Question

When the cardiac glycoside, digitoxin, is used for heart failure, the effects include:

I. inhibition of phosphate hydrolysis from E₂ of the (Na⁺–K⁺)–ATPase pump
II. activation of the (Na⁺–K⁺)–ATPase pump
III. increase in intracellular Na⁺ concentration
IV. increase in Ca²⁺ concentration in the sarcoplasmic reticulum

Answer 1

III. increase in intracellular Na+ concentration.

Cardiac glycosides are a class of organic compounds that increases the pumping activity (output force) of the heart thereby increasing its rate of contractions by inhibiting the activity of a sodium-potassium pump(Na-K pump).This pump mediates the active transport of sodium ions out and potassium ions into the cell membrane.

Cardiac glycosides inhibits this pump by stabilizing an E2-P transition state.Due to this stabilization the sodium ions cannot be dissociated or extruded out.As a result of this Na+ concentration gets accumulated.This in turn leads to the accumulation of intracellular calcium in the heart which are then released by a sarcoplasmic reticulum.