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

The present study examined Ca2+ sensitivity of diaphragm muscle (Dia,) fibers expressing different myosin heavy chain (MHC) isoforms. We hypothesized that Dia, fibers expressing the MHCslow isoform 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, fibers. The Ca2+ concentration at which 50% maximal force was generated (pCa(50)) was determined for each fiber. SDS-PAGE and Western analyses were used to determine the MHC and Tn isoform composition of single fibers. The pCa(50) for Dia, fibers expressing MHCslow was 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, fibers expressing MHC,I,, contained the fast TnC isoform. These results suggest that the combination of TnT, TnI, and TnC isoforms may determine Ca2+ sensitivity in Dia(m) fibers.