Renal excretion and reabsorption of organic cations are mediated by electrogenic and electroneutral organic cation transporters, which belong to a recently discovered family of polyspecific transporters. These transporters are electrogenic and exhibit differences in substrate specificity. In rat, the renal expression of the polyspecific cation transporters rOCT1 and rOCT2 was investigated. By in situ hybridization, significant amounts of both rOCT1 and rOCT2 mRNA were detected in S1, S2, and S3 segments of proximal tubules. By immunohistochemistry, expression of the rOCT1 protein was mainly observed in S1 and S2 segments of proximal tubules, with lower expression levels in the S3 segments. At variance, rOCT2 protein was mainly expressed in the S2 and S3 segments. Both transporters were localized to the basolateral cell membrane. Neither rOCT1 nor rOCT2 was detected in the vasculature, the glomeruli, and nephron segments other than proximal tubules. The data suggest that rOCT1 and rOCT2 are responsible for basolateral cation uptake in the proximal tubule, which represents the first step in cation secretion.

In a systematic approach to the study of Saccharomyces cerevisiae genes of unknown function, 150 deletion mutants were constructed (1 double, 149 single mutants) and phenotypically analysed. Twenty percent of all genes examined were essential. The viable deletion mutants were subjected to 20 different test systems, ranging from high throughput to highly specific test systems. Phenotypes were obtained for two-thirds of the mutants tested. During the course of this investigation, mutants for 26 of the genes were described by others. For 18 of these the reported data were in accordance with our results. Surprisingly, for seven genes, additional, unexpected phenotypes were found in our tests. This suggests that the type of analysis presented here provides a more complete description of gene function.

tion of thiazide-sensitive Na ϩ -Cl Ϫ cotransport and associated gene products in mouse DCT. Am J Physiol Renal Physiol 281: F1028-F1035, 2001. First published August 8, 2001; 10.1152/ ajprenal.00148.2001.-The mammalian distal nephron develops a complex assembly of specialized cell types to accomplish the fine adjustment of urinary electrolyte composition. The epithelia of the distal convoluted tubule (DCT), the connecting tubule (CNT), and the cortical collecting duct (CCD) show an axial structural heterogeneity that has been functionally elucidated by the localization of proteins involved in transepithelial ion transport. We compared the distribution of the thiazide-sensitive Na ϩ -Cl Ϫ cotransporter (TSC), basolateral Na ϩ /Ca 2ϩ exchanger (Na/Ca), cytosolic calcium-binding proteins calbindin D28K and parvalbumin, and the key enzyme for selective aldosterone actions, 11␤-hydroxysteroid-dehydrogenase 2 (11HSD2), in the distal convolutions of the mouse. In the mouse, as opposed to the rat, we found no clear subsegmentation of the DCT into a proximal (DCT1) and a distal (DCT2) portion. The TSC was expressed along the entire DCT. Na/Ca and calbindin D28K were similarly expressed along most of the DCT, with minor exceptions in the initial portion of the DCT. Both were also present in the CNT. Parvalbumin was found in the entire DCT, with an occasional absence from short end portions of the DCT, and was not present in CNT. 11HSD2 was predominantly located in the CNT and CCD. Short end portions of DCT only occasionally showed the 11HSD2 signal. We also observed an overlap of 11HSD2 immunoreactivity and mRNA staining. Our observations will have implications in understanding the physiological effects of gene disruption and targeting experiments in the mouse. distal convoluted tubule; connecting tubule; sodium-calcium exchanger; calbindin; parvalbumin; 11␤-hydroxysteroid-dehydrogenase type 2 MAMMALIAN KIDNEYS ARE CAPABLE of adapting urinary sodium and chloride excretion according to intake across a wide range of salt intakes with high precision. The fine adjustments take place in the distal nephron. Successive portions of the renal tubule are formed to adapt to this function, and an axial heterogeneity of the distal segments has been defined (11, 13). In the cortex, the thick ascending limb of Henle's loop (TAL), the distal convoluted tubule (DCT), the connecting tubule (CNT), and the collecting duct (CCD) have been identified. The specific transport properties of these epithelia are accomplished by the expression of proteins representing cotransporters, exchangers, and ion channels. These proteins govern ion movement from one side of the cell to the other. The distribution, ontogeny, and functional aspects of these proteins in the mammalian distal nephron have been reviewed elsewhere (2, 23). In the rat, the electroneutral cationchloride cotransporter (NaϪ ) has been localized to the TAL (11). The thiazide-sensitive Na ϩ -Cl Ϫ cotransporter (TSC; also termed NCC) has been localized to the more proximal (DCT1) and di...

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