Synergism between caffeine and dl-phenylpropanolamine on brown adipose tissue thermogenesis in the adult rat.

Caffeine produces enhanced oxygen consumption, an effect that may reflect an action of caffeine on brown adipose thermogenesis. In Experiment 1, adult male rats were anesthetized with 1.2 g/kg urethane and treated (IP) with either 0.9% saline or 10, 20 or 40 mg/kg caffeine (n = 4 each group). Interscapular BAT (IBAT) and rectal temperatures were recorded every minute for 10 minutes prior to and 30 minutes following drug injection. Stable IBAT and rectal temperatures were observed prior to and after saline injection whereas rats treated with 20 and 40 mg/kg caffeine exhibited moderate increases in IBAT, but not rectal, temperature. In Experiment 2, adult male rats were treated with either 0.9% saline or 10 mg/kg caffeine, anesthetized with urethane (1.2 g/kg) and treated (30 minutes after pretreatment injections) with either 0.9% saline or 10 mg/kg dl-phenylpropanolamine (dl-PPA). A combination of caffeine and dl-PPA produced significantly greater BAT thermogenesis than just dl-PPA alone. The implications of these data for the inclusion of caffeine in over-the-counter diet-pills are discussed.