IN-SITU CHARACTERIZATION OF OXALATE TRANSPORT ACROSS THE BASOLATERAL MEMBRANE OF THE PROXIMAL TUBULE

Oxalate transport across the contraluminal membrane of the proximal tu bule was studied in vivo using the ''capillary stopped flow microperfu sion method'' (Pflugers Arch 400:250-256, 1984). Cellular uptake of ox alate was characteristic of a carrier-mediated transport process (J(ma x) = 1.6 +/- 0.6 pmol/s per cm proximal tubular length, K-m = 2.03 +/- 0.77 mmol/l). Sulphate inhibited oxalate transport in a dose-dependen t manner (K-i-value = 1.53 +/- 0.38 mmol/l). Sulphate transport across the basolateral membrane was also characteristic of a carrier-mediate d transport process (J(max) = 1.83 +/- 0.56 pmol/s per cm proximal tub ular length, K-m = 1.37 +/- 0.57 mmol/l). Oxalate inhibited the sulpha te transport in a dose-dependent manner (K-i = 2.06 +/- 0.82 mmol/l). No significant differences were found between the K-i values and the K -m values of the two substances, indicating that oxalate and sulphate are transported by the same carrier across the basolateral membrane of the proximal tubule. Oxalate transport was not dependent on the extra cellular sodium or potassium concentration. Bicarbonate competitively inhibited the oxalate transport. Chloride significantly inhibited the oxalate transport, but not dose dependently. It is, therefore, suggest ed that oxalate is transported into the cell of the proximal tubule in exchange for sulphate or bicarbonate. The dose-independent inhibition by chloride is suggested to be mediated by the coupling of the sulpha te (bicarbonate)/oxalate exchanger with the chloride/bicarbonate excha nger at the basolateral membrane of the proximal tubule. This, further more, suggests that the transport of oxalate or sulphate across the ba solateral membrane might be indirectly coupled with the reabsorption o f chloride at this membrane side.