Characterization of the endogenous carnitine transport and expression of a rat renal Na(+)-dependent carnitine transport system in Xenopus laevis oocytes.
L-Carnitine transport was characterized in Xenopus laevis oocytes before and after injection of mRNA isolated from rat renal cortex. Non-injected oocytes revealed endogenous Na(+)-dependent transport of L-carnitine. After injection of 15 ng of rat kidney mRNA, the Na(+)-dependent L-carnitine transport increased 2-3-fold, reaching maximal activity after 5-6 days. The expressed carnitine transport was maximal at pH 7.5, whereas the endogenous transport showed no clear maximum between pH 6.0 and 8.5. Kinetic analysis revealed apparent Km values for L-carnitine of 66 microM for the endogenous and 149 microM for the expressed transport. Trimethyl-lysine and D-carnitine inhibited both the endogenous and the expressed transport. In contrast, L-acetylcarnitine, L-isovalerylcarnitine, L-palmitoylcarnitine and butyrobetaine inhibited predominantly the expressed transport, whereas glycinebetaine had no inhibitory effect on either transport system. Size-fractionated rat renal-cortex mRNA (median size 2 kb) induced a 3-fold higher L-carnitine transport than did unfractionated mRNA. These studies demonstrate that Xenopus laevis oocytes exhibit Na(+)-dependent L-carnitine transport and provide the basis for expression-cloning of a rat renal Na(+)-dependent L-carnitine transport system.
Berardi
S
S
Department of Internal Medicine, University Hospital Zürich, Switzerland.
Hagenbuch
B
B
Carafoli
E
E
Krähenbühl
S
S
D000818:Animals; D001692:Biological Transport; D002331:Carnitine / Q000235:genetics / Q000378:metabolism*; D007672:Kidney Cortex / Q000378:metabolism*; D008297:Male; D009865:Oocytes / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism; D014982:Xenopus laevis
D012333:RNA, Messenger; D012964:Sodium; D002331:Carnitine