MOLECULAR PHYSIOLOGY IN THE TRANSPORT SYSTEM Proximal Tubular Transport of Organic Compounds Polyspecific cation transporters in the proximal tubule

The homeostasis of endogeneous organic cations, such transported by rOCT1 (unpublished data). OCT1, as choline, monoamine neurotransmitters, nucleosides, OCT2 and OCT3 may also be inhibited by non-charged cationic drugs and cationic xenobiotics, is controlled compounds such as corticosteroids and by anions such by polyspecific cation transporters in small intestine, as probenecid or a-ketoglutarate (unpublished data). liver, kidney, brain and heart [1–3]. In 1994, some of The substrate and inhibitor specificity of OCT1, OCT2 us cloned the polyspecific cation transporter rOCT1 and OCT3 overlaps. For some cations, significant from rat kidney, which proved to be the first member differences in affinity and maximal transport rates of a rapidly growing family of polyspecific transporters between OCT1, OCT2 and OCT3 within identical with 12 predicted a-helical transmembrane domains species have been observed. For example, after expresand a large extracellular loop between the first and sion of rOCT1 and rOCT3 in Xenopus laevis oocytes, second presumed transmembrane domain [4,5]. This apparent m values of 95 mM (rOCT1) and 2.5 mM family includes a subfamily of electrogenic cation (rOCT3) were determined for the uptake of transporters (OCT1, OCT2 and OCT3), a subfamily [14C]tetraethylammonium (TEA) [4,9]. The IC50 containing cation transporters and the sodium carnitvalues for the inhibition of [14C]TEA uptake by procaiine co-transporter (OCTN1 and OCTN2), a subfamily namide were 29 mM and 0.42 mM for rOCT1 and of anion transporters (OAT1, OAT2 and OAT3) and rOCT2, respectively (unpublished data). In different various integral membrane proteins with non-identified species, the affinity of substrates and inhibitors for the functions [5]. OCT2 and OCT3 have ~70 and 50% same transporter subtype may differ significantly. For amino acid identity to rOCT1, respectively; OCTN1/2 example, for the inhibition of [14C]TEA uptake by have ~40% amino acid identity to rOCT1, whereas mepiperphenidol, IC50 values of 0.83 mM (rOCT2) OAT1–3 have ~30% amino acid identity to rOCT1. and 4.8 mM (hOCT2) were determined. Also, the 50 The functional properties of OCT1, OCT2 and OCT3 values for corticosterone inhibition of [14C]TEA from rat and human have been studied in detail uptake mediated by rOCT3 or hOCT3 were 4.9 and [4,6–9]. These transporters are sodium-independent 0.12 mM, respectively [10,11]. The species differences facilitated diffusion systems which may operate in both in substrate affinity of the cation transporters highlights directions [6,7]. Since cation uptake by these transthe limitations and risks of using animal models for porters can be driven by the membrane potential studying the excretion and toxicity of cationic drugs and/or by the chemical cation gradient, under normal during drug development. Another recent observation metabolic conditions, the influx of cations into cells is may be of great biomedical importance. We observed mediated much more effectively than the efflux of that rOCT1 stably expressed in human embryonic cations. However, in depolarized cells, cation efflux kidney (HEK) cells is up-regulated when protein may become important. kinase C is stimulated, and that this up-regulation is OCT1, OCT2 and OCT3 are polyspecific transcombined with an ~10-fold increase in affinity of porters which translocate a variety of organic cations rOCT1 for TEA [12]. with different structures and are inhibited by some To understand the physiological function of the hydrophobic cations which are not transported. organic cation transporters in the kidney, their nephron Inhibitory cations are, for example, quinine, tetrabudistribution and membrane localization must be tylammonium and cyanine 863. However, N-methylknown. Throughout the last 20 years, functional measquinine, which contains a constant positive charge at urements have been performed with microperfused variance to quinine, and tributylmethylammonium are proximal tubules and isolated plasma membrane vesicles, trying to distinguish and characterize the different Correspondence and offprint requests to: H. Koepsell, University of polyspecific cation transport systems [1,2,5]. In Würzburg, Department of Anatomy, Koellikerstrasse 6, D-97070, Würzburg, Germany. Figure 1, transport systems which have been character-