In heart muscle cells (cardiomyocytes), calcium (Ca2+) is used as a secondary messenger in signaling cascades. Cardiomyocytes have a specialized organelle, the sarcoplasmic reticulum (SR), which serves as a high-concentration Ca2+ reservoir, as depicted on the right. The positive charges from all the Ca2+ inside the SR are balanced by Cl-, and the SR contains a low concentration of sodium, Na+ (compared to the sodium concentration in other compartments/spaces). There is no charge difference across the SR membrane (between the cytosol and the SR interior). Ca2+ can also enter the cardiomyocyte cytosol through ion channels inthe plasma membrane. The concentration of ions (K+, Na+, etc.) in the cytosol and extracellular space are such that the inside of the cell (cytosol) is more negative than the outside of the cell (extracellular space). All intracellular compartments have and maintain an abundance of ATP. Once Ca2+, even just a small amount, is triggered to rush into the cytosol from the SR and the extracellular space it must be promptly removed. The removal of calcium is performed by two membrane proteins, a Ca2+- ATPase pump and a Na+-Ca2+ transporter. One protein is found exclusively in the SR membrane and the other only in the plasma membrane.

a. Ca2+-ATPase pump and the Na+-Ca2+ transporter are working to remove Ca2+ from the cytosol. Which is preforming active and which is performing passive transport?

b. Which membrane transporter is likely found in the SR membrane and which is likely found in the plasma membrane? Explain.

c. Based on the answer to b, is the Na+-Ca2+ transporter a uniporter, a symporter, or an antiporter? Explain.

d. Why do you think it is important to quickly remove released Ca2+ from the cytosol and return the concentrations to their initial state?

Extracellular Space Nucleus SR Cytosol Cellular Compartment Ca2+ concentration (mM) Na+ concentration (mm) Cytosol 0.0001 SR Extracellular Space 1 1 5 5 150