Kinetic properties of myosin heavy chain isoforms in single fibers from human skeletal muscle.

The head portion of the myosin heavy chain is essential in force generation. As previously shown, Ca2+-activated muscle fibers from rat and rabbit display a strong correlation between their myosin heavy chain isoform composition and the kinetics of stretch activation, corresponding to an order of velocity: myosin heavy chain Ib > myosin heavy chain IId(x) > myosin heavy chain IIa >> myosin heavy chain I. Here, we show a similar correlation for human muscle fibers (myosin heavy chain IIb > myosin heavy chain IIa >> myosin heavy chain I), suggesting isoform-specific differences between the kinetics of force-generating power strokes. The kinetics of myosin heavy chain I are similar in human and rodents. This holds also true for myosin heavy chain IIa, but human myosin heavy chain IIb is slower than rodent myosin heavy chain IIb. It is similar to rodent myosin heavy chain IId(x).