An elevation of intracellular calcium concentrations to about 1 uM is an absolute requirement for smooth muscle contraction, and a reduction in smooth muscle calcium concentration is considered mechanism of smooth muscle relaxation.
Calcium sits at a critical location on the muscle fiber, facilitating the interaction of the muscle proteins myosin and actin.
Stimulation of muscle fibers by nerve impulses, hormones, or stretch in the fiber increases the amount of calcium in the muscle cells and causes the muscle to contract. Calcium binds to calmodulin, causing structural changes that allow calmodulin to bind and activate myosin light chain kinase.
In the presence of calcium and calmodulin, this kinase phosphorylates a light chain of myosin. This result in structural changes in the thick filament, which then allow its interaction with actin, actomyosin ATPase activity and smooth muscle contraction.
As the cells pump calcium ions back outside, the muscle relaxes.
Calcium and mechanism of muscle contraction
