The mechanism of smooth muscle contraction differs from skeletal muscle in several key ways. Which statement is TRUE regarding regulation of smooth muscle contraction?
- A Smooth muscle uses troponin-tropomyosin complex on actin for Ca2+ regulation, identical to skeletal muscle
- B Ca2+ directly activates myosin ATPase without need for calmodulin or kinase
- C Smooth muscle contraction requires troponin C, which has greater Ca2+ affinity than skeletal troponin C
- D Ca2+ binds calmodulin → Ca2+-calmodulin activates myosin light chain kinase (MLCK) → phosphorylates MLC at Ser19 → cross-bridge cycling ✓
Explanation
Smooth muscle lacks troponin; its contraction is regulated by phosphorylation of myosin rather than changes in actin. The sequence is: Ca2+ (from SR release or extracellular influx) → binds calmodulin (4 Ca2+ per calmodulin) → Ca2+/calmodulin complex activates MLCK → MLCK phosphorylates Ser19 on the regulatory myosin light chain (MLC20) → activated myosin ATPase drives cross-bridge cycling. Relaxation occurs when MLCP (myosin light chain phosphatase) dephosphorylates MLC. PKC and Rho-kinase modulate this pathway (Ca2+ sensitization) allowing maintained tone at lower [Ca2+].
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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