MECHANICAL-PROPERTIES AND MYOSIN HEAVY-CHAIN ISOFORM COMPOSITION OF SKINNED SKELETAL-MUSCLE FIBERS FROM A HUMAN BIOPSY SAMPLE

Experiments were conducted to investigate the mechanics of contraction of chemically skinned muscle fibre segments of a biopsied sample of s ingle human quadriceps muscle. Subsequently, the isoforms of the myosi n heavy chain (MHC) were analysed by sodium dodecyl sulphate (SDS) gel electrophoresis. Of the 41 fibres, 26 contained MHCI (type I), 11 of the fibres contained MHCIIa (type IIA), and 4 of the fibres contained both MHCI and MHCIIa (of which MHCIIa was always slightly predominant (type IIC)). Distinct differences between fibre types were found in te rms of the kinetics of force responses following stepwise length chang es (order of velocity: IIA > IIC > I). The differences in maximal shor tening velocity and in the kinetics of Ca2+-dependent activation were of the same order, but much less pronounced. Type I fibres had signifi cantly greater fibre diameters than type IIA fibres. No significant di fferences were found among different fibre types in terms of isometric tension, resting sarcomere length or the length change needed to disc harge the elasticity of maximally Ca2+-activated fibres (y(o) value). The distribution of shortening velocity and kinetics of stretch activa tion values suggest that two muscle fibre subtypes may exist in human type I fibres.