Glucose turnover in chronic uremia: increased recycling with diminished oxidation of glucose

Carbohydrate metabolism in uraemia

PURPOSE OF REVIEW

Most uraemic patients are insulin resistant. This review focuses on the occurrence, mechanisms and consequences of this insulin resistance. Hypoglycaemia is also possible in a minority of uraemic patients; its causes are discussed at the end of the review.

RECENT FINDINGS

Insulin resistance is detectable when the glomerular filtration rate is below 50 ml/min per 1.73 m in non-diabetic uraemic individuals. Uraemia can alter insulin sensitivity even in diabetic patients; familial insulin resistance may favour the occurrence of diabetic nephropathy. Although reduced glucose non-oxidative disposal is the most evident defect of carbohydrate metabolism, abnormal glucose oxidation, endogenous glucose production and insulin secretion are also contributors. The accumulation of nitrogenous compounds is the most important mechanism of a specific state of insulin resistance in uraemia. Their identification is progressing, particularly in the field of carbamoylated amino acids. The consequences of chronic renal failure such as anaemia, metabolic acidosis and secondary hyperparathyroidism also indirectly play a role.

SUMMARY

The treatment of uraemia by renal replacement therapies or low-protein diets improves insulin sensitivity. However, patients still have a high cardiovascular risk. The identification of the accumulating molecular species that specifically alter insulin sensitivity is therefore of great interest. The favourable effect of non-specific insulin sensitizers such as glitazone may also help to reduce this risk.

Splanchnic tissues play a crucial role in uremic glucose intolerance

OBJECTIVE

Determine the mechanism of glucose intolerance in chronically uremic subjects.

DESIGN

Comparison of doubly labeled oral glucose tolerance tests.

SUBJECTS

Seven nondialyzed chronically uremic subjects (creatinine, 420 +/- 104 micromol/L) and 7 healthy subjects, matched for age and body mass index.

INTERVENTION

Plasma glucose was labeled by an infusion of dideuterated glucose started 120 minutes before ingestion of 1 g/kg of naturally 13C-enriched corn starch glucose. Glucose levels and oxidation were monitored for 330 minutes after glucose ingestion.

RESULTS

Uremic subjects had normal fasting plasma glucose levels and impaired glucose tolerance with high plasma insulin (P <.001 versus controls). Glucose tolerance was impaired because of an increased total rate of appearance of glucose (cumulated on 330 minutes: uremic, 1,231 +/- 42 mg/kg/330 min, controls, 1,031 +/- 64; P <.05). Peripheral glucose uptake was increased (P <.05) because of an increased nonoxidative disposal (P =.051).

CONCLUSIONS

Although peripheral glucose uptake resisted the stimulatory effect of the high insulin levels, glucose tolerance was impaired through splanchnic metabolic disturbances: reduced splanchnic glucose uptake and increased endogenous glucose production. The respective contribution of these abnormalities remains to be determined.

Comparison of leucine kinetics in endurance-trained and sedentary humans

Whole body leucine kinetics was compared in endurance-trained athletes and sedentary controls matched for age, gender, and body weight. Kinetic studies were performed during 3 h of rest, 1 h of exercise (50% maximal oxygen consumption), and 2 h of recovery. When leucine kinetics were expressed both per unit of body weight and per unit of fat-free mass, both groups demonstrated an increase in leucine oxidation during exercise (P < 0.01). Trained athletes had a greater leucine rate of appearance during exercise and recovery compared with their sedentary counterparts (P < 0.05) and an increased leucine oxidation at all times on the basis of body weight (P < 0.05). However, all of these between-group differences were eliminated when leucine kinetics were corrected for fat-free tissue mass. Therefore, correction of leucine kinetics for fat-free mass may be important when cross-sectional investigations on humans are performed. Furthermore, leucine oxidation, when expressed relative to whole-body oxygen consumption during exercise, was similar between groups. It is concluded that there was no difference between endurance-trained and sedentary humans in whole body leucine kinetics during rest, exercise, or recovery when expressed per unit of fat-free tissue mass.

Defective nonoxidative leucine degradation and endogenous leucine flux in cirrhosis during an amino acid infusion

The metabolic fate of leucine's first and second carbon may be different depending on the tissue in which leucine is metabolized, as well as the prevailing hormonal milieu of that tissue. However, previous studies of leucine kinetics in humans have used only leucine labeled (as tracer) at the first carbon position. Because cirrhosis is associated with factors (such as insulin resistance and altered fuel substrate utilization) that may influence how leucine is degraded, the kinetics of leucine's first and second carbon using a simultaneous infusion of [1-14C] leucine and [2-13C] leucine were studied in the postabsorptive state and during an amino acid infusion in 6 stable cirrhotic patients and 6 matched controls. The data were normalized for different body compartments that were quantified from the dilution of H2 [180] and bromide. The body cell mass, but not body weight or fat-free body mass, was decreased in cirrhosis (P < .001). In response to the amino acid infusion, total leucine appearance from proteolysis and leucine's incorporation into protein increased significantly in both groups, but were higher in cirrhotic patients. Endogenous protein breakdown decreased in normals but remained unchanged in cirrhosis. These alterations in leucine metabolism became more prominent when data were expressed based on the body cell mass rather than on body weight. The oxidation of leucine's first carbon (C1) was decreased in cirrhosis, but the oxidation of leucine's second carbon (C2) did not differ between groups during both the postabsorptive period and the amino acid infusion, while nonoxidative leucine degradation [the difference between the oxidation of leucine's (C1) and (C2)] was also decreased in cirrhosis. In addition, there was a positive correlation between nonoxidative leucine degradation (which represents leucine incorporation into fat), and the respiratory quotient obtained from indirect calorimetry (r = .87; P < .001). These data suggest that the extent of leucine carbon oxidation is dependent on whether fat or carbohydrate is the prevailing fuel substrate. In addition, cirrhotic patients have decreased nonoxidative leucine degradation and are unable to suppress endogenous protein breakdown normally in response to amino acid administration. These abnormalities may contribute to the diminished fat stores and body cell mass commonly observed in cirrhosis.

Acute changes in leucine and phenylalanine kinetics produced by parenteral nutrition in premature infants

To determine the effect of parenteral nutrition on the balance and catabolism of leucine (by oxidation) and phenylalanine (by hydroxylation) and to assess any acute changes in proteolysis and/or protein synthesis, leucine and phenylalanine kinetics were measured by stable isotope tracer infusions in nine 32-wk gestation premature infants under both basal conditions and in response to an i.v. infusion of glucose, lipid, and amino acids. Leucine and phenylalanine balance both changed from negative to positive during parenteral nutrition. However, leucine and phenylalanine catabolism were differently affected by parenteral nutrition; the rate of leucine oxidation increased 2-fold, whereas the rate of phenylalanine hydroxylation was unchanged from basal values. Phenylalanine utilization for protein synthesis and leucine utilization for protein synthesis (based on both plasma leucine and alpha-ketoisocaproic acid enrichments) increased significantly during parenteral nutrition. The endogenous rates of release of leucine (based on plasma leucine enrichment) and phenylalanine (both reflecting proteolysis) were significantly reduced during parenteral nutrition. The endogenous rate of release of leucine (based on alpha-ketoisocaproic acid enrichment) was slightly but not significantly lower during parenteral nutrition. The substantial increase in leucine oxidation without changes in phenylalanine hydroxylation suggests a possible limitation in the phenylalanine/tyrosine supply during parenteral nutrition. In addition, these results suggest that premature infants respond to parenteral nutrition with acute increases in whole body protein synthesis as well as a probable reduction in proteolysis.

Low protein diet in uremia: effects on glucose metabolism and energy production rate

Low-protein diets (LPD) increase insulin-mediated glucose disposal in chronic renal failure (CRF), but the fate of the better utilized glucose and the effect on energy production rate are unknown. Using a two-step (1 and 5 mU x kg(-1) x min(-1)) euglycemic hyperinsulinemic clamp combined with indirect calorimetry, we studied the effects of a LPD (0.3 g x kg(-1) x day(-1), supplemented with essential amino acids and ketoanalogs) in six patients suffering from chronic renal failure. After three months of diet, no significant change was observed concerning glomerular filtration rate, body wt, or arterial pH. In the postabsorptive state, plasma glucose and insulin levels were significantly lower, and energy production rose from 15.72 +/- 0.48 to 17.16 +/- 0.67 Cal x kg(-1) x min(-1) (P < 0.05). Insulin-stimulated glucose oxidation (2.36 +/- 0.29 vs. 3.37 +/- 0.35 mg x kg(-1) x min(-1); P < 0.05 at first clamp step) and nonoxidative disposal (P < 0.05 at both clamp steps) increased after LPD. This confirms that LPD ameliorates insulin sensitivity in CRF, even for low plasma insulin concentrations. Since energy production rate is increased by LPD, the caloric intake should be increased when protein intake is restricted.

Effect of parenteral amino acids on leucine and urea kinetics in preterm infants

OBJECTIVE

To examine the effect of two commonly used parenteral alimentation amino acid mixtures on whole-body leucine and urea kinetics.

METHODS

Ten stable preterm infants were studied during the first 4 weeks after birth. Six infants received a mixture containing higher branched-chain amino acids, lower glycine plus added dicarboxylic acids formulation (Trophamine), and four received a standard amino acid mixture (Aminosyn). Whole-body protein turnover was measured with (1-13C)leucine tracer, and the rate of oxidation of protein was calculated by quantifying the appearance of carbon 13 from leucine in carbon dioxide as well as from rates of urea synthesis estimated by using (15N2)urea tracer. Energy consumption and substrate oxidation were quantified by indirect respiratory calorimetry. Infants were given similar quantities of energy (approximately 61 kcal/kg per day), glucose (approximately 10.7 mg/kg per minute) and protein (approximately 2.1 gm/kg per day).

RESULTS

There was no significant difference in the rate of appearance of leucine in the two groups. However, the fraction of leucine oxidized (p = 0.002) and total rte of oxidation of leucine was higher in the Trophamine group. Thus additional branched-chain amino acids resulted in an increased contribution of C-1 of leucine to expired CO2. The rate of urea N synthesis was also similar in the two groups (Trophamine: 2.92 +/- 0.87 mg N/kg per hour; Aminosyn: 2.70 +/- 1.18 mg N/kg per hour).

CONCLUSIONS

Although the use of Trophamine normalizes the blood amino acid pattern, it does not appear to improve nitrogen/protein kinetics. Furthermore, the additional branched-chain amino acids are disposed of by increased oxidation.

Protein turnover during puberty in normal children

To investigate whether insulin resistance of puberty involves protein metabolism, we compared whole body leucine kinetics in 20 prepubertal Tanner I (TI), and 21 pubertal Tanner II-IV (TII-IV) healthy children. Leucine flux (LRa), oxidation (LOX), and nonoxidative disposal (NOXLD) were measured during primed constant infusion of [1-13C]leucine at baseline and during a stepwise hyperinsulinemic (10 and 40 mU.m-2.min-1)euglycemic clamp in combination with indirect calorimetry. At baseline LRa and LOX were lower in TII-IV vs. TI [LRa: 3.59 +/- 0.17 vs. 4.05 +/- 0.18 mumol.min-1.kg-1 fat-free mass (FFM), P = 0.036; LOX: 0.45 +/- 0.03 vs. 0.59 +/- 0.04 mumol.min-1. FFM-1, P = 0.005], but NOXLD was similar. Insulin-like growth factor I (IGF-I) levels correlated inversely with LRa, NOXLD, and LOX. Energy expenditure correlated positively with LRa, LOX, and NOXLD. During the clamp absolute and percent suppression in LRa were significantly lower in TII-IV than TI. In conclusion, 1) proteolysis and protein oxidation are lower during puberty compared with prepuberty, whereas protein synthesis is unchanged; 2) insulin action in inhibiting proteolysis is decreased during puberty; and 3) increased pubertal IGF-I levels may play a role in decreased protein degradation.

Quantification of carbohydrate oxidation by respiratory gas exchange and isotopic tracers

Estimates of glucose oxidation measured by indirect respiratory calorimetry and by [U-13C]glucose tracer were compared as a function of respiratory exchange ratio (RER) in 14 studies performed on 9 healthy adult subjects. RER was varied between 0.7 and 1.04, either by fasting or by infusing glucose. 13C enrichment of plasma glucose and expired CO2 were measured by mass spectrometry. The two methods gave similar results when the nonprotein respiratory quotient (NPRQ) was between 0.76 and 0.90. Glucose oxidation by the tracer method was quantified to be higher than that by respiratory calorimetry when NPRQ was < 0.76; it was lower than the respiratory calorimetry estimate when NPRQ was > 0.90. The discrepancy between the two methods at low RER may represent the contribution of gluconeogenesis, whereas, at high RER, the discrepancy may be the consequence of lipogenesis. We conclude that respiratory calorimetry and [13C]glucose tracer give comparable results only in a narrow range of RER. These data are important when the disposal of glucose is compared using these techniques in different metabolic states with varying respiratory quotients.

Leucine kinetics during a brief fast in diabetes in pregnancy

The effect of diabetes in pregnancy on leucine turnover and oxidation was examined in 12 insulin-dependent diabetic (IDDM) subjects and 12 gestationally diabetic (GDM) subjects during the third trimester of pregnancy. The data were compared with those in normal pregnant women studied during the same time period and reported previously. Eight of the IDDM subjects were on continuous subcutaneous insulin infusion (insulin pump), and four were on conventional twice-daily insulin treatment. Of the GDM group, seven were on insulin therapy and five were on dietary management. Leucine kinetics were quantified using [1-13C]leucine tracer in combination with respiratory calorimetry and measurement of lean body mass using the H2[18O] dilution method. In addition, glucose kinetics were measured in insulin-treated subjects using [6,6(2)H2]glucose tracer. Despite rigorous metabolic control, fasting plasma glucose (IDDM 5.5 +/- 1.9 mmol/L [P < .05], GDM 4.7 +/- 1.3 [P < .01], controls 3.6 +/- .6, mean +/- SD) and hemoglobin A1 ([HbA1] IDDM 7.9 +/- 1.9%, GDM 7.5% +/- 2.1%) levels were higher in diabetic subjects. Although total insulin levels were higher in insulin-treated diabetic subjects, free-insulin concentrations were similar in all groups. Rates of excretion of urinary urea nitrogen and respiratory quotients were also similar. The rate of glucose turnover was lower in insulin-treated subjects compared with normals. Leucine flux, a measure of the rate of protein breakdown, and leucine oxidation were higher in IDDM and insulin-treated GDM subjects. The rate of leucine oxidation was increased in conventionally managed IDDM and insulin-treated GDM subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose metabolism in a term infant with transient hyperinsulinism and high carbohydrate intake

Transient hyperinsulinaemia is a well-known cause of hypoglycaemia in newborn infants. The hypoglycaemia may be caused by a decreased glucose production and/or an increased glucose uptake. Whether the increased uptake is caused by increased glucose oxidation or increased non-oxidative disposal is not known. The aim of this study was to investigate the fate of the large amount of glucose infused in a term infant who developed hypoglycaemia due to transient hyperinsulinaemia shortly after birth and was treated with high glucose infusions. On day 6 an indirect calorimetry study was performed, together with a glucose turnover study. Carbohydrate intake was 13.6 mg/kg per minute (19.6 g/kg per day). Both studies were repeated on day 11, when carbohydrate intake was normalised to 7.8 mg/kg per minute (11.2 g/kg per day). Glucose oxidation was 28% higher and non-oxidative glucose disposal was 257% higher on day 6 as compared to day 11. Our results indicate that hypoglycaemia during hyperinsulinism is the result of increased non-oxidative disposal of glucose and not increased glucose oxidation. The results indicate a remarkable capacity of the newborn for lipogenesis during high carbohydrate intake.

In vivo differences between the turnover rates of leucine and leucine's ketoacid in stable cirrhosis

Based on urinary nitrogen excretion, previous studies have indicated increased protein breakdown rates in cirrhosis. However, studies using [1-13C]-leucine infusion methodology have found normal protein breakdown rates. Because abnormal partitioning between extracellular and intracellular leucine exists in cirrhosis, plasma enrichment of leucine's keto acid (KIC), a marker of intracellular leucine, may more accurately reflect protein metabolism than plasma [1-13C]leucine enrichment. Therefore, protein breakdown and oxidation were calculated using both [1-13C]leucine and [1-13C]KIC and compared with urinary nitrogen excretion in seven cirrhotics and seven matched controls after an overnight fast. The ratio of KIC and leucine plasma enrichment was decreased (P less than 0.001) in cirrhosis because of lower KIC enrichment (P less than 0.006). Cirrhotics had increased rates of protein breakdown (P less than 0.006) and protein oxidation (P less than 0.05) based on KIC (P less than 0.006) but not leucine enrichment. In controls, protein oxidation calculated from urinary nitrogen excretion did not differ from KIC results (0.88 +/- 0.08 vs. 0.83 +/- 0.06) but was higher than the leucine method (0.88 +/- 0.08 vs. 0.73 +/- 0.05; P less than 0.01). However, in cirrhotics protein oxidation based on urinary nitrogen was lower than the KIC methodology (P less than 0.01). Therefore, cirrhotics have accelerated rates of protein breakdown and oxidation associated with increased extrarenal nitrogen loss. Furthermore, these results suggest abnormal leucine transport across cell membranes.

Leucine kinetics and fuel utilization during a brief fast in human pregnancy

To examine proteolysis, protein and leucine oxidation, and fuel utilization during a brief fast (approximately 17 hours) in human pregnancy, we determined leucine kinetics, urea nitrogen excretion, and respiratory quotient (RQ) in 11 pregnant subjects during the second half of gestation, and in 11 normal nonpregnant controls. The total rate of appearance (Ra) of leucine was similar in the pregnant and control groups (pregnant 4.99 +/- 0.60 v control 5.25 +/- 1.60 mmol/h [mean +/- SD]). However, leucine Ra per kilogram was significantly lower in pregnant subjects (pregnant 68 +/- 7 v control 82 +/- 13 mumol/kg/h, P less than .01). In addition, urinary urea nitrogen excretion was also significantly less in pregnant subjects (pregnant 3.74 +/- 1.09 v control 5.58 +/- 1.6 mg/kg/h, P less than .01). The RQ measured in the pregnant group was significantly higher than controls (0.82 +/- 0.05 v 0.76 +/- 0.04, P = .01), resulting in higher calculated carbohydrate oxidation rates during fasting in pregnancy. These data suggest that total rates of proteolysis (reflected by leucine flux) are similar in pregnant and nonpregnant subjects after an overnight fast. When normalized to body weight, proteolysis is lower in pregnant subjects. Urea excretion rates are also lower in pregnancy. These findings support the hypothesis that there is a pregnancy-induced adaptation to conserve maternal protein stores during a brief fast. The higher rate of carbohydrate oxidation during fasting in pregnancy may be a reflection of the fetal-placental unit's use of glucose as its predominant oxidative substrate.

Measurements of total body and extracellular water in cirrhotic patients with and without ascites

Using H2[18O] tracer isotope dilution and corrected bromide space as standard reference techniques, we determined total body water and extracellular water in cirrhotic patients with (four men and four women) and without (seven men and six women) ascites and compared them with a normal control group (eight men and six women). These results were then compared with calculations of total body and extracellular water determined by the bioelectrical impedance analysis technique. According to H2[18(O)] dilution, total body water was similar in cirrhotic patients without ascites and in controls (60.8% +/- 2.1% vs. 60.3% +/- 1.4% body wt), but was increased in patients with ascites (69.7% +/- 1.2% body wt; p less than 0.002). Correlation was excellent between the H2[18(O)] dilution and bioelectrical impedance measurements of total body water in controls and cirrhotic patients without ascites (r = 0.98; p less than 0.0001). However, this correlation was poor in cirrhotic patients with ascites (r = 0.17; not significant). According to the bromide space, extracellular water (expressed as a percentage of total body water) was increased in cirrhotic patients with (57.8% +/- 1.8%; p less than 0.001) and without (44.0% +/- 1.2%; p less than 0.001) ascites compared with controls (36.6% +/- 1.0%). A poor correlation (r = 0.41; p less than 0.13) was seen for extracellular water measurements between the bromide space method and the bioelectrical impedance method, which failed to detect the differences among the three groups observed with the bromide space technique. Furthermore, bioelectrical impedance failed to detect any change in total body or extracellular water after paracentesis, with a degree of inaccuracy that increased linearly as the amount of ascitic fluid removed increased (r = 0.97; p less than 0.001). All these intergroup comparisons remained the same, whether the analysis was of both men and women combined or for each gender individually. However, we saw differences between men and women in the control group and cirrhotic group without ascites. These results demonstrate that abnormalities in water homeostasis and compartmentalization between intracellular (the difference between total body and extracellular water fluid) and extracellular water may exist in cirrhosis whether or not fluid accumulation is clinically evident. These data further indicate that alterations in the metabolically active body cell mass (as represented by intracellular water) in cirrhosis may occur independently of total body water and calculated fat-free body mass. In addition, gender is an important variable to control for in studies of this type.(ABSTRACT TRUNCATED AT 400 WORDS)

Determination of [U-13C]glucose turnover into various metabolite pools for the differential diagnosis of lactic acidemias

Stable-isotope tracer experiments performed in vitro are evaluated for their utility in differentiating between pyruvate dehydrogenase and cytochrome oxidase deficiencies, two of several enzyme defects commonly associated with the lactic acidemias. Fibroblasts of enzyme-deficient individuals and of age-matched controls are grown in medium containing [U-13C]glucose. Direct analysis of cells and conditioned culture medium provides only minor differences in the organic acid/amino acid GC-MS profiles, making differentiation of enzyme defects difficult by this method. However, differences have been found in the glucose turnover into various cell metabolites, making differentiation of these two enzyme defects possible. The cellular pool of glutamic acid experiences 13C-enrichment in both the control and cytochrome oxidase deficient lines, but not in the pyruvate dehydrogenase-deficient line. The cellular pool of an unknown, possibly an aminopentose sugar, on the other hand, experiences 13C-enrichment in the pyruvate dehydrogenase and control lines, but not in the cytochrome oxidase line. These observations, as well as other differences in the extent of enrichment into various metabolite pools, suggest that this stable-isotope approach, in vitro, is feasible and may allow these two enzyme defects to be differentiated in a definitive manner. Such stable-isotope experiments are easy to carry out with cultured cells and are inexpensive. Applications of the technique to other genetic disorders might be appropriate.

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)

Leucine metabolism in human newborns

The present study was designed to 1) determine whether a relationship exists between newborn birth weight and leucine metabolism and 2) compare leucine and energy metabolism in a period of rapid growth and development (i.e., newborn) with a constant nongrowth period (i.e., adult). Leucine kinetics and energy expenditure were measured in the postabsorptive state in 12 normal full-term newborns in early neonatal life and in 11 normal adults using a primed constant L-[1-13C]leucine infusion combined with respiratory calorimetry. A significant positive correlation between newborn birth weight and leucine flux was observed. Leucine flux per kilogram body weight was significantly greater in newborns compared with adults, as was leucine oxidation and energy expenditure. When normalized to metabolic body weight (W0.75), newborns and adults had similar rates of energy expenditure, leucine flux, and oxidation. The allometric exponent relating leucine flux to body weight (0.80) was nearly equivalent to that describing the energy expenditure-to-body weight relationship (0.79) and both were similar to the 0.75 metabolic weight exponent. These data suggest the following. 1) A relationship exists between newborn birth weight and protein metabolism, as reflected by the correlation between leucine flux when expressed as micromoles per kilogram per hour and birth weight. It is speculated that this relationship may be the result of differences in previous protein and energy supplies. 2) The high rate of leucine flux measured in newborns probably reflects the rapid remodeling of protein that occurs in this period of development, even during fasting.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic contribution to glucagon-induced glucose production and insulin secretion in uremia

Spontaneous or propranolol-induced hypoglycemia can occur in uremic humans. We studied glucose kinetics (using [3-3H]glucose) in five uremic humans 24 h after hemodialysis and in seven normal controls. The effect of glucagon infusion at rates of 3, 6, 12, and 18 ng X kg-1 X min-1 at 60-min intervals was compared with either saline or beta-adrenergic blockade (propranolol infusion). In uremics, plasma glucose increased by 20-25% and by 40-50% at the 3 and 6 ng X kg-1 X min-1 glucagon doses, respectively, with no further increases at higher infusion rates. Glucose production increased transiently and in tandem with glucose uptake at each glucagon increment (P less than 0.0001). During beta-adrenergic blockade, the effect of glucagon in stimulating glucose production was blunted by 14-24% at the 6-18 ng X kg-1 X min-1 doses (P less than 0.05). During saline infusion, plasma insulin concentrations increased progressively to peak levels fourfold above basal at the 18 ng X kg-1 X min-1 dose. This increase in plasma insulin was virtually abolished by concomitant beta-adrenergic blockade (P = 0.0002). In contrast to uremic subjects, normal controls exhibited lesser degrees of hyperglycemia and hyperinsulinemia at all glucagon infusion rates. Propranolol infusion had no effect on the increments in glucose production and uptake nor on the plasma insulin response. These results suggest that in uremic humans propranolol independently reduces the hepatic response to glucagon and the insulin secretory response to hyperglycemia and/or hyperglucagonemia. These observations provide a possible mechanism for the adrenergic regulation of glucose homeostasis in uremia.

Leucine metabolism in stable cirrhosis

Alterations in protein and amino acid metabolism have been postulated to explain the frequent observations of muscle wasting and decreased plasma branched-chain amino acid concentrations in cirrhosis. In order to investigate the changes in protein metabolism, we have measured the rates of leucine turnover and oxidation in six stable, biopsy-proven cirrhotics and six age and sex-matched healthy control subjects after an overnight fast, using [1-13C]leucine tracer. Following a primed constant-rate infusion of [1-13C]leucine, the 13C enrichments of plasma leucine and expired CO2 were used to estimate leucine turnover and oxidation, respectively. Fat-free body mass was estimated from the measurements of total body water as quantified by H2[18O] tracer dilution. The rates of CO2 production and oxygen consumption were measured hourly during the study period, using open-circuit respiratory calorimetry. Urinary urea, ammonia and total nitrogen excretion rates were quantified from timed urine samples. Even though the plasma leucine levels were lower in cirrhotics as compared with controls (100.5 +/- 17.1 vs. 138.3 +/- 20.4 mumoles per liter, mean +/- S.D., p less than 0.001), the rates of leucine turnover were not significantly different in the two groups (89.4 +/- 19.0 vs. 87.8 +/- 19.0 mumoles per kg X hr). In contrast, the rates of leucine oxidation were significantly reduced in cirrhosis (8.1 +/- 2.5 vs. 12.7 +/- 3.1 mumoles per kg X hr, p less than 0.01). When all subjects were considered, the leucine oxidation rate was correlated with plasma leucine concentration (r = 0.62, p less than 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)