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The thermogenic function of brown adipose tissue (BAT) is known to be mainly regulated by a signal transduction cascade via β-adrenoceptor. However, recent studies indicated that the α-adrenoceptor and its downstream signal transduction cascade, causing elevation of the cellular Ca2+ level, are also important for the regulation of this function of BAT. In the present study, expression profiles of 3 isoforms of the inositol 1,4,5-trisphosphate (IP3) receptor, known as one of the major components of the machinery regulating the intracellular Ca2+ concentration in the BAT of rats, were investigated by Northern analysis. Of these three IP3 receptor isoforms, the type 2 one was found to be the most abundant of the three in BAT. Furthermore, when rats were exposed to the cold, under which condition the thermogenic function of BAT is known to be stimulated, the expression levels of types 1 and 2 isoforms of IP3 receptor were remarkably elevated. The results of Western analysis supported the predominant expression of the type 2 isoform in BAT. However, different from the results of Northern analysis, the expression levels of types 1 and 2 isoforms of IP3 receptor protein in BAT were not influenced by exposure of the animals to the cold.