METHODS

Female lean (+/+ or+/ob) and genetically obese (ob/ob) mice were obtained at an age of 5 wk from Jackson Laboratories, Bar Harbor, ME. They were maintained at 26OC in plastic cages, four mice per cage, with free access to food (Purina rat chow 5012) and water and a 12: 12 lighting schedule. They were studied at an age of 12-13 wk.

Three days before a first series of four experiments, animals were individually caged. In each experiment seven mice (all lean or all obese) acclimated to 26 it lC were injected subcutaneously on the back near the tail with L-[2, 5, 6,-3H] noradrenaline (44.7 Ci/mmol), previously diluted to 25@/ml with 0.9% NaCl containing 0.01% ascorbic acid, at a dose of 250 pCi/kg for lean mice or 125 &i/kg for obese mice. After exactly 1 h, one mouse was killed, three mice were placed at 4 t 2” C, and three mice were left at 26 & lC (injections were staggered in time so that timing of exposure was exact). Pairs of mice (one at 26OC and one at 4 C) were killed, 1, 2, and 3 h after the first mouse. In a second series of four experiments animals were placed in individual. cages at 16 t 2OC or two mice per cage at 33 tlC for a period of 2 wk before the experiments. In each experiment four lean and four obese mice acclimated to 16 or 33OC were injected with [3H] noradrenaline (as described above). Mice were killed in pairs (one lean and one obese) at l-hour intervals, starting 1 h after injection of [3H] noradrenaline. Animals were killed by cervical dislocation and the interscapular plus subscapular brown adipose tissue, the heart, and the spleen removed and placed in an ice-cold solution containing 0.25 M sucrose, 1 mM HEPES (N-2. hydroxyethylpiperazine-N’-2-ethanesulfonic acid) and