Answer:
Calcium ions.
Step-by-step explanation:
Calcium ions is stored in the sarcoplasmic reticulum of the muscle fibers. Calcium plays an important role in the muscle contraction.
Smooth muscles use calcium for the phosphorylation of myosin that couples the electrical excitation and initiates muscle contraction. The process of cross bridge cycle occurs in the muscle contraction.
Thus, the answer is calcium.