Fig. 3. Correspondence between myosin heavy chain (MHC) protein composition and other myofibrillar isoforms identified through SDS-PAGE and western blotting. (A) 8% silver-stained gel demonstrating different MHC isoforms present in various muscles. MHC isoforms could not be distinguished between the fast [deep abdominal flexor (DA) and central cutter (CCT)] and S₁ [ventral cutter (VCT) and crusher (CR)] muscles. However, the S₂ fibers [distal cutter (DCT) and superficial tail flexors (SF)] contained an additional isoform that migrated more slowly than the fast or S₁ isoforms (arrowheads mark positions of the isoforms). Some slow muscles contained a mixture of the S₁ and S₂ MHC isoforms (DCT), while others contained only the S₂ isoform (SF). The double band at the far left is rabbit MHC used as a control (Sigma). (B) 10% silver-stained gel showing other myofibrillar isoforms from the samples identified in A. Some of the dominant bands correspond to MHC, paramyosin isoforms P₁ and P₂, P75, troponin T (TnT), actin (A), tropomyosin (Tm) and troponin I (TnI). Fast fibers characteristically possess paramyosin isoforms P₁ and P₂, P75, TnT₂, TnI₁, TnI₃ and TnI₅. S₁ fibers possess P₂, TnT₃, TnI₂ and TnI₄. S₂ fibers are characterized by P₂, TnT₁, TnT₃, TnI₁ and TnI₂. (C) Composite of three western blots of the above samples probed with antibodies to P75, TnT and TnI. Most of the above isoform differences are observed when the blot is labeled with these antibodies.