Glycogen metabolism in rat interscapular brown adipose tissue

It is generally considered that the main substrate oxidized by brown adipose tissue is fatty acids derived from endogenous triacyglycerol. However, the tissue from coldadapted rats fed a typical high-carbohydrate laboratory diet contains a high activity of pyruvate dehydrogenase (McCormack & Denton, 1977) and also high activities of hexokinase, phosphofructokinase and pyruvate kinase (Cooney & Newsholme, 1982; Sale, 1985). This suggest that at least under these conditions, glucose may also be an important source of acetyl-CoA for oxidation. A view further strengthened by our recent finding that stimulation of the tissue by noradernaline in vivo is associated with a ten-fold rise in the proportion of pyruvate dehydrogenase in its active non-phosphorylated form (Gibbins et al., 1985). In the present communication we report that this tissue also contains a substantial amount of glycogen together with high activities of both glycogen synthase and phosphorylase. Moreover, these enzymes are shown to respond to changes in circulating levels of insulin and noradrenaline. Female albino Wistar rats were transferred to 4-6°C 1 week after weaning and used 3 4 weeks later. The rats has free access to food and were transferred to room temperature (20-25°C) 1 h before each experiment. Glycogen content was assayed either by the method of Montgomery (1957) or that of Randle et al. (1964) in which glycogen glucose was assayed using the glucose oxidase method (Hugget & Nixon, 1957). Both methods yielded values close to 3mg of glycogen/g wet wt. of tissue. When allowance is made for the 50% fat content of brown adipose tissue, this level is similar to that found in rat heart and skeletal muscle and approaches half that found in rat liver (see Russell & Bloom, 1955). Phosphorylase a activity was measured from the incorporation of ['4C]glucoseI-phosphate (Gilboe et al., 1972) with the activity ratio being defined as the ratio of phosphorylase a activity to the sum of phosphorylase a and b activities (assayed by addition of 5 mM-ADP and 8% (v/v) dimethoxyethane). Glycogen synthase was measured by following the incorporation of [U-'4C]UDPglucose (Thomas et al., 1968), with the activity ratio being defined as the ratio of activities in the absence and presence of 10 mM-glucose 6-phosphate. Total phosphorylase and glycogen synthase activities were 10.0 f 1.3 and 0.37 k 0.044 units/g wet wt. respectively (results are given as mean f S.E.M. for 79 observations, where a unit of enzyme activity forms the equivalent of I pmol of product/min at 30°C). These activities compare with 22 and 1.2 units/g wet wt. for phosphorylase and glycogen synthase respectively in rat liver (Gilboe et al., 1972; Thomas et al., 1985). Circulating concentrations of insulin were manipulated within the physiological range by injections of either anti-insulin serum or glucose. The activity of phosphorylase was very similar in animals treated with either antiinsulin serum or glucose, whereas the activity of glycogen synthase was markedly increased under conditions of high circulating insulin (Table I). In contrast, injection of noradrenaline was found to significantly increase the phosphorylase activity ratio while resulting in a substanTable I . Effects of alterations of circulating insulin and noradrenaline on activities of glycogen synthase andphosphorylase in brown adipose tissue of cold-adapted rats