Which of the following is most directly required to initiate the coupling of myosin to actin in the sliding filament theory?

• A. Calcium ions
• B. Sodium ions
• C. Potassium ions
• D. ATP

Answer: (A)

Verified step by step guidance

1. Understand the sliding filament theory: This theory explains how muscles contract by the sliding of actin and myosin filaments past each other, which shortens the muscle fiber.
2. Identify the role of calcium ions: Calcium ions are crucial in muscle contraction as they bind to troponin, a regulatory protein on the actin filament, causing a conformational change that exposes the binding sites for myosin on the actin filaments.
3. Recognize the role of ATP: ATP is necessary for the detachment of myosin heads from actin after a power stroke and for the re-cocking of the myosin heads, but it is not directly responsible for the initial coupling of myosin to actin.
4. Consider the roles of sodium and potassium ions: These ions are primarily involved in generating action potentials and maintaining the resting membrane potential, rather than directly facilitating the coupling of myosin to actin.
5. Conclude that calcium ions are most directly required: Since calcium ions are responsible for exposing the binding sites on actin for myosin attachment, they are the most directly required ions for initiating the coupling of myosin to actin in muscle contraction.