A rise in the intracellular Ca²⁺ concentration causes muscle cells to contract. In addition to an ATPdriven Ca²⁺ pump, muscle cells that contract quickly and regularly, such as those of the heart, have an additional type of Ca²⁺ pump—an antiport that exchanges Ca²⁺ for extracellular Na⁺ across the plasma membrane. The majority of the Ca²⁺ ions that have entered the cell during contraction are rapidly pumped back out of the cell by this antiport, thus allowing the cell to relax. Ouabain and Digitalis are used for treating patients with heart disease because they make heart muscle cells contract more strongly. Both drugs function by partially inhibiting the Na+ pump in the plasma membrane of these cells. Can you propose an explanation for the effects of the drugs in the patients? What will happen if too much of either drug is taken?