NBCn1 Increases NH4 + Reabsorption Across Thick Ascending Limbs, the Capacity for Urinary NH4 + Excretion, and Early Recovery from Metabolic Acidosis

• Published in: Journal of the American Society of Nephrology Vol. 32; no. 4; pp. 852 - 865
• Main Authors: Olsen, Jeppe S. M., Svendsen, Samuel, Berg, Peder, Dam, Vibeke S., Sorensen, Mads V., Matchkov, Vladimir V., Leipziger, Jens, Boedtkjer, Ebbe
• Format: Journal Article
• Language: English
• Published: American Society of Nephrology 01.04.2021
• Subjects: Basic Research
• ISSN: 1046-6673; 1533-3450
• DOI: 10.1681/ASN.2019060613

Significance Statement Recovery from metabolic acidosis necessitates increased renal net acid excretion through urinary elimination of NH 4 + . Renal thick ascending limbs (TALs) contribute to a medullary shortcut, where NH 4 + originating from proximal tubules is ultimately secreted in collecting ducts. NH 4 + transfer across TALs requires a basolateral exit pathway for H + to avoid intracellular accumulation. Experiments with knockout mice show that the Na + /HCO 3 − cotransporter NBCn1 mediates basolateral HCO 3 − uptake and increases NH 4 + reabsorption in TAL, amplifies the corticomedullary NH 4 + gradient, elevates the capacity for urinary NH 4 + excretion, and accelerates recovery of arterial blood pH and [HCO 3 − ] during metabolic acidosis. NBCn1 is crucial for acid-base handling in TALs, and for early renal compensation of systemic acid-base disturbances. Background The electroneutral Na + /HCO 3 − cotransporter NBCn1 (Slc4a7) is expressed in basolateral membranes of renal medullary thick ascending limbs (mTALs). However, direct evidence that NBCn1 contributes to acid-base handling in mTALs, urinary net acid excretion, and systemic acid-base homeostasis has been lacking. Methods Metabolic acidosis was induced in wild-type and NBCn1 knockout mice. Fluorescence-based intracellular pH recordings were performed and NH 4 + transport measured in isolated perfused mTALs. Quantitative RT-PCR and immunoblotting were used to evaluate NBCn1 expression. Tissue [NH 4 + ] was measured in renal biopsies, NH 4 + excretion and titratable acid quantified in spot urine, and arterial blood gasses evaluated in normoventilated mice. Results Basolateral Na + /HCO 3 − cotransport activity was similar in isolated perfused mTALs from wild-type and NBCn1 knockout mice under control conditions. During metabolic acidosis, basolateral Na + /HCO 3 − cotransport activity increased four-fold in mTALs from wild-type mice, but remained unchanged in mTALs from NBCn1 knockout mice. Correspondingly, NBCn1 protein expression in wild-type mice increased ten-fold in the inner stripe of renal outer medulla during metabolic acidosis. During systemic acid loading, knockout of NBCn1 inhibited the net NH 4 + reabsorption across mTALs by approximately 60%, abolished the renal corticomedullary NH 4 + gradient, reduced the capacity for urinary NH 4 + excretion by approximately 50%, and delayed recovery of arterial blood pH and standard [HCO 3 − ] from their initial decline. Conclusions During metabolic acidosis, NBCn1 is required for the upregulated basolateral HCO 3 − uptake and transepithelial NH 4 + reabsorption in mTALs, renal medullary NH 4 + accumulation, urinary NH 4 + excretion, and early recovery of arterial blood pH and standard [HCO 3 − ]. These findings support that NBCn1 facilitates urinary net acid excretion by neutralizing intracellular H + released during NH 4 + reabsorption across mTALs.