SDH AND ACTOMYOSIN ATPASE ACTIVITIES OF DIFFERENT FIBER TYPES IN RAT DIAPHRAGM MUSCLE

In the rat diaphragm muscle, the histochemical classification of type I, IIa, IIb, or IIx fibers was correlated with myosin heavy chain (MHC ) immunoreactivity. Expression of MHC isoforms in single dissected fib ers was also assessed electrophoretically. Most fibers (similar to 86% ) expressed a single MHC isoform, and when present, coexpression of MH C-SX and MHC-2B isoforms was most prevalent. Type I and IIa fibers wer e the smallest, type IIb fibers were the largest, and type IIx fibers were intermediate. Succinate dehydrogenase (SDH) and calcium-activated myosin adenosinetriphosphatase (actomyosin ATPase) activities were me asured with quantitative histochemical procedures. Type I and IIa fibe rs had the highest SDH activities, followed in rank order by type IIx and IIb fibers. Type I fibers had the lowest actomyosin ATPase activit y, followed in rank order by type IIa, IIx, and IIb fibers. Across all fibers, there was an inverse relationship between fiber SDH activity and cross-sectional area and a positive correlation between fiber acto myosin ATPase activity and cross-sectional area. The SDH and actomyosi n ATPase activities of muscle fibers were also inversely correlated. T hese phenotypic differences in SDH and ATPase activities may be import ant in determining the contractile and fatigue properties of different fiber types in the rat diaphragm muscle.