Exercise training induces transitions of myosin isoform subunits within histochemically typed human muscle fibres.

Fibre type composition based on histochemical myosin ATPase reaction was studied in cross sections of biopsies from the vastus lateralis muscle of men. In addition, protein composition as well as peptide patterns of isolated myosin heavy chains were examined in batches of individually classified fibres from the same biopsies. High intensity endurance training during 8 weeks induces significant decreases by 31-70% of the type IIB fibre population in 3 of 4 subjects (in one case no change was observed). These decreases were offset by corresponding increases in either type I or type IIA fibres with the type IIC fibres remaining always below 3%. A total of 13 professional cyclists with training periods over several years have a 20 times lower content of type IIB fibres than 4 sedentary controls and a concomitant high content of 80% of type I fibres. The content of type I and type IIB fibres of 8 sprinter athletes did almost not differ from that of controls. Thus the type IIB fibres respond most sensitively with a decrease to aerobic endurance training. Since both type IIA and IIB fibres were identical in protein composition containing the same fast variety of myosin light chains and heavy chains as well as troponin-I, their interconversion could not be seen at the molecular level. However, the slow variety of myosin light chains and of troponin-I started accumulating after 8 weeks of training in type IIA fibres. Furthermore, the myosin heavy chain isoform started shifting by producing new peptide patterns that resemble the digestion pattern of slow myosin heavy chains in fibres which still classified as type IIA. These changes on the molecular level in type IIA fibres mark the beginning of their transition over the intermediate and variable type IIC fibres, towards the slow type I fibre.