Dietary hco3 reduces distal tubule acidification by increasing cellular hco3 secretion. Wesson, Donald E. Texas Tech University Health Sciences Center, Lubbock, TX and Veterans Affairs Medical Center, Baylor College of Medicine, Houston, TX
APStracts 3:0035F, 1996.
We examined the components of net HCO3 reabsorption (H+/HCO3 secretion and transepithelial HCO3 permeability) in in vivo-perfused distal tubules of anesthetized rats to determine the mechanisms by which dietary HCO3 reduces acidification in this nephron segment. Animals eating a minimum electrolyte diet drank either 80 mM NaHCO3, 80 mM NaCl, or 40 mM Na2SO4 for 7-10 days were compared to controls drinking distilled H2O. Perfusing with a HCO3 and Cl--containing solution, net HCO3 reabsorption was lower than control in only the NaHCO3 animals (14.4 +/- 1.3 vs. 4.1 +/- 0.5 pmol/mm.min, p &LT 0.001). Perfusing with a zero HCO3 , zero Cl- solution, distal tubule luminal HCO3 accumulation (JHCO3) was higher in NaHCO3 animals than control (-13.7 +/- 1.3 vs. -4.7 +/- 0.7 pmol/mm.min p &LT 0.002). Despite having higher JHCO3, estimated transepithelial HCO3 permeability in the NaHCO3 animals was similar to control (0.52 +/- 0.06 vs. 0.36 +/- 0.04 x 10-7 cm2/sec, p = NS). Luminal acetazolamide (Az) reduced JHCO3 in NaHCO3 animals to a level similar to control ( -6.2 +/- 0.6 vs. -4.0 +/- 0.5 pmol/mm.min, p = NS) in this nephron segment containing cells with cytoplasmic but no luminal carbonic anhydrase activity. Including Cl- in the initial perfusate increased JHCO3 in NaHCO3 animals only (-20.8 +/- 1.9 vs. -13.7 +/- 1.3 pmol/mm.min, p &LT 0.02), and this increase was inhibited by luminal Az. Calculated H+ secretion was similar among groups. Together, the data indicate that dietary HCO3 reduces distal tubule acidification by increasing Az-sensitive generation of HCO3 by distal tubule cells that enters the lumen by a mechanism augmented by luminal Cl-. 

Received 26 July 1995; accepted in final form 9 February 1996.
APS Manuscript Number F247-5.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 13 March 96