1. Investigación

Para estudiar el efecto de la gestación y la diabetes sobre la actividad y la expresión (ARNm) de la lipoproteína lipasa (LPL) y de la lipasa sensible a las hormonas (HSL) en tejidos, las ratas recibieron 40 mg de estreptozotocina/kg y se sacrificaron al día 20 de gestación. En el tejido adiposo blanco {T AB), la actividad y el ARNm de la LPL eran más bajos en las ratas preñadas controles (PC) que en las vírgenes controles (VC), siendo también inferior la actividad en las diabéticas (D) que en las controles(C), tanto vírgenes (V) como preñadas (P). En la glándula mamaria (GM), la actividad y el ARNm de la LPL estaban más altos en las PC que en las VC, y disminuyeron en las PD y VD respecto de las C. En el T AB, la actividad HSL estaba más alta en las P que en las V y era similar en las PC y PD, sin cambios en los niveles de ARNm. En la GM, la actividad y el ARNm de la HSL estaban más bajos en las P que en las V y eran menores en las D que en las C. Estos resultados permiten concluir que los mecanismos de regulación de la actividad y de la expresión molecular (ARNm) de la LPL y de la HSL durante la gestación y la diabetes son específicos de cada tejido. El paralelismo en los cambios de la actividad y el nivel de ARNm de la LPL en el T AB y GM, así como entre la actividad y el ARNm de la HSL en la GM, indican que la regulación a nivel transcripcional de la expresión de estos genes es un mecanismo importante para la captación y movilización de los TO en estos tejidos.

I. During the first two thirds of gestation, coinciding with a minimal accretion by the conceptus, the mother is in an anabolic state which is supported by her hyperphagia and the more efficient conservation of exogenous nutrients when she eats. During this phase maternal fat deposits are accumulated thanks to the enhancement in adipose tissue lipogenic and glycerolgenic activity. In contrast, in the latter part of gestation, the rapid fetal growth is sustained by the intense transfer of nutrients from maternal circulation. 2. Glucose is quantitatively the most abundant of the several substrates that cross the placenta and despite increased maternal gluconeogenesis this transfer is responsible for the maternal tendency to hypoglycemia. This causes a switch to a net catabolic state which is especially evident in the net breakdown of fat depots. 3. Enhanced release of adipose tissue Iipolytic products, free fatty acids (FFA) and glycerol, facilitates the liver synthesis of triglycerides and their later release into circulation associated to very low-density lipoprotein (VLDL). Glycerol is also used as an important gluconeogenic substrate and FF As are broken down through 13-oxidation for ketone body synthesis. Flow through these pathways becomes increased when food is withheld and this actively contributes to the availability of fuels to the fetus which becomes partially preserved from maternal metabolic insult. Increased liver production of VLDL-triglycerides and decreased extrahepatic lipoprotein lipase contribute to exaggerated maternal hypertriglyceridemia which, besides being a floating metabolic reserve for emergency conditions such as starvation, constitutes an essential substrate for milk synthesis around parturition in preparation for lactation. 4. While the maternal anabolic tendencies found during the first two-thirds of gestation seem to be facilitated by hyperinsulinemia in the presence of a normal responsiveness to the hormone, it is proposed that most of the metabolic changes taking place during the last third of gestation seem to be caused by the insulin-resistant state which is consistently present at this stage, since its reversion caused by sustained exaggerated hyperinsulinemia also reverts several of these metabolic adaptations.

Since during pregnancy the mother switches from an anabolic to a catabolic condition, the present study was addressed to determine the effect of 48 h food deprivation on days 7, 14 and 20 of pregnancy in the rat as compared to age matched virgin controls. Body weight, free of conceptus, decreased with food deprivation more in pregnant than in virgin rats, with fetal weight (day 20) also diminishing with maternal starvation. The decline of plasma glucose with food deprivation was greatest in 20 day pregnant rats. Insulin was highest in fed 14 day pregnant rats, and declined with food deprivation in all the groups, the effect being not significant in 7-day pregnant rats. Food deprivation increased plasma glycerol only in virgin and 20 day pregnant rats. Plasma NEFA and 3-hydroxybutyrate increased with food deprivation in all groups, the effect being highest in 20 day pregnant rats. Food deprivation decreased plasma triacylglycerols in 14 day pregnant rats but increased in 20 day pregnant rats. In 20-day fetuses, plasma levels of glucose, NEFA and triacylglycerols were lower than in their mothers when fed, and food deprivation caused a further decline in plasma glucose, whereas both NEFA and 3-hydroxybutyrate increased. Liver triacylglycerols concentration did not differ among the groups when fed, whereas food deprivation caused an increase in all pregnant rats and fetuses, the effect being highest in 20-day pregnant rats. Lipoprotein lipase (LPL) activity in adipose tissue was lower in 20 day pregnant rats than in any of the other groups when fed, and it decreased in all the groups with food deprivation, whereas in liver it was very low in all groups when fed and increased with food deprivation only in 20 day pregnant rats. A significant increase in liver LPL was found with food deprivation in 20 day fetuses, reaching higher values than their mothers. Thus, the response to food deprivation varies with the time of pregnancy, being lowest at mid pregnancy and greatest at late pregnancy, and although fetuses respond in the same direction as their mothers, they show a specific response in liver LPL activity

Human very-low-density lipoproteins (VLDL) were subfractionated by heparin-Sepharose chromatography into an unbound (:\I and three bound (B, C and D) populations at increasing ionic strengths. Subfractions were characterized regarding their chemic;,[ composition and efficiency of triacylglycerol hydrolysis by rat adipose tissue LPL. The triacylglycerol content decreased, whcrca, the cholesterol and protein contents increased from subfractions A and B to subfraction D. VLDL-D showed the highest ap<' E/apo C ratio, though all the subfractions contained appreciable apo E. Appearance of VLDL-A resulted from exceeding the binding capacity of the column, since practically all its particles eluted at positions of bound VLDL under re-chromatograph~ Subfractions B, C and D stimulated LPL activity on emulsified tri[ 14C]oleoylglycerol to a similar extent, indicating that their 3P'' C-11 content was equally effective activating LPL. Incubation of tri[ 14C]oleoylglycerol labeled VLDL subfractions with fat pad pieces in the presence or absence of heparin resulted in greater hydrolysis and fatty acid uptake for VLDL-B and -C than f.•1 VLDL-D, a pattern observed over a wide range of LPL activities in the media. We conclude: (1) any VLDL particle can intcr;t,1 with heparin, which is consistent with the presence of apo E in all the subfractions, and (2) triacylglycerols in apo E-rich VLDI are less efficiently hydrolyzed by LPL than those in apo E-poor particles. We propose that richness in apo E impairs LPL acu"r upon VLDL and decreases the rate of delivery of fatty acids to peripheral tissues.

The mechanism that induces maternal hypertriglyceridemia in late normal pregnancy, and its physiologic significance are reviewed as a model of the effects of sex steroids on lipoprotein metabolism. In the pregnant rat, maternal carcass fat content progressively increases up to day 19 of gestation, then declines at day 21. The decline may be explained by the augmented lipolytic activity in adipose tissue that is seen in late pregnancy in the rat. This change causes maternal circulating free fatty acids and glycerol levels to rise. Although the liver is the main receptor organ for these metabolites, liver triglyceride content is reduced. Circulating triglycerides and very-low-density lipoprotein (VLDL)-triglyceride levels are highly augmented in the pregnant rat, indicating that liver-synthesized triglycerides are rapidly released into the circulation. Similar increments in circulating VLDL-triglycerides are seen in pregnant women during the third trimester of gestation. This increase is coincident with a decrease in plasma postheparin lipoprotein lipase activity, indicating a reduced removal of circulating triglycerides by maternal tissues or a redistribution in their use among the different tissues. During late gestation in the rat, tissue lipoprotein lipase activity varies in different directions; it decreases in adipose tissue, the liver, and to a smaller extent the heart, but increases in placental and mammary gland tissue. These changes play an important role in the fate of circulating triglycerides, which are diverted from uptake by adipose tissue to uptake by the mammary gland for milk synthesis, and probably by the placenta for hydrolysis and transfer of released nonesterified fatty acids to the fetus. After 24 hours of starvation, lipoprotein lipase activity in the liver greatly increases in the rat in late pregnancy; this change is not seen in virgin animals. This alteration is similar to that seen in liver triglyceride content and plasma ketone body concentration in the fasted pregnant rat. In the fasting condition during late gestation, heightened lipoprotein lipase activity is the proposed mechanism through which the liver becomes an acceptor of circulating triglycerides, allowing their use as ketogenic substrates, so that both maternal and fetal tissues may indirectly benefit from maternal hypertriglyceridemia. Changes in the magnitude and direction of lipoprotein lipase activity in different tissues during gestation actively contribute both to the development of hypertriglyceridemia and to the metabolic fate of circulating triglycerides. Any deviation in these metabolic adaptations occurring in the human mother may have consequences that modify her lipoprotein profile, even postpartum. Hormone-induced changes in pregnancy mirror those seen with oral contraceptive steroids and provide a teleologic rationale for the lipoprotein changes induced by sex steroids.

Background: Routine total parenteral nutrition (TPN) in neonatal care can result in hepatic dysfunction in 40–60% of patients, most commonly as fatty liver, but little work has been conducted on the underlying mechanisms causing hepatic dysfunction. Objective: To use a piglet model for the premature human neonate on TPN, supplemented with lipid emulsions, to investigate hepatic responses. Method: Piglets were delivered 2 days prematurely. Six control piglets were fed enterally (E), whilst twelve animals were maintained on TPN. TPN piglets received the standard TPN solution plus the lipid emulsion as either ClinOleic (C, n = 6) or Intralipid (I, n = 6). Hepatic lipid content and the fatty acid composition of liver triacylglyercol (TAG) as well as hepatic lipase (HL) activity were determined. Lipoprotein lipase (LPL) activity was measured in the liver, muscle and adipose tissue. The plasma concentrations of choline, bilirubin, TAG and non-esterified fatty acids (NEFA) were also measured. Results: Liver lipid was significantly increased in piglets on TPN and the tissue fatty acid profiles reflected the lipid emulsion. HL and LPL activities were reduced in liver but LPL increased in adipose tissue during TPN. Plasma concentrations of choline, bilirubin, TAG and NEFA were similar across the treatments. Conclusions: The results suggest fatty liver occurs in neonates receiving TPN and the source of the accumulated lipid appears to be the lipid emulsion used. The factors regulating lipase activity during TPN require further study. The piglet can be used as a model for neonatal TPN.

To understand the mechanism responsible for maternal hyperlipidemia, 25 healthy pregnant women were studied longitudinally during the three trimesters of gestation and at post-partum, and 11 were studied again at post-lactation. Triglyceride and cholesterol levels increased with gestation in all the lipoprotein fractions. However, the greatest change appeared in low density (LDL) and high density (HDL) lipoproteins, both of which showed an increase in their triglyceride/ cholesterol ratio. The proportional distribution of HDL subfractions showed that the HDL2b fraction was the only one that increased with gestation, whereas both HDL.sa and HDLsb had the greatest decrease. Cholesteryl ester transfer protein activity increased during the second trimester of gestation. While postheparin lipoprotein lipase activity decreased during the third trimester, postheparin hepatic lipase activity progressively decreased from the first trimester. The 17J3-estradiol, progesterone, and prolactin hormones progressively increased from the first trimester of gestation. The lipoprotein-triglyceride values correlated linearly and negatively with the logarithm of either postheparin lipase activities, HDlrtriglycerides showing the highest correlation coefficient when plotted against the hepatic lipase values (r = -0.757). It appeared that the highest correlation between any of the HDL subclasses and the activity of the enzymes was for hepatic lipase activity versus HDL2b (r = -0.456) or HDLsa (r = 0.519). A significant lineal correlation also appeared between the postheparin hepatic lipase activity and the logarithm of any of the sex hormones studied, the highest value corresponding to estradiol (r = -0.783). Ill Therefore, during gestation, the effect of estrogen in enhancing very low density lipoprotein (VLDL) production and decreasing hepatic lipase activity plays a key role in the accumulation of triglycerides in lipoproteins of density higher than VLDL.