In smooth muscle, what is the critical kinase responsible for initiating contraction, and how is it regulated differently from skeletal muscle crossbridge cycling?
- A Myosin light chain kinase (MLCK): activated by Ca²⁺-calmodulin complex → phosphorylates MLC20 → activates myosin ATPase. Unlike skeletal muscle (troponin-C Ca²⁺ regulation), smooth muscle regulation is primarily myosin-based via MLCK/MLC phosphatase balance ✓
- B Titin kinase: phosphorylated by Ca²⁺ to activate thin filament regulation identical to skeletal muscle via troponin
- C Rho-kinase (ROCK): activated directly by Ca²⁺ to phosphorylate MLC20 as the primary Ca²⁺-dependent mechanism
- D PKC: directly activated by Ca²⁺ to phosphorylate myosin heavy chain, initiating crossbridge cycling
Explanation
Smooth muscle lacks troponin; Ca²⁺ regulation is myosin-dependent. Increased intracellular Ca²⁺ (from SR release or L-type channels) binds calmodulin, and the Ca²⁺-calmodulin complex activates MLCK. MLCK phosphorylates the 20 kDa myosin regulatory light chain (MLC20) at Ser19, activating myosin ATPase and enabling crossbridge cycling with actin. Relaxation requires MLC phosphatase to dephosphorylate MLC20. Ca²⁺ sensitization via RhoA/ROCK pathway inhibits MLC phosphatase, maintaining contraction at lower [Ca²⁺] (pharmacomechanical coupling). This dual Ca²⁺-dependent and Ca²⁺-sensitization mechanism makes smooth muscle uniquely suited for sustained tonic contractions.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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