Regulation of leucine metabolism in man: a stable isotope study.

Leucine catabolism is regulated by either of the first two degradative steps: (reversible) transamination to the keto acid or subsequent decarboxylation. A method is described to measure rates of leucine transamination, reamination, and keto acid oxidation. The method is applied directly to humans by infusing the nonradioactive tracer, L-[15N,1-13C]leucine. Leucine transamination was found to be operating several times faster than the keto acid decarboxylation and to be of equal magnitude in adult human males under two different dietary conditions, postabsorptive and fed. These results indicate that decarboxylation, not transamination, is the rate-limiting step in normal human leucine metabolism.

Role of counterregulatory hormones in the catabolic response to stress.

Patients with major injury or illness develop protein wasting, hypermetabolism, and hyperglycemia with increased glucose flux. To assess the role of elevated counterregulatory hormones in this response, we simultaneously infused cortisol (6 mg/m2 per h), glucagon (4 ng/kg per min), epinephrine (0.6 microgram/m2 per min), and norepinephrine (0.8 micrograms/m2 per min) for 72 h into five obese subjects receiving only intravenous glucose (150 g/d). Four obese subjects received cortisol alone under identical conditions. Combined infusion maintained plasma hormone elevations typical of severe stress for 3 d. This caused a sustained increase in plasma glucose (60-80%), glucose production (100%), and total glucose flux (40%), despite persistent hyperinsulinemia. In contrast, resting metabolic rate changed little (9% rise, P = NS). Urinary nitrogen excretion promptly doubled and remained increased by approximately 4 g/d, reflecting increased excretion of urea and ammonia. Virtually all plasma amino acids declined. The increment in nitrogen excretion was similar in three additional combined infusion studies performed in 3-d fasted subjects not receiving glucose. Cortisol alone produced a smaller glycemic response (20-25%), an initially smaller insulin response, and a delayed rise in nitrogen excretion. By day 3, however, daily nitrogen excretion was equal to the combined group as was the elevation in plasma insulin. Most plasma amino acids rose rather than fell. In both infusion protocols nitrogen wasting was accompanied by only modest increments in 3-methylhistidine excretion (approximately 20-30%) and no significant change in leucine flux. We conclude: (a) Prolonged elevations of multiple stress hormones cause persistent hyperglycemia, increased glucose turnover, and increased nitrogen loss; (b) The sustained nitrogen loss is no greater than that produced by cortisol alone; (c) Glucagon, epinephrine, and norepinephrine transiently augment cortisol-induced nitrogen loss and persistently accentuate hyperglycemia; (d) Counterregulatory hormones contribute to, but are probably not the sole mediators of the massive nitrogen loss, muscle proteolysis, and hypermetabolism seen in some clinical settings of severe stress.

Mechanism of increased gluconeogenesis in noninsulin-dependent diabetes mellitus. Role of alterations in systemic, hepatic, and muscle lactate and alanine metabolism.

To assess the mechanisms responsible for increased gluconeogenesis in noninsulin-dependent diabetes mellitus (NIDDM), we infused [3-14C]lactate, [3-13C]alanine, and [6-3H]glucose in 10 postabsorptive NIDDM subjects and in 9 age- and weight-matched nondiabetic volunteers and measured systemic appearance of alanine and lactate, their release from forearm tissues, and their conversion into plasma glucose (corrected for Krebs cycle carbon exchange). Systemic appearance of lactate and alanine were both significantly greater in diabetic subjects (18.2 +/- 0.9 and 5.8 +/- 0.4 mumol/kg/min, respectively) than in the nondiabetic volunteers (12.6 +/- 0.7 and 4.2 +/- 0.3 mumol/kg/min, respectively, P less than 0.001 and P less than 0.01). Conversions of lactate and alanine to glucose were also both significantly greater in NIDDM subjects (8.6 +/- 0.5 and 2.4 +/- 0.1 mumole/kg/min, respectively) than in nondiabetic volunteers (4.2 +/- 0.4 and 1.8 +/- 0.1 mumol/kg/min, respectively, P less than 0.001 and P less than 0.025). The proportion of systemic alanine appearance converted to glucose was not increased in NIDDM subjects (42.7 +/- 1.9 vs. 44.2 +/- 2.9% in nondiabetic volunteers), whereas the proportion of systemic lactate appearance converted to glucose was increased in NIDDM subjects (48.3 +/- 3.8 vs. 34.2 +/- 3.8% in nondiabetic volunteers, P less than 0.025); the latter increased hepatic efficiency accounted for approximately 40% of the increased lactate conversion to glucose. Neither forearm nor total body muscle lactate and alanine release was significantly different in NIDDM and nondiabetic volunteers. Therefore, we conclude that increased substrate delivery to the liver and increased efficiency of intrahepatic substrate conversion to glucose are both important factors for the increased gluconeogenesis of NIDDM and that tissues other than muscle are responsible for the increased delivery of gluconeogenic precursors to the liver.

Lipoproteins containing the truncated apolipoprotein, Apo B-89, are cleared from human plasma more rapidly than Apo B-100-containing lipoproteins in vivo.

We have reported previously on two truncations of apolipoprotein B (apo B-40 and apo B-89) in a kindred with hypobetalipoproteinemia. Premature stop codons were found to be responsible for both apo B-40 and apo B-89, but the physiologic mechanisms accounting for the reduced plasma concentrations of these proteins have not been determined in vivo. This study investigates the metabolism of apo B-89 in two subjects heterozygous for apo B-89/apo B-100 and in one apo B-40/apo B-89 compound heterozygote. In both heterozygotes total apo B concentration is approximately 30% of normal and apo B-89 is present in lower concentrations in plasma than apo B-100. After the administration of [1-13C]leucine as a primed constant infusion over 8 h, 13C enrichments of plasma leucine as well as enrichments of VLDL-, IDL-, and LDL-apo B-89 leucine and VLDL-, IDL-, and LDL-apo B-100 leucine were measured over 110 h. Enrichment values were subsequently converted to tracer/tracee ratios and a multicompartmental model was used to estimate metabolic parameters. In both apo B-89/apo B-100 heterozygotes apo B-89 and apo B-100 were produced at similar rates. Respective transport rates of apo B-89 and apo B-100 for subject 1 were 2.13 +/- 0.18 and 2.56 +/- 0.13 mg.kg-1.d-1, and for subject 2, 6.59 +/- 0.18 and 8.23 +/- 0.39 mg.kg-1.d-1. However, fractional catabolic rates of VLDL, IDL, and LDL particles containing apo B-89 were 1.4-3 times higher than the rates for corresponding apo B-100-containing particles. Metabolic parameters of apo B-89 in the apo B-40/apo B-89 compound heterozygote compared favorably with those established for apo B-89 in apo B-89/apo B-100 heterozygotes. Thus, the enhanced catabolism of VLDL, IDL, and LDL particles containing the truncated apolipoprotein is responsible for the relatively low levels of apo B-89 seen in these subjects.

Lipid transport in the human newborn. Palmitate and glycerol turnover and the contribution of glycerol to neonatal hepatic glucose output.

Free fatty acid (FFA) transport was measured in 11 and glycerol turnover in 5 newborns with continuous tracer infusion of [1-(13)C]palmitate or [2-(13)C]glycerol, respectively. In addition, simultaneous determination of glucose production in the latter group with [6,6-(2)H(2)]glucose tracer and measurement of the appearance rate of [(13)C]glucose derived from [(13)C]glycerol allowed calculation of gluconeogenesis from glycerol.The average FFA inflow rate was 11.5+/-1.7 mumol kg(-1)min(-1), 2.5-4.5 h after the last feeding, and 16.7+/-2.8 mumol kg(-1)min(-1), 5-12 h after the last meal. These rates are comparable to those found in adults only after 8-16 h and approximately 72 h of fasting, respectively. FFA inflow in the newborn was directly correlated with time of fasting, plasma FFA level, and plasma glycerol level. Palmitate clearance and fractional removal were inversely related to palmitate level. Glycerol flux averaged 4.4+/-0.5 mumol kg(-1)min(-1), a value three- to fourfold that of the postabsorptive adult. Approximately 75% of transported glycerol was converted to glucose and represented 5.0+/-0.6% of hepatic glucose production. Furthermore, there was a direct relationship between glycerol turnover and the fraction of glucose coming from glycerol. Despite the absolutely elevated neonatal FFA and glycerol transport rates, these were quantitatively similar to values found in adults with comparable elevated substrate levels. Furthermore, other similarities with the adult in the relationships between inflow transport and substrate values, and between transport and fractional removal suggest that the regulatory aspects of lipid transport in man are already well developed by the first day of life.

Ketone body transport in the human neonate and infant.

Using a continuous intravenous infusion of D-(-)-3-hydroxy[4,4,4-2H3]butyrate tracer, we measured total ketone body transport in 12 infants: six newborns, four 1-6-mo-olds, one diabetic, and one hyperinsulinemic infant. Ketone body inflow-outflow transport (flux) averaged 17.3 +/- 1.4 mumol kg-1 min-1 in the neonates, a value not different from that of 20.6 +/- 0.9 mumol kg-1 min-1 measured in the older infants. This rate was accelerated to 32.2 mumol kg-1 min-1 in the diabetic and slowed to 5.0 mumol kg-1 min-1 in the hyperinsulinemic child. As in the adult, ketone turnover was directly proportional to free fatty acid and ketone body concentrations, while ketone clearance declined as the circulatory content of ketone bodies increased. Compared with the adult, however, ketone body turnover rates of 12.8-21.9 mumol kg-1 min-1 in newborns fasted for less than 8 h, and rates of 17.9-26.0 mumol kg-1 min-1 in older infants fasted for less than 10 h, were in a range found in adults only after several days of total fasting. If the bulk of transported ketone body fuels are oxidized in the infant as they are in the adult, ketone bodies could account for as much as 25% of the neonate's basal energy requirements in the first several days of life. These studies demonstrate active ketogenesis and quantitatively important ketone body fuel transport in the human infant. Furthermore, the qualitatively similar relationships between the newborn and the adult relative to free fatty acid concentration and ketone inflow, and with regard to ketone concentration and clearance rate, suggest that intrahepatic and extrahepatic regulatory systems controlling ketone body metabolism are well established by early postnatal life in humans.

The effect of hemodialysis on protein metabolism. A leucine kinetic study.

To assess the effect of hemodialysis on protein metabolism, leucine flux was measured in seven patients before, during, and after high efficiency hemodialysis using cuprophane dialyzers and bicarbonate dialysate during a primed-constant infusion of L-[1-13C]leucine. The kinetics [mumol/kg per h, mean +/- SD] are as follows: leucine appearance into the plasma leucine pool was 86 +/- 28, 80 +/- 28, and 85 +/- 25, respectively, before, during, and after dialysis. Leucine appearance into the whole body leucine pool, derived from plasma [1-13C]alpha-ketoisocaproate enrichment, was 118 +/- 31, 118 +/- 31, and 114 +/- 28 before, during, and after dialysis, respectively. In the absence of leucine intake, appearance rate reflects protein degradation, which was clearly unaffected by dialysis. Leucine oxidation rate was 17.3 +/- 7.8 before, decreased to 13.8 +/- 7.8 during, and increased to 18.9 +/- 10.3 after dialysis (P = 0.027). Leucine protein incorporation was 101 +/- 26 before, was reduced to 89 +/- 23 during, and returned to 95 +/- 23 after dialysis (P = 0.13). Leucine net balance, the difference between leucine protein incorporation and leucine release from endogenous degradation, was -17.3 +/- 7.8 before, decreased to -28.5 +/- 11.0 during, and returned to -18.9 +/- 10.3 after dialysis (P < 0.0001). This markedly more negative leucine balance during dialysis was accountable by dialysate leucine loss, which was 14.4 +/- 6.2 mumol/kg per h. These data suggest that hemodialysis using a cuprophane membrane did not acutely induce protein degradation. It was, nevertheless, a net catabolic event because protein synthesis was reduced and amino acid was lost into the dialysate.

Epinephrine plasma metabolic clearance rates and physiologic thresholds for metabolic and hemodynamic actions in man.

To determine the plasma epinephrine thresholds for its metabolic and hemodynamic actions and plasma epinephrine metabolic clearance rates, 60-min intravenous epinephrine infusions at nominal rates of 0.1, 0.5, 1.0, 2.5, and 5.0 microgram/min were performed in each of six normal human subjects. These 30 infusions resulted in steady-state plasma epinephrine concentrations ranging from 24 to 1,020 pg/ml. Plasma epinephrine thresholds were 50-100 pg/ml for increments in heart rate, 75-125 pg/ml for increments in blood glycerol and systolic blood pressure, 150-200 pg/ml for increments in plasma glucose (the resultant of increments in glucose production and decrements in glucose clearance), blood lactate, blood beta-hydroxybutyrate, and diastolic blood pressure, and greater than 400 pg/ml for early decrements in plasma insulin. Changes in blood alanine, plasma glucagon, plasma growth hormone, and plasma cortisol were not detected. At steady-state plasma epinephrine concentrations of 24-74 pg/ml, values overlapping the basal normal range, the mean (+/-SE) plasma metabolic clearance rate of epinephrine was 52 +/- 4 ml x min-1 x kg-1; this value rose to 89 +/- 6 ml x min-1 x kg-1 (P less than 0.01) at steady-state epinephrine concentrations of 90-1,020 pg/ml. We conclude that in human subjects: (a) the plasma epinephrine thresholds for its hemodynamic and metabolic actions lie within the physiologic range, (b) epinephrine and norepinephrine accelerate their own metabolic clearance, and (c) epinephrine is 10 times more potent than norepinephrine.

Branched-chain amino acid nitrogen transfer to alamine in vivo in dogs. Direct isotopic determination with [15N]leucine.

To investigate the contribution of branched-chain amino acids as a nitrogen source for alanine in vivo, dogs were infused with l-[(15)N]leucine, l-[U-(14)C]leucine, l-[2,3,3,3-(2)H(4)]alanine, and d-[6,6-(2)H(2)]-glucose. (14)C and (15)N isotopic equilibrium in plasma leucine, and deuterium enrichment in arterial and femoral plasma glucose and alanine were achieved within 3 h of initiation of the respective isotope infusion in all animals. The average flux of leucine determined by [(15)N]leucine was 5.4 mumol.kg(-1).min(-1), whereas using [(14)C]leucine it was 3.7 mumol.kg(-1).min(-1). Turnover rates for alanine and glucose were 11.0 and 17.2 mumol.kg(-1).min(-1), respectively.[(15)N]alanine was detected as early as 30 min, but nitrogen isotopic equilibrium in alanine was not achieved until 6 h. The absolute rate of leucine nitrogen transfer to alanine was 1.92 mumol.kg(-1).min(-1), which represented 41-73% (mean 53%) of leucine's nitrogen and 15-20% (mean 18%) of alanine's nitrogen. Fractional extraction of alanine and leucine by the dog hindlimb was 35 and 24%, respectively. Average net alanine balance was -6.7 mumol.leg(-1).min(-1), reflecting a release rate (17.4 mumol.kg(-1).min(-1)) that exceeded the rate of uptake (10.8 mumol.leg(-1).min(-1)). Of the leucine taken up by the hindlimb, 34% transferred its nitrogen to alanine and 8% was oxidized to CO(2). Since the latter value reflects transamination as well as irreversible catabolism, the nitrogen derived from the oxidation of leucine by the hindlimb could account for only 25% of the observed (15)N incorporation into alanine. The significantly faster flux of leucine nitrogen when compared with leucine carbon suggests significant recycling of the leucine alpha-ketoacid. These studies demonstrate that leucine is a major donor of nitrogen to circulating alanine in vivo.

Effects of physiologic levels of glucagon and growth hormone on human carbohydrate and lipid metabolism. Studies involving administration of exogenous hormone during suppression of endogenous hormone secretion with somatostatin.

To study the individual effects of glucagon and growth hormone on human carbohydrate and lipid metabolism, endogenous secretion of both hormones was simultaneously suppressed with somatostatin and physiologic circulating levels of one or the other hormone were reproduced by exogenous infusion. The interaction of these hormones with insulin was evaluated by performing these studies in juvenile-onset, insulin-deficient diabetic subjects both during infusion of insulin and after its withdrawal. Infusion of glucagon (1 ng/kg-min) during suppression of its endogenous secretion with somatostatin produced circulating hormone levels of approximately 200 pg/ml. When glucagon was infused along with insulin, plasma glucose levels rose from 94 +/- 8 to 126 +/- 12 mg/100 ml over 1 h (P less than 0.01); growth hormone, beta-hydroxy-butyrate, alanine, FFA, and glycerol levels did not change. When insulin was withdrawn, plasma glucose, beta-hydroxybutyrate, FFA, and glycerol all rose to higher levels (P less than 0.01) than those observed under similar conditions when somatostatin alone had been infused to suppress glucagon secretion. Thus, under appropriate conditions, physiologic levels of glucagon can stimulate lipolysis and cause hyperketonemia and hyperglycemia in man; insulin antagonizes the lipolytic and ketogenic effects of glucagon more effectively than the hyperglycemic effect. Infusion of growth hormone (1 mug/kg-h) during suppression of its endogenous secretion with somastostatin produced circulating hormone levels of approximately 6 ng/ml. When growth hormone was administered along with insulin, no effects were observed. After insulin was withdrawn, plasma beta-hydroxybutyrate, glycerol, and FFA all rose to higher levels (P less than 0.01) than those observed during infusion of somatostatin alone when growth hormone secretion was suppressed; no difference in plasma glucose, alanine, and glucagon levels was evident. Thus, under appropriate conditions, physiologic levels of growth hormone can augment lipolysis and ketonemia in man, but these actions are ordinarily not apparent in the presence of physiologic levels of insulin.

Epinephrine plasma thresholds for lipolytic effects in man: measurements of fatty acid transport with [l-13C]palmitic acid.

To determine the plasma epinephrine thresholds for its lipolytic effect, 60-min epinephrine infusions at nominal rates of 0.1, 0.5, 1.0, 2.5, and 5.0 micrograms/min were performed in each of four normal young adult men while they also received a simultaneous infusion of [1-13C]palmitic acid to estimate inflow transport of plasma free fatty acids. These 20 infusions resulted in steady-state plasma epinephrine concentrations ranging from 12 to 870 pg/ml. Plasma epinephrine thresholds for changes in blood glucose, lactate, and beta-hydroxybutyrate were in the 150--200-pg/ml range reported by us previously (Clutter, W. E., D. M. Bier, S. D. Shah, and P. E. Cryer. 1980. J. Clin. Invest. 66: 94--101.). Increments in plasma glycerol and free fatty acids and in the inflow and outflow transport of palmitate, however, occurred at lower plasma epinephrine thresholds in the range of 75 to 125 pg/ml. Palmitate clearance was unaffected at any steady-state epinephrine level produced. These data indicate that (a) the lipolytic effects of epinephrine occur at plasma levels approximately threefold basal values and (b) lipolysis is more sensitive than glycogenolysis to increments in plasma epinephrine.

Insulin-mediated reduction of whole body protein breakdown. Dose-response effects on leucine metabolism in postabsorptive men.

In vivo effects of insulin on plasma leucine and alanine kinetics were determined in healthy postabsorptive young men (n = 5) employing 360-min primed, constant infusions of L-[1-13C]leucine and L-[15N]alanine during separate single rate euglycemic insulin infusions. Serum insulin concentrations of 16.4 +/- 0.8, 29.1 +/- 2.7, 75.3 +/- 5.0, and 2,407 +/- 56 microU/ml were achieved. Changes in plasma 3-methyl-histidine (3-MeHis) were obtained as an independent qualitative indicator of insulin-mediated reduction in proteolysis. Hepatic glucose output was evaluated at the lowest insulin level using D-[6,6-2H2]glucose. The data demonstrate a dose-response effect of insulin to reduce leucine flux, from basal values of 77 +/- 1 to 70 +/- 2, 64 +/- 3, 57 +/- 3, and 52 +/- 4 mumol(kg X h)-1 at the 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.01). A parallel, progressive reduction in 3-MeHis from 5.8 +/- 0.3 to 4.3 +/- 0.3 microM was revealed. Leucine oxidation estimated from the 13C-enrichment of expired CO2 and plasma leucine (12 +/- 1 mumol[kg X h]-1) and from the 13C-enrichment of CO2 and plasma alpha-ketoisocaproate (19 +/- 2 mumol[kg X h]-1) increased at the 16 microU/ml insulin level to 16 +/- 1 and 24 +/- 2 mumol(kg X h)-1, respectively (P less than 0.05 for each), but did not increase at higher insulin levels. Alanine flux (206 +/- 13 mumol(kg X h)-1) did not increase during the clamp, but alanine de novo synthesis increased in all studies from basal rates of 150 +/- 13 to 168 +/- 23, 185 +/- 21, 213 +/- 29, and 187 +/- 15 mumol(kg X h)-1 at 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.05). These data indicate the presence of insulin-dependent suppression of leucine entry into the plasma compartment in man secondary to a reduction in proteolysis and the stimulation of alanine synthesis during euglycemic hyperinsulinemia.

Glutamine: a major gluconeogenic precursor and vehicle for interorgan carbon transport in man.

To compare glutamine and alanine as gluconeogenic precursors, we simultaneously measured their systemic turnovers, clearances, and incorporation into plasma glucose, their skeletal muscle uptake and release, and the proportion of their appearance in plasma directly due to their release from protein in postabsorptive normal volunteers. We infused the volunteers with [U-14C] glutamine, [3-13C] alanine, [2H5] phenylalanine, and [6-3H] glucose to isotopic steady state and used the forearm balance technique. We found that glutamine appearance in plasma exceeded that of alanine (5.76 +/- 0.26 vs. 4.40 +/- 0.33 mumol.kg-1.min-1, P < 0.001), while alanine clearance exceeded glutamine clearance (14.7 +/- 1.3 vs. 9.3 +/- 0.8 ml.kg-1.min-1, P < 0.001). Glutamine appearance in plasma directly due to its release from protein was more than double that of alanine (2.45 +/- 0.25 vs. 1.16 +/- 0.12 mumol.kg-1.min-1, P < 0.001). Although overall carbon transfer to glucose from glutamine and alanine was comparable (3.53 +/- 0.24 vs 3.47 +/- 0.32 atoms.kg-1.min-1), nearly twice as much glucose carbon came from protein derived glutamine than alanine (1.48 +/- 0.15 vs 0.88 +/- 0.09 atoms.kg-1.min-1, P < 0.01). Finally, forearm muscle released more glutamine than alanine (0.88 +/- 0.05 vs 0.48 +/- 0.05 mumol.100 ml-1.min-1, P < 0.01). We conclude that in postabsorptive humans glutamine is quantitatively more important than alanine for transporting protein-derived carbon through plasma and adding these carbons to the glucose pool.

The Prader-Willi syndrome. Regulation of fat transport.

Studies of fat mobilization and transport are reported in six patients with the Prader-Willi syndrome. Two patients had carbohydrate intolerance. One of these had a low and the other an augmented insulin response to glucose challenge. Following challenge with glucose, three of the four nondiabetics had normal insulin responses or increased responses consistent with their obesity; the other nondiabetic had insulinopenia. Measurements of the effects of norepinephrine, insulin, glucose, and 5-methylpyrazole-3-carboxylic acid on plasma levels of FFA, glycerol, and ketones provide no evidence for abnormal regulation of mobilization of fat from adipose tissue. Measurements of plasma lipids and postheparin lipolytic activity are consistent with normal uptake of fat into adipose tissue, and normal fatty acid composition of adipose tissue gives no evidence for abnormal lipogenesis.

Leucine metabolism in type II diabetes mellitus.

Severe muscle wasting is a well-recognized characteristic of untreated insulin-deficient diabetes mellitus, a condition in which leucine turnover and oxidation are accelerated. To ascertain whether a similar circumstance exists in type II diabetes when insulin is present but with reduced efficacy, we investigated leucine turnover and oxidation in five obese type II diabetic women by tracer infusion of L-[1-13C,15N]leucine in the postabsorptive state both before and after intensive insulin therapy. With conventional treatment, the type II diabetic women received 61 +/- 33 (SD) U/day of insulin, and their fasting plasma glucose averaged 194 +/- 41 (SD) mg/dl. Leucine carbon flux (QC), nitrogen flux (QN), and oxidation (C) averaged 6.4 +/- 1.2, 15.6 +/- 4.6, and 1.4 +/- 0.3 mmol/h, respectively. These values were not different from the respective values of 6.6 +/- 1.3, 17.0 +/- 8.3, and 1.0 +/- 0.2 mmol/h in matched obese nondiabetic controls, suggesting that leucine metabolism is not altered in insulin-treated type II diabetics. After a week of intensive insulin therapy in which the same diabetic subjects received 94 +/- 36 U/day of insulin, postabsorptive plasma glucose declined to 117 +/- 26 mg/dl. Leucine QC (6.2 +/- 1.0), QN (14.8 +/- 3.7), and C (1.5 +/- 0.5 mmol/h) were unaltered by the increased insulin therapy. Thus, obese type II diabetics had normal leucine kinetics but were hyperglycemic while receiving conventional insulin therapy. Additional intensive insulin therapy in these diabetic subjects improved plasma glucose but did not alter leucine kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Stable-label intravenous glucose tolerance test minimal model.

The minimal model approach to estimating insulin sensitivity (Sl) and glucose effectiveness in promoting its own disposition at basal insulin (SG) is a powerful tool that has been underutilized given its potential applications. In part, this has been due to its inability to separate insulin and glucose effects on peripheral uptake from their effects on hepatic glucose inflow. Prior enhancements, with radiotracer labeling of the dosage, permit this separation but are unsuitable for use in pregnancy and childhood. In this study, we labeled the intravenous glucose tolerance test (IVGTT) dosage with [6,6-2H2]glucose, [2-2H]glucose, or both stable isotopically labeled glucose tracers and modeled glucose kinetics in six postabsorptive, nonobese adults. As previously found with the radiotracer model, the tracer-estimated S*l derived from the stable-label IVGTT was greater than Sl in each case except one, and the tracer-estimated SG* was less than SG in each instance. More importantly, however, the stable-label IVGTT estimated each parameter with an average precision of +/- 5% (range 3-9%) compared to average precisions of +/- 74% (range 7-309%) for SG and +/- 22% (range 3-72%) for Sl. In addition, because of the different metabolic fates of the two deuterated tracers, there were minor differences in basal insulin-derived measures of glucose effectiveness, but these differences were negligible for parameters describing insulin-stimulated processes. In conclusion, the stable-label IVGTT is a simple, highly precise means of assessing insulin sensitivity and glucose effectiveness at basal insulin that can be used to measure these parameters in individuals of all ages, including children and pregnant women.

Gluconeogenesis in very low birth weight infants receiving total parenteral nutrition.

Very low birth weight (VLBW) infants are dependent on total parenteral nutrition (TPN) to prevent hypoglycemia and provide a sufficient energy intake. However, diminished tolerance for parenteral glucose delivered at high rates frequently provokes hyperglycemia. We hypothesized that when their glucose supply is reduced to prevent hyperglycemia, VLBW infants can maintain normoglycemia via gluconeogenesis from glycerol and amino acids. Twenty infants born at 27 +/- 0.2 (mean +/- SE) gestational weeks and having a birth weight of 996 +/- 28 g, received lipids (1.6 +/- 0.1 mg x kg(-1) x min(-1)), protein (2.2 +/- 0.1 mg x kg(-1) x min(-1)), and glucose (3.1 +/- 0.1 mg x kg(-1) x min(-1) [17.1 +/- 0.2 micromol x kg(-1) x min(-1)]) parenterally over a period of 8-12 h on day 5.0 +/- 0.2 of life. Gluconeogenesis was estimated using [U-13C]glucose (n = 8) or [2-(13)C] glycerol (n = 6) and mass isotopomer distribution analysis (MIDA), or 2H2O (n = 6) and the rate of deuterium incorporation in carbon 6 of glucose. Blood glucose averaged 3.0 +/- 0.1 mmol/l; plasma glucose appearance rate (glucose Ra), 28.8 +/- 1.1 micromol x kg(-1) x min(-1); and glucose production rate (GPR), 10.7 +/- 1.0 micromol x kg(-1) x min(-1). The [U-13C]glucose and [2-(13)C]glycerol tracers provided similar estimates of gluconeogenesis, averaging 28 +/- 2 and 26 +/- 2% of glucose Ra and 72 +/- 5 and 73 +/- 9% of GPR, respectively. Glycerol contributed 64 +/- 5% of total gluconeogenesis. Gluconeogenesis measured by 2H2O, which does not include the contribution from glycerol, was comparable to the nonglycerol fraction of gluconeogenesis derived by the [2-(13)C]glycerol MIDA. We conclude that in VLBW infants receiving TPN, normoglycemia was maintained during reduced glucose infusion by glucose production primarily derived from gluconeogenesis, and that glycerol was the principal gluconeogenic substrate.

Measurement of "true" glucose production rates in infancy and childhood with 6,6-dideuteroglucose.

"New" glucose production has been measured in 54 infants and children for the first time by continuous three-to-four-hour influsion of the safe, nonradioactive tracer 6,6-dideuteroglucose. The use of combined gas chromatography--mass spectrometry with monitoring of selected ions allowed deuterium enrichment in blood glucose to be measured on microliter samples with an error of less than 2 per cent. In the young child, glucose production increased in a slightly curvilinear manner from 1 kg. to 25 kg. body weight, when it reached 140 mg. per minute, almost the adult value of 173 mg. per minute (2.28 +/- 0.23 mg./kg.-min., mean +/- S.E.). Normalized for weight, glucose production in premature infants was 5.46 +/- 0.31 mg./kg.-min., in term neonates averaged 6.07 +/- 0.27 mg./kg.-min., in children below the age of six years was 7.1 +/- 0.27 mg./kg.-min., and in late childhood averaged 5.4 +/- 0.28 mg./kg.-min. Relative to estimated brain weight, however, glucose production was essentially linear from the 1-kg. premature infant to the 80-kg. adult. These data, the first measurements of "new" glucose production in childhood, suggest that brain size may be a principal determinant of those factors that regulate hepatic glucose output throughout life.

Whole body de novo amino acid synthesis in type I (insulin-dependent) diabetes studied with stable isotope-labeled leucine, alanine, and glycine.

Dynamic aspects of whole body alanine and glycine metabolism have been explored in insulin-dependent (type I) diabetic subjects. Using a primed, continuous intravenous (i.v.) infusion of [2H3]alanine and [15N]glycine given simultaneously with [1-13C]leucine, whole body alanine and glycine fluxes and their rates of de novo synthesis were measured in 6 diabetic young men. Subjects were studied in the postabsorptive state, after blood glucose was clamped overnight at 15.2 +/- 0.3 mM, and then, on the following night, at 5.9 +/- 0.2 mM (insulin infusion rates of 0.24 +/- 0.09 and 1.65 +/- 0.20 U/h, respectively). In the normoglycemic state, leucine, alanine, and glycine fluxes averaged 88 +/- 4, 378 +/- 39, and 155 +/- 8 mumol X kg-1 X h-1, respectively. Based on the leucine flux, alanine and glycine de novo synthesis rates were 264 +/- 36 and 67 +/- 8 mumol X kg-1 X h-1. In the hyperglycemic state, leucine flux increased 23% (P less than 0.01), alanine flux rose slightly (+5%) but significantly (P less than 0.05), while alanine de novo synthesis and glycine flux remained unchanged and glycine de novo synthesis decreased by 33% (P less than 0.001). These results show that small alterations in peripheral alanine inflow in the hyperglycemic state reflect increased proteolysis and suggest that increased circulating plasma glucose does not contribute to de novo alanine synthesis in the absence of adequate insulin effect and/or augmented glucose tissue uptake. These observations also reveal the importance of insulin in the maintenance of whole body leucine economy, since a lower rate of insulin administration was associated with an increased rate of leucine oxidation.

Quantitative aspects of glucose production and metabolism in healthy elderly subjects.

The metabolic basis for the reduced glucose tolerance that occurs during aging in humans has been explored with the aid of a primed constant intravenous infusion method of labeled glucose (6-3H; 6,6,2H- and U-13C-glucose). Healthy young adult men and women (24 +/- 3 yr) and elderly men and women (75 +/- 4 yr) participated in a series of studies designed to quantify rates of plasma glucose appearance, oxidation, and recycling while subjects were in the postabsorptive (basal) state and to determine rates of hepatic glucose production and glucose disappearance in response to intravenous glucose at approximately 1 and 2 mg x kg-1min-1 and also 4 mg x kg-1min-1 without or with a simultaneous infusion of insulin to maintain normoglycemia. Basal rates of glucose production were 2.41 +/- 0.06 and 2.18 /+/- 0.05 mg x kg-1min-1 in the young adults and elderly, respectively (P less than 0.05). Recycling of glucose carbon and glucose oxidation rates did not differ significantly between the two age groups. Infusion of unlabeled glucose reduced hepatic glucose production to the same extent in the two groups, indicating that the mechanisms responsible for altered hepatic glucose production with intravenous glucose administration remain intact during human aging. Plasma insulin changes were similar in young adult and elderly subjects receiving 4 mg x kg-1min-1 unlabeled glucose except that the higher plasma glucose levels in the elderly were associated with higher insulin levels. For elderly subjects, the amount of exogenous insulin required to maintain normoglycemia at the 4 mg x kg-1min-1 glucose infusion rate was about twice that necessary in young adults.