Which mechanism releases Ca2+ from the sarcoplasmic reticulum?

• A. Voltage-sensitive proteins in T tubules stimulate Ca2+ release from terminal cisternae of SR
• B. Ca2+ pumped into the cytoplasm by Ca2+-ATPase
• C. Passive diffusion of Ca2+ through the sarcolemma
• D. Binding of Ca2+ to ATP triggers release

Answer: (A)

Depolarization of the muscle fiber activates voltage-sensitive receptors in the T tubules, which are mechanically linked to calcium-release channels on the sarcoplasmic reticulum (terminal cisternae). When these channels open, Ca2+ pours from the SR into the cytosol, raising intracellular calcium to initiate contraction. This release is driven by the electrical signal via the T-tubule receptors activating the SR channels, not by Ca2+ moving across the membrane from outside or by ATP binding. The other ideas either describe removing Ca2+ from the cytosol, letting Ca2+ diffuse across the membrane, or incorrectly tying ATP binding to calcium release, none of which explains the SR Ca2+ release mechanism.