ALPHA-ADRENERGIC STIMULATION OF THERMOGENESIS IN A RAT KANGAROO (MARSUPIALIA, BETTONGIA-GAIMARDI)

The Tasmanian bettong (Bettongia gaimardi) is a small rat kangaroo wit hout detectable brown adipose tissue (BAT). In view of our previous fi ndings of norepinephrine-mediated increase in O-2 consumption (Vo(2)) in the perfused hindlimb of this species, the present study examined t he effect of alpha-adrenoceptors on the thermogenesis of conscious bet tongs at rest by infusing adrenergic agents via an indwelling catheter in the tail vein. The resting Vo(2) was 22.9+/-1.9 mmol . kg(-1). h(- 1) Norepinephrine (10-80 mu g . kg(-1). min(-1)) stimulated V-o2 in a dose-dependent manner with the maximal increment of 46.7%. Naphazoline (an alpha(1),alpha(2)-adrenergic agonist) and phenylephrine (an alpha (1)-adrenergic agonist) also elicited increases in V-o2 with maximal v alues of 29.6 and 34.8%, respectively In contrast, the alpha(2)-adrene rgic agonist clonidine had no significant effects. Both alpha- and bet a-adrenergic blockers were used to antagonize the submaximal increase in Vo(2) elicited by norepinephrine. At a dose of 10 mu g . kg(-1). mi n(-1), the alpha-adrenergic blocker phentolamine abolished the effects of naphazoline and phenylephrine and reduced norepinephrine-induced V -o2 by 45.5%. The beta-adrenergic blocker propranolol inhibited the no repinephrine-induced V-o2 by 58.8% at 20 mu g . kg(-1). min(-1). A com bination of the two antagonists blocked 82.5% of the norepinephrine-in duced Vo(2). Pretreatment of the animal with indomethacin (1 mg/kg), a known inhibitor of prostaglandin cyclooxygenase, had no effect on phe nylephrine-elicited Vo(2). Taken together, these results indicate that alpha(1)-adrenoceptors are directly involved in norepinephrine-induce d thermogenesis in non-BAT tissue(s).