP. Relaxation is initiated by Ca2+ reuptake back into the SR via the sarcoplasmic/endoplasmic reticulum ATPase

Activation of mitochondrial enzymes by [Ca2+]m

Oxidative phosphorylation in mitochondria provides most of the ATP in nearly all cell types, and accounts for over 90% of ATP production in the heart[98]. It was originally proposed that changes in the ADP/ATP ratio were the main regulator of ATP synthesis, and this was demonstrated in isolated mitochondria [99]. However, the role of a drop in ATP and increase in ADP as the trigger for stimulating ATP synthesis in vivo had to be re-evaluated when studies demonstrated that high energy phosphate

Mitochondrial Ca2+ — protective or harmful?

Mitochondria have the capacity to take up huge amounts of Ca2+[13], [14] and thus could potentially remove toxic levels of Ca2+ from the cytosol. Unfortunately, such accumulation of Ca2+ can eventually damage mitochondria both by competing for ATP production and more importantly by inducing the mitochondrial permeability transition pore (MPTP). It has been known for many years that Ca2+-induced mitochondrial dysfunction during ischaemia is associated with the transition from reversible to

Therapeutic implications

As well as the possible benefits of modulating [Ca2+]m during ischaemia/reperfusion injury discussed above, there is evidence that both RuR and Ru360 can reduce ventricular fibrillation (VF): In a pacing-induced model of VF in the isolated rat heart, perfusion with RuR (5 μM), or Ru360 (10 μM), resulted in conversion of VF to ventricular tachycardia (VT) [185]. This is potentially beneficial since VT commonly occurs before VF, and sudden cardiac death [186].

However, the benzodiazepines like

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