Absorption of d- and l-carnitine by the intestine and kidney tubule in the rat

The process by which l- and d-carnitine are absorbed was investigated using the live rat and the isolated vascularly perfused intestine. A lumenal dose of 2–6 nmol in the perfused intestine resulted in less than 5% transport of either isomer to the perfusate in 30 min. The l-isomer was taken up by the intestinal tissue about twice as rapidly as the d-isomer by both the perfused intestine (52.8% and 21.6%, respectively) and the live animal (80% and 50%, respectively) in 30 min. After 1 h 90% of the l-carnitine had accumulated in the intestinal tissue and was released to the circulation over the next several hours. Accumulation of d-carnitine reached a maximum of 80% in 2 h and release to the circulations was similar to that of l-carnitine. Uptake of both l-[14Clearnitine and acetyl-l-[14Clearnitine was more rapid in the upper jejunal segment than in other portions of the small intestine. Acetylation occurred in all segments, resulting in nearly 50% conversion to this derivative in 5 min. Increasing the dose of l-carnitine reduced the percent acetylation. The uptake of both isomers was a saturable process and high concentrations of d-carnitine, acetyl-l-carnitine and trimethylaminobutyrate inhibited l-carnitine uptake. In the live animal after 5 h, the distribution of isotope from l-[14C]earnitine and d-[3H]earnitine differed primarily in the muscle where 29.5% of the l-carnitine and 5.3% of the d-carnitine was found and in the urine where 2.9% of the l-carnitine and 7.1% of the d-carnitine was found. The renal threshold for l-carnitine was 80 μM and for d-carnitine 30 μM, in the isolated perfused kidney. Approx. 40% of the l-carnitine but none of the d-carnitine excreted in the urine was acetylated. l-Carnitine and d-carnitine competed for tubular reabsorption.