W. F. Schwenk

Underestimation of glucose turnover measured with [6-3H]- and [6,6-2H]- but not [6-14C]glucose during hyperinsulinemia in humans.

Recent studies indicate that hydrogen-labeled glucose tracers underestimate glucose turnover in humans under conditions of high flux. The cause of this underestimation is unknown. To determine whether the error is time-, pool-, model-, or insulin-dependent, glucose turnover was measured simultaneously with [6-3H]-, [6,6-2H2]-, and [6-14C]glucose during a 7-h infusion of either insulin (1 mU · kg−1 · min−1) or saline. During the insulin infusion, steady-state glucose turnover measured with both [6-3H]glucose (8.0 ± 0.5 mg · kg−1 · min−1) and [6,6-2H2]glucose (7.6 ± 0.5 mg kg−1 min−1) was lower (P < .01) than either the glucose infusion rate required to maintain euglycemia (9.8 ± 0.6 mg · kg−1 · min−1) or glucose turnover determined with [6-14 C]glucose and corrected for Cori cycle activity (9.8 ± 0.7 mg · kg−1 · min1). Consequently “negative” glucose production rates (P <.01) were obtained with either [6-3 H]- or [6,6-2 H2]-but not [6-14 C]glucose. The difference between turnover estimated with [6-3 H]glucose and actual glucose disposal (or 14C glucose flux) did not decrease with time and was not dependent on duration of isotope infusion. During saline infusion, estimates of glucose turnover were similar regardless of the glucose tracer used. High-performance liquid chromatography of the radioactive glucose tracer and plasma revealed the presence of a tritiated nonglucose contaminant. Although the contaminant represented only 1.5% of the radioactivity in the [6-3H]glucose infusate, its clearance was 10-fold less (P <.001) than that of [6-3H]glucose. This resulted in accumulation in plasma, with the contaminant accounting for 16.6 ± 2.09 and 10.8 ± 0.9% of what customarily is assumed to be plasma glucose radioactivity during the insulin or saline infusion, respectively (P < .01). When corrected for the presence of the contaminant, glucose turnover determined with [6-3H]glucose during insulin infusion (9.5 ± 0.6 mg · kg−1 · min−1) no longer differed from either the glucose infusion rate or that determined with [6-14C]glucose. Therefore, the underestimation of glucose turnover during insulin infusion and negative glucose production rates observed with traditional methods to analyze plasma radioactivity and commercially available tracers is the result of an artifactual increase in [6-3H]glucose specific activity. The etiology of the underestimation of glucose turnover with [6,6-2H2]glucose remains to be determined.

Decreased Uptake of Glucose by Human Forearm During Infusion of Leucine, Isoleucine, or Threonine

Competition between glucose and free fatty acids as metabolic fuels is supported by both in vitro and in vivo data, but whether amino acids can also compete with glucose as a source of energy in vivo remains to be established. To determine the effect of increased availability of an amino acid on whole-body glucose flux and glucose carbon uptake by the human forearm, five groups of overnight-fasted normal subjects were infused with either saline, leucine (at 0.5 or 1.0μmol · kg−1 · min−1), isoleucine (0.5 μmol · kg−1 · min−1), or threonine (0.5 μmol · kg−1 · min−1). Plasma glucose concentrations and glucose flux decreased similarly in all groups. No significant changes in forearm output of leucine carbon, isoleucine carbon, or threonine were seen during saline infusion. In contrast, during leucine infusion there was a dose-dependent increase (r = .86, P < .001) in leucine carbon uptake with increased arterial leucine and a-ketoisocaproate concentrations. During infusions of isoleucine and threonine, increases (P < .05) in isoleucine carbon uptake and threonine uptake, respectively, were observed. Glucose uptake by forearm tissues did not change during the saline infusion, but it decreased (P < .05) in all four groups receiving an amino acid infusion. Changes in leucine carbon uptake were strongly correlated (r = −.76, P < .001) with changes in glucose uptake. Therefore, amino acids affect glucose uptake in human forearm tissue and presumably compete as oxidative fuels.

Optimal rate of enteral glucose administration in children with glycogen storage disease type I.

Rates of administration of enteral carbohydrate to maintain the plasma glucose concentration and suppress organic acidemia in young children with glycogen storage disease Type I have not been clearly established. Therefore, we studied six children with the disease during sequential nasogastric infusions of carbohydrate at four different rates (10.5, 8.6, 5.8, and 3 mg of carbohydrate per kilogram of body weight per minute). The rates at which total and endogenous glucose appeared in the plasma were measured with [2H2] glucose. The infusion rates of carbohydrate were linearly correlated (r = 0.88, P less than 0.001) with the plasma glucose concentration, which was about 90 mg per deciliter at a rate of 8.6 mg per kilogram per minute. The mean (+/- SE) rate of appearance of endogenous glucose was 1.4 +/- 0.1 mg per kilogram per minute at a nasogastric infusion rate of 5.8 mg of carbohydrate per kilogram per minute (a rate similar to that of hepatic glucose production in normal children who have fasted overnight), and was completely suppressed at 10.5 mg of carbohydrate per kilogram per minute. Concentrations of plasma lactate, pyruvate, free fatty acids, and ketone bodies were inversely related to the rate of carbohydrate administration below 8.6 mg per kilogram per minute. We conclude that the minimal nocturnal nasogastric infusion rate of carbohydrate needed to maintain plasma glucose concentrations and minimize organic acidemia in young children with glycogen storage disease Type I is approximately 8 to 9 mg per kilogram per minute.

Hepatic and extrahepatic responses to insulin in NIDDM and nondiabetic humans. Assessment in absence of artifact introduced by tritiated nonglucose contaminants.

It is well established that patients with non-insulindependent diabetes mellitus (NIDDM) are resistant to insulin. However, the contribution of hepatic and extrahepatic tissues to insulin resistance remains controversial. The uncertainty may be at least in part due to errors introduced by the unknowing use in previous studies of impure isotopes to measure glucose turnover. To determine hepatic and extrahepatic responses to insulin in the absence of these errors, steady-state glucose turnover was measured simultaneously with [6-3H]- and [6-14C]glucose during sequential 5- and 4-h infusions of insulin at rates of 0.4 and 10 mil · kg−1 · min−1 in diabetic and nondiabetic subjects. At low insulin concentrations, [6-3H]- and [6-14C]glucose gave similar estimates of glucose turnover. Hepatic glucose release was equal to but not below zero in the nondiabetic subjects, but persistent glucose release (P < 0.001) and decreased glucose uptake (P < 0.001) was observed in the diabetic patients. At high insulin concentrations, both isotopes underestimated glucose turnover during the 1st h after initiation of the highdose insulin infusion. More time (P < 0.05) was required to reachieve steady state in NIDDM than nondiabetic subjects. At steady state, [6-3H] but not [6-14C]glucose systematically underestimated (P < 0.05) glucose turnover in both groups due to the presence of a tritiated nonglucose contaminant. The percentage of radioactivity in plasma due to tritiated contaminants was linearly related to turnover. When plasma [6-3H]glucose specific activity was corrected for the presence of the contaminant in each subject, both [6-3H]- and [6-14C]glucose indicated that diabetic patients had hepatic and extrahepatic insulin resistance. We conclude that in these experiments, although the presence of a low percentage of tritiated nonglucose contaminant in the tracer may alter estimates of the severity of hepatic and extrahepatic insulin resistance in patients with NIDDM, it does not obscure their presence

Type I diabetes mellitus does not alter initial splanchnic glucose extraction or hepatic UDP-glucose flux during enteral glucose administration.

Aims/hypothesis. Our aim was to determine whether an alteration in splanchnic glucose metabolism could contribute to postprandial hyperglycaemia in people with Type I (insulin-dependent) diabetes mellitus. Methods. Splanchnic glucose extraction, hepatic glycogen synthesis and endogenous glucose production were compared in 8 Type I diabetic patients and in 11 control subjects. Endogenous hormone secretion was inhibited with somatostatin while insulin ( ∼ 550 pmol/l) and glucagon ( ∼ 130 ng/l) concentrations were matched with exogenous hormone infusions. Glucose containing [3-3H] glucose was infused into the duodenum at a rate of 20 μmol · kg-1· min-1. Plasma glucose concentrations were maintained at about 8.5 mmol/l in both groups by means of a separate variable intravenous glucose infusion. Results. Initial splanchnic glucose uptake, calculated by subtracting the systemic rate of appearance of [3-3H] glucose from the rate of infusion of [3-3H] glucose into the duodenum, did not differ in the diabetic and non-diabetic patients (4.1 ± 0.8 vs 3.0 ± 1.0 μmol/kg/min). In addition, hepatic glycogen synthesis, measured using the acetaminophen glucuronide method did not differ (10.7 ± 2.4 vs 10.1 ± 2.7 μmol · kg-1· min-1). On the other hand, suppression of endogenous glucose production, measured by an intravenous infusion of [6,6-2H2] glucose, was greater (p < 0.05) in the diabetic than in the non-diabetic subjects (1.7 ± 1.6 vs 5.8 ± 1.9 μmol · kg-1· min-1). Conclusion/interpretation. When glucose, insulin and glucagon concentrations are matched in individuals with relatively good chronic glycaemic control, Type I diabetes does not alter initial splanchnic glucose uptake of enterally delivered glucose or hepatic glycogen synthesis. Alterations in splanchnic glucose metabolism are not likely to contribute to postprandial hyperglycaemia in people with well controlled Type I diabetes. [Diabetologia (2001) 44: 729–737]

Assessment of Hepatic Sensitivity to Glucagon in NIDDM: Use as a Tool to Estimate the Contribution of the Indirect Pathway to Nocturnal Glycogen Synthesis

NIDDM is associated with excessive rates of endogenous glucose production in both the postabsorptive and postprandial states. To determine whether this is due to an intrinsic increase in hepatic sensitivity to glucagon, 9 NIDDM and 10 nondiabetic subjects were studied on three occasions. On each occasion, glycogen was labeled the evening before the study with subjects ingesting meals containing [6-3H]galactose. Beginning at 6:00 A.M. on the following morning, somatostatin was infused to inhibit endogenous hormone secretion. Insulin concentrations were maintained constant at basal levels (defined as that necessary to keep glucose at ∼5 mmol/l) in each individual. On one occasion, glucagon was infused at a rate of 0.65 ng ∼ kg−1 · min−1 throughout the experiment, resulting in glucagon concentrations of ∼130 pg/ml and a slow but comparable fall in endogenous glucose production with time in both groups. On the other two occasions, the glucagon infusion was increased at 10:00 A.M. to either 1.5 or 3.0 ng · kg−1 · min−1, resulting in an increase in glucagon concentrations to ∼180 and 310 pg/ml, respectively. The increment in endogenous glucose production (i.e., area above basal) did not differ in diabetic and nondiabetic subjects during either the 1.5 ng · kg−1 · min−1 (0.75 ± 0.055 vs. 0.78 ± 0.048 mmol/kg) or 3.0 ng · kg−1 · min−1 (1.06 ± 0.066 vs. 1.10 ± 0.073 mmol/kg) glucagon infusions. In contrast, the amount of [6-3H]glucose released from glycogen was lower (P < 0.05) in the diabetic than nondiabetic subjects during both glucagon infusions. The specific activity of glycogen, calculated as the integrated release of [6-3H]glucose divided by the integrated release of unlabeled glucose, was lower (P < 0.05) in diabetic subjects than in nondiabetic subjects during both the 1.5 ng · kg−1 · min−1 (19.0 ± 3.9 vs. 41.4 ± 5.7 dpmμmol) and 3.0 ng · kg−1 · min−1 (19.1 ± 3.1 vs. 36.5 ± 7.2 dpm/μmol) glucagon infusions, implying that a greater portion of the glucose released from glycogen was derived from the indirect pathway. We concluded that although NIDDM is not associated with an intrinsic alteration in hepatic sensitivity to glucagon, it does alter the relative contributions of the direct and indirect pathways to nocturnal glycogen synthesis.

Effects of Acute Metabolic Acidosis and Alkalosis on Leucine Metabolism in Conscious Dogs

To determine the effects of acute metabolic acidosis and alkalosis on leucine metabolism in vivo, mongrel dogs were infused with [1-14C] leucine for 8 h, along with NaCI, HCI, or NaHCO3 over the last 4 h. Arterial pH did not change from the basal value during NaCI infusion but decreased (P <.01) and increased (P <.01) during HCI and NaHCO3 infusions, respectively. Total leucine carbon entry did not change from the basal value during saline infusion but increased (P <.01) with acidosis and decreased (P <.05) with alkalosis. Compared with saline controls, acidosis increased (P <.01) leucine oxidation, whereas alkalosis decreased (P <.01) leucine oxidation. During acidosis, total plasma essential and nonessential amino acid concentrations increased (P <.05), whereas during alkalosis, total plasma essential and nonessential amino acid concentrations decreased (P <.05). These studies suggest that acute alterations in arterial pH may affect the regulation of protein metabolism in vivo and must be considered in the interpretation of results from experiments in which alterations of acid-base homeostasis may have occurred.

Familial Insulin Resistance and Acanthosis Nigricans: Presence of a Postbinding Defect

Type A insulin resistance, associated with acanthosis nigricans and menstrual irregularity, has been ascribed to a decreased concentration of insulin receptors. We now report four affected females from one family, a mother and three daughters (including identical twins) who appear to have the type A syndrome. Two of the kindred had no apparent ovarian dysfunction, while the other two had hyperprolactinemia without other findings of polycystic ovary disease, suggesting a genetic disease with variable penetrance. All had normal erythrocyte and monocyte insulin binding. Insulin dose-response studies to assess glucose metabolism and insulinsensitivity were performed in the affected twins. The dose response to insulin was shifted to the right with a decrease in maximal response. These results are consistent with a postbinding defect in insulin action in these patients.

Decreased fasting free fatty acids with L-carnitine in children with carnitine deficiency.

ABSTRACT: At the time of acute presentation, children with carnitine deficiency may have increased free fatty acid concentrations and hypoglycemia. However, whether carnitine replacement affects the plasma concentration of these substrates remains to be determined. Therefore, to evaluate the effect of carnitine replacement on plasma substrate and hormone concentrations, five children with carnitine deficiency (two idiopathic, two secondary to long-chain acyl coenzyme A dehydrogenase deficiency, one secondary to isovaleric acidemia) were fasted overnight before and after treatment with oral carnitine (80 ± 7 mg·kg-1·day-1). During carnitine supplementation, plasma total carnitine (19 ± 4 versus 45 ± 6 nmol/ml, pretreatment versus treatment, respectively) and free carnitine (11 ± 3 versus 31 ± 6 nmol/ml), as well as red blood cell total carnitine (0.057 ± 0.019 versus 0.130 ± 0.019 nmol/mg of hemoglobin) increased (p < 0.05). Fasting plasma glucose (83 ± 4 versus 85 ± 3 mg/dl) and ketone body (0.54 ± 0.18 and 0.56 ± 0.20 mM) concentrations did not change with carnitine supplementation, but plasma free fatty acids (1.28 ± 0.32 versus 0.77 ± 0.07 mM) decreased (p < 0.05). No differences in fasting insulin, growth hormone, or cortisol concentrations were observed. Urinary excretion of free carnitine (0.1 ± 0.0 versus 2.4 ± 0.7 Mmol/mg creatinine), total carnitine (0.3 ± 0.1 versus 3.4 ± 0.9 μmol/mg creatinine) and acyl carnitine (0.2 ± 0.1 versus 0.9 ± 0.3 μmol/mg creatinine) increased (p < 0.05) with carnitine supplementation. The decreased plasma free fatty acid concentrations with carnitine supplementation may be due to more efficient fatty acid oxidation and/or increased urinary excretion of fatty acids as acylcarnitines.