Immunohistochemical identification of the uncoupling protein in rat brown adipose tissue

Brown adipose tissue mitochondria are characterized by the presence of an uncoupling protein that gives them an exceptional capacity for substrate-controlled respiration and thermogenesis. The specific localization of this protein in rat brown adipocytes was demonstrated using an immunohistochemical technique, the peroxidase-antiperoxidase (PAP) method. Light microscopy observations showed that serum antibodies raised against the uncoupling protein selectively reacted with multilocular brown adipocytes. No labeling could be detected in either unilocular adipocytes, capillaries, or muscle fibers (striated and vascular smooth muscle). Staining was more intensive in certain adipocytes than in others, suggesting the presence of cellular heterogeneity. The specificity of the staining technique was demonstrated by showing that treatment of the preparations with antiserum saturated with an excess of uncoupling protein almost entirely inhibited brown adipocyte labeling. The specificity and selectivity of the PAP method allow the clear differentiation of uncoupling protein-containing adipocytes from other cellular types, suggesting that this immunohistochemical technique will represent an extremely useful tool for studying adipocyte function and differentiation.