Which action contributes to the positive inotropic effect of digoxin?

Which action contributes to the positive inotropic effect of digoxin?

• A. Decrease intracellular Na+
• B. Increase availability of intracellular Ca2+ to bind to troponin
• C. Inhibit calcium uptake in the sarcoplasmic reticulum
• D. Increase transport of K+ out of cells

Answer: (B)

The positive inotropic effect of digoxin primarily occurs through its ability to increase the availability of intracellular calcium to bind to troponin, which enhances cardiac muscle contractility. Digoxin is a cardiac glycoside that inhibits the sodium-potassium ATPase pump in the cell membrane of cardiac myocytes. This inhibition leads to an increase in intracellular sodium concentration. As a consequence, the sodium-calcium exchanger, which typically extrudes calcium from the cell in exchange for sodium, becomes less effective. This results in an increase in intracellular calcium levels. Increased intracellular calcium enhances the force of contraction because calcium ions bind to troponin on the actin filaments, promoting the interaction of actin and myosin, which leads to stronger cardiac contractions. Thus, the action of digoxin directly increases the calcium available for muscle contraction, which is crucial for the positive inotropic effect. This mechanism is essential for improving heart function in conditions like heart failure where cardiac output is compromised. The increase in calcium availability is fundamental to understanding how digoxin achieves its therapeutic effects in treating heart conditions.

The positive inotropic effect of digoxin primarily occurs through its ability to increase the availability of intracellular calcium to bind to troponin, which enhances cardiac muscle contractility. Digoxin is a cardiac glycoside that inhibits the sodium-potassium ATPase pump in the cell membrane of cardiac myocytes. This inhibition leads to an increase in intracellular sodium concentration. As a consequence, the sodium-calcium exchanger, which typically extrudes calcium from the cell in exchange for sodium, becomes less effective. This results in an increase in intracellular calcium levels.

Increased intracellular calcium enhances the force of contraction because calcium ions bind to troponin on the actin filaments, promoting the interaction of actin and myosin, which leads to stronger cardiac contractions. Thus, the action of digoxin directly increases the calcium available for muscle contraction, which is crucial for the positive inotropic effect. This mechanism is essential for improving heart function in conditions like heart failure where cardiac output is compromised.

The increase in calcium availability is fundamental to understanding how digoxin achieves its therapeutic effects in treating heart conditions.