Cells in the hypertonic renal medulla maintain their intracellular ion concentration at isotonic levels, despite much higher concentrations of extracellular electrolytes, by accumulating high concentrations of nonperturbing small organic solutes termed osmolytes. Taurine has been identified as a nonperturbing osmolyte in the renal medulla and Madin-Darby canine kidney (MDCK) cells. In hypertonic medium, the increased accumulation of taurine in MDCK cells is the result of increased activity of a Na+-and Cl1-dependent taurine transporter. We have isolated a cDNA encoding a Na+-and Cl-dependent taurine transporter, whose sequence corresponds to a protein of 655 amino acids with igncant amino acid sequence similarity to previously cloned Na+-and Cldependent transporters, incuding the MDCK cell betaine/-aminobutyric acid transporter and several brain neurotransmitter transporters. Northern hybridization indicates that mRNA for the taurine transporter is present in renal cortex and medulla, ileal mucosa, brain, liver, and heart. The abndce of mRNA for the taurine transporter is increased in MDCK cells cultured in hypertonic medium, suggesting that regulation of transport activity by medium hypertonicity occurs at the level of mRNA accumulation. imum velocity (V.,.) of the taurine transporter in the basolateral plasma membrane without change in Km (4).Taurine transport in renal brush border membranes has also been well characterized (5). The activity of the cotransporter in the brush border of the proximal tubule contributes to whole-body homeostasis of taurine; activity increases in animals fed diets deficient in taurine and in sulfur-containing amino acids (6). The addition of 50 pAM taurine to taurine-free medium results in a decrease in the activity of the taurine transporter in MDCK cells (4,7). However, the rate of taurine transport was up-regulated by hypertonicity to the same degree as in cells cultured in taurine-free medium, suggesting that the regulation of taurine transport by hypertonicity and the regulation by medium taurine are independent of each other (4).In this report we describe the cloning of the cDNA for the MDCK cell taurine transporter. The sequence of the cDNAt indicates that the taurine transporter has considerable amino acid sequence similarity to the previously cloned Na+-and Cl--dependent transporters. Northern hybridizations indicate that the abundance of mRNA for the taurine transporter in MDCK cells is regulated by hypertonicity.Taurine (2-aminoethanesulfonic acid) is a major intracellular amino acid in mammals (1). It is involved in a number of important physiological processes, including bile acid conjugation in hepatocytes, modulation of calcium flux and neural excitability, osmoregulation, detoxification, and membrane stabilization (1).The cells of virtually all organisms respond to hypertonicity by the intracellular accumulation ofhigh concentrations of small organic solutes (osmolytes) that, in contrast to high concentrations of electrolytes, do not perturb the function of ...

Myo-inositol and glycinebetaine are compatible osmolytes accumulated in the renal medulla and in MDCK cells cultured in hypertonic media. Both osmolytes are taken up by MDCK cells on Na-coupled transporters. The maximal velocity (Vmax) of both cotransporters is increased by culture in hypertonic medium. When hypertonic MDCK cells are shifted to isotonic medium there is a large transient efflux of osmolytes. To determine the polarity of the cotransporters and the transient efflux, we grew MDCK cells on a porous support to assay transport separately at their apical and basolateral surfaces. In hypertonic cells, basolateral uptake of both osmolytes was 1) more than 10-fold apical uptake, 2) greater than 96% Na dependent, 3) 25- (myo-inositol) and 16-fold (glycinebetaine) uptake in isotonic cells, reaching a maximum 24 h after the switch to hypertonic medium. When medium osmolarity was decreased from hypertonic to isotonic, myo-inositol uptake reversed to the isotonic level within 1 day; glycinebetaine uptake decreased more slowly. When medium osmolarity was decreased from hypertonic to isotonic, there was a large transient increase in basolateral efflux of both osmolytes.

Myo-inositol is a major compatible osmolyte accumulated in the hypertonic renal medulla and in Madin-Darby canine kidney (MDCK) cells cultured in hypertonic media. Myo-inositol is taken up by MDCK cells on a Na(+)-coupled transporter whose activity increases sixfold 24 h after cells are switched to hypertonic medium. To investigate the mechanism of regulation of the cotransporter by hypertonicity, we used the cDNA encoding the canine Na(+)-myo-inositol cotransporter that we recently cloned to measure the abundance of the mRNA for the cotransporter and its rate of transcription after changes in osmolality. When MDCK cells were switched from isotonic to hypertonic medium, cotransporter mRNA abundance rose 10-fold in 16 h. Transcription of the cotransporter gene also rose and 16 h after the switch reached a peak approximately 15-fold that in isotonic cells. When cells were switched back to isotonic medium, mRNA abundance and transcription of the gene returned to isotonic levels in 8 h and transport rate reached isotonic levels in 48 h. Thus transcription appears to be the primary step in regulation of myo-inositol transport by hypertonicity.