Transport of L-carnitine in human corneal and conjunctival epithelial cells.
OBJECTIVE
Previously we demonstrated expression and localization of carnitine/organic cation transporters, OCTN1 and OCTN2, in human corneal and conjunctival epithelia. The present study aimed to examine the characteristics of L-carnitine transporters in cultured human limbal corneal (HCLE) and conjunctival epithelial (HCjE) cells.


METHODS
Time-course, Na(+)-dependence, kinetics, energy- and pH- dependence of L-carnitine transport were investigated by monitoring L-[(3)H]carnitine uptake into HCLE and HCjE cells. To determine the specificity of action, competition and inhibition studies were performed.


RESULTS
The uptake of L-carnitine into HCLE and HCjE cells was saturable and time-dependent. An Eadie-Hofstee plot showed two distinct components: a high- and a low- affinity carnitine transport system in HCLE and/or HCjE cells. L-carnitine transport was significantly inhibited by the metabolic inhibitors (sodium azide, dinitrophenol, iodoacetic acid). The L-carnitine analogs (D-carnitine, acetyl-L-carnitine and γ-butyrobetaine), tetraethylammonium (TEA), 2-amino-2-norbornane carboxylic acid (BCH), strongly inhibited uptake of L-[(3)H]carnitine. Uptake of L-[(3)H]carnitine also required the presence of Na(+) in the external medium and the uptake activity was maximal at pH 5.5. The anti-OCTN2 antibody blocked L-carnitine uptake in both HCLE and HCjE cells whereas the anti-OCTN1 antibody did not significantly block L-carnitine uptake.


CONCLUSIONS
L-carnitine is transported into HCLE and HCjE cells by an active carrier mediated transport system that is time-, Na(+)-, energy- and pH- dependent. The carnitine/organic cation transporter OCTN2 appears to play a dominant role in this process.
Brien Holden Vision Institute, The University of New South Wales, Sydney, NSW, Australia.
Shunjiang
S
Xu
Judith L
JL
Flanagan
Peter A
PA
Simmons
Joseph
J
Vehige
Mark D
MD
Willcox
Qian
Q
Garrett
D026905:Amino Acid Transport Systems / Q000235:genetics / Q000378:metabolism; D000906:Antibodies / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002331:Carnitine / Q000031:analogs & derivatives / Q000378:metabolism*; D002412:Cations / Q000494:pharmacology; D002460:Cell Line; D003228:Conjunctiva / Q000166:cytology*; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism*; D019573:Epithelium, Corneal / Q000166:cytology*; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D007700:Kinetics; D009636:Norbornanes / Q000494:pharmacology; D027701:Organic Cation Transport Proteins / Q000276:immunology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology; D000074058:Solute Carrier Family 22 Member 5; D027981:Symporters; D013696:Temperature; D013997:Time Factors
D026905:Amino Acid Transport Systems; D000906:Antibodies; D002412:Cations; D009636:Norbornanes; D027701:Organic Cation Transport Proteins; D012333:RNA, Messenger; C109807:SLC22A4 protein, human; C492020:SLC22A5 protein, human; D000074058:Solute Carrier Family 22 Member 5; D027981:Symporters; D012964:Sodium; D002331:Carnitine