Glucose metabolism in the developing rat. Studies in vivo

Fish oil diet in pregnancy and lactation reduces pup weight and modifies newborn hepatic metabolic adaptations in rats

PURPOSE

To determine the effects of a diet containing fish oil (FD) during pregnancy and lactation in rats on the metabolic adaptations made by the offspring during early extrauterine life and to compare it to an olive oil diet (OD).

METHODS

Rats were mated and randomly allocated to OD or FD containing 10 % of the corresponding oil. During lactation, litters were adjusted to eight pups per dam. Fetuses of 20 days and pups of 0, 1, 10, 20 and 30 days of age were studied.

RESULTS

Body weight and length were lower in pups of the FD group from birth. The diet, milk, pups' plasma and liver of FD group had higher proportions of n-3 LCPUFA, but the content of arachidonic acid (ARA) was lower. Plasma glucose was higher, but unesterified fatty acids, triacylglycerols (TAG), 3-hydroxybutyrate and liver TAG in 1-day-old pups were lower in the FD group, and differences in some of these variables were also found in pups up to 30 days old. Liver lipoprotein lipase activity and mRNA expression, and the expression of carnitine palmitoyl transferase I, acyl-CoA oxidase and 3-hydroxy 3-methyl glutaryl-CoA synthase increased more at birth in pups of the FD group, but the expression of sterol regulatory element binding protein-1c and Δ6-desaturase mRNA was lower in the FD group.

CONCLUSIONS

Maternal intake of high n-3 LCPUFA retards postnatal development, which could be the result of impaired ARA synthesis, and affects hepatic metabolic adaptations to extrauterine life.

Hypoglycemia with enhanced hepatic glycogen synthesis in recombinant mice lacking hexose-6-phosphate dehydrogenase

Hexose-6-phosphate dehydrogenase (H6PDH) knockout (KO) mice have reduced generation of nicotinamide adenine dinucleotide phosphate (reduced) within the endoplasmic reticulum. As a consequence, 11beta-hydroxysteroid dehydrogenase type 1 enzyme activity switches from a reductase to a dehydrogenase leading to glucocorticoid inactivation. 11beta-Hydroxysteroid dehydrogenase type 1 has emerged as an important factor in regulating hepatic glucose output; therefore, we examined aspects of glucose homeostasis in KO mice. Compared with wild-type mice, KO mice reduced weight gain, displayed peripheral fasting hypoglycemia, improved glucose tolerance, and elevated plasma corticosterone concentrations. Plasma insulin levels in fed and fasted KO mice are normal; however, insulin and plasma glucose levels are reduced 4 h after fasted animals are refed, indicating improved insulin sensitivity. There is preserved induction and activity of the glucocorticoid-responsive gluconeogenic enzymes phosphoenolpyruvate carboxykinase and glucose-6-phosphatase in fasted KO mice. Glycogen storage is elevated in fed KO liver, with fed glycogenesis rates increased in KO mice. There is normal flux of lactate through gluconeogenesis recovered as plasma glucose, coupled with increased glycogen derived from lactate. These data suggest partial retention of glucocorticoid sensitivity at the level of the liver. We therefore postulate that increased glycogen synthesis may reflect increased flux of glucose-6-phosphate (H6PDH substrate) through to glycogen in the absence of H6PDH mediated metabolism.

Glucose turnover in lean and obese rats of the SHR/N-cp and LA/N-cp strains

1. The relationship between hypertension, obesity, non-insulin-dependent diabetes mellitus and various parameters of glucose metabolism was studied. Lean and obese rats of the SHR/N-cp and LA/N-cp congenic strains were studied at four months of age. 2. Tritium and 14C-labeled glucoses were infused in one set of rats while tritiated water and 14C-labeled alanine were infused in a second group. 3. Glucose oxidation, turnover, conversion to glycogen, fatty acid synthesis, and alanine conversion to glucose were determined, as were blood pressure, pulse pressure and heart rate. 4. The presence of obesity influenced body weight, body fat, de novo fatty acid synthesis, organ weights, glucose mass, glucose oxidation, glucose synthesis, glucose carbon turnover and pulse pressure. 5. It had no effect on glycogen synthesis, tissue glycogen levels, blood glucose, glucose space, or blood pressure. 6. Strain differences were observed in final body weight, organ weights, blood pressure, pulse pressure, hepatic fatty acid synthesis, glucose mass, glucose space, glucose synthesis, liver glycogen levels and glucose conversion to muscle glycogen. 7. Strain-phenotype interaction effects were observed on glucose incorporation into hepatic glycogen, Cori cycle activity, hepatic de novo fatty acid synthesis, final body weight, fat pad weight, heart weight, and mean arterial pressure. 8. These results suggest that although obesity and hypertension are genetic traits in these rats, these traits are independent in their influence on the metabolism of glucose and the development of non-insulin-dependent diabetes mellitus.

Radiorespirometric patterns of [14C]-substrates in rats. II. Differences with the nature and administration route of the injection fluid

The effects on the 14CO2 expiratory patterns of the nature and administration route of the injection fluid containing [14C]-glucose were studied. In the case of rapid intravenous injection, no difference in the radiorespirometric pattern was found with different glucose concentrations and volumes of the aqueous injection fluid. With intravenous injection, lower P1 heights and longer plateaus were recorded with the [14C]-glucose blood mixture as compared to the aqueous injection fluid. This result indicates that [14C]-glucose in the aqueous fluid injected into the blood vessel could be supplied into organs or tissues before mixing with the blood fluid in the blood vessel system. It is also suggested that larger amounts of [14C]-glucose are taken up into organs and tissues when the label was in aqueous solution rather than blood. In the case of intraduodenal infusion, a trace of glucose in a small volume (0.05 ml) of aqueous solution gave a similar radiorespirometric pattern to that given by intravenous injection of aqueous fluid. This indicates that there was fast absorption of glucose by the intestinal mucosa, which was not rate-limiting. A relatively large volume and very high concentration of glucose in the infusion fluid caused suppression of the radiorespirometric pattern. This may reflect physical suppression of intestinal peristalsis.

Thyroxine effects on parameters of glucose turnover in BHE rats fed menhaden oil

The effect of hyperthyroidism on glucose turnover in BHE rats fed menhaden oil was studied. Thyroxine-treated rats had a greater glucose mass, a greater absolute glucose synthesis rate, less hepatic and muscle glycogen levels, and greater hepatic and peripheral fat cell lipogenic rates than nontreated rats. No differences in body weight gain were observed, nor were there differences in blood glucose levels, glucose space, or fractional reversible or irreversible glucose use. These observations suggest that thyroxine and menhaden oil were additive in their effects on glucose metabolism in BHE rats, which are genetically programmed to develop non-insulin-dependent diabetes mellitus.

Milk-substitutes comparable to rat's milk; their preparation, composition and impact on development and metabolism in the artificially reared rat

1. Procedures are described to prepare nutritionally adequate rat milk-substitutes by modifying commercially available processed cow's milk, rich in carbohydrate and low in protein and fat compared with rat's milk. 2. Premilk formulas, prepared as intermediates in the preparation of rat milk-substitutes, are rich in protein but low in their concentration of fat, carbohydrate, and minerals when compared with rat's milk. 3. Premilks were supplemented with lactose, vitamins, minerals, fat as oil mixtures, certain amino acids and other constituents to yield rat milk-substitutes which resemble the known composition of rat's milk in their properties and composition. 4. Detailed analyses of the milk-substitutes show them to be comparable to rat's milk in energy content, pH, osmolarity, the concentration of the macronutrients, fat, protein and carbohydrate, and the major minerals. 5. Rat pups were artificially reared from postnatal day 4 or 5 until days 16-18 by fitting them with gastric cannulas through which the milk-substitutes could be infused automatically. 6. The nutritional impact of the milk-substitutes was assessed by a comparison of growth and metabolic characteristics for artificially reared rats with age-matched sucking rats reared by their mother. 7. Indices which were taken to be appropriate included (a) body-weight gain; (b) the concentration in blood of protein, amino acids, ketone bodies, carnitine, glucose, galactose, lactate, insulin, and the electrolytes calcium, sodium, potassium and chloride; (c) the turnover of glucose and 3-hydroxybutyrate; (d) the concentration in brain of protein, cholesterol, cerebroside sulphate and the activities of the enzymes pyruvate dehydrogenase (EC 1.2.4.1), 3-oxo-acid-CoA transferase (EC 2.8.3.5) and acetoacetyl-CoA ligase (EC 6.2.1.16). 8. The studies suggest that milk-substitutes approximating to rat's milk in composition promote acceptable metabolism in the artificially reared rat pup.

Glucose turnover and recycling in unrestrained and unanesthetized 48-h-old fasting or post-absorptive newborn pigs

The metabolism of glucose has been studied in 48-h-old unanesthetized fasting and post-absorptive sucking piglets. Both [6-3H]- and [U-14C]glucose were administered either by a single injection method or by a primed infusion technique. The rates of glucose turnover and recycling were estimated under steady-state conditions. The rates of glucose turnover and recycling in 48-h-old fasting or post-absorptive piglets were not statistically different when measured using the single injection technique or the primed infusion method. The mean (with SE) rate of glucose turnover was 65.8 (2.5) in post-absorptive and 31.1 (1.9) mumol/kg per min in fasted newborn pigs. Glucose utilization was linearly related to blood glucose concentration; regression analysis indicated a y-intercept of 7.2 mumol/kg per min. As tested by arterio-portal differences the gut was not releasing glucose or galactose in 5 h-post-absorptive sucking newborn pigs. Thus, the higher rates of glucose turnover in post-absorptive newborn pigs compared with fasting ones suggest that hepatic glucose production is enhanced in post-absorptive sucking piglets. The mean (with SE) rates of glucose recycling were four times higher in post-absorptive piglets than in fasting ones, i.e. 14.4 (1.6) and 3.7 (0.5)% of [6-3H]glucose turnover respectively. As liver glycogen was exhausted in 48-h-old sucking piglets, this suggests that hepatic glucose production results from gluconeogenesis.

Regulation of the salvage pathway of purine nucleotide synthesis by the oxidation state of NAD+ in rat heart cells

The rate of salvage of purine nucleotides from hypoxanthine in glycolyzing, cultured rat heart cells was found to be decreased when respiration was suppressed. Pyruvate or phenazine methosulfate, acting as hydrogen acceptors, reversed the effect of the respiratory block. The inhibition and the reversal could not be attributed to the limitation of energy supply or of 5-phosphoribosyl-1-pyrophosphate. A causal connection was, however, shown to exist between this inhibition and the concomitant shift in the redox state of NAD+ in favor of NADH. NADH also inhibited the key enzyme of the salvage pathway, hypoxanthine-guanine-phosphoribosyltransferase, in cell-free extracts. Regulation of purine nucleotide synthesis by the redox state of NAD+ in heart cells might gain significance during transition from respiring to hypoxic state and vice versa.

The presence of NADPH-glyceraldehyde 3-phosphate oxidoreductase in macrophages

The existence of an NADPH-oxidoreductase which utilizes D-glyceraldehyde 3-phosphate as substrate has been demonstrated in mouse peritoneal macrophages. D-Glyceraldehyde could also serve as substrate, albeit with a 10-fold lower efficiency. No NADH oxidation could be demonstrated with either substrate. Addition of D-glyceraldehyde to cultured macrophages increased the rate of activity of the hexose monophosphate shunt to about 65% of the level observed in zymosan A-stimulated macrophages. The possible involvement of the oxidoreductase in this phenomenon and in the inhibitory effect of D-glyceraldehyde on the production of oxygen free radicals by zymosan-stimulated cells is discussed.

Postnatal hypoglycaemia and gluconeogenesis in the newborn rat. Delayed onset of gluconeogenesis in prematurely delivered newborns

The concentrations of glucose and lactate in the blood and of liver glycogen, and the phosphoenolpyruvate carboxykinase activity in liver and kidney of term and preterm newborn rats, were studied during the first 6 h post partum. Rates of lactate turnover and gluconeogenesis in vivo from [U-14C]lactate at 3 h and 6 h post partum were also quantified. The development of the prolonged postnatal hypoglycaemia observed after birth in the premature newborn rat is associated with lower rates of glucose production through glycogenolysis and gluconeogenesis; liver glycogenolysis was the main contributing factor to the glucose available during the neonatal period studied in both groups. Delayed induction of liver phosphoenolpyruvate carboxykinase activity was observed in premature newborn rats. Renal phosphoenolpyruvate carboxykinase activity increased 72% from birth in preterm newborns, but only a 25% increase was found in term newborns during the same experimental period. The gluconeogenesis in vivo from [U-14C]lactate paralleled the appearance of cytosolic phosphoenolpyruvate carboxykinase activity in the liver of both groups of newborns. Blood lactate concentrations remained higher in preterm than in term newborns. The postnatal utilization of lactate via the gluconeogenic pathway in either group of newborns was always less than 20% of the total lactate used. The results presented are discussed in relation to the development of postnatal hypoglycaemia and gluconeogenesis in the premature newborn rat.

Control of the activity of phosphoenolpyruvate carboxykinase and the level of its mRNA in livers of newborn rats. Effect of diabetes, glucose load and glucocorticoids

Streptozotocin treatment produces a typical experimental diabetes in neonates exhibiting hyperglycemia, glucosuria, ketonemia and increased level of fatty acids in the blood. The liver is affected as well, with reduced activity of glycogen synthase and a corresponding decrease in the content of liver glycogen. In contrast, the activity of liver cytosolic phosphoenolpyruvate carboxykinase and the level of its mRNA are not affected. Using a cDNA containing P-pyruvate carboxykinase sequence, the relative abundance of the enzyme mRNA was estimated. The level of the mRNA was readily observed increasing by glucocorticoid treatment or decreasing in response to administered load of glucose. These parallel the changes observed in the activity of the enzyme under these treatments, indicating that the level of P-pyruvate carboxykinase mRNA actually determines that of the enzyme. The failure of diabetes to increase the level of enzyme mRNA and the limited response to glucose loading strongly suggest that the mechanisms controlling the level of P-pyruvate carboxykinase mRNA in neonates are relatively resistant to insulin. This is unique to neonates, since in both the adult and the fetal liver. P-pyruvate carboxykinase readily responds to insulin. The minimal levels of glucocorticoids characteristic of neonates may be associated with this phenomenon.

Diurnal variation and response to food withdrawal of lactose synthesis in lactating rats

1. The incorporation of radiolabelled plasma glucose into mammary lactose was used to measure the rate of lactose synthesis in lightly anaesthetized lactating rats. 2. Lactose synthesis showed a diurnal variation with a minimum at 18:00h 3. Food withdrawal for 6h did not affect lactose synthesis in the early morning but greatly decreased it in the afternoon or evening. 4. Plasma glucose, milk sugars and total galactosyltransferase activity (EC 2.4.1.22) did not show the above changes. 5. Measurements of plasma insulin, which varies diurnally, and experiments with injected insulin suggested that variations of insulin within the physiological range do not account for the changes in lactose synthesis described.

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.

Metabolism of glucose, fructose and lactate in vivo in chronically cannulated foetuses and in suckling lambs

1. Chronically cannulated sheep foetuses and suckling lambs were injected with 14C-labelled glucose, fructose or lactate, and sequential blood samples taken under conditions of minimal stress and without anaesthesia. 2. Gluconeogenesis from lactate was not detectable in foetal sheep, but the pathway was active in suckling lambs. 3. Fructose utilization rates were low in foetal sheep, with no measurable conversion into glucose or lactate. 4. The high rates of irreversible loss of both glucose and lactate in the foetus were decreased in suckling lambs. Radioactivity from labelled glucose entered both the lactate and fructose pools in foetal sheep, and entered the lactate pool in suckling lambs. 5. A model is proposed in which carbon flow between glucose, fructose and lactate has been quantified in foetal sheep.

The effects of inhibition of gluconeogenesis in suckling newborn rats

Inhibition of gluconeogenesis with 3-mercaptopicolinate in suckling newborn rats caused a fall in blood [glucose], but no change in their high plasma [free fatty acid] and blood [ketone bodies]. Active gluconeogenesis seems to be more important than sparing of glucose by high concentrations of fat-derived substrates for the maintenance of normal blood [glucose] in suckling newborn rats.

Glucose turnover rate in newborn rats

In starved newborn rats an increase in glucose turnover rate was observed 4 and 6h after birth, but a dramatic fall occurred at 16h. In suckling newborn rats, no decrease in glucose turnover rate was observed at 16h. The metabolic clearance of glucose did not change in fed or starved animals. The results are discussed in relation to metabolic adaptation to extra-uterine life.

Premature induction of glucokinase in the neonatal rat by thyroid hormone

1. It was shown that the development of liver glucokinase in the rat coincided with a peak in the levels of circulating thyroid hormone at about the 16th postnatal day. 2. Administration of thyroid inhibitors blocked the development of the enzyme and administration of thyroid hormone restored activity to normal levels. 3. Glucokinase could be induced prematurely as early as the 2nd postnatal day by the administration of thyroid hormone followed by daily injection of glucose (10 mg/g body weight). 4. Glucocorticoids and corticotropin failed to induce glucokinase activity prematurely. 5. The postnatal increase in circulating thyroid hormone levels together with increased intake of carbohydrate at weaning may be the normal physiological stimulus for induction of this enzyme.

Glucose metabolism in the newborn rat. Hormonal effects in vivo

1. The concentrations of liver glycogen and plasma d-glucose were measured in caesarian-delivered newborn rats at time-intervals up to 3h after delivery after treatment of the neonatal rats with glucagon, dibutyryl cyclic AMP, cortisol or cortisol+dibutyryl cyclic AMP. Glycogenolysis was promoted by glucagon or dibutyryl cyclic AMP in the third hour after birth but not at earlier times. Cortisol and dibutyryl cyclic AMP together (but neither agent alone) promoted glycogenolysis in the second hour after birth, but no hormone combination was effective in the first postnatal hour. 2. The specific radioactivity of plasma d-glucose was measured as a function of time for up to 75 min after the intraperitoneal injection of d-[6-(14)C]glucose and d-[6-(3)H]glucose into newborn rats at delivery and after treatment with glucagon or actinomycin D. Glucagon-mediated hyperglycaemia at this time was due to an increased rate of glucose formation and a decreased rate of glucose utilization. Actinomycin D prevented glucose formation and accelerated the rate of postnatal hypoglycaemia. 3. The specific radioactivity of plasma l-lactate and the incorporation of (14)C into plasma d-glucose was measured as a function of time after the intraperitoneal injection of l-[U-(14)C]lactate into glucagon- or actinomycin D-treated rats immediately after delivery. The calculated rates of lactate formation were unchanged by either treatment, but lactate utilization was stimulated by glucagon administration. Glucagon stimulated and actinomycin D diminished (14)C incorporation into plasma d-glucose. 4. The factors involved in the initiation of glycogenolysis and gluconeogenesis in the rat immediately after birth are discussed.

Glucose metabolism in the newborn rat. Temporal studies in vivo

1. The concentrations of plasma d-glucose, l-lactate, free fatty acids and ketone bodies and of liver glycogen were measured in caesarian-delivered newborn rats at time-intervals up to 4h after delivery. Glucose and lactate concentrations decreased markedly during the first hours after delivery, but there was a delay of 60-90min before significant glycogen mobilization occurred. 2. The specific radioactivity of plasma d-glucose was measured as a function of time for up to 75min after the intraperitoneal injection of d-[6-(14)C]glucose and d-[6-(3)H]glucose into caesarian-delivered rats at 0, 1 and 2h after delivery. Calculations revealed that there was an appreciable rate of glucose formation at all ages studied, but immediately after delivery this was exceeded by the rate of glucose utilization. Around 2h post partum the rate of glucose utilization decreased dramatically and this coincided with a reversal of the immediately postnatal hypoglycaemia. 3. The specific radioactivity of plasma l-lactate and the incorporation of (14)C into plasma d-glucose and liver glycogen was measured as a function of time after the intraperitoneal injection of l-[U-(14)C]lactate into rats immediately after delivery. The logarithm of the specific radioactivity of plasma l-[U-(14)C]lactate decreased linearly with time for at least 60min after injection and the calculated rate of lactate utilization exceeded the rate of lactate formation. 4. (14)C incorporation into plasma d-glucose was maximal from 30-60min after injection of l-[U-(14)C]lactate and the amount incorporated at 60min was 23% of that present in plasma lactate. Although (14)C was also incorporated into liver glycogen the amount was always less than 3% of that present in plasma glucose. 5. The results are discussed in relationship to the adaptation of the newly born rat to the extra-uterine environment and the possible involvement of gluconeogenesis at this time before feeding is established.

The adaptive behaviour of isoenzyme forms of rat liver alanine aminotransferases during development

1. The activities of the mitochondrial and cytosol isoenzyme forms of l-alanine-glyoxylate and l-alanine-2-oxoglutarate aminotransferases were determined in rat liver during foetal and neonatal development. 2. The mitochondrial glyoxylate aminotransferase activity begins to develop in late-foetal liver, increases rapidly at birth to a peak during suckling and then decreases at weaning to the adult value. 3. The cytosol glyoxylate aminotransferase and the mitochondrial and cytosol 2-oxoglutarate aminotransferase activities first appear prenatally, increase further after birth and then rise to the adult values during weaning. 4. In foetal liver the mitochondrial glyoxylate aminotransferase and the cytosol 2-oxoglutarate aminotransferase activities are increased after injection in utero of glucagon, dibutyryl cyclic AMP (6-N,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate) or thyroxine. The cytosol glyoxylate aminotransferase and the mitochondrial 2-oxoglutarate aminotransferase activities are increased after injection in utero of cortisol or thyroxine. 5. After birth the further normal increases in the mitochondrial and cytosol 2-oxoglutarate aminotransferase activities can be hastened by cortisol injection, whereas the increase in cytosol glyoxylate aminotransferase activity requires cortisol treatment together with the intragastric administration of casein. 6. The results are discussed with reference to the metabolic patterns and the changes in regulatory stimuli (hormonal and dietary) that occur during the period of development.

Gluconeogenesis from lactate in the developing rat. Studies in vivo

1. The specific radioactivity of plasma l-lactate and the incorporation of (14)C into plasma d-glucose, liver glycogen and skeletal-muscle glycogen were measured as a function of time after the intraperitoneal injection of l-[U-(14)C]lactate into 2-, 10- and 30-day-old rats. 2. Between 15 and 60min after the injection of the l-[U-(14)C]lactate, the specific radioactivity of plasma lactate decreased with a half-life of 20-33min in animals at all three ages. 3. At all times after injection examined, the specific radioactivity of plasma glucose of the 2- and 10-day-old rats was at least fourfold greater than that of the 30-day-old rats. 4. Although (14)C was incorporated into liver glycogen the amount incorporated was always less than 5% of that present in plasma glucose. 5. The results are discussed with reference to the factors that may influence the rate of incorporation of (14)C into plasma glucose, and it is concluded that the rate of gluconeogenesis in the 2- and 10-day-old suckling rat is at least twice that of the weaned 30-day-old animal.