The roles of cytosolic and intramitochondrial Ca²⁺ and the mitochondrial Ca²⁺-uniporter (MCU) in the stimulation of mammalian oxidative phosphorylation

Szibor et al. (1) concluded that mitochondrial pyruvate oxidation is regulated primarily by cytosolic Ca²⁺ ([Ca²⁺]_cyt) activation of the malate-aspartate shuttle, rather than by mitochondrial Ca²⁺ ([Ca²⁺]_mit) activation of intramitochondrial dehydrogenases. Pyruvate dehydrogenase (PDH) activity largely reflects the ratio of active nonphosphorylated PDH to inactive phosphorylated PDH (PDHP) (2), but Szibor et al. (1) did not measure PDH/PDHP ratios. Moreover, their studies used unphysiological conditions with isolated mitochondria (saturating ADP); with synaptosomes, thymocytes, and fibroblasts (uncoupler and high pyruvate); and with perfused hearts (high pyruvate). These conditions likely suppress ATP-linked PDH kinase activity (inhibited by ADP and pyruvate), resulting in very high PDH/PDHP ratios. This severely limits any potential activation of PDH by the [Ca²⁺]_mit-stimulated PDHP phosphatase, inevitably delivering the results obtained. Under more physiological conditions, where PDH/PDHP ratios are lower, many studies have shown that [Ca²⁺]_mit is a key activator of pyruvate oxidation (3–5).

We suggest that stimulation of the malate-aspartate shuttle by [Ca²⁺]_cyt (increasing mitochondrial oxidation of cytoplasmic NADH) complements regulation of intramitochondrial dehydrogenases by [Ca²⁺]_mit (2). The latter may be regarded as an evolutionary refinement of “intrinsic” mechanisms (also present in lower organisms) increasing ATP production without lowering ATP/ADP ratios (2). Indeed, blockade of the mitochondrial Ca²⁺ uniporter (MCU) using ruthenium red decreases ATP/ADP ratios in stimulated hearts (6), consistent with reduced exercise tolerance in MCU-null mice (7), even though core intrinsic mechanisms are retained. Furthermore,mitochondrial Ca²⁺ influx is not completely suppressed by MCU deletion (7).