Carbohydrate metabolism in pregnancy XVI: longitudinal estimates of the effects of pregnancy on D-(63H) glucose and D-(6-14C) glucose turnovers during fasting in the rat

Alterations in basal glucose metabolism during late pregnancy in the conscious dog

We assessed basal glucose metabolism in 16 female nonpregnant (NP) and 16 late-pregnant (P) conscious, 18-h-fasted dogs that had catheters inserted into the hepatic and portal veins and femoral artery approximately 17 days before the experiment. Pregnancy resulted in lower arterial plasma insulin (11 +/- 1 and 4 +/- 1 microU/ml in NP and P, respectively, P < 0.05), but plasma glucose (5.9 +/- 0.1 and 5.6 +/- 0.1 mg/dl in NP and P, respectively) and glucagon (39 +/- 3 and 36 +/- 2 pg/ml in NP and P, respectively) were not different. Net hepatic glucose output was greater in pregnancy (42.1 +/- 3.1 and 56.7 +/- 4.0 micromol. 100 g liver(-1).min(-1) in NP and P, respectively, P < 0.05). Total net hepatic gluconeogenic substrate uptake (lactate, alanine, glycerol, and amino acids), a close estimate of the gluconeogenic rate, was not different between the groups (20.6 +/- 2.8 and 21.2 +/- 1.8 micromol. 100 g liver(-1). min(-1) in NP and P, respectively), indicating that the increment in net hepatic glucose output resulted from an increase in the contribution of glycogenolytically derived glucose. However, total glycogenolysis was not altered in pregnancy. Ketogenesis was enhanced nearly threefold by pregnancy (6.9 +/- 1.2 and 18.2 +/- 3.4 micromol. 100 g liver(-1).min(-1) in NP and P, respectively), despite equivalent net hepatic nonesterified fatty acid uptake. Thus late pregnancy in the dog is not accompanied by changes in the absolute rates of gluconeogenesis or glycogenolysis. Rather, repartitioning of the glucose released from glycogen is responsible for the increase in hepatic glucose production.

Effect of reduced inspired oxygen on fetal growth and maternal glucose metabolism in rat pregnancy

The effect of prolonged exposure to a reduced fraction of inspired oxygen ([FiO2] 0.17 for 3 days) on maternal glucose kinetics, placental glucose transporters GLUT1 and GLUT3, and fetal growth was examined in rat pregnancy. Arterial and venous catheters were placed 3 days before the study. [3-(3)H]glucose tracer and deuterium labeling of water were used to measure the rates of glucose turnover and gluconeogenesis (GNG), respectively. Glucose uptake by maternal tissues was measured using [14C]2-deoxyglucose. Exposure to a reduced FiO2 resulted in a significant decrease (mean +/- SE) in fetal weight (room air, 4.02 +/- 0.04 g; 0.17 FiO2, 3.27 +/- 0.6 g, P < .02). There was a significant increase in the maternal-fetal glucose gradient (maternal-fetal glucose ratio: room air, 1.48 +/- 0.11; 0.17 FiO2, 2.26 +/- 0.24, P < .05), but there was no change in the maternal or fetal blood lactate concentration. No significant change in maternal blood pH was observed; however, a significant decrease in the blood partial pressure of O2 (PO2) occurred (room air, 97 +/- 0.5 torr; 0.17 FiO2, 81 +/- 1.8) on day 3. There was no change in the rate of turnover of glucose or GNG in the maternal compartment, nor was there any effect on glucose uptake by the maternal tissues. Placental GLUT1 and GLUT3 mRNA were not different in the control or experimental animals. We conclude that a mild reduction in the FiO2 for 3 days in rat pregnancy results in a significant fetal growth restriction that is not related to any observed alteration in maternal glucose metabolism. The lower glucose concentration in the fetal blood may be the consequence of an increase in fetal glucose metabolism, thereby resulting in an increased maternal-fetal gradient of glucose.

Physiological reduction in fasting plasma glucose concentration in the first trimester of normal pregnancy: the diabetes in early pregnancy study

Previous studies indicate that fasting plasma glucose decreases during gestation, but the timing and extent are not consistent from study to study. We had an opportunity to examine this question in the normal pregnancy cohort of women studied in the Diabetes in Early Pregnancy Study. Subjects were monitored to identify pregnancy by human chorionic gonadotropin testing, enrolled within 21 days of conception, and screened to rule out gestational diabetes at the juncture of the second and third trimesters. All subjects were instructed to fast overnight for 10 to 12 hours. Three hundred sixty-one women were studied between 6 and 12 weeks of gestation. A median decrease in plasma glucose of 2 mg/dL was observed between weeks 6 and 10 (P=.007). In a smaller group of subjects evaluated through the third trimester, little further glucose reduction was observed. A reduction in glycosylated hemoglobin levels between 10 and 20 weeks (P=.002) followed the earlier reduction in first trimester glucose levels. Analysis by body mass index (BMI) showed a smaller first trimester reduction with increasing BMI, and none among severely obese women (BMI > 29.9 kg/m2). The decline in fasting plasma glucose in pregnancy begins early in the first trimester, well before fetal glucose requirements can contribute to the decline in the glucose level. Thereafter, plasma glucose levels decrease little. These results suggest that in the setting in which this study was performed (an overnight fast) maternal physiologic adjustments account for a reduction in plasma glucose early in the first trimester of pregnancy, and possibly even later in gestation as well.

Glucose turnover and gluconeogenesis in human pregnancy

The rate of appearance (Ra) of glucose in plasma and the contribution of gluconeogenesis were quantified in normal pregnant women early ( approximately 10 wk) and late ( approximately 34 wk) in gestation. Their data were compared with those of normal nonpregnant women. Glucose Ra was measured using the [U-13C]glucose tracer dilution method. Gluconeogenesis was quantified by the appearance of 2H on carbon 5 and 6 of glucose after deuterium labeling of body water pool. Weight-specific glucose Ra was unchanged during pregnancy (nonpregnant, 1.89+/-0.24; first trimester, 2.05+/-0.21; and third trimester 2.17+/-0.28 mg/kg.min, mean+/-SD), while total glucose Ra was significantly increased (early, 133.5+/-7.2; late, 162.6+/-16.4 mg/min; P = 0.005). The fractional contribution of gluconeogenesis via pyruvate measured by 2H enrichment on C-6 of glucose (45-61%), and of total gluconeogenesis quantified from 2H enrichment on C-5 of glucose (i.e. , including glycerol [68-85%]) was not significantly different between pregnant and nonpregnant women. Inasmuch as total glucose Ra was significantly increased, total gluconeogenesis was also increased in pregnancy (early pregnancy, 94.7+/-15.9 mg/min; late pregnancy, 122.7+/-9.3 mg/min; P = 0.003). These data demonstrate the ability of the mother to adapt to the increasing fetal demands for glucose with advancing gestation. The mechanism for this unique quantitative adjustment to the fetal demands remains undefined.

The set point for maternal glucose homeostasis is lowered during late pregnancy in the rat: the role of the islet beta-cell and liver

The aim of this study was to determine the effects of late pregnancy on the ability of insulin to suppress maternal hepatic glucose production in the rat. Unlike in most previous studies, suppression of hepatic glucose production was measured at levels of glycaemia above the relatively hypoglycaemic basal pregnant level. Glucose kinetics were measured using steady-state tracer methodology in chronically catheterised, conscious virgin control and pregnant rats, firstly, during basal and low-dose hyperinsulinaemic euglycaemic clamp conditions and secondly, during a three-step glucose infusion protocol (glucose infusion rates of 0, 60 and 150 mumol.kg-1. min-1). During the clamps, plasma glucose levels were not different (6.1 +/- 0.4 vs 6.5 +/- 0.3 mmol/l, pregnant vs virgin; N.S.), but plasma insulin levels were higher in the pregnant rats (242 +/- 30 vs 154 +/- 18 pmol/l. pregnant vs virgin; p < 0.05) most probably due to stimulated endogenous insulin release in this group. Hepatic glucose production was suppressed from basal levels by 41% in virgin and 90% in pregnant rats. During the glucose infusion studies, at matched insulin levels (147 +/- 10 vs 152 +/- 14 pmol/l), but at plasma glucose levels which were much lower in the pregnant rats (5.5 +/- 0.2 vs 8.4 +/- 0.6 mmol/l, pregnant vs virgin; p < 0.0001), hepatic glucose production was shown to be suppressed by a similar degree in both groups (41 +/- 5 vs 51 +/- 5% from basal, pregnant vs virgin; N.S.). Both the plasma insulin and percentage suppression of hepatic glucose production dose responses to plasma glucose were markedly shifted to the left indicating that the plasma glucose set point is lowered in pregnancy. In conclusion, suppression of hepatic glucose production by insulin is not impaired and the set point for plasma glucose homeostasis is lowered during late pregnancy in the rat.

The feto-placental glucose steal phenomenon is a major cause of maternal metabolic adaptation during late pregnancy in the rat

The aim of this study was to determine the extent to which a feto-placental glucose steal phenomenon contributes to the process of maternal metabolic adaptation to late pregnancy. Glucose metabolism was studied in virgin control, pregnant rats and virgin rats with a phlorizin-induced model of the feto-placental glucose steal phenomenon. Whole body glucose kinetics and glucose uptake into individual tissues were measured in anaesthetised rats basally and during hyperinsulinaemic euglycaemic clamps. The basal glucose metabolism of the pregnant rats was closely mimicked by the phlorizin-treated rats. Basal plasma glucose was 39% and 38% lower (p < 0.0001 for both); hepatic glucose production was 21% and 26% higher (p < 0.05 for both); and plasma glucose clearance was 109% and 104% higher (p < 0.0001 for both) in the pregnant and phlorizin-treated rats, respectively, compared to the control rats. Basal glucose uptake into peripheral tissues was lower in both the pregnant and phlorizin-treated compared to the control rats, being most evident in heart (p < 0.01 for both) and brown adipose tissue (p < 0.001 for both). In the clamp studies, impairment of glucose uptake into skeletal muscle was observed in both the pregnant and phlorizin-treated rats compared to the control rats. In conclusion, the feto-placental glucose steal phenomenon is a major contributing factor to postabsorptive glucose metabolism in late pregnancy. This phenomenon also contributes to the impairment of maternal insulin-stimulated peripheral glucose uptake.

Glucose metabolism during fasting through human pregnancy: comparison of tracer method with respiratory calorimetry

Glucose turnover and glucose oxidation were quantified in six normal pregnant women serially throughout pregnancy, using [U-13C]glucose tracer in combination with open-circuit indirect respiratory calorimetry. Five normal nonpregnant women were studied for comparison. With advancing gestation and increase in maternal body weight, there was a proportionate increase in the rate of appearance (Ra) of glucose so that Ra expressed per kilogram body weight did not change from the first to third trimester. The tracer measured rate of glucose oxidation expressed per kilogram body weight also did not change significantly throughout pregnancy. Oxygen consumption (VO2) in pregnant subjects did not differ from that in nonpregnant subjects. However, the respiratory exchange ratio (RER) increased significantly during pregnancy (0.88 +/- 0.53 3rd trimester and 0.76 +/- 0.50 nonpregnant, P < 0.01). The estimated contribution of carbohydrate to VO2 measured by respiratory calorimetry was greater than that measured by the tracer method. This discrepancy became wider as the respiratory quotient increased in late pregnancy. These data suggest that maternal glucose metabolism adjusts throughout pregnancy to meet the increased demands of the conceptus. The discrepancy between tracer method and respiratory calorimetry was probably due to the contribution of (fetal) lipogenesis and (maternal) gluconeogenesis to RER.

Progressive suppression of muscle glucose utilization during pregnancy

Glucose utilization indices (GUIs) were measured in heart and a range of skeletal muscles in conscious, unrestrained, virgin or pregnant rats in the absorptive and post-absorptive phases. A clear effect of pregnancy to diminish muscle GUIs was identified, the magnitude of which was greatest in late gestation in the absorptive phase. Differences in the time courses and magnitudes of the response to pregnancy were observed between individual muscles. The effects of pregnancy are discussed in relation to an increased availability of lipid fuels and to decreased insulin and glucose concentrations.

Effect of partial starvation on in vitro spontaneous activity and glycogen levels of uterine smooth muscle from pregnant and non pregnant rats

Effects of a restricted-diet (50% of the normal feeding) given during 14 days, on the isometric developed tension (IDT) of uterine horns isolated from pregnant and non pregnant (diestrous) rats, incubated in a KRB-medium without glucose, were explored. In 14 days-pregnant rats, dietary restriction did not alter the contractile activity with respect to normal-fed controls. Besides, levels of uterine glycogen, immediately after killing the animals or after 60 min incubation, remained unaltered. In advanced pregnancy partial starvation led to decay of spontaneous contractile activity after 60 min incubation. However, the considerable increment in the levels of tissue glycogen at 0 time was not modified, nor its decrease at the end of the in vivo experimental period. In non-pregnant rats, a reduced feeding did not alter the development of contractile tension, but exerted a pronounced effect on the glycogen levels: these were significantly lower than controls at 0 time but suffered no changes after 60 min on in vitro activity. Indomethacin appeared to have no effect on the spontaneous contractile activity of 14 days-pregnant rats. It significantly depressed contractility in 21 days-pregnant rats. Indomethacin did not modify the levels of glycogen in any of the experimental groups.

Effects of ethanol on glucose turnover in pregnant rats

Glucose turnover was measured in term pregnant rats fed ethanol (30% of caloric intake) throughout gestation. Ethanol ingestion significantly reduced maternal weight gain and term fetal body weight when compared to pair-fed or ad libitum-fed controls. At term the blood glucose level and 6-3H-glucose turnover were reduced when compared to either control group. The rate of gluconeogenic recycling, indicated by the difference between 6-3H and 6-14C-glucose turnover determinations, was reduced by ethanol ingestion to half that of the control groups. Glucose turnover correlated with both conceptus weight and blood glucose level. Impaired maternal glucose homeostasis, including a reduced gluconeogenic response to the metabolic demands of late pregnancy, may thus contribute to the effects of ethanol on intrauterine growth.

Glucose-alanine relationship in normal human pregnancy

In order to quantify the glucose-alanine relationship in normal human pregnancy, the turnover rates of alanine and the incorporation of alanine carbon into glucose were quantified in 15 pregnant women during the last 4 weeks of gestation following a ten-hour fast. Eight nonpregnant women of similar age group were studied as controls. L-[2,3-13C2]Alanine and D[6,6-2H2]glucose were infused as tracers. The 13C enrichment of alanine, lactate, and glucose and the deuterium enrichment of glucose were measured by gas chromatography-mass spectrometry. In five pregnant and five nonpregnant women, the contribution of alanine carbon to expired CO2 directly and via glucose was estimated by combining indirect respiratory calorimetry with the tracer infusions. The alanine turnover rates in the pregnant and nonpregnant women were similar (pregnant, 4.43 +/- 0.82 mumol/kg x min; nonpregnant, 4.11 +/- 1.08 mumol/kg x min, mean +/- SD). However, the fraction of alanine incorporated into glucose was significantly lower during pregnancy (23.5 +/- 8.3% v 30.8 +/- 8.2%, P less than .04). In pregnant women, 20% of lactate pool was derived from alanine as compared with 28% in nonpregnant subjects (P less than .02). Twenty-four percent of alanine turnover was converted to CO2 in both pregnant and nonpregnant women. The plasma insulin concentration was increased significantly during pregnancy (P less than .05). These data suggest that gluconeogenesis from alanine is attenuated during pregnancy. This decrease in gluconeogenesis is not the result of decreased alanine flux, but due to intrinsic intrahepatic mechanism such as decreased deamination of alanine mediated by the predominant insulin effect or a decreased hepatic uptake of alanine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of feeding a high-fat diet during pregnancy on glucose metabolism in the rat

To alter glucose homeostasis in a period of great glucose demand, pregnant rats were submitted to a high-fat diet and compared to virgin rats. In virgin rats, blood glucose, ketone bodies, plasma insulin, and free fatty acids were not affected by the diet consumed. Glucose turnover measured in the postabsorptive period was slightly decreased in virgin rats fed a high-fat diet compared to rats fed a standard diet. Assuming that the glucose turnover rate is representative for the 24-hour average endogenous glucose production, in rats fed a standard diet the daily carbohydrate intake (9.2 +/- 0.7 g/d) exceeded the glucose turnover rate (4 +/- 0.2 g/d) and could meet the glucose requirement. In rats fed a high-fat diet the carbohydrate intake (2.7 +/- 0.2 g/d) was lower than the glucose turnover rate (3.8 +/- 0.2 g/d), which demonstrated the need for an active endogenous glucose production. Blood glucose, ketone bodies, plasma insulin, and free fatty acid concentrations followed the same patterns during pregnancy in rats fed a standard diet compared to rats fed a high-fat diet. The glucose turnover rate in the postabsorptive period was no more decreased by the high-fat diet in pregnant rats compared to virgin rats despite the greater glucose demand. In late pregnancy the glucose turnover rate was increased up to 70%.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the availability of substrates on hepatic and renal gluconeogenesis in the fasted late pregnant rat

Studies were conducted to examine the role of gluconeogenetic substrate availability on glucose production in the fasted late pregnant rat. Virgin and 21-day pregnant rats were studied after 24 hours' food deprivation. Pregnant animals showed decreased circulating glucose and gluconeogenic amino acid and increased plasma glycerol concentration. Glucose formation was studied in vivo two, five, and ten minutes after the intravenous administration of two concentrations of 14C-alanine, 14C-pyruvate, or 14C-glycerol. Concentrations of 0.2 mmols of 14C-glycerol or 14C-pyruvate, but not of 14C-alanine, enhanced 14C-glucose production in pregnant rats, whereas 1 mmol of any of the three 14C-substrates always enhanced 14C-glucose production in these rats. Both 1 mmol/L and 5 mmol/L 14C-alanine increased 14C-glucose formation in 90-minute-incubated liver slices of fasted pregnant rats, in spite of decreased cytosolic activity of alanine aminotransferase. The three substrates enhanced "in vitro" renal gluconeogenesis in pregnant rats. Under all experimental conditions studied, labeled glycerol was converted more efficiently into glucose than equivalent amounts of any other substrate used, and this difference was greater in pregnant, than in virgin animals. Results indicate that, in spite of enhanced gluconeogenetic activity, maternal glucose production in the fasted state at late gestation is limited by the deficiency of certain substrates, such as amino acids. It is proposed that glycerol derived from enhanced maternal adipose tissue lipolysis constitutes a preferential gluconeogenetic substrate in comparison with others, such as alanine, that are more efficiently transferred through the placenta to the fetus.

Carbohydrate metabolism in the fetus and neonate and altered neonatal glucoregulation

Altered glucose homeostasis in the neonate often results from antecedent events during fetal life. This article describes the normal and altered development of glucoregulatory capabilities during perinatal life and relates it to problems of hypo- and hyperglycemia in the neonate.