AJP - Renal Physiology, Vol 248, Issue 1 113-F121, Copyright © 1985 by American Physiological Society
Load dependence of HCO3 and H2O reabsorption in the early proximal tubule of the Munich-Wistar rat
D. A. Maddox and F. J. Gennari
Studies were undertaken in Munich-Wistar rats to evaluate the influence of
variations in the filtered load of bicarbonate (FLHCO3) and water [single
nephron glomerular filtration rate (SNGFR)] on the pattern of reabsorption
along the accessible proximal tubule. SNGFR and FLHCO3 were varied by
examining animals under conditions of hydropenia, plasma and extracellular
volume expansion (VE), and VE plus aortic constriction. Water and HCO-3
reabsorption rates were measured at intervals along the proximal tubule,
from very early segments to late segments, and these values compared with
previous measurements in euvolemic rats. The earliest accessible portion of
the proximal tubule reabsorbed HCO3 and water avidly; 40-55% of FLHCO3 and
18-20% of SNGFR were reabsorbed within the first millimeter. Moreover, when
FLHCO3 was increased to as high as 2,400 pmol/min, HCO3 reabsorption rate
in the first millimeter of the tubule increased concomitantly, reaching
values as high as 1,000 pmol X mm-1 X min-1. In a similar fashion, water
reabsorption in the first millimeter increased in direct relation to
increases in SNGFR, reaching values as high as 13 nl X mm-1 X min-1 at
SNGFR values of 70 nl/min. These results indicate that the early proximal
tubule has much higher HCO3 and water reabsorptive rates and a stronger
load dependence than has been found in later segments of the proximal
tubule. The early proximal tubule thus appears to play a critical role in
the maintenance of glomerulotubular balance.
