PERIPHERAL MECHANISMS OF THERMOGENESIS INDUCED BY EPHEDRINE AND CAFFEINE IN BROWN ADIPOSE-TISSUE

The peripheral mechanisms by which ephedrine and caffeine influence thermogenesis were investigated in innervated rat interscapular brown adipose tissue (IBAT) by assessing its rate of oxygen consumption (MO2) in vitro. Dose-response measurements with tissues from intact or sympathectomized (6-OHDA) animals indicate that the thermogenic effects of low concentrations of ephedrine and also of caffeine are entirely dependent upon the presence of intact sympathetic nerve endings, and thus depend on presynaptic mechanisms. Direct postsynaptic stimulation of thermogenesis is only apparent at much higher concentrations, namely > 1-mu-M for ephedrine and > 2mM for caffeine. At subminimal concentrations that neither ephedrine nor caffeine influenced basal tissue respiration, they induced a 4-5-fold increase in basal MO2 when administered in combination, a synergistic response prevented by pre-treatment of the rat with 6-OHDA. Synergistic increases in IBAT respiration were also obtained when subminimal concentration of ephedrine was added to 3-propylxanthine (a specific inhibitor of phosphodiesterase), to 8-phenyltheophylline (a potent adenosine receptor antagonist) or to adenosine deaminase (for enzymatic inactivation of endogenous adenosine). Conversely, the marked synergism in thermogenic response with ephedrine + caffeine was reduced in the presence of 2-chloroadenosine (an adenosine analogue). In tissues from fasted rats, the ephedrine + caffeine synergism in thermogenic response, although attenuated, was nevertheless present. These studies therefore demonstrate that ephedrine, at doses comparable with therapeutic use, stimulates thermogenesis in BAT via sympathetically released NA. In addition, a synergistic interaction between caffeine and ephedrine on BAT thermogenesis is explained by ephedrine's enhancement of sympathetic neuronal release of NA, together with caffeine's dual ability to antagonize adenosine and to inhibit cellular phosphodiesterase activity.