Involvement of OCTN2 in the transport of acetyl-L-carnitine across the inner blood-retinal barrier.
OBJECTIVE
To elucidate the mechanisms of acetyl-L-carnitine transport across the inner blood-retinal barrier (inner BRB).


METHODS
In vivo integration plot and retinal uptake index (RUI) analyses were used to examine acetyl-L-[(3)H]carnitine transport in the retina across the inner BRB in rats. RUI was determined from the ratio of acetyl-L-[(3)H]carnitine and [(14)C]n-butanol, a freely diffusible internal reference, in the retina divided by the same ratio in the solution injected in the carotid artery. The transport mechanism was characterized in a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells), as an in vitro inner BRB model.


RESULTS
The apparent influx permeability clearance (K(in)) per gram retina of acetyl-L-[(3)H]carnitine was found to be 2.31 microL/(minute . g retina). The K(in) of acetyl-L-[(3)H]carnitine was 3.7-fold greater than that of [(3)H]D-mannitol, a nonpermeable paracellular marker. Acetyl-L-[(3)H]carnitine uptake by the retina was found to be significantly inhibited by L-carnitine and acetyl-L-carnitine, supporting a carrier-mediated influx transport of acetyl-L-carnitine at the inner BRB. L-[(3)H]carnitine and acetyl-L-[(3)H]carnitine uptake by TR-iBRB2 cells was Na(+)- and concentration-dependent, with a K(m) of 29 and 26 microM, respectively. These forms of transport were significantly inhibited by organic cation/carnitine transporter (OCTN) substrates and inhibitors such as L-carnitine and acetyl-L-carnitine, tetraethylammonium, quinidine, and betaine. These transport properties are consistent with those of carnitine transport by OCTN2. OCTN2 was predominantly expressed in TR-iBRB2 cells and isolated rat retinal vascular endothelial cells.


CONCLUSIONS
The findings suggest that OCTN2 is involved in the transport of acetyl-L-carnitine from the circulating blood to the retina across the inner BRB.
Tachikawa
Masanori
M
Department of Pharmaceutics, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Toyama, Japan.
Takeda
Yoko
Y
Tomi
Masatoshi
M
Hosoya
Ken-ichi
K
D000108:Acetylcarnitine / Q000378:metabolism*; D026962:Amino Acid Transport System ASC / Q000502:physiology; D000818:Animals; D001692:Biological Transport; D001813:Blood-Retinal Barrier / Q000378:metabolism*; D002331:Carnitine / Q000378:metabolism; D002352:Carrier Proteins / Q000502:physiology; D002460:Cell Line; D004730:Endothelium, Vascular / Q000378:metabolism*; D008297:Male; D008565:Membrane Proteins / Q000502:physiology; D050479:Neurotransmitter Transport Proteins; D027701:Organic Cation Transport Proteins / Q000502:physiology*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012160:Retina / Q000378:metabolism*; D012171:Retinal Vessels / Q000166:cytology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000074058:Solute Carrier Family 22 Member 5; D000070590:Solute Carrier Proteins; D027981:Symporters
D026962:Amino Acid Transport System ASC; D002352:Carrier Proteins; D008565:Membrane Proteins; D050479:Neurotransmitter Transport Proteins; D027701:Organic Cation Transport Proteins; D012333:RNA, Messenger; C000705567:Slc22a4 protein, rat; C115441:Slc22a5 protein, rat; C515774:Slc6a14 protein, rat; D000074058:Solute Carrier Family 22 Member 5; D000070590:Solute Carrier Proteins; D027981:Symporters; D000108:Acetylcarnitine; D002331:Carnitine