Final answer:
Calmodulin activates myosin light-chain kinase, leading to the phosphorylation of myosin heads and cross-bridge formation in smooth muscle contraction. Myosin phosphatase causes relaxation by dephosphorylating myosin heads, resulting in cross-bridge detachment. The correct option is A.
Step-by-step explanation:
In smooth muscle cells, cross-bridge formation is regulated by a cascade involving various proteins since these cells lack troponin. Calmodulin plays a crucial role by binding to calcium ions that have entered through sarcolemma calcium channels or released from the sarcoplasmic reticulum.
This Ca2+-calmodulin complex then activates myosin light-chain kinase (MLCK), which phosphorylates the myosin heads, converting ATP to ADP and Pi, and causing the heads to bind to actin-binding sites. This process enables the myosin heads to pull on the thin filaments against the thick filaments, leading to smooth muscle contraction.
Myosin phosphatase counteracts this contraction by dephosphorylating the myosin heads, which leads to cross-bridge detachment and muscle relaxation.