Diaphragm muscle fibers expressing the MHC(slow) isoform had significantly greater Ca(2+) sensitivity than fibers expressing fast MHC isoforms, associated with slow troponin complex isoforms.
In rat diaphragm muscle fibers, calcium sensitivity is greater in fibers expressing slow MHC isoforms, which is associated with the expression of slow troponin complex isoforms.
The present study examined Ca(2+) sensitivity of diaphragm muscle (Dia(m)) fibers expressing different myosin heavy chain (MHC) isoforms. We hypothesized that Dia(m) fibers expressing the MHC(slow) isoform have greater Ca(2+) sensitivity than fibers expressing fast MHC isoforms and that this fiber-type difference in Ca(2+) sensitivity reflects the isoform composition of the troponin (Tn) complex (TnC, TnT, and TnI). Studies were performed in single Triton-X-permeabilized Dia(m) fibers. The Ca(2+) 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(m) fibers expressing MHC(slow) was significantly greater than that of fibers expressing fast MHC isoforms, and this greater Ca(2+) sensitivity was associated with expression of slow isoforms of the Tn complex. However, some Dia(m) fibers expressing MHC(slow) contained the fast TnC isoform. These results suggest that the combination of TnT, TnI, and TnC isoforms may determine Ca(2+) sensitivity in Dia(m) fibers.
Geiger et al. (Mon,) reported a other. MHC(slow) isoform expression vs. Fast MHC isoforms was evaluated on Ca(2+) concentration at which 50% maximal force was generated (pCa(50)). Diaphragm muscle fibers expressing the MHC(slow) isoform had significantly greater Ca(2+) sensitivity than fibers expressing fast MHC isoforms, associated with slow troponin complex isoforms.