Force-calcium relationship depends on myosin heavy chain and troponin isoforms in rat diaphragm muscle fibers

Abstract

The present study examined Ca²⁺sensitivity of diaphragm muscle (Dia_m) fibers expressing different myosin heavy chain (MHC) isoforms. We hypothesized that Dia_mfibers expressing the MHC_slowisoform have greater Ca²⁺sensitivity than fibers expressing fast MHC isoforms and that this fiber-type difference in Ca²⁺sensitivity reflects the isoform composition of the troponin (Tn) complex (TnC, TnT, and TnI). Studies were performed in single Triton-X-permeabilized Dia_mfibers. The Ca²⁺concentration at which 50% maximal force was generated (pCa₅₀) was determined for each fiber. SDS-PAGE and Western analyses were used to determine the MHC and Tn isoform composition of single fibers. The pCa₅₀for Dia_mfibers expressing MHC_slowwas significantly greater than that of fibers expressing fast MHC isoforms, and this greater Ca²⁺sensitivity was associated with expression of slow isoforms of the Tn complex. However, some Dia_mfibers expressing MHC_slowcontained the fast TnC isoform. These results suggest that the combination of TnT, TnI, and TnC isoforms may determine Ca²⁺sensitivity in Dia_mfibers.