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

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.