Mechanism of action of digitalis is -

Ans. is 'a' i.e., Inhibits Na+ K+ ATPase pump o Digitalis inhibits Na+ 1C ATPase by selectively binding to its extracellular face.o Following figure will help you to understand the mechanism of action of digoxin.o There are two important pumps in myocytes:i) 3Na+/l Ca+2 exchanger# This pump transports Ca+2 in exchange of Na+.# When Na+ concentration inside the myocyte is high, Na+/Ca+2 exchanger cause efflux of Na+ out of the myocytes and in exchange it causes influx of Ca+2 inside the myocytes.# NaVCa+2 exchanger does not require ATP to function, ions move along their concentration gradient.ii) Na+/K+ ATPase# This pump causes efflux of Na+ in exchange of influx of K+.o Digitalis inhibits Na+ K+ ATPase. Therefore, efflux of Na+ by Na+K+ ATPase is inhibited. This results in increased concentration of Na+ inside the myocytes.o Increased Na+ concentration causes increased efflux of Na+ by Na+/Ca+2 exchanger with an increased influx of Ca+2 inside of myocytes.o Thus, intracellular concentration of Ca+2 is increased which results in increased force of cardiac contractiono It is worth noting that Na+ efflux is affected in two ways:i) Na+ efflux is dcreased by Na+/K+ ATPase (due to inhibition of Na+K+ ATPase by digoxin).ii) Na+ efflux is increased by Ca+2/Na+ exchanger (due to increased intracellular Na+ as a result of decreased Na+ efflux by Na+/K+ ATPase)