Uptake of L-carnitine by rat jejunal brush border microvillous membrane vesicles. Evidence of passive diffusion.

We have previously described apparent active transport of carnitine into rat intestinal mucosa with intracellular accumulation against a concentration gradient in a process dependent upon the presence of sodium ions, oxygen, and energy. In the work described here, we sought to define the interaction between carnitine and the brush border membrane, which we presumed contained the transport mechanism. Using isolated rat jejunal brush border microvillous membrane vesicles, we found evidence of passive diffusion alone. We found no evidence of carrier-mediated transport--in particular no saturation over a concentration range, inhibition by structural analogs, transstimulation phenomenon, and no influence of sodium ions, potential difference or proton gradients. We conclude that a carnitine transporter does not exist in the brush border membrane of enterocytes and that other cellular mechanisms are responsible for the apparent active transport observed.