Which drug directly stimulates contraction of cardiac muscle -

Ans. is 'a' i.e., Digitalis o Cardiac glycosides (digitalis) act directly on the heart to increase the force of contraction.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.# Na+/Ca+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)