Effect of electroneutral luminal and basolateral lactate transport on intracellular pH in salamander proximal tubules.
We used microelectrodes to examine the effects of organic substrates, particularly lactate (Lac-), on the intracellular pH (pHi) and basolateral membrane potential (Vbl) in isolated, perfused proximal tubules of the tiger salamander. Exposure of the luminal and basolateral membranes to 3.6 mM Lac- caused pHi to increase by approximately 0.2, opposite to the decrease expected from nonionic diffusion of lactic acid (HLac) into the cell. Addition of Lac- to only the lumen also caused alkalinization, but only if Na+ was present. This alkalinization was not accompanied by immediate Vbl changes, which suggests that it involves luminal, electroneutral Na/Lac cotransport. Addition of Lac- to only the basolateral solution caused pHi to decrease by approximately 0.08. The initial rate of this acidification was a saturable function of [Lac-], was not affected by removal of Na+, and was reversibly reduced by alpha-cyano-4-hydroxycinnamate (CHC). Thus, the pHi decrease induced by basolateral Lac- appears to be due to the basolateral entry of H+ and Lac-, mediated by an H/Lac cotransporter (or a Lac-base exchanger). Our data suggest that this transporter is electroneutral and is not present at the luminal membrane. A key question is how the addition of Lac- to the lumen increases pHi. We found that inhibition of basolateral H/Lac cotransport by basolateral CHC reduced the initial rate of pHi increase caused by luminal Lac-. On the other hand, luminal CHC had no effect on the luminal Lac(-)-induced alkalinization. These data suggest that when Lac- is present in the lumen, it enters the cell from the lumen via electroneutral Na/Lac cotransport and then exists with H+ across the basolateral membrane via electroneutral H/Lac cotransport. The net effect is transepithelial Lac- reabsorption, basolateral acid extrusion, and intracellular alkalinization.
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510.
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Siebens
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Boron
D000557:Ambystoma; D000818:Animals; D001692:Biological Transport; D003373:Coumaric Acids / Q000494:pharmacology; D004566:Electrodes; D005260:Female; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D007773:Lactates / Q000378:metabolism*; D008954:Models, Biological; D012964:Sodium / Q000502:physiology
D003373:Coumaric Acids; D007773:Lactates; C007175:alpha-cyano-4-hydroxycinnamate; D012964:Sodium