In the context of muscle contraction, how does cotransport contribute to calcium ion regulation?

• A. Cotransport directly synthesizes calcium ions within muscle cells.
• B. Cotransport facilitates passive diffusion of calcium ions.
• C. Cotransport involves the movement of calcium ions in the same direction as their gradient.
• D. Cotransport helps regulate calcium ions by exchanging them with other ions across the sarcoplasmic reticulum membrane.

Answer: (D)