The latch state in smooth muscle allows sustained force generation with low energy expenditure. What is the molecular basis of the latch state?
- A Caldesmon binds and stabilises actin-myosin cross-bridges in a rigor-like state without ATP hydrolysis
- B Dephosphorylation of myosin light chains while cross-bridges remain attached; MLCK is inactivated but MLCP dephosphorylates myosin, slowing the ATPase cycle and creating slowly cycling, dephosphorylated cross-bridges ✓
- C Calmodulin remains bound to actin filaments, preventing cross-bridge detachment after initial Ca2+ transient
- D Titin-like proteins in smooth muscle physically lock myosin heads in the attached position
Explanation
The smooth muscle latch state is a prolonged maintenance of force at low intracellular Ca2+ and low metabolic cost. After initial Ca2+-CaM-MLCK-mediated phosphorylation of myosin light chains (Ser19) and cross-bridge formation, Ca2+ can fall. As Ca2+ drops, MLCK becomes inactive, and myosin light chain phosphatase (MLCP/PP1M) dephosphorylates the myosin heads. Dephosphorylated myosin cross-bridges (latched bridges) detach very slowly (low ATPase activity), generating sustained tension without continuous ATP hydrolysis. This mechanism is important for tonic smooth muscle (e.g., blood vessels, bronchioles) maintaining vascular tone economically. Caldesmon modulates the latch state by competing with MLCK but is not the primary mechanism.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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