Basolateral membrane Na+/H+ antiport, Na+/base cotransport, and Na+-independent Cl-/base exchange in the rabbit S3 proximal tubule

Abstract

The basolateral membrane Na+ and Cl--dependent acid-base transport processes were studied in the isolated perfused rabbit S3 proximal straight tubule. Intracellular pH (pH(i)) was measured with 2'7'-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) and a microfluorometer coupled to the tubule perfusion apparatus. Reduction of basolateral HCO3- from 25 to 5 mM caused pH(i) to decrease at a rate of 0.81 pH/min. Approximately 50% of this rate was Na+-dependent, 30% Cl--dependent and 20% Na+ and Cl--independent. Two basolateral Na+-dependent acid base transport pathways were detected: (a) an amiloride-sensitive Na+/H+ antiporter and (b) a stilbene-sensitive Na+/base cotransporter. No evidence was found for a Na+-dependent Cl-/base exchanger. The Cl--dependent component of basolateral base efflux was mediated by a stilbene-sensitive Na+-independent Cl-/base exchange pathway. The results suggest that the acid base transport pathways of the basolateral membrane of the S3 proximal tubule differ from more proximal nephron segments.