The positive inotropic effect of digitalis is due to inhibition of Na+/K+ ATPase pump in cardiac muscle cell membrane leading to:
## **Core Concept**
The positive inotropic effect of digitalis (a cardiac glycoside) is primarily attributed to its action on the cardiac muscle cell membrane, specifically through the inhibition of the **Na+/K+ ATPase pump**. This pump is crucial for maintaining the sodium and potassium gradients across the cell membrane. Inhibition of this pump leads to a cascade of effects that ultimately result in increased contractility of the heart.
## **Why the Correct Answer is Right**
The correct answer, , involves the inhibition of the Na+/K+ ATPase pump, which leads to an increase in intracellular sodium levels. This increase in sodium concentration reduces the activity of the **Na+/Ca2+ exchanger**, a mechanism that normally helps to extrude calcium from the cell. As a result, there is an increase in intracellular calcium levels. The increased intracellular calcium enhances the contractility of the heart by increasing the amount of calcium available to bind with troponin and tropomyosin in the cardiac muscle cells, thus initiating contraction. This mechanism explains the positive inotropic effect of digitalis.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the direct effect of digitalis on the Na+/K+ ATPase pump does not immediately lead to an increase in intracellular potassium; rather, it leads to a decrease in the gradient that helps maintain potassium inside the cell.
- **Option B:** This option is incorrect because while digitalis does affect calcium handling indirectly, the primary and most direct effect related to its positive inotropic action is through the increase in intracellular calcium due to reduced Na+/Ca2+ exchange activity, not through a direct increase in calcium influx.
- **Option C:** This option seems related but does not accurately describe the mechanism; the increase in contractility is not due to an increase in the rate of depolarization.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the therapeutic effects of digitalis, including its positive inotropic effect and its effect on heart rate (notably slowing the heart rate in atrial fibrillation), are achieved at doses close to its toxic dose. This narrow therapeutic window makes monitoring and dose adjustment critical.
## **Correct Answer:** . Increased intracellular calcium concentration.