Oral L-carnitine supplementation is commonly used in sports nutrition and in medicine; however, there is controversy regarding the mechanisms that mediate intestinal L-carnitine transport. We have previously reported that the Na+/L-carnitine transporter OCTN2 is present in the small intestinal apical membrane. Herein we aimed to find out if this step of intestinal L-carnitine absorption is ontogenically regulated, and if so, to determine the molecular mechanism(s) involved. L-[3H]-Carnitine uptake was measured in the jejunum and ileum of fetuses (E17 and E21), newborn (1 day-old), suckling (15 day-old), weaning (1 month-old) and adult (2 and 6 month-old) Wistar rats. Both, Na+-dependent and Na+-independent L-carnitine uptake rates, normalized to intestinal weight, significantly increased during the late gestation period, and then declined during the suckling period. After weaning, the rate of Na+-dependent L-carnitine uptake is no longer measurable. In E21- fetuses and newborn rats, L-carnitine uptake was higher in the ileum than in the jejunum. The decline in Na+-dependent L-carnitine uptake with maturation was mediated via a decrease in the V max of the uptake process with no change in its apparent K m. Semi-quantitative RT-PCR assays showed that OCTN2 mRNA levels were significantly higher in E21-fetuses and newborn rats compared to suckling rats, which were in turn significantly higher than that in adult rats. Neither retardation of weaning nor L-carnitine supplementation prevented the down-regulation of Na+/L-carnitine transport activity. The results demonstrate for the first time that intestinal Na+-dependent L-carnitine uptake activity is under genetic regulation at the transcriptional level.
The Journal of Membrane Biology – Springer Journals
Published: Jan 1, 2005
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