Glucose metabolism in the newborn rat. Hormonal effects in vivo

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Postnatal profiles of glycogenolysis and gluconeogenesis are modified in rat pups by maternal dietary glucose restriction

Because glucose is an important metabolic fuel during perinatal development, the effect of restriction of maternal dietary glucose on the developmental profile of neonatal glucoregulatory pathways was investigated. Pregnant rats were fed isoenergetic diets (0, 12, 24 or 60% glucose) and offspring were killed at seven postpartum time periods: 0-2, 4-6, 12-16 and 24 h, and 3, 6 and 15 d. Failure of the most restricted pups (0%) to survive 24 h was explained by persistent hypoglycemia resulting from the following: 1) insufficient tissue glycogen reserves at birth; 2) lower liver glycogen mobilization; 3) delayed phosphorylase a induction; and 4) low phosphoenolpyruvate carboxykinase (PEPCK) gene expression, all of which occurred despite the lower insulin:glucagon ratio. Differences in liver glycogen stores, which had been exhausted in all dietary groups by 16 h, could not account for the high d 1 pup mortality in the moderately restricted (12 and 24% glucose) groups. However, a certain metabolic distress was suggested because these moderately restricted neonates had significantly higher liver PEPCK gene expression at 12-16 h but significantly lower plasma glucose at 24 h. The high d 3 mortality, confirmed by analysis of deviance, was not supported by significant differences in any of the measured glucoregulatory indices. We conclude that dietary glucose during pregnancy is required for neonatal survival; its restriction not only lowers tissue glycogen reserves, but can disrupt the normal gene expression of liver PEPCK and the neonatal profile of phosphorylase a activity. Importantly, these observations show that the development of neonatal glucoregulatory mechanisms is modified by the availability of maternal dietary glucose.

Effect of maternal ethanol ingestion during pregnancy and lactation on the structure and function of the postnatal rat liver plasma membrane. Assessement with [3H]prazosin binding to the hepatic alpha 1-adrenergic receptors

A liquid low-fat nutritionally adequate Metrecal diet in which alcohol contributed 37% of the total calories was given to pregnant rats and maintained during lactation. Control rats were pairfed with an isocaloric sucrose-Metrecal diet. After birth, litters were killed at different ages (days 1-30), and the results showed that growth and survival of progeny from the alcohol-treated rats were adversely affected. Likewise, the wet weights of livers from such pups were consistently less than from the pair-fed controls. The yield of hepatic plasma membrane protein per wet liver weight was constant and independent of either age or diet. Using [3H]prazosin as radioligand, equilibrium binding studies were carried out to monitor changes in the structure and function of the plasma membrane in the new-born pups concomitant with the development of alpha 1-adrenergic receptors. Results obtained with the alcohol-fed pups showed that the binding affinity (KD) was not altered throughout. However, the receptor density (Bmax) was decreased significantly. This decrease ranged from 60 to 70% in pups 6- to 15-days-old; 45% at 20 days; and 30% in pups at 25 and 30 days of age. These observations suggest that maternal ethanol ingestion affected the postnatal development of rat liver plasma membranes. Furthermore, by using the hepatic alpha 1-adrenergic receptor as a metabolic probe, we deduce that a possible impairment exists in the capacity of the alcoholic progeny to respond to the hormonal action of epinephrine. Such a defect may contribute to impaired growth and metabolism in these young animals.

Hormonal control of glucose homeostasis during development and ageing in mice

Age-related changes in the hormonal control of glucose homeostasis were examined in immature (5-wk-old), young adult (10-wk-old) and older adult (30 and 60-wk-old mice). In immature mice, basal plasma glucose and insulin concentrations were higher than young adult mice. Glucose tolerance was impaired in immature mice, although the plasma insulin response to glucose and other secretagogues (arginine and glucagon) was well developed, and the hypoglycaemic effect of exogenous insulin was not impaired. Glucagon and epinephrine evoked a greater acute hyperglycaemia in immature mice, suggesting that these hormones exert a stronger rapid glucose-raising effect in preadult life. Basal plasma glucose and insulin concentrations were lowest in young adult mice, and increased with advancing adult age. Glucose tolerance was best in young adult mice and deteriorated with age, while plasma insulin concentrations after administration of glucose, arginine and glucagon were lowest in young adult mice and increased with age. However, in response to these secretagogues, the percentage increase in plasma insulin above basal was reduced in older adult mice. This indicates a defect of stimulus-recognition-secretion-coupling in the B-cells of older adult mice. The raised plasma glucose concentrations of older adult mice could not be attributed to an increase in the acute hyperglycaemic action of glucagon or epinephrine. The hypoglycaemic response to exogenous insulin decreased with age, suggesting that tissue sensitivity to insulin was impaired. Treatment with growth hormone and cortisone for 5 days produced a greater antagonism of insulin in older mice than young adult or immature mice. Growth hormone impaired glucose tolerance at each age, but only produced a marked hyperinsulinaemia in older adult mice. In contrast, cortisone produced a marked hyperinsulinaemia at each age, but only impaired glucose tolerance in older adult mice.

Rat liver glycogen metabolism in the perinatal period

The correlation between blood glucose levels, the concentration of glycogen, the activities of glycogen synthase and phosphorylase and their respective kinases and phosphatases was examined in liver of rat fetuses between day 18 of gestation and one day after birth. Between day 18 and 21 there is a rapid increase in the concentration of glycogen and in the activity of synthase a and a much slower increase in the activity of phosphorylase a. The activity of the respective kinases increased rapidly during this period and reached maximum on day 21. The activity of synthase phosphatase and phosphorylase phosphatase increased after day 18, to reach a maximum on day 19 and 20, respectively, but decreased again towards day 21. The possibility that the changes in glycogen concentration and enzyme activities were related to an effect of glucose or AMP on the respective phosphatases was considered. It was found that the Km of phosphorylase phosphatase for glucose in the prenatal period was 5--7 mM, as in the adult. Since the level of blood glucose during this period was constant (2.8 mM), an effect of glucose on phosphatase activity seems unlikely. AMP concentration increased between day 18 and 21 from 6--15 nmol/g. In view of the low level of phosphorylase a activity during this period, the increase in AMP concentration is not considered to be important in the regulation of glycogen breakdown at this time. Immediately after birth blood glucose levels dropped to 5 mg/dl. This was accompanied by a rapid decrease in glycogen concentration and in the activity of glycogen synthase and a rise in phosphorylase activity. Blood glucose levels returned to the initial level within 1 h after birth, whereas the changes in glycogen concentration and enzyme activities continued for at least 3 h after birth. On day 22 all parameters examined had reached the level found in adult rat liver. It is suggested that the rapid changes observed immediately after birth are due to an effect of gypoglycemia mediated by hormones and cannot be ascribed to direct effects of metabolites on the enzyme systems involved.

Glucose metabolism in the newborn rat: the role of insulin

The effect of the administration of anti-insulin serum to newborn rats, surgically delivered under ether anaesthesia at term, was examined with respect to liver glycogen concentration and plasma concentrations of glucose, lactate and free fatty acids. Newborn rats thus treated showed decreased liver glycogen concentrations and elevated plasma concentrations of glucose, lactate and free fatty acids compared to untreated control animals one hour later. These effects were dose-dependent with respect to the amount of anti-insulin serum administered. The simultaneous administration of glucagon with anti-insulin serum at birth was no more effective in mobilising glycogen stores than anti-insulin serum alone, although plasma glucose concentrations in these animals were higher and plasma lactate concentrations were lower. Either anti-insulin serum or glucagon abolished the postnatal hypoglycaemia observed in untreated neonatal rats. The rate of fall in plasma lactate concentrations after birth was stimulated in glucagon-treated rats but was retarded in rats treated with anti-insulin serum. Hormonal control over the initiation of glycogenolysis and gluconeogenesis in the newborn rat appears to be different, a fall in plasma insulin being the prime factor involved in triggering glycogen mobilization and a rise in plasma glucagon the prime event that initiates gluconeogenesis.

Postmaturity in the rat: impairment of insulin, glucagon, and glycogen stores

Prolonged gestation (2 extra days in utero) was obtained by daily subcutaneous injection of progesterone (2.5 mg) to pregnant rats from day 20.5 post coitum (p.c.) throughout day 22.5 p.c. after reduction of the litter to 6 fetuses on day 14.5 p.c. Exogenous progesterone per se or litter reduction were without effect of fetal pancreas or fetal liver. Plasma insulin, insulin and glucagon in the pancreas, and liver glycogen stores have been systematically measured in postmature animals and in controls during the perinatal period. In 23.5 day-old postmature as compared to 21.5 day-old normal fetuses, the intrauterine mortality was increased (26%), the body weight was increased by 30%, the liver weight was decreased by 20%, the glycogen content of liver was dramatically depleted (1.1 +/- 0.2 mg/g body weight on day 23.5 p.c. against 6.7 +/- 0.3 on day 21.5 p.c.), the plasma insulin was lowered by 63% and the blood glucose level was normal. In postmature neonates during the first day of life the mortality rate was considerable (40%) and a dramatic fall of blood glucose was observed 6 hours after birth. The accumulation of insulin and glucagon in the pancreas, which normally occurs in the two first days after birth, was much lower in the postmature fetuses: in 23.5 day-old fetuses as compared to 2 day-old normal newborns of the same gestational age the insulin content was only 50% and the glucagon content 69%. The deficit of insulin accumulation in the postmature pancreas lasted at least five days. The ability of the endocrine pancreas to recover from this alteration as well shown by the lack of diabetes when the animals were examined three weeks later by a glucose tolerance test. These findings suggest that the drop of plasma insulin is a prime factor in causing the lack of glycogen stores in prolonged fetuses and the impairement of glycogen stores appear to be an important feature of postmaturity, since neonates exhibit, in these conditions, a lethal drop of blood glucose as glycogenolysis operates on very low glycogen stores.

Development of glucagon sensitivity in neonatal rat liver

The ontogenesis of the hepatic glucagon-sensitive adenylate cyclase system has been studied in the rat. With a partially purified liver membrane preparation, fetal adenylate cyclase was less responsive to glucagon than the enzyme from neonatal or adult livers. Similar results were obtained in gently prepared liver homogenates, suggesting that destruction of essential components of the fetal liver membrane did not account for the relative unresponsiveness of the adenylate cyclase enzyme to glucagon. Investigation of other factors that might account for diminished fetal hepatic responsiveness to glucagon indicate (a) minimal glucagon degradation by fetal membranes relative to 8-day or adult tissue; and (b) available adenylate cyclase enzyme, as suggested by a 13-fold increase over basal cyclic AMP formation with NaF in fetal liver membranes. These results indicate that neither enhanced glucagon degradation nor adenylate cyclase enzyme deficiency accounts for the relative insensitivity of the fetal hepatic adenylate cyclase system to glucagon. In early neonatal life, hepatic adenylate cyclase responsiveness to glucagon rapidly developed and was maximal 6 days after birth. These changes were closely paralleled by a fivefold increase in glucagon binding and the kinetically determined Vmax for cyclic AMP formation. These observations suggest that (a) fetal hepatic unresponsiveness to glucagon may be explained by a limited number of glucagon receptor sites; (b) during the neonatal period, the development of glucagon binding is expressed primarily as an increase in adenylate cyclase Vmax; (c) the ontogenesis of hepatic responsiveness to glucagon may be important in the resolution of neonatal hypoglycemia.

Effects of exogenous hormones and glucose on plasma levels and hepatic metabolism of amino acids in the fetus and in the newborn rat

The present study examines the role of insulin, glucagon and cortisol in the regulation of gluconeogenesis from lactate and amino acids in fetal and newborn rats. Injection of glucagon in the full-term fetal rat caused a rise in glucose (and insulin) and a fall in blood levels of most individual amino acids, stimulated hepatic accumulation of 14C-amino isobutyric acid and 14C-cycloleucine and increased the conversion of 14C lactate, alanine and serine to glucose in vivo and in vitro (liver slices). Such changes were equivalent to the changes seen in 4 h old newborn rats. When glucagon was administered at birth, little difference was observed between control and treated animals in plasma amino acids and a smaller increment in conversion of 14C substrate to glucose occurred. By contrast, insulin injection at birth caused hypoglycemia, suppression of levels of certain amino acids and inhibition of conversion of 14C substrates into glucose. Glucose injection at birth caused elevated glycemia and plasma insulin and suppression of most amino acid levels and of conversion of 14C substrate into glucose. Cortisol injection at birth caused a marked, generalized by hyperaminoacidemia, a stimulation of glucagon secretion and of conversion of 14C substrates into glucose. These observations support the thesis that glucagon plays a major role in the induction of hepatic gluconeogenesis and that insulin acts as an antagonist hormone.

Glycogen metabolism in the liver of the foetal rat

1. The glycogen present in the liver of rat foetuses was labelled by injecting a trace amount of [6-(3)H]glucose into the mother at 19.5 days of gestation. The radioactivity incorporated in the glycogen 4h after the administration of the label was still present 38h later. A large proportion of this radioactivity was on the outer chains of the polysaccharide. These results indicate that there is normally almost no glycogen degradation in the foetal liver. In contrast, glycogen breakdown occurs very rapidly in the livers of foetuses whose mother is anaesthetized. 2. Glycogen synthetase is present in the liver at day 16 of gestation at a concentration as high as 30% of that in the adult, but essentially as an inactive (b) enzyme. The appearance of synthetase phosphatase between days 18 and 19 corresponds to that of synthetase a and to the beginning of glycogen synthesis. From day 19 to 21.5 the amount of synthetase a present in the foetal liver is just sufficient to account for the actual rate of glycogen deposition. 3. The content of total phosphorylase in the foetal liver increases continuously from day 16 to birth. However, a precise measurement of the a and b forms of the enzyme in the liver of non-anaesthetized foetuses is not possible. Taking the rate of glycogenolysis as an appropriate index of phosphorylase activity, we conclude that this enzyme is almost entirely in the inactive form in the foetal liver under normal conditions. 4. The accumulation of glycogen in the liver during late pregnancy may therefore be explained by a relatively slow rate of synthesis and a nearly total absence of degradation.