Unloaded shortening velocity and myosin heavy chain and alkali light chain isoform composition in rat skeletal muscle fibres.

1. This study aims to assess the role of myosin heavy chain (MHC) and alkali myosin light chain (MLC) isoforms in determining maximum velocity of shortening in fast skeletal muscle fibres. 2. The maximum velocity of shortening as determined by the slack test (VO) was tested for its relationship with MHC composition and with alkali MLC isoform ratio of fast fibres of known MHC composition. 3. MHC isoform composition was determined using sodium dodecyl sulphate- polyacrylamide gel electrophoresis (SDS-PAGE) and monoclonal antibodies against MHCs, and combining the results obtained using the two methods. Three groups of fast fibres containing only one MHC isoform were identified: IIA, IIX and IIB fibres containing respectively IIA MHC, IIX MHC and IIB MHC. Fibres containing more than one MHC isoform were discarded. 4. The mean V. value of IIA fibres was 2-33 + 0-29 muscle lengths per second (L s'; mean + S.D.), this was significantly lower than that for IIX fibres (3 07 + 0 70 L s1) which in turn had a mean V. value significantly lower than that for IIB fibres (3-69 + 1.01 L s'). 5. The relative proportion of alkali MLC isoforms (MLC3f, MLCIf) was determined by means of electrophoretic separation and densitometric quantification and was expressed as MLC3f/MLC2f with reference to the dithio-nitrobenzoic acid (DTNB) light chain (MLC2f). The mean value of the MLC3f/MLC2f ratio was significantly lower in IIA than in IIX and IIB fibres. 6. V. was found to be proportional to the relative content of MLC3f in IIA, IIX and IIB fibres. The slope of the regression line of VO versus MLC3f/MLC2f was significantly higher for fibres containing IIB MHC than for fibres containing IIX MHC. 7. These results show that both MHC content and the proportion of MLC3f are important determinants of unloaded shortening velocity among fast fibres. The sensitivity of VO to variations in alkali MLC ratio depends on which MHC isoform is present. Variations in alkali MLC isoforms have much greater effects on V. in IIB than in IIX fibres.