Fractional absorption of L-carnitine after oral administration in rats: evaluation of absorption site and dose dependency

We evaluated the fractional absorption of L-carnitine, a gamma-amino acid essential cofactor for the transfer of long-chain fatty acids, in rats in vivo after oral administration to determine its absorption behavior. At both low (0.05 micromol/rat) and high (100 micromol/rat) doses, L-carnitine was recovered only from the region of the cecum and below at 10 h after administration. During a major shift in distribution from cecum at 10 h to feces at 24 h, there was no significant change in the total recovery at each dose, suggesting that L-carnitine absorption is negligible in the cecum and the large intestine (colon and rectum). However, the recovery of L-carnitine was incomplete and the fraction recovered was larger at the high dose than at the low dose. The fractions absorbed were estimated to be 96.7 and 33.0% for the low and high doses, respectively, as these were the fractions that disappeared from the gastrointestinal tract. These values were comparable with 100 and 42%, respectively, of bioavailability values by the pharmacokinetic analysis of plasma concentration data in our preceding study [Matsuda et al., Biopharmaceutics & Drug Disposition, in press]. These results suggest that L-carnitine is significantly absorbed only in the small intestine, without undergoing first-pass degradation, and in a dose-dependent manner presumably due to the involvement of saturable transport by L-carnitine carriers. Consistent with the suggestions in vivo, L-carnitine absorption in the closed intestinal loop in situ was concentration-dependent in the small intestine but not in the large intestine, and the apparent membrane permeability in the large intestine was smaller by an order of magnitude than that of passive transport in the small intestine. These findings support our preceding kinetic modeling strategy assuming the small intestine to be the sole absorption site, and should be of help in guiding studies on development of more efficient oral L-carnitine delivery strategies.