Demonstration of a Na(+)-dicarboxylate cotransporter in bovine adrenocortical cells

Localization of the calcium-regulated citrate transport process in proximal tubule cells

Urinary citrate is an important inhibitor of calcium-stone formation. Most of the citrate reabsorption in the proximal tubule is thought to occur via a dicarboxylate transporter NaDC1 located in the apical membrane. OK cells, an established opossum kidney proximal tubule cell line, transport citrate but the characteristics change with extracellular calcium such that low calcium solutions stimulate total citrate transport as well as increase the apparent affinity for transport. The present studies address several fundamental properties of this novel process: the polarity of the transport process, the location of the calcium-sensitivity and whether NaDC1 is present in OK cells. OK cells grown on permeable supports exhibited apical >basolateral citrate transport. Apical transport of both citrate and succinate was sensitive to extracellular calcium whereas basolateral transport was not. Apical calcium, rather than basolateral, was the predominant determinant of changes in transport. Also 2,3-dimethylsuccinate, previously identified as an inhibitor of basolateral dicarboxylate transport, inhibited apical citrate uptake. Although the calcium-sensitive transport process in OK cells is functionally not typical NaDC1, NaDC1 is present in OK cells by Western blot and PCR. By immunolocalization studies, NaDC1 was predominantly located in discrete apical membrane or subapical areas. However, by biotinylation, apical NaDC1 decreases in the apical membrane with lowering calcium. In sum, OK cells express a calcium-sensitive/regulated dicarboxylate process at the apical membrane which responds to variations in apical calcium. Despite the functional differences of this process compared to NaDC1, NaDC1 is present in these cells, but predominantly in subapical vesicles.

Relationship between aging and renal high-affinity sodium-dependent dicarboxylate cotransporter-3 expression characterized with antifusion protein antibody

Sodium-dependent dicarboxylate cotransporter (NaDC), which is responsible for the transportation of intermediates of the Krebs cycle, has been implicated in extending the life span of drosophila. In the present study, we cloned an intracellular domain segment of human kidney NaDC-3, which is 75% identical to that of the rat, constructed a polyclonal antibody against fusion protein of glutathione S-transferase (GST)-NaDC-3, and detected its renal expression changes with aging in both Wistar rats and normal humans. Western blot and immunohistochemistry confirmed the specificity of the antibody, and its location was found to be on the basolateral membrane of the renal proximal tubule. In addition, Western and Northern blots showed that NaDC-3 in kidneys significantly increased with age in Wistar rats. In healthy humans, renal NaDC-3 abundance also increased with age. Our results demonstrated that NaDC-3 expression was increased in aged Wistar rats and aged people, indicating that NaDC-3 may have a role in the process of kidney aging.