SF2523 Continuity of the outer mitochondrial membrane with tubular profiles of endoplasmic reticulum has also been described in different cell types, such as rat hepatocytes, the ciliate Tetrahymmena pyriformis, in fungi, and neural tissue. There is increasing biochemical and morphological evidence demonstrating similarities between the ER and mitochondrial outer membranes, as well as transfer and exchange of materials between the ER and mitochondria. The dynamic interactions of these membranes comprise the phenomena of membrane flow and transformation. It has been proposed that the ER could provide new membranes for mitochondrial growth, and, thus, the role of the ER would be to provide new membrane lipids. Several reports indicated that certain mitochondrial phospholipids were formed in the ER and then transferred to the mitochondrion in liver cells. The Thio-TEPA localized regions of membrane interaction could raise intermittent bridges, through which cellular macromolecules may be exchanged. To date, the establishment of a physical connection between the ER and mitochondria in BAT was not previously described. In these cells, the globular ER structures touch the mitochondria. The two structures are apparently pulled together, propitiating the fusion of mitochondrial and ER membranes. This is different from what was observed in striated muscle where there seems to be no membrane fusion. Small tubular units hold the mitochondria and ER together, and communication between the two sub cellular compartments would then be mediated by the tethering structures. A link between BAT and skeletal muscle has been recently reported by Seale et al.. These authors found that the transcriptional regulator PRDM 16 controls a bidirectional differentiation between skeletal myoblasts and brown adipocytes. The finding of SERCA 1 in BAT mitochondria has led us to study a possible role of Ca2+ in BAT mitochondria thermogenesis. It is well established that SERCA 1 uses the energy derived from ATP hydrolysis to simultaneously pump Ca2+ across a membrane and to produce heat. In previous report, the effect of Ca2+ was studies activating BAT mitochondria with 1 mM ATP.