Cytoplasmic and mitochondrial Ca levels in brown adipocytes

We elucidated the mitochondrial functions of brown adipocytes in intracellular signalling, paying attention to mitochondrial activity and noradrenaline- and forskolin-induced Ca(2+) mobilizations in cold-acclimated rats.

A confocal laser-scanning microscope of brown adipocytes from warm- or cold-acclimated rats was employed using probes rhodamine 123 which is a mitochondria-specific cationic dye, and the cytoplasmic and mitochondrial Ca(2+) probes fluo-3 and rhod-2. X-ray microanalysis was also studied.

The signal of rhodamine 123 in the cells was decreased by antimycin A which effect was less in cold-acclimated cells than warm-acclimated cells. Cytoplasmic and mitochondrial Ca(2+) in cold-acclimated brown adipocytes double-loaded with fluo-3 and rhod-2 were measured. Noradrenaline induced the rise in cytoplasmic Ca(2+) ([Ca(2+)](cyto)) followed by mitochondrial Ca(2+) ([Ca(2+)](mito)), the effect being transformed into an increase in [Ca(2+)](cyto) whereas a decrease in [Ca(2+)](mito) by antimycin A or carbonyl cyanide m-chlorophenylhydrazone (CCCP). Antimycin A induced small Ca(2+) release from mitochondria. CCCP induced Ca(2+) release from mitochondria only after the cells were stimulated with noradrenaline. Further, forskolin also elicited an elevation in [Ca(2+)](cyto) followed by [Ca(2+)](mito) in the cells. The Ca measured by X-ray microanalysis was higher both in the cytoplasm and mitochondria whereas K was higher in the mitochondria of cold-acclimated cells in comparison to warm-acclimated cells.

These results suggest that noradrenaline and forskolin evoked an elevation in [Ca(2+)](cyto) followed by [Ca(2+)](mito), in which H(+) gradient across the inner membrane is responsible for the accumulation of calcium on mitochondria. Moreover, cAMP also plays a role in intracellular and mitochondrial Ca(2+) signalling in cold-acclimated brown adipocytes.