Cross-bridge cycling kinetics in human muscle fibers expressing different MHC isoforms

A correlation exists between maximum unloaded shortening velocity (V_o) 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 (MHC_Slow 35%; MHC_2A 38%; MHC_2X 7%), while 18% co-expressed MHC_2A and MHC_2X isoforms, and 2% co-expressed MHC_2A and MHC_Slow isoforms. Mechanical measurements in single triton-X permeabilized fibers during maximum Ca²⁺ activation (pCa 4.0) at 15°C included: V_o measured using the "slack" test; rate of force redevelopment (k_tτ) after a rapid release (15% L_o) and restretch; and ratio of stiffness (0.2%L_o length oscillations at 1kHz) during maximal activation to rigor (α_fs). From these measures, apparent forward (f_app) and reverse (g_app) rate constants for cross-bridge cycling were estimated (k_tτ=f_app+g_app and α_fs=f_app/f_app+g_app). Both V_o and k_tτ were slower in fibers expressing the MHC_Slow isoform compared to fibers expressing fast MHC isoforms, while α_fs was comparable across fiber types. These results indicate that both f_app and g_app are slower in human fibers expressing the MHC_Slow isoform compared to fibers expressing fast MHC isoforms. The impact of MHC isoform co-expression could only be evaluated in fibers co-expressing MHC_2A and MHC_2X isoforms where no effect on cross-bridge cycling kinetics was observed.