Properties and Regulation of Organic Cation Transport in Freshly Isolated Human Proximal Tubules

Abstract

The kidney, and more specifically the proximal tubule, is the main site of elimination of cationic endogenous metabolites and xenobiotics. Although numerous studies exist on renal organic cation transport of rat and rabbit, no information is available from humans. Therefore, we examined organic cation transport and its regulation across the basolateral membrane of isolated human proximal tubules. mRNA for the cation transporters hOCT1 and hOCT2 as well as hOCTN1 and hOCTN2 was detected in these tubules. ؉ uptake by 29 ؎ 3% (n ‫؍‬ 10). hANP (10 nM) or 8-bromo-cGMP (100 M) also decreased ASP ؉ uptake by 17 ؎ 3 (n ‫؍‬ 9) or 32 ؎ 5% (n ‫؍‬ 10), respectively. We show for the first time that organic cation transport across the basolateral membrane of isolated human proximal tubules, most likely mediated via hOCT2, is electrogenic and regulated by protein kinase C, the cAMP-and the cGMP-dependent protein kinases.The proximal tubule is the site of secretion and reabsorption of endogenous metabolites and xenobiotics in the kidney. Many of these substances are organic cations. As several drugs are among these organic cations, specific knowledge about properties of organic cation transport in the human proximal tubule is of great importance. The first organic cation transporter was cloned from rat (rOCT1) in 1994 (1). The first human organic cation transporters (hOCT1 and hOCT2) were cloned 3 years later (2, 3), and three other members of this family (hOCTN1, hOCTN2, and hOCT3, originally named EMT) followed (4 -6). Previous functional studies (7, 8) on organic cation transport in the proximal tubule of the rat showed the differences between transport across the luminal or basolateral membrane. After cloning of the transporters data from functional investigations of these transporters together with those obtained by microperfusion experiments and transport studies with membrane vesicles (8 -12) led to a model of organic cation transport in the proximal tubule. OCTN1 was shown to be a H ϩ /organic cation exchanger (4, 13), whereas OCT1 has been characterized as a functionally different potential driven uniporter. Immunohistochemistry localized rOCT1 and rOCT2 to the basolateral membrane of proximal tubules (14 -16). The OCTN1 is most likely located in the luminal membrane of proximal tubules (17, 18). Thus, organic cation transporters expressed in the luminal and basolateral membranes of the proximal tubule are molecularly and functionally very different members of this protein family.After cloning of the first transporters, studies were performed to gain information on properties of these organic cation transporters expressed in Xenopus laevis oocytes or cell lines (19 -21). From these studies we have information on their electrogeneity, substrate specificities, and inhibitors with K m and K i values for the distinct cation transporters from rat and man. The homologous transporters of these two species differ in K m and K i values for the investigated cations (19 -24). Properties found for the cloned rOCT1 and rOCT2 do not...

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