**Question:** The biochemical mechanism of action of digitalis is associated with:

A. Inhibition of Na+/K+-ATPase
B. Inhibition of Ca2+ channels
C. Activation of Na+/K+-ATPase
D. Inhibition of Ca2+ pumps

**Core Concept:** Digitalis is a group of cardiac glycosides derived from the foxglove plant (Digitalis purpurea), which has been used for centuries to treat heart failure and arrhythmias. Digitalis exerts its effects by modulating the activity of the sarcoplasmic reticulum and cardiac sarcolemma.

**Why the Correct Answer is Right:** The correct answer is A, Inhibition of Na+/K+-ATPase. Digitalis inhibits the Na+/K+-ATPase enzyme, which is located in the cardiac sarcolemma, specifically in the sarcoplasmic reticulum and plasma membrane. This inhibition leads to a decrease in ATPase activity, resulting in the following effects:

1. Reduced active transport of Na+ ions into the cell, leading to increased intracellular Na+ concentration.
2. Increased active transport of K+ ions out of the cell, leading to increased intracellular K+ concentration.
3. Increased intracellular Ca2+ concentration due to the increased Na+ concentration, which subsequently causes positive inotropy, lusitropy, and chronotropy, improving cardiac contractility and relaxation.

**Why Each Wrong Option is Incorrect:**

B. Inhibition of Ca2+ channels is incorrect because digitalis does not directly inhibit Ca2+ channels but rather alters the ion concentrations within the cardiac cell, leading to the observed effects on Ca2+ channels indirectly.

C. Activation of Na+/K+-ATPase is incorrect because digitalis actually inhibits Na+/K+-ATPase, as mentioned above.

D. Inhibition of Ca2+ pumps is incorrect, as digitalis does not directly target Ca2+ pumps but rather affects Ca2+ ion concentrations, which subsequently influences Ca2+ pumps.

**Clinical Pearl:** Understanding the mechanism of action of digitalis is essential for its proper use and dosing in clinical practice. The goal of digitalis therapy is to improve cardiac contractility, lusitropy, and chronotropy while avoiding the side effects, such as digitalis toxicity, which can lead to arrhythmias, electrolyte imbalances, and cardiac failure.

**Correct Answer:** A. Inhibition of Na+/K+-ATPase

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Digitalis exerts its effects by inhibiting the Na+/K+-ATPase enzyme, leading to the changes in ion concentrations within the cardiac cell. This inhibition of Na+/K+-ATPase results in reduced active transport of sodium ions into the cell, increased potassium ions out of the cell, and increased calcium ions into the cell. These changes subsequently result in positive inotropy, lusitropy, and chronotropy, improving cardiac contractility, relaxation, and heart rate, respectively.

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Digitalis is a cardiac glycoside derived from the foxglove plant (Dig