Ion transport and morphological changes of mitochondria in brown adipocytes of warm- and cold-acclimatized obese Zucker rats.

Brown adipose tissue plays the dominant role in response to cold acclimatization through its capacity to produce heat. To demonstrate the cellular function for thermogenesis induced by cold acclimation in the brown adipose tissue of obese Zucker rats, we examined the changes for the area as well as the Na, K, Cl, and Ca concentrations in the mitochondria of brown adipocytes after the warm (25 degrees C, WG) and the cold acclimations (10 degrees C, CG). Moreover, the respiratory quotients (RQs) of these rats were measured. After the acclimations, the RQ in the CG was decreased and the oxygen consumption increased. A morphometric analysis of electron micrographs of brown adipocytes from the two groups of rats showed a marked increase in the area of the mitochondria in the CG. An electron probe X-ray microanalysis showed an increase in the Ca concentration and decreases in the Na and K concentrations in the matrix of the mitochondria of the cells in the CG. These results suggest that the reduction in the RQ of obese Zucker rats acclimated to cold is the consequence of the metabolism of a large quantity of lipid in the brown adipocytes. Our data also indicate that the observed change in the mitochondrial area and the increase for Ca in the mitochondria were associated with the cold-induced thermogenesis in brown adipocytes of obese Zucker rats.