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Certain steroids obtained from plants are potent inhibitors of the Sodium-Potassium pump. Digitoxigenin and ouabain are members of this class of inhibitors, which are known as cardiotonic steroids because of their effects on the heart. These compounds inhibit the dephosphorylation of the E2-P form of the ATPase when applied on the extracellular face of the membrane. Digitalis is a mixture of cardiotonic steroids derived from the dried leaf of the foxglove plant (Digitalis purpurea). The compound increases the force of contraction produced by the heart, and is the drug of choice in heart failure. The inhibition of the sodium potassium pump by Digitalis leads to a higher level of Na+ inside the cell. The diminished Na+ gradient results in a slower intrusion of Ca2+ by the sodium-calcium exchanger. The increase of Ca2+ enhances the ability of the cardiac muscle to contract. Digitalis was being used long before its discovery through foxglove extract. People were using it to cure people with congestive heart failure.
Berg, Jeremy M. Biochemistry. 7th ed. (377)