Disparate effects of fenfluramine on thermogenesis in brown adipose tissue in the rat.

It has been suggested that fenfluramine, a clinically used appetite suppressant, can also promote weight loss by augmenting energy expenditure, as indicated by increased whole-body O2 consumption (VO2) and mitochondrial GDP binding in brown adipose tissue (BAT) of fenfluramine-treated rats. To further investigate a possible involvement of BAT in the drug's metabolic effects, 113Sn-labelled microspheres were injected into the left cardiac ventricle of conscious rats 70-80 min after intraperitoneal delivery of 20 mg/kg fenfluramine (DL-mixture) or saline vehicle. At 28 degrees C ambient temperature, fenfluramine augmented resting whole-body VO2 and increased the microsphere entrapment in BAT, indicating enhanced blood flow and metabolism. At 20 degrees C ambient temperature, the expected increase in BAT blood flow associated with nonshivering thermogenesis was observed in control rats, but in fenfluramine-treated rats the increase in BAT blood flow was severely attenuated, and VO2 and body temperature were reduced. The stimulatory effect of fenfluramine on BAT metabolism was not prevented by urethane anesthesia but did not occur if the tissue was denervated. These blood flow measurements corroborate previous reports, based on GDP-binding assays, that fenfluramine treatment can augment thermogenesis in BAT by effects mediated through the innervation of the tissue. However, the data also indicate that this calorigenic effect is dependent on ambient temperature being near thermoneutrality and that in a cool environment the drug inhibits BAT thermogenesis.