EFFECT OF DEVELOPMENT AND NUTRITIONAL STATE ON THE UPTAKE, METABOLISMAND RELEASE OF FREE AND ACETYL-L-CARNITINE BY THE RODENT SMALL-INTESTINE

Intestinal carnitine levels and the incorporation and release of exoge nous, [C-14]carnitine were compared in intestine from adult rat and gu inea pig. Total carnitine levels were 4-fold higher in rat as compared to guinea pig intestine. Retention of label was also 4-fold greater, 4 h after placing carnitine (7 nmol) in the lumen. Carnitine was detec ted in rat chow (64 nmol/g) but not in guinea pig chow. Intestinal car nitine was reduced 2-fold in rats fed a carnitine-free diet for 2 week s, suggesting the importance of dietary carnitine in determining intes tinal carnitine levels. Two conditions where fatty acid oxidation is i ncreased (fasting and suckling) resulted in elevated carnitine levels and retention. In the 3-day fasted guinea pig, intestinal carnitine in creased by 40% and retention of a lumenal dose of [C-14]carnitine incr eased about 7-fold after 4 h. During suckling, carnitine levels peaked after 3 days (792 nmol/g) and decreased to near adult levels after 7 days (108 nmol/g). Retention of a lumenal dose of carnitine was greate r after 4 h in 1-day old neonatal, than in adult intestine (82% vs. 7% of a 7 nmol dose, respectively). This reflects, in part, the larger i ntestinal carnitine pool on day 1 (352 nmol/g) than on day 29 (91 nmol /g). The calculated efflux of total intestinal carnitine after 4 h was similar for adults and neonates (72 vs. 58 nmol/g) suggesting that ef flux relative to pool size was greater in the adult than in the neonat e. Uptake of [C-14]acetylcarnitine was similar to [C-14]carnitine in 1 -day old animals, but was retained to a lesser extent (36% vs. 82%, re spectively) after 4 h. The calculated efflux of total intestinal carni tine when acetylcarnitine was the substrate was about 4-fold that when carnitine was the substrate. Incorporation of [C-14]carnitine into en terocytes isolated from 3-day old animals was 4-fold greater than into enterocytes isolated from adults (152 vs. 36 pmol/mg protein after 60 min). Active transport of carnitine into enterocytes from neonates, b ut not from adults is suggested, since labeled free intracellular carn itine reached 4-fold the calculated equilibrium value in neonatal ente rocytes, but did not exceed the equilibrium value in adult enterocytes .