By Regulating Mitochondrial Ca(2+)-Uptake UCP2 Modulates Intracellular Ca(2+)

INTRODUCTION: The possible role of UCP2 in modulating mitochondrial Ca(2+)-uptake (mCa(2+)-uptake) via the mitochondrial calcium uniporter (MCU) is highly controversial. METHODS: Thus, we analyzed mCa(2+)-uptake in isolated cardiac mitochondria, MCU single-channel activity in cardiac mitoplasts, dual Ca(2+)-transients from mitochondrial ((Ca(2+))m) and intracellular compartment ((Ca(2+))c) in the whole-cell configuration in cardiomyocytes of wild-type (WT) and UCP2(-/-) mice. RESULTS: Isolated mitochondria showed a Ru360 sensitive mCa(2+)-uptake, which was significantly decreased in UCP2(-/-) (229.4±30.8 FU vs. 146.3±23.4 FU, P<0.05). Single-channel registrations confirmed a Ru360 sensitive voltage-gated Ca(2+)-channel in mitoplasts, i.e. mCa1, showing a reduced single-channel activity in UCP2(-/-) (Po,total: 0.34±0.05% vs. 0.07±0.01%, P<0.05). In UCP2(-/-) cardiomyocytes (Ca(2+))m was decreased (0.050±0.009 FU vs. 0.021±0.005 FU, P<0.05) while (Ca(2+))c was unchanged (0.032±0.002 FU vs. 0.028±0.004 FU, P>0.05) and transsarcolemmal Ca(2+)-influx was inhibited suggesting a possible compensatory mechanism. Additionally, we observed an inhibitory effect of ATP on mCa(2+)-uptake in WT mitoplasts and (Ca(2+))m of cardiomyocytes leading to an increase of (Ca(2+))c while no ATP dependent effect was observed in UCP2(-/-). CONCLUSION: Our results indicate regulatory effects of UCP2 on mCa(2+)-uptake. Furthermore, we propose, that previously described inhibitory effects on MCU by ATP may be mediated via UCP2 resulting in changes of excitation contraction coupling.