Indomethacin can antagonize the diuretic action of loop diuretics by –
**Core Concept**
Indomethacin is a nonsteroidal anti-inflammatory drug (NSAID) that inhibits cyclooxygenase (COX) enzymes, leading to a decrease in prostaglandin synthesis. Prostaglandins play a crucial role in renal physiology, particularly in mediating the diuretic response to loop diuretics.
**Why the Correct Answer is Right**
Prostaglandins, specifically PGE2 and PGF2α, are synthesized in the renal medulla and play a key role in maintaining renal blood flow and glomerular filtration rate. Indomethacin, by inhibiting COX enzymes, reduces the production of these prostaglandins, leading to vasoconstriction of the afferent arterioles and reduced glomerular filtration rate. This, in turn, antagonizes the diuretic action of loop diuretics, which work by inhibiting the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle. The reduced glomerular filtration rate leads to decreased delivery of sodium and chloride to the distal tubules, where loop diuretics exert their effect.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the mechanism by which indomethacin antagonizes loop diuretics. While indomethacin does affect renal function, its primary mechanism is not through the inhibition of the renin-angiotensin-aldosterone system (RAAS).
**Option B:** This option is incorrect because it suggests that indomethacin increases the diuretic action of loop diuretics, which is the opposite of the correct answer.
**Option C:** This option is incorrect because it does not accurately describe the mechanism by which indomethacin antagonizes loop diuretics. While indomethacin does affect renal function, its primary mechanism is not through the inhibition of the sympathetic nervous system.
**Clinical Pearl / High-Yield Fact**
Indomethacin and other NSAIDs can lead to a decrease in renal blood flow and glomerular filtration rate, particularly in patients with pre-existing renal impairment. This is a critical consideration when using NSAIDs in patients with kidney disease.
**Correct Answer: C. Indomethacin inhibits prostaglandin synthesis, which is necessary for maintaining renal blood flow and glomerular filtration rate, thereby antagonizing the diuretic action of loop diuretics.**