Which intracellular sequence correctly describes contraction of vascular smooth muscle after an alpha-1 agonist binds its receptor?

• A Ca2+ influx binds troponin C, exposing actin-myosin binding sites
• B Ca2+ directly activates myosin ATPase independent of regulatory proteins
• C Ca2+ calmodulin complex activates myosin light chain kinase, phosphorylating myosin light chains ✓
• D Phospholamban inhibition enhances calcium uptake and triggers contraction

Explanation

Vascular smooth muscle lacks troponin. Alpha-1 stimulation releases IP3-mediated calcium from the sarcoplasmic reticulum and opens membrane calcium channels. Calcium binds calmodulin, and the Ca2+ calmodulin complex activates myosin light chain kinase, which phosphorylates the regulatory light chain of myosin, allowing actin interaction and crossbridge cycling. Troponin B is specific to striated muscle, so option A is wrong. Phospholamban regulates cardiac sarcoplasmic reticulum calcium reuptake, promoting relaxation not contraction.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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