Cytosolic and mitochondrial Ca concentrations in primary hepatocytes change with ageing and in consequence of an mtDNA mutation.

Mitochondrial Ca flux is crucial for the regulation of cell metabolism. Ca entry to the mitochondrial matrix is mediated by VDAC1 and MCU with its regulatory molecules. We investigated hepatocytes isolated from conplastic C57BL/6NTac-mt mice (mtNOD) that differ from C57BL/6NTac mice (controls) by a point mutation in mitochondrial-encoded subunit 3 of cytochrome c oxidase, resulting in functional and morphological mitochondrial adaptations. Mice of both strains up to 12 months old were compared using mitochondrial GEM-GECO1 and cytosolic CAR-GECO1 expression to gain knowledge of age-dependent alterations of Ca concentrations. In controls we observed a significant increase in glucose-induced cytosolic Ca concentration with ageing, but only a minor elevation in mitochondrial Ca concentration. Conversely, glucose-induced mitochondrial Ca concentration significantly declined with ageing in mtNOD mice, paralleled by a slight decrease in cytosolic Ca concentration. This was consistent with a significant reduction of the MICU1 to MCU expression ratio and a decline in MCUR1. Our results can best be explained in terms of the adaptation of Ca concentrations to the mitochondrial network structure. In the fragmented mitochondrial network of ageing controls there is a need for high cytosolic Ca influx, because only some of the isolated mitochondria are in direct contact with the endoplasmic reticulum. This is not important in the hyper-fused elongated mitochondrial network found in ageing mtNOD mice which facilitates rapid Ca distribution over a large mitochondrial area.