MAXIMUM VELOCITY OF SHORTENING IN RELATION TO MYOSIN ISOFORM COMPOSITION IN SINGLE FIBERS FROM HUMAN SKELETAL-MUSCLES

1. Maximum velocity of shortening (V(max)) and compositions of myosin heavy chain (MHC) and myosin light chain (MLC) isoforms were determine d in single fibres from the soleus or the lateral region of the quadri ceps (vastus lateralis) muscles in man. Muscle samples were obtained b y percutaneous biopsy, and membranes were permeabilized by glycerol tr eatment (chemical skinning) or by freeze-drying. 2. Types I, IIA and I IB MHCs were resolved from single fibre segments by 6% sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and five differ ent fibre types were identified: fibres containing type I MHC, types I and IIA MHCs, type II A MHC, types II A and II B MHCs, and type II B MHC. Only a few fibres co-expressed types I and II A MHCs but 28 % of all quadriceps fibres expressed both IIA and IIB MHCs in variable prop ortions. Fibres co-expressing types I and IIB MHCs were not found. 3. Alkali (MLC, and MLC3) and dithio nitrobenzoic acid (DTNB) (MLC2) myos in light chains were observed in all type II fibres in variable propor tions. MLC (MLC1s and MLC2.) isoforms from type I fibres had lower mig ration rates than the corresponding isoforms from type II fibres (MLC1 f and MLC2f). More than half of type I fibres in both soleus (65%) and quadriceps (68%). muscles also expressed 'fast' MLC3 and 36% of the t ype II fibres from quadriceps muscle expressed the slow isoform of MLC 2. 4. Differences were observed in some mechanical characteristics of freeze-dried versus chemically skinned fibres. Maximum tension (P0) an d specific tension were lower in freeze-dried types I and IIA fibres t han in chemically skinned, while no differences were observed in the I IA/B fibres. The numbers of types I/IIA and IIB fibres were too low to allow statistical comparisons. In chemically skinned fibres, mean spe cific tension (0.20 +/- 0.01 N/mm2) did not vary with fibre type. In f reeze-dried fibres, on the other hand, specific tensions varied accord ing to MHC type: higher (P < 0.01) specific tensions were observed in types IIB (0.19 +/- 0.01 N/mm2) and type IIA/B fibres (0.18 +/- 0.04 N /mm2) than in type I fibres (0.12 +/- 0.02 N/mm2). The specific tensio n of type II A fibres (0.12 +/- 0.05 N/mm2) did not differ significant ly from the other fibre types. Cross-sectional areas and mean V(max) d id not differ between freeze-dried and chemically skinned fibres, eith er when all fibres were pooled or within respective fibre types. V(max ) data from all fibres of a given type, irrespective of membrane perme abilization technique, have therefore been pooled. 5. V(max) and MHC c omposition were measured in one set of 103 single human fibres taken f rom the soleus (n = 17) and quadriceps (n = 86) muscles. A wide range in V(max) was observed in quadriceps fibres (0.2-4.1 muscle lengths pe r second (ML/s)), and a narrower range in the soleus fibres (0.2-1.0 M L/s). These muscle-specific ranges were related to a homogeneous MHC c omposition (i.e. type 1) in the soleus fibres, while different MHCs (t ypes I, II A or II B) and MHC combinations were observed in the quadri ceps fibres. 6. V(max) was highly dependent on MHC composition and sig nificant differences (P < 0.001) were observed between fibre types. V( max) values in types I, II A and II B fibres from quadriceps were 0.3 +/- 0.1 (n = 32), 1.0 +/- 0.4 (n = 22) and 3.1 +/- 0.8 ML/s (n = 7), r espectively. Mean V(max) in type I fibres from soleus (0.3 +/- 0.2 ML/ s) were identical to values from quadriceps. 7. Fibres co-expressing t ypes II A and II B MHCs (II A/B) had a range of V(max) values that ove rlapped the ranges for II A and II B fibres; mean V(max) (1.3 +/- 0.7 ML/s) in IIA/B fibres was intermediate between II A and II B fibres. T he variation in V(max) within the type IIA/B group was related (P < 0. 001) to the proportion of IIA and II B MHCs. In two fibres having comb inations of type I and II A MHCs, V(max) values were 0.3 and 1.0 ML/s and these corresponded to high proportions of type I and type II A MHC , respectively. 8. Considerable variability in V(max) was observed wit hin fibre types classified according to MHC composition. Analysis of M LC composition of type II fibres yielded no significant correlations b etween V(max) and proportions of fast type alkali MLCs. However, V(max ) was lower in type II fibres expressing a combination of the slow and fast isoform of MLC2 than in fibres lacking MLC2s. 9. The relative pr oportions of types I, II A and II B MHCs in human single skeletal musc le fibres appear to be the major determinant of mechanical V(max). Thi s study also demonstrates that important contractile properties togeth er with myosin isoform composition can be studied in human short fibre segments obtained by percutaneous muscle biopsy.