Cardiac myocytes, as well as many other cells, have a Na+-Ca++ exchanger (not an active energy-requiring pump) that is essential for maintaining sodium and calcium homeostasis. The exact mechanism by which this exchanger works is unclear.  It is known that calcium and sodium can move in either direction across the sarcolemma. Furthermore, three sodium ions are exchanged for each calcium, therefore an electrogenic potential is generated by this exchanger.  The direction of movement of these ions (either inward or outward) depends upon the membrane potential and the chemical gradient for the ions. We also know that an increase in intracellular sodium concentration competes for calcium through this exchange mechanism leading to an increase in intracellular calcium concentration. As intracellular sodium increases, the concentration gradient driving sodium into the cell across the exchanger is reduced, thereby reducing the activity of the exchanger, which decreases the movement of calcium out of the cell. Therefore, mechanisms that lead to an accumulation of intracellular sodium cause a subsequent accumulation of intracellular calcium because of decreased exchange pump activity.