NBCe1-A is required for the renal ammonia and K⁺response to hypokalemia

Abstract

Hypokalemia increases ammonia excretion and decreases K+ excretion. The present study examined the role of the proximal tubule protein NBCe1-A in these responses. We studied mice with Na+-bicarbonate cotransporter electrogenic, isoform 1, splice variant A (NBCe1-A) deletion [knockout (KO) mice] and their wild-type (WT) littermates were provided either K+ control or K+-free diet. We also used tissue sections to determine the effect of extracellular ammonia on NaCl cotransporter (NCC) phosphorylation. The K+-free diet significantly increased proximal tubule NBCe1-A and ammonia excretion in WT mice, and NBCe1-A deletion blunted the ammonia excretion response. NBCe1-A deletion inhibited the ammoniagenic/ammonia recycling enzyme response in the cortical proximal tubule (PT), where NBCe1-A is present in WT mice. In the outer medulla, where NBCe1-A is not present, the PT ammonia metabolism response was accentuated by NBCe1-A deletion. KO mice developed more severe hypokalemia and had greater urinary K+ excretion during the K+-free diet than did WT mice. This was associated with blunting of the hypokalemia-induced change in NCC phosphorylation. NBCe1-A KO mice have systemic metabolic acidosis, but experimentally induced metabolic acidosis did not alter NCC phosphorylation. Although KO mice have impaired ammonia metabolism, experiments in tissue sections showed that lack of ammonia does impair NCC phosphorylation. Finally, urinary aldosterone was greater in KO mice than in WT mice, but neither expression of epithelial Na+ channel α-, β-, and γ-subunits nor of H+-K+-ATPase α1- or α2-subunits correlated with changes in urinary K+. We conclude that NBCe1-A is critical for the effect of diet-induced hypokalemia to increase cortical proximal tubule ammonia generation and for the expected decrease in urinary K+ excretion.

Role of the renal androgen receptor in sex differences in ammonia metabolism

“…NBCe1-A immunolabel intensity was less in cortical proximal tubule segments in KS-AR-KO mice than in WT mice. NBCe1-A immunolabel was not detectable in the OM PST, as previously reported (47,48). The first major finding in this study is that AR expression mediates sex differences in renal size.…”

“…NBCe1-A immunolabel intensity was less in cortical proximal tubule segments in KS-AR-KO mice than in WT mice. NBCe1-A immunolabel was not detectable in the OM PST, as previously reported(47,48).D: quantitative immunohistochemistry (qIHC) for NBCe1-A. Male KS-AR-KO mice had significantly less NBCe1-A expression in the PCT and cortical PST than did WT mice.…”

“…The A variant of NBCe1 is the predominant variant present in the proximal tubule (50), and NBCe1-A has been shown to regulate ammonia metabolism (47,48). In male mice, KS-AR-KO decreased cortical NBCe1-A expression significantly; in female mice, expression was unaltered (Fig.…”

“…NBCe1 is a proximal tubule integral membrane protein with a major role in filtered bicarbonate reabsorption that also regulates ammonia metabolism (47)(48)(49). Two splice variants of NBCe1, NBCe1-A and NBCe1-B, are present in the mouse kidney (50,51).…”

Effects of extreme potassium stress on blood pressure and renal tubular sodium transport

“…NBCe1A, which facilitates the bicarbonate reclamation that accompanies luminal H ϩ secretion (39), exhibited a significant but less pronounced increase in response to K ϩ depletion (Fig. 5B) (35). KCl loading also had a trend toward increased NHE3 and NBCe1A expression, albeit to a lesser degree.…”