Question

How do calcium ions stimulate contraction in nonmuscle cells and smooth muscle cells?

Choose one:
A. Calcium ions bind to troponin and lead to a conformational change in tropomyosin that exposes the myosin binding sites on the actin filament.
B. Calcium ions released into the cell promote the release of secretory vesicles that send neurotransmitters to neighboring cells to stimulate an action potential.
C. Calcium ions trigger an action potential in the cell, leading to gene expression changes that influence contraction.
D. Calcium ions lead to activation of a kinase that phosphorylates nonmuscle myosin to alter its conformation and enable actin binding.

Answer 1

Calcium ions cause contractions in nonmuscle and smooth muscle cells by activating a kinase that phosphorylates nonmuscle myosin thus allowing it to bind to actin and enable contraction.

Step-by-step explanation:

Calcium ions stimulate contraction in nonmuscle cells and smooth muscle cells by leading to the activation of a kinase that phosphorylates nonmuscle myosin, altering its conformation to enable actin binding. This is represented by option D. The mechanism of Ca++ action differs in smooth muscle compared to skeletal muscle; instead of binding to troponin, calcium binds to calmodulin.

Then, the Ca++-calmodulin complex activates myosin light-chain kinase, which in turn phosphorylates the light chains of the myosin heads. This phosphorylation changes the conformation of myosin, allowing it to bind to actin and initiate contraction with the aid of ATP.