Carnitine transport in human intestinal biopsy specimens. Demonstration of an active transport system.

Although carnitine is present in a variety of foods, the mechanism of its absorption has not been previously studied in humans. We investigated the absorption of carnitine by studying uptake into human intestinal mucosal biopsy specimens. We found evidence of active transport in the duodenum and ileum, but not in the colon. We demonstrated that intracellular concentrations exceeded concentrations in the incubation media at steady states and that uptake against a concentration gradient was abolished by anoxia and by replacement of sodium ion with potassium. Studies of initial rate of uptake over a range of concentrations revealed a curve consistent with a two-component system: a saturable system with a KT of 558 microM and a linear component probably representing passive diffusion. Addition of D-carnitine and L-acetylcarnitine resulted in diminished uptake of L-carnitine, suggesting that these substrates utilize the same transport mechanism. These studies demonstrate the presence of an active intestinal transport system for L-carnitine in human intestinal mucosa.