Temperature dependence of activation of thick and thin filaments in skeletal and cardiac muscles.

Contraction of striated muscle is initiated by an increase in the cytosolic Ca2+ concentration, which is regulated by tropomyosin and troponin on thin filaments in sarcomeres; viz., the binding of Ca2+ to troponin C shifts the "on-off" equilibrium of the thin filament state toward the "on" state, allowing myosin-binding to actin. Also, accumulating evidence shows that in both skeletal and cardiac muscles of mammals, increasing ambient temperature to within the body temperature range not only accelerates the actomyosin ATPase activity but also shifts the "on-off" equilibrium of the thin filament state toward the "on" state, even in the absence of Ca2+. Especially noteworthy is that in an in vitro motility assay, the skeletal contractile system, compared with the cardiac contractile system, shows greater temperature dependence of activation, despite a higher threshold temperature for the onset of sliding (by ∼2°C). In this short review, we discuss the differential effects of heating on the skeletal vs. cardiac contractile system, focusing especially on the temperature-dependent shift of the "on-off" equilibrium of the thin filament state in sarcomeres.