Human skeletal muscle myosin function at physiological and non-physiological temperatures

Aim: The aim of the study was to assess the function of human skeletal muscle myosin across a wide range of temperatures, including physiological. Methods: We used a single fibre in vitro motility assay. The in vitro motility speed of actin filaments propelled by myosin extracted from fibres expressing type I myosin heavy chain (MyHC; n = 9), IIa MyHC (n = 6), IIax MyHC (n = 4) and I/IIa MyHC (n = 1) was measured at 15, 20, 25, 30 and 35 °C. Results: The motility speed between groups of fibres expressing different MyHC differed significantly (P ? 0.001). The increase in motility speed with an increase in temperature was statistically significant (P ? 0.001) between all temperatures. The relative difference in motility speed between the slow type I and the fast IIax MyHC fibres decreased with increasing temperature, i.e. a 7.5‐fold difference at 15 °C was reduced to twofold at 35 °C. Furthermore, the twofold difference in motility speed between type IIa and IIax MyHC at 15 °C disappeared completely at 35 °C. The activation energy, E_A, and temperature coefficient, Q₁₀, over the 15–35 °C temperature range was higher for type I MyHC, 54.47 ± 4.37 kJ mol^?1 and 2.09 ± 0.12, respectively, than for type IIa MyHC, 45.41 ± 3.12 kJ mol^?1 (P < 0.001) and 1.85 ± 0.08 (P < 0.001), or IIax MyHC, 34.71 ± 1.75 kJ mol^?1 (P ? 0.001) and 1.60 ± 0.04 (P ? 0.001). Conclusion: The present results suggest a significantly reduced difference in shortening velocity between different human muscle fibre types at physiological temperature than previously reported at lower temperatures (12 or 15 °C) with implications for human in vivo muscle function.