Sdh and actomyosin atpase activities of different fiber types in the rat diaphragm muscle. Sieck, Gary C., Wen-Zhi Zhan, Y. S. Prakash, Molly J. Daood, and Jon F. Watchko. Departments of Anesthesiology and Physiology & Biophysics, Mayo Clinic and Mayo Medical School, Rochester, Minnesota 55905 and Department of Pediatrics, Magee Womens Hospital, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
APStracts 2:0281A, 1995.
In the rat diaphragm muscle, different muscle fiber types were classified histochemically as type I, IIa, IIb, or IIx based on differences in staining intensity for myofibrillar adenosine triphosphatase (mATPase) after acid and alkaline preincubations. The histochemical classification of different fiber types was confirmed by immunoreactivitiy to antibodies for different myosin heavy chain (MHC) isoforms. The expression of different MHC isoforms in single dissected muscle fibers was also assessed electrophoretically. Most fibers (86%) expressed a single MHC isoform. Among those fibers coexpressing MHC isoforms, coexpression of MHC-2X and 2B isoforms was most prevalent, with the expression of one MHC isoform usually predominating. Based on histochemical and immunohistochemical classification, type I fibers were the most abundant (37%) in the rat diaphragm, followed in rank order by type IIx (24%), IIa (31%) and IIb (8%) fibers. Type-dependent differences in fiber cross-sectional area were found, such that types I and IIa fibers were the smallest, type IIx fibers were intermediate and type IIb fibers were the largest. Given these differences in fiber type proportions and cross -sectional areas, the relative contribution of type IIx fibers to total diaphragm mass was nearly twice that of any other fiber type. Succinate dehydrogenase (SDH) and calcium-activated myosin ATPase (actomyosin ATPase) activities of different fiber types were measured using quantitative histochemical procedures. Type I and IIa fibers had the highest SDH activities followed in rank order by type IIx and IIb fibers. Type I fibers had the lowest actomyosin ATPase activity 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 actomyosin ATPase activity and cross-sectional area. The SDH and actomyosin ATPase activities of muscle fibers were also inversely correlated. These phenotypic differences in metabolic and morphometric properties of muscle fibers in the rat diaphragm may be important in determining the contractile and fatigue properties of the different fiber types. 

Received 29 March 1995; accepted in final form 15 June 1995.
APS Manuscript Number A346-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 11 July 1995.