title: Furosemide-induced urinary acidification is caused by pronounced H+ secretion in the thick ascending limb.
abstract: The loop diuretic furosemide inhibits NaCl reabsorption in the thick ascending limb (TAL). In addition, furosemide acidifies the urine, which is traditionally explained by increased Na+ loading to the distal tubule causing an activation of H+ secretion via H+-ATPase in α-intercalated cells. The inability to acidify urine in response to furosemide serves to diagnose distal renal tubular acidosis (dysfunction of α-intercalated cells). Since the TAL is important for acid/base regulation, we speculated that it is involved in furosemide-induced urinary acidification. Luminal furosemide (100 μM) caused major, stable, and reversible intracellular alkalization (7.27 ± 0.06 to 7.6 ± 0.04) in isolated perfused murine medullary TAL and pronounced H+ secretion. This H+ secretion was fully inhibited with luminal amiloride (1 mM) and the Na+/H+ exchanger (NHE)3-specific antagonist #4167 (1 μM). Moreover, furosemide triggered a substantial drop of intracellular Na+ concentration in the medullary TAL. These results suggest that the furosemide-induced H+ secretion is a consequence of a drop in intracellular Na+ concentration, increasing the driving force for NHE3. Intriguingly, in whole animal experiments, furosemide-induced urinary acidification and net acid excretion were markedly reduced by specific NHE3 inhibition. Furthermore, the furosemide-induced urinary acidification was partially preserved during epithelial Na+ channel inhibition with benzamil. These results provide new insights in the mechanism of furosemide-induced urinary acidification and emphasize the role of the TAL in renal acid/base handling.
authors.forename: Pauline I A
authors.initials: PI
authors.lastname: de Bruijn
authors.forename: Casper K
authors.initials: CK
authors.lastname: Larsen
authors.forename: Sebastian
authors.initials: S
authors.lastname: Frische
authors.forename: Nina
authors.initials: N
authors.lastname: Himmerkus
authors.forename: Helle A
authors.initials: HA
authors.lastname: Praetorius
authors.forename: Markus
authors.initials: M
authors.lastname: Bleich
authors.forename: Jens
authors.initials: J
authors.lastname: Leipziger
mesh_terms: D000141:Acidosis, Renal Tubular / Q000175:diagnosis; D000584:Amiloride; D000818:Animals; D053503:Epithelial Sodium Channels / Q000378:metabolism; D005665:Furosemide / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008138:Loop of Henle / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D011522:Protons; D012964:Sodium / Q000378:metabolism; D049994:Sodium Potassium Chloride Symporter Inhibitors / Q000494:pharmacology*; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors; D014556:Urine / Q000737:chemistry*
chemical_list: D053503:Epithelial Sodium Channels; D011522:Protons; C000613386:Slc9a3 protein, mouse; D049994:Sodium Potassium Chloride Symporter Inhibitors; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D005665:Furosemide; D012964:Sodium