NBCn1 Increases NH4 + Reabsorption Across Thick Ascending Limbs, the Capacity for Urinary NH4 + Excretion, and Early Recovery from Metabolic Acidosis
Authors: Olsen Jeppe S. M. , Svendsen Samuel , Berg Peder , Dam Vibeke S. , Sorensen Mads V. , Matchkov Vladimir V. , Leipziger Jens , Boedtkjer Ebbe 
Published: January 2021

Significance Statement
Recovery from metabolic acidosis necessitates increased renal net acid excretion through urinary elimination of NH4
+. Renal thick ascending limbs (TALs) contribute to a medullary shortcut, where NH4
+ originating from proximal tubules is ultimately secreted in collecting ducts. NH4
+ transfer across TALs requires a basolateral exit pathway for H+ to avoid intracellular accumulation. Experiments with knockout mice show that the Na+/HCO3
− cotransporter NBCn1 mediates basolateral HCO3
− uptake and increases NH4
+ reabsorption in TAL, amplifies the corticomedullary NH4
+ gradient, elevates the capacity for urinary NH4
+ excretion, and accelerates recovery of arterial blood pH and [HCO3
−] 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+/HCO3
− 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 NH4
+ transport measured in isolated perfused mTALs. Quantitative RT-PCR and immunoblotting were used to evaluate NBCn1 expression. Tissue [NH4
+] was measured in renal biopsies, NH4
+ excretion and titratable acid quantified in spot urine, and arterial blood gasses evaluated in normoventilated mice.

Results
Basolateral Na+/HCO3
− cotransport activity was similar in isolated perfused mTALs from wild-type and NBCn1 knockout mice under control conditions. During metabolic acidosis, basolateral Na+/HCO3
− 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 NH4
+ reabsorption across mTALs by approximately 60%, abolished the renal corticomedullary NH4
+ gradient, reduced the capacity for urinary NH4
+ excretion by approximately 50%, and delayed recovery of arterial blood pH and standard [HCO3
−] from their initial decline.

Conclusions
During metabolic acidosis, NBCn1 is required for the upregulated basolateral HCO3
− uptake and transepithelial NH4
+ reabsorption in mTALs, renal medullary NH4
+ accumulation, urinary NH4
+ excretion, and early recovery of arterial blood pH and standard [HCO3
−]. These findings support that NBCn1 facilitates urinary net acid excretion by neutralizing intracellular H+ released during NH4
+ reabsorption across mTALs.