Effects of hypothyroidism on maximum specific force in rat diaphragm muscle fibers

We hypothesized that 1) hypothyroidism (Hyp) decreases myosin heavy chain (MHC) content per half-sarcomere in diaphragm muscle (Dia_m) fibers, 2) Hyp decreases the maximum specific force (F_max) of Dia_m fibers because of the reduction in MHC content per half-sarcomere, and 3) Hyp affects MHC content per half-sarcomere and F_max to a greater extent in fibers expressing MHC type 2X (MHC_2X) and/or MHC type 2B (MHC_2B). Studies were performed on single Triton X-permeabilized fibers activated at pCa 4.0. MHC content per half-sarcomere was determined by densitometric analysis of SDS-polyacrylamide gels and comparison with a standard curve of known MHC concentrations. After 3 wk of Hyp, MHC content per half-sarcomere was reduced in fibers expressing MHC_2X and/or MHC_2B. On the basis of electron-microscopic analysis, this reduction in MHC content was also reflected by a decrease in myofibrillar volume density and thick filament density. Hyp decreased F_max across all MHC isoforms; however, the greatest decrease occurred in fibers expressing fast MHC isoforms (∼40 vs. ∼20% for fibers expressing slow MHC isoforms). When normalized for MHC content per half-sarcomere, force generated by Hyp fibers expressing MHC_2A was reduced compared with control fibers, whereas force per half-sarcomere MHC content was higher for fibers expressing MHC_2X and/or MHC_2B in the Hyp Dia_m than for controls. These results indicate that the effect of Hyp is more pronounced on fibers expressing MHC_2X and/or MHC_2B and that the reduction of F_max with Hyp may be at least partially attributed to a decrease in MHC content per half-sarcomere but not to changes in force per cross bridge.