AJP - Regulatory, Integrative and Comparative Physiology, Vol 269, Issue 2 339-R349, Copyright © 1995 by American Physiological Society
Urate transport in Homarus americanus hepatopancreas: studies on membrane vesicles and R cells
A. T. Nies, R. K. Kinne, E. Kinne-Saffran and M. K. Grieshaber
Universitat Dusseldorf, Institut fur Tierphysiologie, Germany.
[2-14C]urate uptake was studied in hepatopancreatic basolateral membrane
vesicles and in R cell suspensions of the American lobster by Millipore
filtration techniques. Unspecific binding of urate to the vesicular
membrane was 25.5 +/- 3.0% of equilibrium. Vesicular uptake showed a
diffusional and a saturable component (Km) 0.37 +/- 0.04 mM and maximal
velocity (Vmax) 16.5 +/- 1.2 pmol urate.mg protein-1.s-1). [2-14C]urate
uptake was significantly trans-stimulated by urate. Purine analogues,
probenecid, p-aminohippuric acid, pyrazinoic, and oxonic acid cis-inhibited
urate transport. Urate uptake was not affected by Na+ or K+ transmembrane
gradients but stimulated by 1 mM 2-oxoglutarate at the cis-side in
Na(+)-containing media. Cellular urate uptake was inhibited by pyrazinoic
acid. Uptake was saturable (Km 0.53 +/- 0.11 mM and Vmax 3.7 +/- 0.4 pmol
urate.mg protein-1.s-1) and Na(+)-independent. However, 2-oxoglutarate
stimulated uptake in Na(+)-containing media. These results suggest that
urate uptake across the basolateral membrane occurs via a specific,
Na(+)-independent transport system that may operate in the exchange mode
accepting 2-oxoglutarate as countertransported substrate. In vivo, urate
uptake thereby would be a tertiary active system driven by a 2-oxoglutarate
gradient established across the cell membrane by the operation of a
Na(+)-2-oxoglutarate cotransport system.
