Distribution of adipocyte-related cells in skeletal muscle of rainbow trout Oncorhynchus mykiss

Adiponectin is one of the adipokines secreted mainly from adipocytes in mammals. In rainbow trout, however, adiponectin is highly expressed in skeletal muscle. Although it has been suggested that fish skeletal muscle contains adipocytes, their endocrine function and distribution are poorly understood. Recently, an EST analysis of rainbow trout found that heart-type fatty acid binding protein (H-FABP), a member of the intracellular fatty acid binding protein family, encodes an adipose-specific gene. In this study, we produced anti-adiponectin and H-FABP antibodies and investigated the distribution of adipocytes and related cells in skeletal muscle of rainbow trout. The adiponectin signal was detected at around 75 kDa in muscle in Western blotting. Since the molecular mass of rainbow trout adiponectin is around 25 kDa, this 75 kDa band would be a trimer. For H-FABP, the signal band was detected at around 15 kDa. Immunohistochemistry of skeletal muscle sections indicated that adiponectin and H-FABP signals were present outside of muscle cells and throughout the muscle tissues, suggesting the existence of adipocyte-related cells in these regions. These results will contribute to our understanding of energy metabolism in fish skeletal muscle.