Localization of organic cation transporters OCT1 and OCT2 in rat kidney.

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.