URATE TRANSPORT IN HOMARUS-AMERICANUS HEPATOPANCREAS - STUDIES ON MEMBRANE-VESICLES AND R-CELLS

[2-C-14]urate uptake was studied in hepatopancreatic basolateral membr ane vesicles and in R cell suspensions of the American lobster by Mill ipore filtration techniques. Unspecific binding of urate to the vesicu lar membrane was 25.5 +/- 3.0% of equilibrium. Vesicular uptake showed a diffusional and a saturable component (K-m) 0.37 +/- 0.04 mM and ma ximal velocity (V-max) 16.5 +/- 1.2 pmol urate mg protein(-1) . s(-1)) . [2-C-14]urate uptake was significantly trans-stimulated by urate. Pu rine analogues, probenecid,p-aminohip-puric acid, pyrazinoic, and oxon ic acid cis-inhibited urate transport. Urate uptake was not affected b y Na+ or KC transmembrane gradients but stimulated by 1 mM 2-oxoglutar ate at the cis-side in Na+-containing media. Cellular urate uptake was inhibited by pyrazinoic acid. Uptake was saturable (K-m 0.53 +/- 0.11 mM and V-max 3.7 +/- 0.4 pmol urate . mg protein(-1) . s(-1)) and Nai independent. However, 2-oxoglutarate stimulated uptake in Na+-contain ing media. These results suggest that urate uptake across the basolate ral membrane occurs via a specific, Na+-independent transport system t hat may operate in the exchange mode accepting 2-oxoglutarate as count ertransported substrate. In vivo, urate uptake thereby would be a tert iary active system driven by a 2-oxoglutarate gradient established acr oss the cell membrane by the operation of a Na+-2-oxoglutarate cotrans port system.