Assessment of brown adipose tissue activity in rats by 99mTc-sestamibi uptake

Brown adipose tissue ( BAT) physiology and imaging have recently attracted considerable attention. BAT is characterized both by enhanced perfusion and increased mitochondrial activity. Tc-99m-sestamibi is a lipophilic cationic tracer that concentrates in mitochondria. Data on the accumulation of Tc-99m-sestamibi in BAT are currently lacking. This study investigates the in vivo Tc-99m-sestamibi uptake in rat BAT. Tc-99m-sestamibi was administered in male Wistar rats of various age and body size. Tc-99m-sestamibi uptake was measured in vitro in BAT and white fat (WF) together with cytochrome c oxidase activity. Both Tc-99m-sestamibi uptake and cytochrome c oxidase activity were higher in BAT than in WF (P < 0.05). Tc-99m-Sestamibi uptake in both BAT and WF was negatively related to body weight (r = -0.96 and -0.89, respectively) as was the BAT/ WF uptake ratio (r = -0.85). These data show a higher Tc-99m-sestamibi uptake in BAT compared to WF, in agreement with the high mitochondrial content and respiratory activity of the former. The strong negative correlation between Tc-99m-sestamibi uptake in BAT and body weight ( negative allometry), is in accordance to increased needs of thermogenesis in smaller animals. Implications of increased Tc-99m-sestamibi uptake in BAT in radionuclide imaging are also discussed.