Coordinated Down-regulation of NBC-1 and NHE-3 in Sodium and Bicarbonate Loading

Background: Bicarbonate reabsorption in the kidney proximal tubule is predominantly mediated via the apical Na+/H+ exchanger (NHE-3) and basolateral Na+: HCO(-3) cotransporter (NBC-1). The purpose of these studies was to examine the effects of Na+ load and altered acid-base status on the expression of NHE-3 and NBC-1 in the kidney.

Methods: Rats were placed on 280 mmol/L of NaHCO(3), NaCl, or NH(4)Cl added to their drinking water for 5 days and examined for the expression of NHE-3 and NBC-1 in the kidney.

Results: Serum [HCO(-3)] was unchanged in NaHCO(-3) and NaCl-loaded animals versus control (P> 0.05). However, a significant hyperchloremic metabolic acidosis was developed in NH4Cl-loaded animals. A specific polyclonal antibody against NBC-1 recognized a 130 kD band, which was exclusively expressed in the basolateral membrane of proximal tubules. Immunoblot studies indicated that the protein abundance of NBC-1 and NHE-3 in the cortex decreased by 74% (P < 0.04) and 66% (P < 0.03), respectively, in NaHCO(3) loading and by 72% (P < 0.003) and 55% (P < 0.04), respectively, in NaCl loading. Switching from NaHCO(3) to distilled water resulted in rapid recovery of NHE-3 and NBC-1 protein expression toward normal levels. Metabolic acidosis increased the abundance of NHE-3 (P < 0.0001) but not NBC-1 (P> 0.05).

Conclusions: NaHCO(-3) or NaCl loading coordinately down-regulates the apical NHE-3 and basolateral NBC-1 in rat kidney proximal tubule, presumably due to increased Na+ load. We propose that the down-regulation of these two Na+- and HCO(3)-absorbing transporters is, to a large degree, responsible for enhanced excretion of excess of Na+ and alkaline load and prevention of metabolic alkalosis in rats subjected to NaHCO(-3) loading.