AJP - Cell Physiology, Vol 263, Issue 1 C220-C225, Copyright © 1992 by American Physiological Society
Citrate transport in proximal cell line
D. Law, K. S. Hering-Smith and L. L. Hamm
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110.
Citrate uptake into kidney proximal tubules occurs via an apical
dicarboxylate transporter and a poorly characterized process in the
basolateral membrane. We used OK cells, a cell line derived from opossum
kidney, to study citrate transport in proximal tubule-like cells. Citrate
uptake into cell monolayers was studied using [14C]citrate with
[3H]mannitol as a volume marker. Citrate uptake into these cells was sodium
dependent and saturable with increasing concentrations of citrate. In
contrast to previous models, citrate transport was altered minimally by
changes in pH from 6.2 to 7.0 and increased at pH 7.4 to 7.8. A variety of
di- and tricarboxylates were tested for interaction with citrate transport.
The dicarboxylates succinate, malate, and oxaloacetate at 1 mM
concentration inhibited citrate uptake minimally (uptake at least 80% of
control); one dicarboxylate, alpha-ketoglutarate, did inhibit citrate
uptake significantly. In contrast, the tricarboxylates isocitrate and
tricarballylate inhibited citrate uptake significantly, indicating probable
competitive inhibition with the transport process. These characteristics
are distinctly different from those of the apical membrane dicarboxylate
transporter. 1,2,3-Benzenetricarboxylic acid, an inhibitor of the
mitochondrial tricarboxylate transporter, did not alter citrate uptake. In
conclusion, the OK proximal cell line exhibits a novel citrate transport
process compared with the apical transport of citrate described in most
proximal systems. This transport process probably involves the trivalent
species of citrate in contrast to the usual predominant transport of
divalent citrate. This transport process may represent a process similar to
that in the basolateral membrane of the proximal tubule.
