A correlation exists between maximum unloaded shortening velocity (Vo) and myosin heavy chain (MHC) isoform expression in muscle fibers. The present study examined the association between MHC isoform expression and estimates of cross-bridge cycling kinetics in single human vastus lateralis muscle fibers. MHC isoform expression was determined by SDS PAGE. 80% of all fibers singularly expressed MHC isoforms (MHCSlow 35%; MHC2A 38%; MHC2X 7%), while 18% co-expressed MHC2A and MHC2X isoforms, and 2% co-expressed MHC2A and MHCSlow isoforms. Mechanical measurements in single triton-X permeabilized fibers during maximum Ca2+ activation (pCa 4.0) at 15°C included: Vo measured using the "slack" test; rate of force redevelopment (ktτ) after a rapid release (15% Lo) and restretch; and ratio of stiffness (0.2%Lo length oscillations at 1kHz) during maximal activation to rigor (αfs). From these measures, apparent forward (fapp) and reverse (gapp) rate constants for cross-bridge cycling were estimated (ktτ=fapp+gapp and αfs=fapp/fapp+gapp). Both Vo and ktτ were slower in fibers expressing the MHCSlow isoform compared to fibers expressing fast MHC isoforms, while αfs was comparable across fiber types. These results indicate that both fapp and gapp are slower in human fibers expressing the MHCSlow isoform compared to fibers expressing fast MHC isoforms. The impact of MHC isoform co-expression could only be evaluated in fibers co-expressing MHC2A and MHC2X isoforms where no effect on cross-bridge cycling kinetics was observed.
|Original language||English (US)|
|State||Published - Mar 20 1998|
ASJC Scopus subject areas
- Molecular Biology