Cold-stimulated sympathetic activity in brown adipose tissue of obese (ob/ob) mice

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Abstract
Whether the known failure of brown adipose tissue thermogenesis in the cold-exposed genetically obese (ob/ob) mouse might be due to a defect in the usual cold-induced increase in sympathetic activity in this tissue was studied. Turnover of noradrenaline (norepinephrine) in brown adipose tissue, heart and spleen was measured in lean and in genetically obese mice from the rate of disappearance of radioactivity after injection of a tracer amount of [3H]noradrenaline and the noradrenaline content, as measured by a radioenzymatic method. Mice were acclimated to 33.degree., 26.degree. or 16.degree. C or were acclimated to 26.degree. C and exposed to 4.degree. C for 3 h. Noradrenaline content of brown adipose tissue of obese mice was either the same as or higher than that of lean mice at all temperatures studied. Noradrenaline turnover rates in brown adipose tissue were similar in lean and obese mice at 33.degree. C and at 16.degree. C and a similar stimulation of noradrenaline synthesis and secretion occurred in both types of mouse when they were exposed to 4.degree. C. Noradrenaline turnover rates in heart and spleen were similar in lean and obese mice at all temperatures studied. The only major difference between lean and obese mice was a lower noradrenaline turnover rate in brown adipose tissue of the obese mouse acclimated to 26.degree. C. The sympathetic innervation of brown adipose tissue is present in the obese mouse, and there is no major defect in cold-induced stimulation of the sympathetic innervation to brown adipose tissue in these animals.

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