Coordinated down-regulation of NBC-1 and NHE-3 in sodium and bicarbonate loading

Background. Bicarbonate reabsorption in the kidney proximal tubule is predo minantly mediated via the apical Na+/H+ exchanger (NHE-3) and basolateral N a+:HCO3- 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 NaHCO3, NaCl, or NH4Cl added to their drinking water for 5 days and examined for the expression of NHE-3 an d NBC-1 in the kidney. Results. Serum [HCO3-] was unchanged in NaHCO3- and NaCl-loaded animals ver sus control (P>0.05). However, a significant hyperchloremic metabolic acido sis was developed in NH4Cl-loaded animals. A specific polyclonal antibody a gainst NBC-1 recognized a 130 kD band, which was exclusively expressed in t he basolateral membrane of proximal tubules. Immunoblot studies indicated t hat 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 NaHCO3 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. NaHCO3- or NaCl loading coordinately downregulates the apical NHE-3 and basolateral NBC-1 in rat kidney proximal tubule, presumably due t o increased Na+ load. We propose that the down-regulation of these two Na+- and HCO3-absorbing transporters is, to a large degree, responsible for enh anced excretion of excess of Na+ and alkaline load and prevention of metabo lic alkalosis in rats subjected to NaHCO3- loading.