Correlation between histochemically assessed fiber type distribution and isomyosin and myosin heavy chain content in porcine skeletal muscles

Highly sensitive enzyme assays developed to differentiate skeletal muscle f ibers allow the recognition of three main fiber types: slow-twitch oxidativ e (SO), fast-twitch oxidative glycolytic (FOG), and fast-twitch glycolytic (FG). Myosin, the predominant contractile protein in mammalian skeletal mus cle, can be separated based on the electrophoretic mobility under nondissoc iating conditions into SM2, SM1, IM, FM3, and FM2 isoforms, or under dissoc iating conditions into myosin heavy chain (MHC) I, IIb, IIx/d, and IIa. The purpose of the present study was to determine whether the histochemical me thod of differentiation of fiber types is consistent with the electrophoret ically identified isomyosin and MHC isoforms. These comparisons were made u sing serratus ventralis (SV), gluteus medius (GM), and longissimus muscles (LM) from 13 pigs. Two calculation methods for the histochemical assessed f iber type distribution were adopted. The first method incorporated the numb er of fibers counted for each fiber type and calculated a percentage of the total fiber number (fiber number percentage: FNP). The second method expre ssed the cross-sectional area of each fiber type as a percentage of the tot al fiber area measured per muscle (fiber area percentage: FAP). Independent of the calculation methods, correlation analyses revealed in al l muscles a strong relation between SO fibers, the slow isomyosin (SM1 and SM2), and MHCI, as well as between the FG fibers, the fast isomyosin (FM3 a nd FM2), and MHCIIx/b content(P < .05). There were no correlations between FOG fiber population assessed by histochemical analysis and intermediate is oform (IM) or MHCIIa content. The present results did not provide conclusiv e evidence as to which of the calculation methods (FNP or FAP) was more clo sely related to myosin composition of skeletal muscles. Despite some incomp atibility between the methods, the present study shows that histochemical a s well as electrophoretic analyses yielded important information about the composition of porcine skeletal muscle. The combination of the two methods may be essential to accurately characterize porcine skeletal muscles.