By Regulating Mitochondrial Ca2+-Uptake UCP2 Modulates Intracellular Ca2+

Introduction The possible role of UCP2 in modulating mitochondrial Ca2+-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 Ca2+-transients from mitochondrial ((Ca2+)m) and intracellular compartment ((Ca2+)c) in the whole-cell configuration in cardiomyocytes of wildtype (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 Ca2+-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 (Ca2+)m was decreased (0.050 +/- 0.009 FU vs. 0.021 +/- 0.005 FU, P<0.05) while (Ca2+)c was unchanged (0.032 +/- 0.002 FU vs. 0.028 +/- 0.004 FU, P>0.05) and transsarcolemmal Ca2+-influx was inhibited suggesting a possible compensatory mechanism. Additionally, we observed an inhibitory effect of ATP on mCa(2+)-uptake in WT mitoplasts and (Ca2+)m of cardiomyocytes leading to an increase of (Ca2+)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.