Fractional contributions to total carnitine in the neonatal rat

This study was designed to determine the fraction of tissue total carnitine derived from the mother in utero, from milk, and from fetal and neonatal de novo synthesis. Female rats were offered drinking water ad libitum containing either tritium or [14C]-labeled carnitine (0.1 mCi/mmol, 5 mM). The rats were mated, and after birth, the pups of females given the tritium-labeled water were switched with the pups of rats given the [14C]-labeled carnitine (cross-fostering). Analyses of neonatal tissue total carnitine in five tissues were determined on d 0, 1, 2, 5, 10, and 20 after birth. Carnitine derived from neonatal de novo synthesis remained relatively constant from birth throughout the 20 d period, ranging from 26% in liver to 50% in the heart at d 0. The contribution from milk carnitine rose rapidly after birth until d 5, at which time it became relatively constant, ranging from 40.8% in heart to 55.4% in kidney at d 20. The fetus is a major contributor to its own tissue carnitine. Nevertheless, the infant rat is dependent upon external sources of carnitine in order to maintain tissue carnitine levels.