Correlation between myosin heavy chain expression and actomyosin atpase activity in human muscle fibers

The myosin heavy chain (MHC) is the site of ATP hydrolysis during crossbridge cycling; therefore, a correlation should exist between MHC isoform expression and actomyosin ATPase activity in human muscle fibers. Needle biopsies were obtained from the quadriceps femoris of 5 sedentary subjects (21-43 yr). Fibers were classified histochemicalîy and also on the basis of MHC isoform immunoreactivity. Calcium-activated actomyosin ATPase activity of muscle fibers (20 fibers per type) was determined in alternate serial sections using a quantitative histochemical procedure (Blanco & Sieck, Histochem J. 24: 431-444, 1992}. The mean actomyosin ATPase activity (mmol Pi/ liter tissue/min) of type I fibers (52.8±6.1) was two-fold lower (p<0.05) than that of type Ha (102.1 + 13.3) and type IIx (122.1 + 13.5) fibers. The variability in actomyosin ATPase activity was not different across fiber types (CV%, type M2.1±3.0, IIa=9,8±4.0, and 11x10.8+3.7%). These results suggest that MHC isoform expression is the major determinant of actomyosin ATPase activity in human muscle fibers. The similar variance in actomyosin ATPase activity across fiber types further suggests that activation history does not play a major role in determining actomyosin ATPase activity. These results suggest differences in velocity of shortening among fiber types in human muscle.