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

Abstract

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. 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. 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. 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.