Key result
Smooth muscle adaptation to longer lengths involves recruitment of myosin monomers and oligomers into the actin filament lattice, allowing constant maximal force generation over a twofold length range.
Recruitment of myosin monomers and oligomers into the actin filament lattice occurs during smooth muscle adaptation to longer lengths, explaining its ability to maintain contractility over a broad length range.
May inform vascular smooth muscle models; leaves open clinical translation to tone disorders.
Smooth muscle is able to function over a much broader length range than striated muscle. The ability to maintain contractility after a large length change is thought to be due to an adaptive process involving restructuring of the contractile apparatus to maximize overlap between the contractile filaments. The molecular mechanism for the length-adaptive behavior is largely unknown. In smooth muscle adapted to different lengths we quantified myosin monomers, basal and activation-induced myosin light chain (MLC) phosphorylation, shortening velocity, power output and active force. The muscle was able to generate a constant maximal force over a two fold length range when it was allowed to go through isometric contraction/relaxation cycles after each length change (length adaptation). In the relaxed state, myosin monomer concentration and basal MLC phosphorylation decreased linearly, while in the activated state activation-induced MLC phosphorylation and shortening velocity/power output increased linearly with muscle length. The results suggest that recruitment of myosin monomers and oligomers into the actin filament lattice (where they form force-generating filaments) occurs during muscle adaptation to longer length, with the opposite occurring during adaptation to shorter length.
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Chitano et al. (2017) studied Smooth muscle physiology. Muscle length adaptation was evaluated on Myosin monomers, MLC phosphorylation, shortening velocity, power output and active force. Smooth muscle adaptation to longer lengths involves recruitment of myosin monomers and oligomers into the actin filament lattice, allowing constant maximal force generation over a twofold length range.
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