Cholesterol synthesis and accretion within various tissues of the fetal and neonatal rat.

The rate of cholesterol synthesis is reported to be higher in fetal relative to adult rats. Along with the observation that maternal diets high in fat and cholesterol are unable to alter the rate of cholesterol synthesis in the fetus, this has been taken as indirect evidence that the fetal rat meets its cholesterol needs through de novo synthesis. This study quantified the rates of cholesterol synthesis and accumulation in the liver, brain, intestine, and carcass of the fetal and neonatal rat and the placenta to determine whether these developing tissues are able to support their own cholesterol needs without the uptake of plasma lipoprotein cholesterol. The rate of cholesterol synthesis was determined in vivo using [3H]water. The rate of cholesterol accumulation was determined by calculating the difference in tissue cholesterol content between 2 subsequent days of development. Total fetal body cholesterol synthesis was sufficient to support the rate of cholesterol accumulation. Fetal and neonatal liver synthesized cholesterol at a rate in excess of cholesterol accumulation, suggesting hepatic secretion of cholesterol into the plasma. Before the onset of suckling, the rates of de novo cholesterol synthesis in the intestine, brain, and carcass were also sufficient but not higher than the need for cholesterol accretion. After the establishment of suckling, the rate of cholesterol accumulation in the intestine and carcass was in excess of synthesis, suggesting that neonatal tissues derive some of their cholesterol from dietary milk or liver. These studies suggest that the perinatal rat does not require exogenous cholesterol to support tissue cholesterol accretion. However, the fetal liver may support cholesterol accretion in other tissues through rates of synthesis in excess of accumulation and secretion into plasma. The placenta may derive some cholesterol from the maternal and/or fetal plasma.

Hepatic cholesterol and fatty acid synthesis in pregnant and fetal rats: effect of maternal dietary fat and cholestyramine.

Previous studies from this laboratory have demonstrated that 20% rather than 5% (wt/wt) safflower oil or addition of 5% (wt/wt) cholestyramine to the diet of pregnant rats leads to an increase in the activity of 3-hydroxy-3-methylglutaryl CoA reductase, the rate-limiting enzyme of cholesterol synthesis, in the fetal liver. Total cholesterol, however, was not altered in fetal plasma or liver. The effect of these diets on cholesterol and fatty acid synthesis in vivo was therefore studied in fetal and maternal liver. In fetuses of rats fed a reference nonpurified diet, rates of hepatic cholesterol and fatty acid synthesis decreased from gestation d 20 to 21. In contrast, total and active 3-hydroxy-3-methyl-glutaryl CoA reductase activity increased. Adding cholestyramine to the diet or modifying the quantity of safflower oil fed had no effect on fetal hepatic lipogenesis. Maternal hepatic cholesterol synthesis was greater in rats fed cholestyramine, whereas fatty acid synthesis was lower in the dams fed the diet containing 20% compared with 5% safflower oil. The results suggest near-term fetal liver 3-hydroxy-3-methylglutaryl CoA reductase activities do not reflect fetal cholesterol synthesis in vivo.

Perinatal development of hepatic cholesterol synthesis in the rat.

Rates of cholesterol synthesis and HMG CoA reductase activity in rat liver, have been reported to be high before and low after birth. The timing of the decline in perinatal rates of cholesterol synthesis, however, is uncertain. These studies, therefore, determined in vivo rates of cholesterol synthesis using [3H]water and hepatic reductase activity in vitro in perinatal rats. The lipid composition of the plasma, liver and its microsomal subfraction were also determined. Reductase activity increased during late gestation, remained high immediately after birth, then decreased with the commencement of suckling. Rates of cholesterol synthesis increased from gestation day 18 to 20, but in contrast to reductase activity, decreased on the day before birth. Plasma cholesterol and triacylglycerol levels increased to gestation day 19, then decreased to term. By the 6th h after birth, plasma and liver cholesterol and triacylglycerol levels had increased markedly. By 48 h after birth, the high hepatic cholesterol content was associated with an increase in the cholesteryl ester fraction. The microsomal cholesterol/phospholipid molar ratio decreased from gestation day 16 until 12 h after birth, then increased markedly from 36 to 48 h. There was an apparent inverse relationship between the change in microsomal cholesterol/phospholipid molar ratio and the fatty acid unsaturation index from gestation day 16 to 36 h after birth. The results suggest that in late gestation and before suckling, the low in vivo rate of hepatic cholesterol synthesis may not be due to low activity of HMG CoA reductase.

Effect of dietary fat content and composition during pregnancy on fetal hepatic HMG CoA reductase activities and lipids in rats.

The effects of diets containing 20% (wt/wt) safflower, olive or palm oil or 5% (wt/wt) safflower oil when fed throughout gestation on hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity and on plasma and liver lipids were studied in fetal rats at d 21 of gestation. Hepatic total and active HMG CoA reductase activity, plasma free cholesterol and liver triglyceride and phospholipid 18:2(n-6) levels were higher in fetuses of rats fed 20% than in those of rats fed 5% safflower oil. Fetuses of rats fed olive oil had higher active HMG CoA reductase levels than fetuses of rats fed 20% safflower or palm oil; their total reductase activity was similar to that of the safflower oil group and higher than that for the palm oil group. Fetal liver and plasma cholesterol and triglyceride, as well as liver phospholipid concentrations, were not altered by the type of oil fed. The diets containing safflower oil resulted in higher 18:2(n-6) and lower 18:1 levels in fetal liver phospholipid and triglycerides than did the diets containing palm or olive oil. These studies demonstrate that fetal liver HMG CoA reductase activity is influenced by the maternal diet fat content and composition although the effect of specific fatty acids is unknown.

Effects of cholestyramine feeding on tissue lipase activities and plasma fatty acids in the pregnant rat.

Rats fed a non-absorbable bile acid binding resin (cholestyramine) throughout gestation had decreased activities of adipose tissue lipoprotein lipase (LPL), hepatic triacylglycerol lipase and a heparin-releasable placental lipase distinct from LPL, when assayed at near-term gestation. The fetal plasma and liver triacylglycerol concentrations were not altered. The fetal liver total lipid and plasma triacylglycerol, however, had reduced levels of n-6 and n-3 series fatty acids, suggesting decreased availability of maternal dietary-derived essential fatty acids. These studies suggest that cholestyramine feeding may alter triacylglycerol flux and the quantity or type of maternal fatty acids available for placental transfer. The resin has application for in vivo study of the effects of maternal lipid transfer on the regulation of fetal hepatic lipid synthesis.

Effects of cortisol or corticotropin administration on hepatic 3-hydroxy-3-methylglutaryl-CoA reductase activity and plasma lipids in the pregnant rat and fetuses.

Pregnant rats were given pharmacological doses of cortisol or ACTH or no hormone from gestation day 9 to 19 and maternal and fetal hepatic 3-hydroxy-3-methylglutaryl-CoA reductase activity and plasma cholesterol studied on gestation day 20. Reductase activity was also studied in the maternal and fetal adrenal of the rats given cortisol or no hormone. Cortisol administration increased the maternal and fetal plasma cholesterol but had no effect on the hepatic active (phosphorylated) 3-hydroxy-3-methylglutaryl-CoA reductase activity when compared to untreated rats. Total (active + inactive) 3-hydroxy-3-methylglutaryl-CoA reductase activity, however, was reduced in maternal liver but not altered in the fetal liver by cortisol. The maternal cortisol treatment decreased the fetal, but not maternal, adrenal 3-hydroxy-3-methylglutaryl-CoA reductase total enzyme activity. The data support a hypothesis that utilization of plasma cholesterol for adrenal steroidogenesis may be an important determinant of plasma cholesterol homeostasis in the rat fetus. Maternal ACTH administration increased the foetal but not maternal plasma cholesterol, whilst active 3-hydroxy-3-methylglutaryl-CoA reductase activity was increased in the pregnant rat but not her fetuses. This result may suggest coordination of hepatic active reductase activity with adrenal cholesterol utilization in the pregnant rat. The reason for the fetal hypercholesterolaemia caused by ACTH, which is not known to cross the placenta, is uncertain. The studies, however, indicate that fetal cholesterol homeostasis and the rate limiting enzyme of cholesterol synthesis is influenced by maternal glucocorticoid administration.

Induction of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in foetal rats by maternal cholestyramine feeding.

Late gestation foetus from rats fed a non-absorbable bile acid binding resin (cholestyramine) have increased hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase activity. This was due to increased unphosphorylated (active) as well as total reductase and was accompanied by higher fatty acid synthetase activity. No increase in foetal hepatic cystathionase or tyrosine aminotransferase activity, or changes in plasma insulin, corticosterone or thyroxine were found. The studies demonstrate that foetal hepatic cholesterol metabolism is sensitive to drug-induced perturbation of maternal lipoprotein metabolism. The data suggest induction of foetal cholesterol and fatty acid synthesis by a specific mechanism not involving generalized hormone-induction of hepatic enzyme systems. Cholestyramine appears to have application for in vivo study of the regulation of foetal cholesterologenesis and its coordination to maternal and foetal steroid requirements.

Effect of chronic modification of diet fat and cholesterol during gestation on plasma hormones and hepatic enzyme activities in rat fetus.

Diet-induced elevation of cholesterol and corticosterone in pregnant rats had no effect on fetal plasma cholesterol or corticosterone, 3-hydroxy-3-methylglutaryl coenzyme A or hepatic reductase activities. The data suggest that increasing the maternal cholesterol pool has no effect on cholesterol transfer to, or metabolism in, the developing offspring, and that the late gestation fetus can maintain corticosterone levels appropriate to gestational age despite diet-induced elevations in the maternal plasma.