Transfer of base across the basolateral membrane of cortical tubules of rat kidney.

The transfer of HCO3-/OH- across the peritubular membrane of rat cortical tubules was studied by measuring capillary pH during stopped-flow microperfusion of peritubular capillaries with electrolyte solutions containing 3 mM HCO3-. The rate of alkalinization of these solutions was significantly delayed when 10(-4) M acetazolamide, 5 X 10(-4) M SITS or 2 mM Ba2+ were added, as well as when chloride was substituted by gluconate. Under these conditions, stationary capillary pH was slightly but significantly increased. In another series, the lumen of proximal tubules was perfused with alkaline solutions while pH was measured in adjacent capillaries. During perfusion, capillary pH rose to a level 0.4 units higher than in free-flow conditions, returning after filling the lumen with oil; the rate of capillary pH return to baseline is a measure of base extrusion from cells, in the absence of influx from the lumen. This rate is also significantly delayed by acetazolamide. The data show that peritubular base extrusion is dependent on carbonic anhydrase, on basolateral membrane voltage, and on interstitial chloride, and delayed by the anion exchange inhibitor SITS; they are compatible with both basolateral HCO3-/Cl- exchange and conductive bicarbonate transfer.