Kinetic properties of cardiac myosin heavy chain isoforms in rat

The head portion of the myosin heavy chain (MHC) is essential in force generation. As previously shown, Ca2+-activated fibres of mammalian skeletal muscle display a strong correlation between their MHC isoform complement and the kinetics of stretch activation, suggesting isoform-specific differences in kinetic properties of myosin heads. Using the same methodology on muscle strips of atria and ventricles of hyper- and hypothyroid rats, this study showed that the kinetics of cardiac alphaMHC are 3 times faster than those of cardiac betaMHC under isometric conditions and maximal Ca2+ activation. Comparison of rat heart and skeletal muscle fibres revealed that 100% alphaMHC heart muscle strips exhibited faster stretch activation kinetics (time parameter t(3): 108+/-18 ms, mean+/-SD) than rat type-IIA fibres (t(3): 157+/-19 ms), but slower than type-IID fibres (t(3): 55 10 ms). The kinetics of 100% betaMHC heart muscle strips (t3: 351+/-44 ms) were faster than that of type-I fibres in rat skeletal muscle (t3: 901+/-348 ms). This difference between the two muscle types calls in question the generally accepted identity of betaMHC and MHCIbeta.