title: Bicarbonate reabsorption and NHE-3 expression: abundance and activity are increased in Henle's loop of remnant rats.
abstract: BACKGROUND
  The bulk of bicarbonate reabsorption along the loop of Henle (LOH) is localized at the level of the thick ascending limb (TAL) and is mainly dependent on the presence of luminal Na+-H+ exchanger (NHE-3). We investigated whether the reduction of renal mass is associated with alterations in LOH bicarbonate transport coupled to changes in NHE-3 gene expression and in vivo activity.
  
  
  METHODS
  Sham-operated and remnant rats (4/6 nephrectomy) were studied 15 days after the surgery. To measure net bicarbonate reabsorption (JHCO3-) superficial loops were perfused by in vivo micropuncture. Perfusate was an end-like proximal solution containing 3H-methoxy-inulin. NHE-3 gene expression was quantified by competitive PCR using an internal standard of cDNA that differed from the wild-type NHE-3 by a deletion of 76 bp. Western blot experiments were performed on TAL suspension using anti-NHE-3 antibodies.
  
  
  RESULTS
  At various LOH bicarbonate loads, JHCO3- was constantly larger in remnant rats as compared to sham-operated animals. NHE-3 mRNA abundance was estimated to be 0.339 +/- 0.031 attomoles (amol)/ng-1 total RNA in sham-operated (N = 5) and it increased to 0.465 +/- 0.023 in remnant rats (N = 5, P < 0.01). Western blot experiments showed a significant increase of NHE-3 protein abundance in TAL of remnant rats as compared to sham-operated animals. Finally, by means of a specific NHE-3 inhibitor, S-3226, in vivo microperfusion experiments demonstrated that NHE-3 in vivo activity along the LOH was substantially increased in remnant rats in addition to the non-NHE-3 bicarbonate transport.
  
  
  CONCLUSIONS
  These data indicate that the reduction of renal mass increases mRNA, protein abundance and in vivo activity of NHE-3 along the TAL. This may explain, at least in part, the augmented transepithelial bicarbonate transport along the LOH. Such an effect will counterbalance the increased glomerular bicarbonate load, thus preventing urinary bicarbonate loss and mitigating the ensuing metabolic acidosis.
authors.affiliation: Nephrology and Research Centre for Cardiovascular Diseases, Second University of Naples, Naples, Italy. gb.capasso@unina2.it
authors.forename: Giovambattista
authors.initials: G
authors.lastname: Capasso
authors.forename: Maria
authors.initials: M
authors.lastname: Rizzo
authors.forename: Angelo
authors.initials: A
authors.lastname: Pica
authors.forename: Francesco S
authors.initials: FS
authors.lastname: Di Maio
authors.forename: Orson W
authors.initials: OW
authors.lastname: Moe
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Alpern
authors.forename: Natale G
authors.initials: NG
authors.lastname: De Santo
mesh_terms: D000136:Acid-Base Equilibrium / Q000502:physiology; D000138:Acidosis / Q000378:metabolism*; D000471:Alkalosis / Q000378:metabolism*; D000818:Animals; D001639:Bicarbonates / Q000652:urine*; D015153:Blotting, Western; D015870:Gene Expression / Q000502:physiology; D006146:Guanidines / Q000494:pharmacology; D008138:Loop of Henle / Q000378:metabolism*; D008297:Male; D008689:Methacrylates / Q000494:pharmacology; D009392:Nephrectomy; D016133:Polymerase Chain Reaction; D051381:Rats; D017207:Rats, Sprague-Dawley; D012079:Renal Circulation / Q000502:physiology; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism*
chemical_list: C118956:3-(2-(3-guanidino-2-methyl-3-oxo-propenyl)-5-methylphenyl)-N-isopropylidene-2-methyl-acrylamide dihydrochloride; D001639:Bicarbonates; D006146:Guanidines; D008689:Methacrylates; C000613387:Slc9a3 protein, rat; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers