Fiber-type differences in muscle mitochondrial profiles

1 Department of Biology, Queen's University, Kingston, Ontario K7L 3N6; 2 Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1; and 3 Hopkins Marine Station, Stanford University, Pacific Grove, California 93950-3094 Submitted 30 January 2003 ; accepted in final form 3 June 2003 Although striated muscles differ in mitochondrial content, the extent of fiber-type specific mitochondrial specializations is not well known. To address this issue, we compared mitochondrial structural and functional properties in red muscle (RM), white muscle (WM), and cardiac muscle of rainbow trout. Overall preservation of the basic relationships between oxidative phosphorylation complexes among fiber types was confirmed by kinetic analyses, immunoblotting of native holoproteins, and spectroscopic measurements of cytochrome content. Fiber-type differences in mitochondrial properties were apparent when parameters were expressed per milligram mitochondrial protein. However, the differences diminished when expressed relative to cytochrome oxidase (COX), possibly a more meaningful denominator than mitochondrial protein. Expressed relative to COX, there were no differences in oxidative phosphorylation enzyme activities, pyruvate-based respiratory rates, H 2 O 2 production, or state 4 proton leak respiration. These data suggest most mitochondrial qualitative properties are conserved across fiber types. However, there remained modest differences ( 50%) in stoichiometries of selected enzymes of the Krebs cycle, -oxidation, and antioxidant enzymes. There were clear differences in membrane fluidity (RM > cardiac, WM) and proton conductance (H + /min/mV/U COX: WM > RM > cardiac). The pronounced differences in mitochondrial content between fiber types could be attributed to a combination of differences in myonuclear domain and modest effects on the expression of nuclear- and mitochondrially encoded respiratory genes. Collectively, these studies suggest constitutive pathways that transcend fiber types are primarily responsible for determining most quantitative and qualitative properties of mitochondria. skeletal muscle; oxidative phosphorylation; reactive oxygen species; membrane fluidity; proton leak; energy metabolism Address for reprint requests and other correspondence: C. D. Moyes, Dept. of Biology, Queen's Univ., Kingston, Ontario, Canada K7L 3N6 (E-mail: moyesc{at}biology.queensu.ca ).