Question

Smooth muscle is unique in its ability to generate various degrees of tension at a constant concentration of intracellular calcium. This change in calcium sensitivity of smooth muscle can be attributed to differences in the activity of MLCP. Smooth muscle contracts when the myosin light chain is phosphorylated by the actions of myosin light chain kinase (MLCK). MLCP is a phosphatase that can dephosphorylate the myosin light chain, rendering it inactive and therefore attenuating the muscle contraction. Which of the following statements is true regarding smooth muscle contraction?

1) Both actin and myosin are important components of the smooth muscle contractile apparatus much like that of skeletal muscle and cardiac muscle, but these do not play a role in calcium sensitivity.
2) ATP is required for smooth muscle contraction. Decreased ATP levels would be expected to decrease the ability of smooth muscle to contract even in the face of high calcium levels.
3) The calcium-calmodulin complex binds with MLCK, which leads to phosphorylation of the myosin light chain. A decrease in the calcium-calmodulin complex should attenuate the contraction of smooth muscle.
4) The binding of calcium ions to calmodulin is an initial step in the activation of the smooth muscle contractile apparatus.
5) Smooth muscle contraction is regulated by the activity of MLCP, which can dephosphorylate the myosin light chain, rendering it inactive and attenuating the muscle contraction.

Answer 1

Smooth muscle contraction is controlled by the interplay between MLCK and MLCP activities, with the former phosphorylating myosin light chains to facilitate contraction and the latter dephosphorylating them to induce relaxation, thereby altering calcium sensitivity.

Step-by-step explanation:

Smooth muscle contraction requires ATP and calcium ions (Ca++), similar to other muscle types, but with unique mechanisms of action. Calcium ions enter smooth muscle fibers through voltage-gated calcium channels or are released from the sarcoplasmic reticulum (SR), which then bind to calmodulin. This forms a Ca++-calmodulin complex that activates myosin light chain kinase (MLCK), leading to phosphorylation of the myosin light chain, ultimately allowing the myosin heads to pull on actin filaments and cause contraction.

Contraction persistence in smooth muscle, even as Ca++ levels decrease, can be explained by latch-bridges maintaining cross-bridges between myosin heads and actin, allowing for sustained muscular tone with minimal energy consumption. The process depends on a balance of kinase activity and phosphatase activity, particularly myosin light chain phosphatase (MLCP), which dephosphorylates myosin light chains and contributes to muscle relaxation.

Thus, the true statement regarding smooth muscle contraction is that MLCP activity influences calcium sensitivity by dephosphorylating myosin light chains to attenuate contraction, even in the presence of constant intracellular calcium levels. Though both actin and myosin are crucial in the contraction mechanism of smooth muscle, they are not directly related to the variability in calcium sensitivity.