Although metformin, a cationic agent for type II diabetes, shows its pharmacological effect in the liver, the drug is mainly eliminated into urine. The tissue selectivity based on the function of drug transporters is unclear. In the present study, the transport of metformin was examined using HEK293 cells transiently transfected with five human renal organic ion transporter cDNAs. Human OCT1 and OCT2, but not OAT1, OAT3 or OCT2-A, stimulated the uptake. A kinetic analysis of metformin transport demonstrated that the amount of plasmid cDNA for transfection was also important parameter to the quantitative elucidation of functional characteristics of transporters, and both human and rat OCT2 had about a 10- and 100-fold greater capacity to transport metformin than did OCT1, respectively. In male rats, the mRNA expression level of rOCT2 in the whole kidneys was 8-fold greater than that of rOCT1 in the whole liver. The in vivo distribution of metformin in rats revealed that the expression level of renal OCT2 was a key factor in the control of the concentrative accumulation of metformin in the kidney. These findings suggest that metformin is a superior substrate for renal OCT2 rather than hepatic OCT1, and renal OCT2 plays a dominant role for metformin pharmacokinetics.

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These results suggest that hOAT3 plays an important role for anionic drug secretion in patients with renal diseases and that the expression levels of drug transporters may be related to the alteration of renal drug secretion.

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Chloride ion has a stimulatory effect on the transport of organic anions across renal basolateral membranes. However, the exact mechanisms at molecular levels have been unclear as of yet. Human organic anion transporters hOAT1 and hOAT3 play important roles in renal basolateral membranes. In this study, the effects of Cl(-) on the activities of these transporters were evaluated by using HEK293 cells stably expressing hOAT1 or hOAT3 (HEK-hOAT1 or HEK-hOAT3). The uptake of p-[(14)C]aminohippurate by HEK-hOAT1 and [(3)H]estrone sulfate by HEK-hOAT3 was greater in the presence of Cl(-) than in the presence of SO(4)(2-) or gluconate. Additionally, the uptake of various compounds by HEK-hOAT1 and HEK-hOAT3 was significantly higher in the Cl(-)-containing medium than the gluconate-containing medium, suggesting that the influences of Cl(-) are not dependent on substrate and that Cl(-) directly stimulates the functions of hOAT1 and hOAT3. The substitution of gluconate with Cl(-) did not change the K(m) value for the uptake of p-[(14)C]aminohippurate by HEK-hOAT1 but caused an approximately threefold increase in the maximal uptake rate (V(max)) value. On the other hand, replacement of gluconate with Cl(-) decreased the K(m) value for the uptake of [(3)H]estrone sulfate and cefotiam by HEK-hOAT3 to about one-third, while it did not change the V(max) value. In summary, Cl(-) upregulates the activities of both hOAT1 and hOAT3, but its effects on transport kinetics differ between these transporters. It was suggested that Cl(-) participates in the trans-location process for hOAT1, and the substrate recognition process for hOAT3.