Chloride/formate exchange with formic acid recycling: a mechanism of active chloride transport across epithelial membranes.
The pathways for transport of Cl- and formate in microvillus membrane vesicles isolated from rabbit renal cortex were evaluated. An outward formate gradient stimulated the uptake of Cl-, and an outward Cl- gradient stimulated the uptake of formate, indicating Cl-/formate exchange. In addition, an inside alkaline pH gradient induced the accumulation of formate, consistent with nonionic diffusion of formic acid. Although an inward Na+ gradient also stimulated uphill formate accumulation, suggesting Na+/formate cotransport, this effect was abolished when ionophores were used to prevent the generation of a transmembrane pH gradient, indicating an indirect coupling of formic acid transport to Na+/H+ exchange. An inside alkaline pH gradient only minimally stimulated the uptake of 82Br-, used as tracer for Cl-, confirming the absence of appreciable Cl-/OH- exchange. However, the same pH gradient in the presence of a physiologic formate concentration (0.2 mM) markedly stimulated 82Br- influx. These data suggest that Cl-/formate exchange with recycling of formic acid by nonionic diffusion is a potential mechanism for active Cl- absorption across the luminal membrane in the proximal tubule and perhaps in other epithelia.
D000818:Animals; D001693:Biological Transport, Active; D001965:Bromides / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D004848:Epithelium / Q000378:metabolism; D005561:Formates / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007672:Kidney Cortex / Q000378:metabolism; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D008297:Male; D008871:Microvilli / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000378:metabolism
D001965:Bromides; D002712:Chlorides; D005561:Formates; C030544:formic acid; D012964:Sodium