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.
Amlal
H
H
Department of Medicine, University of Cincinnati Medical Center, 231 Albert Sabin Way, Cincinnati, OH 45267-0585, USA.
Chen
Q
Q
Greeley
T
T
Pavelic
L
L
Soleimani
M
M
D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D015536:Down-Regulation; D007150:Immunohistochemistry; D007668:Kidney / Q000378:metabolism*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism*; D027982:Sodium-Bicarbonate Symporters / Q000032:analysis* / Q000235:genetics; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000032:analysis* / Q000235:genetics
D001639:Bicarbonates; C422004:Slc4a4 protein, rat; C000613387:Slc9a3 protein, rat; D027982:Sodium-Bicarbonate Symporters; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D012964:Sodium