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Question 5 - Answer: (D) MLCP (myosin light chain phosphatase)

Explanation:
Smooth muscle contraction is triggered by a rise in intracellular Ca²⁺, which binds calmodulin. The Ca²⁺-calmodulin complex activates myosin light-chain kinase (MLCK), which phosphorylates myosin light chains and permits cross-bridge cycling. Relaxation requires the phosphatase MLCP to dephosphorylate the myosin light chains again.
The key point tested here is Ca²⁺-independent regulation of the contractile apparatus (also called Ca²⁺ sensitization/desensitization):
  • If MLCP activity is inhibited/decreased while intracellular Ca²⁺ stays the same, fewer myosin light chains get dephosphorylated, so more cross-bridges stay active and more tension is produced for the same level of Ca²⁺ - this is Ca²⁺-sensitization.
  • Conversely, activating MLCP produces Ca²⁺-desensitization (less tension for the same Ca²⁺ level).
This is the basis of the "latch state" seen in tonic/vascular smooth muscle during sustained (steady-state) contraction: pathways such as RhoA/Rho-kinase inhibit MLCP, which increases the apparatus's sensitivity to calcium and allows the muscle to maintain force economically (lower ATP and Ca²⁺ turnover) rather than through any change in actin, calmodulin itself, or the Ca²⁺-calmodulin complex concentration.
"In addition to the contractile mechanisms in smooth muscle that depend on changes in intracellular Ca2+ concentration, the degree of contraction also can be regulated by Ca2+-independent mechanisms... if there is activation of myosin-light-chain phosphatase, fewer cross-bridges will cycle and less tension will be produced (Ca2+-desensitization)" - Costanzo Physiology, 7th Ed.
So the increase in calcium sensitivity in the steady state is attributable to a change (decrease) in MLCP activity, not to actin, calmodulin, or the Ca²⁺-calmodulin complex - making (D) MLCP the correct answer. (A), (B), and (C) are part of the Ca²⁺-dependent activation pathway, not the mechanism that alters sensitivity independent of Ca²⁺ levels.
If you'd like, I can also go through the other marked/uncertain questions on this sheet (e.g., Q1 ECG/MEA, Q3, Q7 on peritoneal dialysis, or Q8 on Botox) - just let me know which ones.
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