Kinetics of bicarbonate transport in the early proximal convoluted tubule

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.