What is the mechanism of loop diuretics in CHF and their main therapeutic effect?

• A. Inhibit Na-Cl transporter in distal tubule, causing mild diuresis.
• B. Inhibit Na-K-2Cl transporter in the thick ascending limb, promoting rapid diuresis and preload reduction.
• C. Block aldosterone receptors, reducing sodium retention.
• D. Directly dilate venous capacitance vessels.

Answer: (B)

Loop diuretics work by blocking the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb of the loop of Henle. This prevents reabsorption of sodium, potassium, and chloride, so more of these ions and water are excreted in the urine. The result is a rapid diuresis and a fall in circulating volume, which lowers venous return to the heart (preload). In congestive heart failure, reducing preload helps relieve congestion and pulmonary edema, improving symptoms like dyspnea and edema.

This mechanism sets loop diuretics apart from other options: inhibiting the distal tubule Na-Cl transporter yields only milder diuresis; blocking aldosterone receptors reduces sodium retention via the collecting ducts; and directly dilating venous capacitance vessels is not the primary action of loop diuretics.