Uncoupling protein-2 modulates myocardial excitation-contraction coupling

Ca2+ dynamics in the mitochondria - state of the art

The importance of [Ca2+] in the mitochondrial matrix, [Ca2+]mito, had been proposed by early work of Carafoli and others [1 ], [2 ] and [3 ]. The key suggestion in the 1970s [4 ] was that regulatory [Ca2+]mito played a role in controlling the rate of activation of tricarboxylic acid cycle dehydrogenases, important in the regulation of ATP production by the electron transport chain (ETC) during oxidative phosphorylation. This view is now established [5 ] and [6 ] and the key questions currently debated are to what extent do the mitochondria acquire and release Ca2+, and what impact do mitochondria have on the dynamic Ca2+ signal in the cardiac ventricular myocyte [7 ]. Although investigations of Ca2+ dynamics in mitochondria have been problematic, disparate and inconclusive, they have also been both provocative and exciting. A recent special issue of this journal presented contrasting perspectives on the speed, extent and mechanisms of changes in [Ca2+]mito, and how these changes may influence cellular spatio-temporal [Ca2+]i dynamics [8 ]. An audio discussion is also available online [9 ]. The uncertain nature of the signaling pathways is noted in Table 1 (see below) which shows mitochondrial proteins and processes that are of current focus and which remain contentious. Each of the “items” listed is largely unsettled, or is a “work in progress”. There may be advocates for opposing positions noted or recent discoveries that must still be tested at multiple levels by diverse laboratories. Currently, the first item, the mitochondrial sodium/calcium exchanger (NCLX) [10 ], appears the most solid with respect to the molecular identification and physiological function, whereas, the recently described candidates of the mitochondrial Ca2+ uniporter (MCU) [11 ] and [12 ] still need to be verified and broadly examined by the scientific community.

Chronic and acute exposure of mouse hearts to fatty acids increases oxygen cost of excitation-contraction coupling

The aim of the present study was to evaluate the underlying processes involved in the oxygen wasting induced by inotropic drugs and acute and chronic elevation of fatty acid (FA) supply, using unloaded perfused mouse hearts from normal and type 2 diabetic ( db/db) mice. We found that an acute elevation of the FA supply in normal hearts, as well as a chronic (in vivo) exposure to elevated FA as in db/db hearts, increased myocardial oxygen consumption (MV̇o2unloaded) due to increased oxygen cost for basal metabolism and for excitation-contraction (EC) coupling. Isoproterenol stimulation, on top of a high FA supply, led to an additive increase in MV̇o2unloaded, because of a further increase in oxygen cost for EC coupling. In db/db hearts, the acute elevation of FA did not further increase MV̇o2. Since the elevation in the FA supply is accompanied by increased rates of myocardial FA oxidation, the present study compared MV̇o2 following increased FA load versus FA oxidation rate by exposing normal hearts to normal and high FA concentration (NF and HF, respectively) and to compounds that either stimulate (GW-610742) or inhibit [dichloroacetate (DCA)] FA oxidation. While HF and NF + GW-610742 increased FA oxidation to the same extent, only HF increased MV̇o2unloaded. Although DCA counteracted the HF-induced increase in FA oxidation, DCA did not reduce MV̇o2unloaded. Thus, in normal hearts, acute FA-induced oxygen waste is 1) due to an increase in the oxygen cost for both basal metabolism and EC coupling and 2) not dependent on the myocardial FA oxidation rate per se, but on processes initiated by the presence of FAs. In diabetic hearts, chronic exposure to elevated circulating FAs leads to adaptations that afford protection against the detrimental effect of an acute FA load, suggesting different underlying mechanisms behind the increased MV̇o2 following acute and chronic FA load.