The Ca2+ pumps and the Na+/Ca2+ exchangers.

The Ca2+ ATPases or Ca2+ pumps transport Ca2+ ions out of the cytosol, by using the energy stored in ATP. The Na+/Ca2+ exchanger uses the chemical energy of the Na+ gradient (the Na+ concentration is much higher outside than inside the cell) to remove Ca2+ from the cytosol, Ca2+ pumps are found in the plasma membrane and in the endoplasmic reticulum of the cells. The pumps are probably present in the membrane of other organelles, but little experimental information is available on this matter. The Na+/Ca2+ exchangers are located on the plasma membrane. A Na+/Ca2+ exchanger was found in the mitochondria, but very little is known on its structure and sequence. These transporters control the Ca2+ concentration in the cytosol and are vital to prevent Ca2+ overload of the cells. Their activity is controlled by different mechanisms, that are still under investigation. A number of the possible isoforms for both types of proteins has been detected.