The connecting tubule is the main site of the furosemide-induced urinary acidification by the vacuolar H+-ATPase

Abstract

Final urinary acidification is achieved by electrogenic vacuolar H(+)-ATPases expressed in acid-secretory intercalated cells (ICs) in the connecting tubule (CNT) and the cortical (CCD) and initial medullary collecting duct (MCD), respectively. Electrogenic Na(+) reabsorption via epithelial Na(+) channels (ENaCs) in the apical membrane of the segment-specific CNT and collecting duct cells may promote H(+)-ATPases-mediated proton secretion by creating a more lumen-negative voltage. The exact localization where this supposed functional interaction takes place is unknown. We used several mouse models performing renal clearance experiments and assessed the furosemide-induced urinary acidification. Increasing Na(+) delivery to the CNT and CCD by blocking Na(+) reabsorption in the thick ascending limb with furosemide enhanced urinary acidification and net acid excretion. This effect of furosemide was abolished with amiloride or benzamil blocking ENaC action. In mice deficient for the IC-specific B1 subunit of the vacuolar H(+)-ATPase, furosemide led to only a small urinary acidification. In contrast, in mice with a kidney-specific inactivation of the alpha subunit of ENaC in the CCD and MCD, but not in the CNT, furosemide alone and in combination with hydrochlorothiazide induced normal urinary acidification. These results suggest that the B1 vacuolar H(+)-ATPase subunit is necessary for the furosemide-induced acute urinary acidification. Loss of ENaC channels in the CCD and MCD does not affect this acidification. Thus, functional expression of ENaC channels in the CNT is sufficient for furosemide-stimulated urinary acidification and identifies the CNT as a major segment in electrogenic urinary acidification.

Collecting Duct Intercalated Cell Function and Regulation

“…This potential difference would favor H 1 -ATPase function in type A intercalated cells. Wagner and colleagues elucidated in vivo that ENaC function in the connecting segment was sufficient to induce this upregulation (60).…”

“…Together with the glucocorticoid receptor these hormones elicit signaling events in the kidney that regulate salt and potassium homeostasis, intravascular volume (110), and cell metabolism (111)(112)(113). Aldosterone, via the MR, in turn activates ENaC in principal cells, resulting in an increase in paracellular Cl 2 transport, H 1 secretion from intercalated cells, and activation of basolateral Na 1 /K 1 -ATPase (reviewed in references 60,110,114). However, these transcellular transport cascades are mostly relevant to principal cells (11).…”

The Calcium-Sensing Receptor Promotes Urinary Acidification to Prevent Nephrolithiasis

“…8 Urinary acidification occurs along different segments of the nephron and is of crucial importance in acid/base homeostasis. 9,10 In combination with urinary buffers, the activity of acid/ base transporters in intercalated cells (ICs) of CNT and collecting duct system (CD) accomplish the fine-tuning of urinary pH. In type A ICs, the multisubunit vacuolar ATP-driven proton pump (H ϩ -ATPase) is expressed at the apical membrane, which is mainly responsible for proton excretion.…”

Mineralocorticoids Stimulate the Activity and Expression of Renal H+,K+-ATPases

“…Specifically, mineralocorticoids increase expression and activity of the apical epithelial Na ϩ channel and the basolateral Na ϩ ,K ϩ -ATPase in principal cells of the renal collecting duct to drive net Na ϩ reabsorption. 4 Mineralocorticoids also stimulate H ϩ secretion by the collecting duct, in part by stimulating the activity of apical H ϩ -ATPases, 5,6 but the effect of mineralocorticoids on H ϩ ,K ϩ -ATPase proton transport activity and expression in these segments has not been determined.…”