A 55-year-old man presents with shooting groin pain unlike anything he has ever experienced in
his life. He is afebrile and stable. A non-contrast urologic CT shows a small ureteral calculus. The
physician recommends copious fluid intake and simple analgesia with the hopes of spontaneous
passage. The patient doesn’t like this idea and demands medication to decrease the risk of
recurrence. The physician discusses with the patient that occasionally diuretic therapy may be
attempting to decrease recurrence, however he does not recommend it as this time. Which of the
following best describes the target of the diuretic the physician is referring to?
- A) Apical; early-distal convoluted tubule (DCT); Na/Cl antiporter
- B) Apical; early-DCT; Na/Cl symporter
- C) Apical; late-DCT; Na/Cl antiporter
- D) Apical; late-DCT; Na/Cl symporter
- E) Basolateral; early-DCT; Na/Cl antiporter
- F) Basolateral; early-DCT; Na/Cl symporter
- G) Basolateral; late-DCT; Na/Cl antiporter
- H) Basolateral; late-DCT; Na/Cl symporter
0 voters
ANS: B
A general rule about the USMLE is that the more common/quotidian something is (i.e., thiazides
are super-common), the more specific of a question they tend to ask about it.
Symporters move ions or solutes in the same direction across a membrane; antiporters move them
in the opposite direction.1
Apical means on the side of the urine; basolateral means on the side of the blood.2
Thiazide diuretics decrease urinary calcium and are frequently used to prevent recurrence of
nephro- and ureterolithiasis in patients with hypercalciuria.3, 4, 5
They inhibit the Na+/Cl+ symporter on the apical membrane of the early-DCT (distal convoluted
tubule).6
Blockage of the Na+/Cl+ symporter causes an increase in Na+ and water retention in the lumen and
therefore a decrease in DCT intracellular Na+. This causes a compensatory increase in activity of the
basolateral Na+/Ca2+ symporter, resulting in increased calcium reabsorption into the interstitium in
exchange for Na+ secretion into the cell.7, 8
Chlorthalidone, indapamide, and hydrochlorothiazide (HCTZ) are thiazide diuretics used to
decrease stone recurrence