Transport of 99mTc-MAG3 via rat renal organic anion transporter 1.
Recently, complementary DNA (cDNA) encoding a p-aminohippurate (PAH) transporter designated rat organic anion transporter 1 (OAT1) was isolated. OAT1, a multispecific organic anion transporter at the basolateral membrane, is exclusively expressed in the middle segment of the proximal tubule in the rat kidney. It has been proposed that OAT1 is indirectly involved in PAH uptake via the Na(+) dicarboxylate cotransporter. In this study, in molecular biologic experiments using OAT1-expressing Xenopus laevis oocytes, we obtained evidence that (99m)Tc-mercaptoacetylglycylglycylglycine (MAG3) is transported via OAT1.


METHODS
Capped OAT1 complementary RNA (cRNA) was synthesized from library plasmid cDNA linearized with BamHI using in vitro transcription. Defolliculated oocytes were injected with 10 ng of OAT1 cRNA. Two to 3 d after injection, uptake of (99m)Tc-MAG3 was measured using ND96 solution containing 18.5 kBq of (99m)Tc-MAG3. Before the uptake experiments, OAT1-expressing oocytes were preincubated for 2 h with 1 mmol/L glutarate (a dicarboxylate), to generate an outwardly directed glutarate gradient. Then, after incubation for 60 min at room temperature, radioactivity of oocytes was determined. For the inhibition experiments, uptake was assessed in the absence or presence of inhibitor: 2 mmol/L of PAH, o-iodohippurate (OIH), probenecid, 3,5-diiodo-4-pyridone-N-acetate (iodopyracet), furosemide, ethacrynic acid, glucoheptonate, maleic acid, L-Tyr, or tetraethylammonium (TEA) or 0.1 mmol/L of 2,4-dinitrophenol (DNP).


RESULTS
Na(+) had a significant effect on (99m)Tc-MAG3 uptake (P < 0.05). Accumulated glutarate stimulated simultaneous (99m)Tc-MAG3 uptake and glutarate excretion (P < 0.001). The following compounds significantly inhibited (99m)Tc-MAG3 uptake: PAH, 8.5% +/- 16.2% of (99m)Tc-MAG3 uptake in the absence of an inhibitor; OIH, 26.4% +/- 21.7%; probenecid, 29.1% +/- 12.4%; iodopyracet, 15.8% +/- 7.9%; furosemide, 30.5% +/- 15.7%; ethacrynic acid, 21.6% +/- 10.6%; glucoheptonate, 35.6% +/- 22.6%; and maleic acid, 60.1% +/- 18.7%. (99m)Tc-MAG3 accumulation in Xenopus laevis oocytes was not significantly inhibited by TEA, L-Tyr, or DNP.


CONCLUSIONS
The following substances had a cis-inhibitory effect on (99m)Tc-MAG3 transport: PAH, OIH, probenecid, iodopyracet, furosemide, ethacrynic acid, and glucoheptonate. Glutarate had a trans-stimulative effect on (99m)Tc-MAG3 transport. (99m)Tc-MAG3 acts as a substrate of OAT1, an organic anion/dicarboxylate exchanger.
Shikano
Naoto
N
Department of Radiological Sciences, Ibaraki Prefectural University of Health Sciences, Ibaraki, Japan. sikano@ipu.ac.jp
Kanai
Yoshikatsu
Y
Kawai
Keiichi
K
Ishikawa
Nobuyoshi
N
Endou
Hitoshi
H
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D005665:Furosemide / Q000494:pharmacology; D007468:Iodopyracet / Q000494:pharmacology; D008657:Metabolic Clearance Rate; D009865:Oocytes / Q000000981:diagnostic imaging / Q000187:drug effects / Q000378:metabolism*; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011339:Probenecid / Q000494:pharmacology; D011865:Radioisotope Dilution Technique; D011877:Radionuclide Imaging; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D017263:Technetium Tc 99m Mertiatide / Q000493:pharmacokinetics*; D014982:Xenopus laevis
D015221:Potassium Channels; D011994:Recombinant Proteins; D017263:Technetium Tc 99m Mertiatide; D005665:Furosemide; D012964:Sodium; D011339:Probenecid; D007468:Iodopyracet