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.