Plasticity in airway smooth muscle: an update

Area of Science:

• Muscle Physiology
• Biophysics
• Cellular Mechanics

Background:

• Smooth muscles exhibit a wide functional length range, a phenomenon not fully explained by traditional models.
• Previous hypotheses proposed plastic alterations and myosin filament dynamics as key mechanisms for this adaptability.
• The relationship between muscle length, force generation, and shortening velocity requires further elucidation.

Purpose of the Study:

• To present evidence supporting the hypothesis that smooth muscle length adaptability involves plastic myofilament rearrangement.
• To investigate the role of myosin filament evanescence and reformation in muscle plasticity.
• To explore the impact of activation levels and filament dynamics on muscle shortening velocity and force.

Main Methods:

• Adaptation of muscles to a 3-fold range of lengths.
• Measurement of physiological parameters including shortening velocity, maximum power, compliance, and ATPase rate.
• Assessment of thick-filament mass and density changes during activation.
• Analysis of the relationship between force and velocity under varying activation conditions.

Main Results:

• Muscles maintain similar force output across a 3-fold variation in adapted length.
• Shortening velocity, maximum power, compliance, ATPase rate, and thick-filament mass scale with muscle length.
• Thick-filament density increases significantly with rising activation.
• Muscle shortening velocity decreases as force increases, with an inverse proportionality when activation is normalized.

Conclusions:

• The long functional range of smooth muscles is explained by plastic alterations in myofilament arrangement, facilitated by myosin filament dynamics.
• Myosin filament evanescence and reformation are crucial for muscle adaptation to different lengths.
• The observed changes in velocity and force reflect the interplay between filament dynamics, activation, and muscle length.