Furosemide-induced urinary acidification is caused by pronounced H~+ secretion in the thick ascending limb

摘要

The loop diuretic furo-semide 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 antercalated cells. The inability to acidify urine in response to furosemide serves to diagnose distal renal tubular acidosis (dysfunction of alpha-intercalated cells). Since the TAL is important for acid/base regulation, we speculated that it is involved in furosemide-induced urinary acidification. Lumi-nal furosemide (100 muM) caused major, stable, and reversible intra-cellular 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 muM). 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.