1. Investigación

Prolonged hyperinsulinemia in the pregnant rat, caused either by daily treatment with exogenous insulin of the STZ-diabetic rat or a continous infusion of glucose, produces an increase of adipose tissue LPL activity. These findings support the hypothesis that under normal conditions, insulin resistance during late pregnancy is responsible for the decreased LPL activity normally seen in maternal adipose tissue.

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.

To understand the mechanism of exaggerated hypertriglyceridemia in diabetic pregnancy, streptozotocin-treated rats receiving a daily insulin supplement were mated with nom1al males and divided into four groups: i) kept under this regime until the 20th day of gestation (Dl+ll), ii) the same regime until the 12th day of gestation (DI), iii) the insulin treatment was suspended during the first half of gestation (days 0-12) and then restored on a daily basis until the 20th day (DU), and iv) no insulin treatment was given after mating (D). All animals were studied on day 20. Despite increased food intake, maternal conceptus-free body weight was greatly reduced in the D animals as compared with the other groups whose values did not differ. Both the plasma glucose and ~-hydroxybutyrate levels were increased more in D than in DI rats and values in both groups were greater than in the others. Insulin levels showed an opposite trend to that of glucose, but the values in DI+ll rats were higher than in untreated intact control rats (C). The plasma triglyceride concentration was highest in the DI rats, followed by the D group whose values were still significantly higher than in either C or DI+ll rats. Plasma free fatty acid levels were lower in D than in any of the other groups, although they were also lower in Dl+II and DI than in C animals. Adipose tissue lipoprotein lipase activity wa~ highest in DI+II animals and their values were very similar lo those found in DII, whereas the values in the C, D and DI animals were all similar and much lower. Results indicate that reductions in fat accumulation during the first half of gestation impair the activation of lipolytic activity in the severe diabetic mother during late gestation. During this period lipolysis helps sustain maximal hypertriglyceridemia, which develops in animals whose diabetes was circumscribed to the second half of gestation. In general, our findings show that anabolic changes during the first half of gestation affect metabolic events during late gestation.

Etofibrate is the 1,2-ethandiol diester of clofibric acid and nicotinic acid that decreases circulating levels of triacylglycerols and cholesterol. To understand the mechanism by which the drug affects plasma triacylglycerols, normolipemic rats were treated daily with 300 mg of etofibrate /kg body weight or with the medium by a stomach tube. They were decapitated on the 10th day, and showed lower levels of plasma ,8-hydroxybutyrate, glycerol, free fatty acids (FFA), total triacylglycerols and cholesterol and VLDL triacylglycerols and cholesterol, whereas glucose and RIA-determined insulin levels were unmodified. Epididymal fat pad pieces from etofibrate-treated rats incubated in vitro released more glycerol but the same amount of FFA to the medium, and had greater uptake of [U- 14C)glycerol for [14C)acylglycerol formation. In the presence of heparin, they also showed an enhanced release of lipoprotein lipase activity to the medium. The disappearance from plasma of intravenously administered [1- 14C)palmitate was faster in the etofibrate-treated rats, and although they showed a decrease in 14 C-esterified fatty acids of neutral lipids in both liver and plasma VLDL, there was an increase in liver 14 C-labelled water-soluble components. After intravenous [U- 14C)glycerol administration, there was a decrease in plasma VLDL [ 14C)acylglycerol and [ 14C]glucose and in liver [14C]acylglycerol, but an increase in plasma I 14C]lactate. In the liver, etofibrate treatment heightened the cytosolic glycerol-3-phosphate dehydrogenase activity and the total carnitine concentration, whereas it reduced triacylglycerol and cholesterol concentrations. It is proposed that etofibrate enhances the reesterification of fatty acids and glycerol in adipose tissue, which, together with its augmented lipoprotein lipase activity, may facilitate the clearance of circulating triacylglycerols. These effects may act concomitantly with the decreased synthesis of triacylglycerols, secondary to the increased utilization of their precursors, acyl-CoA and glycerol-3-phosphate, in other pathways, causing the reduction of plasma VLDL triacylglycerols produced by etofibrate treatment.

To determine whether changes in uterine blood flow affect placental glucose transfer in the diabetic pregnant rat, on the 7th day of gestation rats were intravenously treated with either streptozotocin (45 mg/kg) (diabetics) or buffer (controls). On the 20th day of gestation, fetal body weight and uterine blood flow appeared reduced whereas fetal/maternal plasma glucose was enhanced and lactate ratios were unchanged in diabetics versus controls. After 20 min of (U- 14C)-D-glucose infusion through the maternal left uterine artery, plasma values of fetuses from left and right uterine horns were higher for 14C-glucose and lower for 14C-lactate in diabetics versus controls, and placental glucose transfer was greatly augmented in diabetics whether or not uterine blood flow was included in its calculation. Whereas a linear correlation existed between placental glucose transfer and maternal plasma glucose concentration, transferred glucose conversion into lactate remained stable even when the maternal glucose level was high. It was concluded that enhanced placental glucose transfer in the pregnant diabetic rat is not modified by reduced uterine blood flow. The limited capacity of the fetus to handle the great incoming flux of glucose through the placenta of a severely diabetic mother produces permanent hyperglycemia which may impair fetal growth.

Reviews these carbohydrate-lipid interactions and analyzes the role of maternal body fat accumulation during the early part of pregnancy in the metabolic adaptations during late gestation. The consequences and intimately related aspects of these interactions are several of the other changes that occur in the maternal/fetal relationship such as the· development of maternal hypertriglyceridemia, changes in the maternal amino acid metabolism, placental metabolite transfer, etc. These topics are reviewed in the other chapters of this book.

Gestational diabetes mellitus (GDM) is a nonhomogenous entity known to affec:t fetal development In different waya In both rat• and human belnga. 'fhe degree of severity of diabetes could affect the maternal-fetal transfer of metabolic fuels and consequently Influence fetal development. To study this hypothesis, pregnant rats were made diabetic by streptozocln (STZ) treatment (45 mg/kg) at day 7 of gestation and were treated with dlffer"1t dally doses of Insulin until the 20th day of gestation, when they were killed and examined. Differences In plasma glucose levels In the groups studled were not accompanied by differences In plasma glycerol, f3-hydroxybutyrate (f3•0HB), or total amino acid levels In mothers or their fetuses. Fetal/maternal ratios of these circulating tuela were not modified by maternal dlabetH, whereas the glucoae level was enhancec: t In diabetic rats not treated with Insulin. Placental glucose transfer was studied directly with a recently reported In situ experimental design and was found to Increase linearly with maternal glycemla, Independently of whether this was modified by Insulin treatment or by acute Intravenous (l.v.) Infusion of glucose In normal animals. Lactate production by the fetal/placental unit decreased In proportion to the glucose level In the maternal circulation. The present data Indicate that the diabetic condition of the mother rat does not modify the mechanisms of placental transfer of metabolic fuels to the fetus, and that the actual tranafer la mainly dependent on the concentrations of these fuels In the maternal circulation. The limited capacity of the fetus to handle the great Influx of glucose through the placenta of a highly hyperglycemlc mother may aggravate the diabetic condition of the fetus, affecting Its subsequent development.