Which pump transports Ca2+ back into the sarcoplasmic reticulum during relaxation?

• A. PMCA pump
• B. Na+/Ca2+ exchanger
• C. SERCA (Ca2+-ATPase)
• D. Ca2+/H+ exchanger

Answer: (C)

Calcium must be removed from the cytosol to allow muscle fibers to relax, and the main way this happens is by actively moving Ca2+ back into the sarcoplasmic reticulum. This is achieved by SERCA, a Ca2+-ATPase situated in the SR membrane. It uses energy from ATP hydrolysis to pump Ca2+ from the cytosol into the SR lumen, lowering cytosolic Ca2+ and stopping actin–myosin interactions so relaxation can occur.

Other pumps work in different places or ways: the PMCA pump moves Ca2+ out of the cell across the plasma membrane; the Na+/Ca2+ exchanger uses the sodium gradient to remove Ca2+ from the cell; and the Ca2+/H+ exchanger exchanges Ca2+ for H+ but does not load Ca2+ back into the SR. Therefore, the SR Ca2+-ATPase (SERCA) is the pump responsible for Ca2+ uptake during relaxation.