Kinetic properties of cardiac myosin heavy chain isoforms in rat

Abstract.

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 αMHC are 3 times faster than those of cardiac βMHC under isometric conditions and maximal Ca²⁺ activation. Comparison of rat heart and skeletal muscle fibres revealed that 100% αMHC 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% βMHC heart muscle strips (t ₃: 351±44 ms) were faster than that of type-I fibres in rat skeletal muscle (t ₃: 901±348 ms). This difference between the two muscle types calls in question the generally accepted identity of βMHC and MHCIβ.