1. Investigación

The profiles of plasma leptin levels in pregnant and lactating rats and their offspring were determined. The plasma leptin levels increased on days 12 and 20 of gestation and declined on day 21 of gestation, remaining at this level during lactation. These changes were similar for lumbar adipose tissue weight, and a significant correlation was found when both variables were plotted with individual values. During the last 2 days of intrauterine life, the plasma leptin levels in the fetuses were in the same range as in their mothers, declining from day 20 to day 21. On the 1st day of life, the leptin levels increased to decline in suckling newborns after 4 days, remaining stable until day 20 of life. The enhancement in maternal white adipose tissue mass that takes place during pregnancy and its decline around parturition and lactation are proposed to contribute actively to the changes in the plasma leptin profile detected at these stages. Besides the contribution of placental leptin for the fetus and milk leptin for the suckling newborn, it is proposed that brown adipose tissue, which is the first form of adipose tissue that appears during development in the rat, is responsible for most of the changes in plasma leptin levels seen around birth, whereas its later decline could be mediated by the hormonal changes occurring after birth.

To determine the effect of under-nutrition during suckling in adults, at delivery female Sprague Dawley rats were allowed to lactate litters of either eight (controls) or sixteen pups each (large litter, LL). The amount of milk taken by LL pups was less than the controls and the concentration of triacylglycerols (TG) in the milk of the former was lower. The increase of both body weight and length in LL was lower than in the controls during suckling. At weaning, pups were allowed to eat ad libitum a standard diet and whereas at 20 months female body weight did not differ between LL and control rats, LL males weighed less than controls. Plasma NEPA were lower in male LL than in controls at 10 months, leptin at 10 and 16 months and TG and VLDL-TG at 20 months, with no differences in females. When 20 months old, lumbar and epididymal adipose tissue weights were lower in male LL than in controls, but not in females. The increase in plasma insulin after oral glucose load was lower in LL than in controls, both in males and females at 4 and 16 months, and only in males at 10 months, whereas the change in plasma glucose remained constant between the groups. Results indicate that both the pancreatic [3-cell function and insulin sensitivity and adipose tissue metabolism are independently programmed as a consequence of under-nutrition during suckling, the effect being more manifest for males than for females.

1.8 or 25 µg of L-thyroxine/100 g body wt. were compared with intact controls (C). The appearance of radioacitvity in fatty acids 30 min after the i.p. injection•of (3-14C)pyruvate was reduced in adipose tissue and enhanced in liver of T+25, being no differences between the other groups and C. ( 14C)- Fatty acids are reduced with 3 h of fasting only in the adipose tissue of T+ 1.8 and C, while 24 h produces a reduction in liver in the T+ 1.8, T+25 and C, and in adipose tissue in the T+l.8 and C animals. The highest percentage of radioactivity was observed in the liver glyceride glycerol fraction, being greater in T+25 than in the other groups. Fasting produces an increment in the ( 14C)-glyceride glycerol fraction. being significant only in the hypothyroid animals in both liver and adipose tissue. The most sensitive parameter to fasting was the formation of ( 14C)-non-saponifiable lipid in both the C and T+ 1.8 animals, while it does not change in T+0 or T+0.I, but is enhanced within 24 h in the adipose tissue of T + 25. It is proposed that most of the observed changes are due to the other endocrine disfunction s which appear in hypo- and hyperthyroidism, as the in vivo results do not comply with in vitro effects of thyroxine on lipogenesis of others.

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 nutri~nts whenever she eats. During this phase maternal fat depots are accumulated thanks to the enhancement in adipose tissue lipogenic and glycerolgenic activity. In the latter part of gestation, on the contrary, the rapid fetal growth is sustained by the intense transfer of nutrients from maternal circulation. Glucose is quantitatively the most abundant of the different substrates that cross the placenta and despite enhanced maternal gluconeogenesis this transfer is the cause of the maternal tendency to hypoglucemia. This causes a switch to a net catabolic state which is specially evident in the net breakdown of fat depots. Enhanced release of adipose tissue lipolytiL- products, FFA and glycerol. facilitates the liver synthesis of triglycerides and their later release into circulation associated to VLDL. Glycerol is also used as an important ,, luconeogenic substrate and FFAs are hroken d~wn through 13-oxidation for ketone body synthesis. These pathwa/s become heightened when food is withheld and actively contribute to the availability of fuels to the fetus which becomes partially preserved from maternal metabolic insult. Enhanced 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.

Pieces of lumbar adipose tissue from 19-day pregnant rats and their virgin controls were incubated in the presence of albumin, glucose, U- 14C-glycerol, and either bovine insulin (200 µU/ml) or adrenaline (2.6 µM). The rate of glycerol release in the medium was augmented in both fed and 48-hour fasted pregnant rats. Insulin reduced this parameter in tissues from pregnant rats but not from controls. Adrenaline enhanced it in all groups, especially in tissues from fed pregnant rats. The rates of CO2 , fatty acids and glyceride glycerol formation from glycerol were higher and the effect of insulin was greater in pregnant than in control rats when fed. Fasting produced a decrease in all these parameters, the effect being greater in pregnant than in control rats. The augmented sensitivity of adipose tissue from the mother allows for a rapid switch from the anabolic to the catabolic state according to the necessities of the fetus.

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.