AJP - Renal Physiology, Vol 253, Issue 5 912-F916, Copyright © 1987 by American Physiological Society
Kinetics of bicarbonate transport in the early proximal convoluted tubule
F. Y. Liu and M. G. Cogan
Department of Medicine, University of California, San Francisco 94143.
Bicarbonate permeability and bicarbonate transport kinetics in the S1
segment of the proximal convoluted tubule (PCT) have not been previously
studied. In vivo microperfusion at a rate of 30 nl/min was performed in
early and late PCT of Munich-Wistar rats. Bicarbonate permeability was
first defined, using a bicarbonate-free, acetazolamide-containing
perfusate, and was over fourfold higher in the early compared with the late
PCT (20.4 +/- 1.8 vs. 4.6 +/- 0.4 X 10(-7) cm2/s, P less than 0.001). Net
bicarbonate absorption was then measured using perfusate bicarbonate
concentrations of 15, 25, 40, and 100 mM at 30 nl/min perfusion rate.
Proton secretory rate was calculated for each group by subtracting the
passive bicarbonate transport component from the net flux. Saturation
kinetics of acidification were observed in both the early and late PCT. The
maximal proton secretory rate at the highest luminal bicarbonate
concentration (Vmax) in the early PCT was about twice that in the late PCT
(504 +/- 37 vs. 265 +/- 15 peq.mm-1.min-1, P less than 0.001). However, the
luminal bicarbonate concentration eliciting half-maximal proton secretion
(apparent Km) was approximately the same (11 mM) in the early and late PCT.
In conclusion, the early PCT has a higher bicarbonate permeability and
proton secretory capacity than the late PCT. Increased Vmax but axial
constancy of Km suggests that there is amplification of similar transport
mechanism(s) affecting bicarbonate absorption in S1 compared with S2 cells.
