Apical membrane chloride/base exchange in the rat proximal convoluted tubule.

Abstract

To examine whether Cl/base exchange is present on the apical membrane of the proximal convoluted tubule, cell pH was measured fluorometrically in the in vivo microperfused rat proximal tubule with (2',7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein.The effect of luminal chloride addition was examined in tubules perfused symmetrically with chloride-free solutions. In the absence of inhibitors, luminal chloride addition did not affect cell pH. However, after inhibition of basolateral membrane anion transport with peritubular 4-acetamido-4'-isothiocyano-(2,2')-disulfonic-stilbene (to amplify effects of apical membrane transport on cell pH), luminal chloride addition caused a small cell acidification (delta pH, = 0.02). When 1 mM formate was added to the solutions, luminal chloride addition caused a larger change in cell pH (delta pH, = 0.06) that was inhibited by (4,4')-diisothiocyano-(2,2')-disulfonic-stilbene. This stimulation of Cl/base exchange was not seen with 1 mM acetate addition. These results demonstrate apical membrane Cl/base exchange, a significant fraction of which is dependent on the presence of formate and probably represents Cl/formate exchange.

Mechanism of apical and basolateral Na(+)-independent Cl-/base exchange in the rabbit superficial proximal straight tubule.

“…superficial S2 proximal tubule is mediated by an apical Na+/ H+ exchanger in parallel with a C1-/OH-exchanger and that the secreted H + and OH -ions form H20 in the tubule lumen. Previous studies have failed to measure appreciable apical Cl -/base exchange in the intact tubule using cell pH measurements in the absence of exogenous formate (26,27). Several factors and methodological differences distinguish the studies of Alpern (26) and Baum (27) from the present study and may account for the marked Cl--induced changes in pHi observed.…”

“…After luminal Cl -removal (solution B), pHi increased by only 0.04 pH units with an EBF of 2.76 pmol/min per mm (Table II). To enhance the measured effect on EBF induced by luminal Cl-removal, an attempt was made to inhibit Na+-dependent transport processes as previously described (26,27). S2 tubules were perfused with 2 mM luminal amiloride and 1 mM basolateral H2DIDS.…”

“…In previous studies demonstrating an effect of exogenous formate on luminal Cl--induced changes in pHi, the tubules had been perfused initially in the absence of CL-, and Cl -was then added to the lumen (26,27). However, in the results described thus far, EBF was calculated after Cl-removal from the lumen in tubules initially perfused and bathed in Cl-containing solutions.…”

“…The authors hypothesized that coupled apical Na+/H' and Cl-/formate exchange with apical formic acid recycling mediated apical NaCl uptake. Alpern (26) and Baum (27), using intracellular pH (pHi)' measurements, detected minimal apical Cl -/base exchange activity in the absence of exogenous formate. However, Sasaki and Yoshiyama (14) failed to find an effect of luminal 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid on rabbit proximal straight tubule intracellular Cl-activity even in the presence of 1 mM luminal formate.…”

Activation of the apical Na+/H+exchanger NHE3 by formate: a basis of enhanced fluid and electrolyte reabsorption by formate in the kidney

“…In the proximal tubule of the rabbit, rat, and mouse kidney, the addition of low concentrations of formate to the luminal and peritubular solutions increased the rate of NaCl absorption (1,2). The formate-stimulated NaCl absorption was electroneutral and inhibited in the presence of DIDS (1-3, 5, 39, 44).…”