Selective substrates for non-neuronal monoamine transporters

Area of Science:

• Pharmacology and Molecular Biology
• Membrane Transport Proteins
• Biochemistry

Background:

• Organic cation transporter 1 (OCT1), OCT2, and extraneuronal monoamine transporter (EMT) are identified as non-neuronal monoamine transporters.
• These transporters accept dopamine, noradrenaline, adrenaline, and 5-hydroxytryptamine as substrates.
• Functional discrimination between these transporters is needed to understand their specific roles.

Purpose of the Study:

• To identify selective transport substrates for OCT1, OCT2, and EMT.
• To functionally discriminate between these non-neuronal monoamine transporters.
• To challenge the concept of polyspecificity in organic cation transporters.

Main Methods:

• Uptake experiments were conducted using stably transfected 293 cell lines, each expressing a single transporter (OCT1, OCT2, or EMT).
• Radiolabeled substrates were used to measure and compare normalized uptake rates.
• Selective substrates including tetraethylammonium, choline, histamine, guanidine, cimetidine, and creatinine were tested.

Main Results:

• Tetraethylammonium is a substrate for OCT1 and OCT2, but not EMT.
• Choline is exclusively transported by OCT1.
• Histamine is transported by OCT2 and EMT, but not OCT1.
• Guanidine, cimetidine, and creatinine are excellent substrates for OCT2, with varying transport rates by OCT1 and EMT.
• These findings reveal distinct substrate recognition and turnover patterns for each transporter.

Conclusions:

• Selective substrates allow for functional discrimination of OCT1, OCT2, and EMT.
• The identified substrate specificities challenge the notion of 'polyspecific' organic cation transporters.
• OCT2 likely corresponds to the organic cation/H+ antiport mechanism in renal brush-border membranes.
• EMT likely corresponds to the guanidine/H+ antiport mechanism in placental and intestinal membranes.