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 K-m values for L-carnitine of 66 mu M for the endogenous and 149 mu M 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.