Journal of Lipid Research, Vol 31, 747-752, Copyright © 1990 by Lipid Research, Inc.
Tracer kinetic studies of the low density lipoprotein metabolism in the fetal rat: an example for estimation of flux rates in the nonsteady state
D Plonne, B Schlag, L Winkler and R Dargel
Institute of Pathological Biochemistry, Friedrich Schiller University, Jena, GDR.
To get insight into the low density lipoprotein (LDL)-apoB flux in the rat
fetus near term and in the early postnatal period, homologous apoE- free
125I-labeled LDL was injected into the umbilical vein of the rat fetus
immediately after Caesarean section. Since the serum LDL-apoB spontaneously
declined after birth, a time-dependent two-pool model was used to calculate
the flux rates in the neonate from the specific activities of LDL-apoB up
to 15 h post partum. An approximate value of LDL-apoB flux in the fetus at
birth was obtained by extrapolation of the kinetic data to the time of
injection of the tracer. The data revealed that the turnover of LDL-apoB in
the fetus (18.6 micrograms LDL-apoB/h per g body weight) exceeded that in
the adult rat (0.4 microgram/h per g body weight) by at least one order of
magnitude. Even 15 h after delivery, the LDL-apoB influx amounted to 2.5
micrograms/h per g body weight. The fractional catabolic rate of LDL-apoB
in the fetus at term (0.39, h-1) slightly exceeded that in the adult animal
(0.15, h-1) and reached the adult level within the first 3 h after birth
and remained constant thereafter. In the rat fetus, LDL-apoB flux greatly
exceeds that of VLDL-apoB. The data support the view of a direct synthesis
and secretion of LDL, most probably by the fetal membranes.
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