A Researcher Is Studying the Electrolytes Changes During Cardiac Muscle

A researcher is studying the electrolytes changes during cardiac muscle contraction and relaxation. He also monitors the intracellular calcium regulation. Microelectrodes placed into cardiac muscle cells detect a rapid decrease in cytoplasmic calcium level immediately preceding relaxation. Which of the following proteins is most likely responsible for the observed change in electrolyte levels?

A) Calmodulin
B) Na⁺/Ca²⁺ exchanger
C) Ryanodine receptors
D) Troponin C
E) Voltage-dependent calcium channels