Smooth muscle, unlike skeletal muscle, can maintain sustained contraction (tone) at very low energy consumption — the so-called 'latch' state. Which molecular mechanism is responsible for the latch state?

• A Dephosphorylated myosin (by myosin light chain phosphatase) slowly detaches from actin but can remain attached in a high-duty-ratio latch state; this maintains force without repeated ATP-consuming cross-bridge cycling ✓
• B Caldesmon and calponin bind actin filaments and are phosphorylated by CaM kinase, permanently preventing actin-myosin dissociation at low Ca²⁺
• C Smooth muscle switches to anaerobic glycolysis during sustained contraction, generating enough ATP for sustained cross-bridge cycling without fatigue
• D The actin isoforms in smooth muscle (γ-smooth muscle actin) have higher affinity for myosin S1 fragments, maintaining cross-bridge attachment without ATPase activity

Explanation

The latch mechanism in smooth muscle was proposed by Dillon and Somlyo: when myosin light chain phosphatase dephosphorylates myosin heads that are attached to actin, these dephosphorylated cross-bridges do not rapidly detach — they 'latch' onto actin filaments. Dephosphorylated attached cross-bridges have very low ATPase activity (the rate-limiting step is product release, not ADP cycling), meaning force is maintained at very low ATP consumption. This allows tonic smooth muscle (e.g., vascular, airway) to sustain contraction for extended periods with minimal energy expenditure. Option B describes regulatory proteins that modulate Ca²⁺ sensitivity, not the latch state per se. Option C (anaerobic glycolysis) is not the mechanism of sustained contraction efficiency. Option D incorrectly attributes the latch state to actin isoform differences.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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