Coexistence of slow and fast isoforms of contractile and regulatory proteins in human skeletal muscle fibres induced by endurance raining

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Abstract
The distribution of fast and slow isoforms of troponin C, I, and T components and myosin heavy chains was investigated in histochemically typed myofibrillar ATPase intermediate (IM) fibres, that is, fibres that stain after both acid and alkaline pre‐incubation in stainings for myofibrillar ATPase. In addition to the previously described IM fibres of types II C and I B, fibres that displayed staining characteristics between types II C and I B were observed and termed type II C–I B. The IM fibres constitute less than 1% of the fibres in normal human limb and abdominal muscles. The IM fibres studied here resulted from extensive endurance training of human triceps brachii muscle (n= 6) and were induced by conversion of a proportion (130) of type II fibres. The immunohistochemical stains of serial sections with antibodies to slow isoforms of troponin I, T, C and myosin heavy chain showed no staining of type II fibres but intense staining of types I and I B fibres, whereas type II C fibres stained with intermediate intensity. The antibodies to fast isoforms of the troponin components and myosin heavy chain did not give rise to staining of type I fibres but dark staining of type II fibres. Type I B fibres stained with intermediate intensity and type II C was either as dark as type II or slightly lighter. Type II C‐I B fibres showed staining intensities intermediate between those observed for types I B and IIC in the immunohistochemical stains. It is therefore concluded that training‐induced myofibrillar ATPase intermediate human skeletal muscle fibres are characterized by the coexistence of slow and fast isoforms of contractile and regulatory proteins. Changes in the distribution of fast and slow isoforms of several of the myofibrillar proteins appeared to be induced in a co‐ordinated manner.