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

Area of Science:

• Muscle physiology
• Skeletal muscle function
• Biophysics

Background:

• Human skeletal muscle comprises different fiber types expressing distinct myosin heavy chains (MyHC).
• Muscle fiber type differences in contractile properties are well-established at lower temperatures.
• The impact of temperature on these differences, especially at physiological levels, requires further elucidation.

Purpose of the Study:

• To investigate the functional performance of human skeletal muscle myosin across a spectrum of temperatures.
• To quantify the in vitro motility speed of actin filaments propelled by different human MyHC isoforms.
• To assess how temperature influences the velocity differences between slow (Type I) and fast (Type II) muscle fibers.

Main Methods:

• Utilized a single fiber in vitro motility assay to measure actin filament speed.
• Myosin was extracted from human muscle fibers expressing Type I, IIa, IIax, and I/IIa MyHC.
• Motility speeds were recorded at temperatures ranging from 15°C to 35°C.

Main Results:

• Significant differences in motility speed were observed between MyHC fiber types (P << 0.001).
• Motility speed significantly increased with temperature across all fiber types (P << 0.001).
• The relative velocity difference between Type I and IIax MyHC fibers decreased from 7.5-fold at 15°C to twofold at 35°C.

Conclusions:

• Human skeletal muscle fiber type differences in shortening velocity are less pronounced at physiological temperatures compared to lower temperatures.
• These findings have significant implications for understanding human in vivo muscle function across varying thermal conditions.
• The temperature-dependent modulation of myosin function suggests adaptive mechanisms in muscle performance.