Flow dependence of bicarbonate transport in the early (S1) proximal convoluted tubule

We previously found, using an in vivo microperfusion pump rate of 30 nl/min, that proton secretion in the early (S1) proximal convoluted tubule (PCT) of the Munich-Wistar rat exhibited saturation kinetics. The maximal transport capacity was very high, approximately 500-600 peq.mm-1.min-1. The present studies assessed the change in early PCT acidification kinetics in response to an increase in microperfusion rate to 45 nl/min. First, bicarbonate permeability in the early PCT was measured and was found to be flow dependent. Proton secretion was then calculated using perfusate bicarbonate concentrations from 8 to 100 mM. Saturation of early proximal acidification (Vmax) still occurred at approximately 500-600 peq.mm-1.min-1, but the bicarbonate concentration effecting half-maximal acidification (apparent Km) decreased (from approximately 11 mM at 30 nl/min perfusion rate to less than 6 mM at 45 nl/min). By increasing luminal perfusion rate further to 60 nl/min at constant luminal bicarbonate concentration (25 mM), we confirmed that luminal flow rate did not affect the maximal level of acidification. Similar flow-dependent changes in acidification kinetics in the late PCT were also found, as has been previously shown. In conclusion, although an increase in luminal flow increased bicarbonate permeability and apparent affinity for substrate transport, there was no effect on maximal acidification rate in the early PCT.