Urinary citrate, which is regulated by proximal tubule reabsorption, is the most important endogenous inhibitor of calcium stone formation. In previous studies, we established the OK (opossum kidney) proximal tubule cell line as the only available continuous cell line expressing measurable citrate transport. An unusual feature in OK cells is calcium inhibition of citrate transport; although this calcium sensitivity parallels changes in urinary citrate with urinary calcium excretion in vivo, this calcium sensitivity has not been described for NaDC-1, the sodium-dependent dicarboxylate transporter thought to be responsible for citrate reabsorption. The present studies examined citrate transport by several species of NaDC-1 in two models, Xenopus oocytes and CUBS, which are HRPE (human retinal pigmented epithelial) cells that have been stably transfected with human NaDC-1. In oocytes expressing opossum NaDC-1, citrate uptake was increased significantly in calcium-free media (282 ± 35 to 705 ± 125 pmole/oocyte/hour); however, succinate uptake was decreased significantly (60.7 ± 5.8 %). (Succinate is used as a test substrate for the dicarboxylate transporter since its ionic concentration is not pH sensitive in the physiologic range and since it is not complexed significantly by calcium.) With rabbit NaDC-1 in oocytes, succinate uptake again decreased in calcium free media (243 ± 17 to 159 ± 15 pmole/oocyte/hour), but there was no significant change in citrate uptake (29 ± 2 to 33 ± 4 pmole/oocyte/hour). In CUBS cells expressing human NaDC-1, neither citrate uptake nor succinate uptake was sensitive to calcium. In addition, in each of these models the affinity for citrate was in the 1-2 mM range in contrast to OK cells in which the affinity for citrate and succinate is in the micromolar range. In sum, these studies suggest that NaDC-1 transport is not sensitive to calcium; therefore the citrate transport system in OK cells is likely a novel calcium sensitive process, which has not been previously described. Based on the similarity of the OK cell calcium sensitivity to in vivo findings, we suggest that citrate transport in vivo may be regulated both by NaDC-1 and another previously unidentified system.
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