Final answer:
The site of calcium regulation in the smooth muscle cell is calmodulin.
Step-by-step explanation:
The site of calcium regulation in the smooth muscle cell is calmodulin. Smooth muscle cells do not contain troponin, so cross-bridge formation is not regulated by the troponin-tropomyosin complex. Instead, calmodulin, a regulatory protein, is responsible for regulating cross-bridge formation and muscle contraction in smooth muscle cells.