The Na/Ca exchanger operates with a stoichiometry of 1 calcium to 3 sodium and can move ions in either direction. Which statement best describes this?

• A. 1 Ca2+ per 3 Na+ in either direction
• B. 1 Ca2+ per 2 Na+ in either direction
• C. 2 Ca2+ per 3 Na+ in either direction
• D. 1 Ca2+ per 4 Na+ in either direction

Answer: (A)

The important concept is that the Na/Ca exchanger moves calcium and sodium in a fixed 3:1 ratio: for every Ca2+ exchanged, 3 Na+ move in or out. This coupling stays the same whether the exchanger is removing Ca2+ from the cell (forward mode) or bringing Ca2+ into the cell (reverse mode). In cardiac cells, this exchanger typically extrudes Ca2+ during relaxation, using 3 Na+ entry for each Ca2+ leaving. It can reverse when the gradients change, such as high intracellular Na+ or a depolarized membrane, causing Ca2+ to enter in exchange for Na+ leaving, but the ratio remains 3 Na+ per 1 Ca2+. The other proposed ratios don’t match this established 3:1 coupling, so they aren’t correct.