Calcium uptake and its effect on respiration and phosphorylation in mitochondria from brown adipose tissue.

The release of controlled respiration by limited concentrations of calcium was investigated in mitochondria from thermogenic brown adipose tissue of guinea pigs (newborn and cold-stressed) and from non-thermogenic tissue (of fetus and weaned animals). In the absence of added nucleotides the mitochondria were found to vary from a loose-coupled to a more tightly-coupled state as judged from the respiratory stimulation elicited by calcium and depending on the thermogenic status of the tissue. The loose-coupled mitochondria from thermogenic tissues were transformed to a more tightly-coupled state by addition of nucleotides. Nucleotides also stimulated the calcium uptake in mitochondria from thermogenic tissue. This stimulation of uptake was not affected by oligomycin. Mitochondria from both thermogenic states of the tissue required phosphate for calcium uptake and for release of controlled respiration by calcium. There were indications that regulation of the degree of coupling in these mitochondria is sited between the resporatory chain itself and the site of energy dissipation for ion translocation. Addition of calcium to brown adipose tissue mitochondria led to uncoupling of the oxidative phosphorylation at a concentration of 150 nmoles/mg protein and loose-coupled the mitochondria at much lower concentrations as judged by the ADP-released respiratory control. On the other hand, optimal calcium-released respiratory control was obtained at 500–1000 nmoles calcium/mg protein, supporting the idea that dissipation of energy prefers calcium transport to ATP synthesis. The calcium content of mitochondria from thermogenic tissue was 12–15 nmoles/mg, and there was a high rate of uptake. In mitochondria from non-thermogenic tissue the content was 81 nmoles/mg and there was a lower rate of uptake. These findings indicate the presence of metabolically active calcium in brown adipose tissue mitochondria and suggest a role for calcium in regulation of the thermogenesis of that tissue.