Inger Magnusson

Increased rate of gluconeogenesis in type II diabetes mellitus. A 13C nuclear magnetic resonance study.

UNLABELLED To quantitate hepatic glycogenolysis, liver glycogen concentration was measured with 13C nuclear magnetic resonance spectroscopy in seven type II diabetic and five control subjects during 23 h of fasting. Net hepatic glycogenolysis was calculated by multiplying the rate of glycogen breakdown by the liver volume, determined from magnetic resonance images. Gluconeogenesis was calculated by subtracting the rate of hepatic glycogenolysis from the whole body glucose production rate, measured using [6-3H]glucose. Liver glycogen concentration 4 h after a meal was lower in the diabetics than in the controls; 131 +/- 20 versus 282 +/- 60 mmol/liter liver (P < 0.05). Net hepatic glycogenolysis was decreased in the diabetics, 1.3 +/- 0.2 as compared to 2.8 +/- 0.7 mumol/(kg body wt x min) in the controls (P < 0.05). Whole body glucose production was increased in the diabetics as compared to the controls, 11.1 +/- 0.6 versus 8.9 +/- 0.5 mumol/(kg body wt x min) (P < 0.05). Gluconeogenesis was consequently increased in the diabetics, 9.8 +/- 0.7 as compared to 6.1 +/- 0.5 mumol/(kg body wt x min) in the controls (P < 0.01), and accounted for 88 +/- 2% of total glucose production as compared with 70 +/- 6% in the controls (P < 0.05). IN CONCLUSION increased gluconeogenesis is responsible for the increased whole body glucose production in type II diabetes mellitus after an overnight fast.

Direct assessment of liver glycogen storage by 13C nuclear magnetic resonance spectroscopy and regulation of glucose homeostasis after a mixed meal in normal subjects.

Despite extensive recent studies, understanding of the normal postprandial processes underlying immediate storage of substrate and maintenance of glucose homeostasis in humans after a mixed meal has been incomplete. The present study applied 13C nuclear magnetic resonance spectroscopy to measure sequential changes in hepatic glycogen concentration, a novel tracer approach to measure postprandial suppression of hepatic glucose output, and acetaminophen to trace the pathways of hepatic glycogen synthesis to elucidate the homeostatic adaptation to the fed state in healthy human subjects. After the liquid mixed meal, liver glycogen concentration rose from 207 +/- 22 to 316 +/- 19 mmol/liter at an average rate of 0.34 mmol/liter per min and peaked at 318 +/- 31 min, falling rapidly thereafter (0.26 mmol/liter per min). The mean increment at peak represented net glycogen synthesis of 28.3 +/- 3.7 g (approximately 19% of meal carbohydrate content). The contribution of the direct pathway to overall glycogen synthesis was 46 +/- 5 and 68 +/- 8% between 2 and 4 and 4 and 6 h, respectively. Hepatic glucose output was completely suppressed within 30 min of the meal. It increased steadily from 60 to 255 min from 0.31 +/- 32 to 0.49 +/- 18 mg/kg per min then rapidly returned towards basal levels (1.90 +/- 0.04 mg/kg per min). This pattern of change mirrored precisely the plasma glucagon/insulin ratio. These data provide for the first time a comprehensive picture of normal carbohydrate metabolism in humans after ingestion of a mixed meal.

Impaired net hepatic glycogen synthesis in insulin-dependent diabetic subjects during mixed meal ingestion. A 13C nuclear magnetic resonance spectroscopy study.

Hepatic glycogen concentration was measured in six subjects with insulin-dependent diabetes mellitus (IDDM) and nine weight-matched control subjects using 13C nuclear magnetic resonance spectroscopy during a day in which three isocaloric mixed meals were ingested. The relative fluxes of the direct and indirect (3 carbon units-->-->glycogen) pathways of hepatic glycogen synthesis were also assessed using [1-13C]glucose in combination with acetaminophen to noninvasively sample the hepatic UDP-glucose pool. Mean fasting hepatic glycogen content was similar in the two groups. After each meal, hepatic glycogen content increased, peaking 4-5 h after the meal in both groups. By 11:00 p.m. the IDDM subjects had synthesized only 30% of the glycogen that was synthesized by the control group [IDDM subjects, net increment = 44 +/- 20 (mean +/- SE) mM; control subjects, net increment = 144 +/- 14 mM; P < 0.05]. After breakfast the flux through the gluconeogenic pathway relative to the direct pathway of hepatic glycogen synthesis was 1.7-fold greater in the IDDM subjects (59 +/- 4%) than in the control subjects (35 +/- 4%, P < 0.0003). In conclusion, under mixed meal conditions, subjects with poorly controlled IDDM have a major defect in net hepatic glycogen synthesis and augmented hepatic gluconeogenesis. The former abnormality may result in an impaired glycemic response to counterregulatory hormones, whereas both abnormalities may contribute to postprandial hyperglycemia.

13C-nuclear magnetic resonance spectroscopy studies of hepatic glucose metabolism in normal subjects and subjects with insulin-dependent diabetes mellitus.

To determine the effect of insulin-dependent diabetes mellitus (IDDM) on rates and pathways of hepatic glycogen synthesis, as well as flux through hepatic pyruvate dehydrogenase, we used 13C-nuclear magnetic resonance spectroscopy to monitor the peak intensity of the C1 resonance of the glucosyl units of hepatic glycogen, in combination with acetaminophen to sample the hepatic UDP-glucose pool and phenylacetate to sample the hepatic glutamine pool, during a hyperglycemic-hyperinsulinemic clamp using [1-13C]-glucose. Five subjects with poorly controlled IDDM and six age-weight-matched control subjects were clamped at a mean plasma glucose concentration of approximately 9 mM and mean plasma insulin concentrations approximately 400 pM for 5 h. Rates of hepatic glycogen synthesis were similar in both groups (approximately 0.43 +/- 0.09 mumol/ml liver min). However, flux through the indirect pathway of glycogen synthesis (3 carbon units-->-->glycogen) was increased by approximately 50% (P < 0.05), whereas the relative contribution of pyruvate oxidation to TCA cycle flux was decreased by approximately 30% (P < 0.05) in the IDDM subjects compared to the control subjects. These studies demonstrate that patients with poorly controlled insulin-dependent diabetes mellitus have augmented hepatic gluconeogenesis and relative decreased rates of hepatic pyruvate oxidation. These abnormalities are not immediately reversed by normalizing intraportal concentrations of glucose, insulin, and glucagon and may contribute to postprandial hyperglycemia.

Contribution of Hepatic Glycogenolysis to Glucose Production in Humans in Response to a Physiological Increase in Plasma Glucagon Concentration

The contribution of net hepatic glycogenolysis to overall glucose production during a physiological increment in the plasma glucagon concentration was measured in six healthy subjects (18–24 years, 68–105 kg) after an overnight fast. Glucagon (∼3 ng · kg−1 · min−1), somatostatin (0.1 μg · kg−1 · min−1), and insulin (0.9 pmol · kg−1 · min−1) were infused for 3 h. Liver glycogen concentration was measured at 15-min intervals during this period using 13C-labeled nuclear magnetic resonance spectroscopy, and liver volume was assessed from magnetic resonance images. The rate of net hepatic glycogenolysis was calculated from the decrease in liver glycogen concentration over time, multiplied by the liver volume. The rate of glucose appearance (Ra) was calculated from [3-3H]glucose turnover data using a two-compartment model of glucose kinetics. Plasma glucagon concentration rose from 136 ± 18 to 304 ± 57 ng/1 and plasma glucose concentration rose from 5.6 ± 0.1 to 10.4 ± 0.9 mmol/1 on initiation of the infusions. Mean baseline Ra was 11.8 ± 0.4 μmol · kg−1 · min−1, increased rapidly after the beginning of the infusions, reaching its highest value after 20–40 min, and returned to baseline by 140 min. Liver glycogen concentration decreased almost linearly (from 300 ± 19 mmol/1 liver at baseline to 192 ± 20 mmol/1 liver at t = 124 min) during 2 h after the beginning of the infusions, and the calculated mean rate of net hepatic glycogenolysis was 21.7 ± 3.6 μmol · kg−1 · min−1. Mean Ra during the same time period was 22.8 ± 2.3 μmol · kg−1 · min−1. Thus, net hepatic glycogenolysis accounted for 93 ± 9% of Ra. In conclusion, during the initial response to a physiological increment in plasma glucagon, 1) net hepatic glycogenolysis accounts for virtually all of the increase in hepatic glucose production, and 2) glucagons evanescent effect on hepatic glucose production is not caused by depletion of hepatic glycogen stores.

Mechanism of impaired insulin-stimulated muscle glucose metabolism in subjects with insulin-dependent diabetes mellitus.

To determine the mechanism of impaired insulin-stimulated muscle glycogen metabolism in patients with poorly controlled insulin-dependent diabetes mellitus (IDDM), we used 13C-NMR spectroscopy to monitor the peak intensity of the C1 resonance of the glucosyl units in muscle glycogen during a 6-h hyperglycemic-hyperinsulinemic clamp using [1-(13)C]glucose-enriched infusate followed by nonenriched glucose. Under similar steady state (t = 3-6 h) plasma glucose (approximately 9.0 mM) and insulin concentrations (approximately 400 pM), nonoxidative glucose metabolism was significantly less in the IDDM subjects compared with age-weight-matched control subjects (37+/-6 vs. 73+/-11 micromol/kg of body wt per minute, P < 0.05), which could be attributed to an approximately 45% reduction in the net rate of muscle glycogen synthesis in the IDDM subjects compared with the control subjects (108+/-16 vs. 195+/-6 micromol/liter of muscle per minute, P < 0.001). Muscle glycogen turnover in the IDDM subjects was significantly less than that of the controls (16+/-4 vs. 33+/-5%, P < 0.05), indicating that a marked reduction in flux through glycogen synthase was responsible for the reduced rate of net glycogen synthesis in the IDDM subjects. 31P-NMR spectroscopy was used to determine the intramuscular concentration of glucose-6-phosphate (G-6-P) under the same hyperglycemic-hyperinsulinemic conditions. Basal G-6-P concentration was similar between the two groups (approximately 0.10 mmol/kg of muscle) but the increment in G-6-P concentration in response to the glucose-insulin infusion was approximately 50% less in the IDDM subjects compared with the control subjects (0.07+/-0.02 vs. 0.13+/-0.02 mmol/kg of muscle, P < 0.05). When nonoxidative glucose metabolic rates in the control subjects were matched to the IDDM subjects, the increment in the G-6-P concentration (0.06+/-0.02 mmol/kg of muscle) was no different than that in the IDDM subjects. Together, these data indicate that defective glucose transport/phosphorylation is the major factor responsible for the lower rate of muscle glycogen synthesis in the poorly controlled insulin-dependent diabetic subjects.

Protein and amino acid metabolism during early starvation as reflected by excretion of urea and methylhistidines

Endogenous excretion of nitrogenous products was studied during early starvation in six healthy, nonobese subjects after six days on a well-defined diet, designed to achieve net protein balance and an adequate calorie supply. The diet contained 0.5 g myofibrillar-free protein and 35 kcal/kg body weight. The subjects then fasted for three days. Urine was collected for 24-hour periods and analyzed for urea, ammonia, 3-methylhistidine, and 1-methylhistidine. Blood glucose and serum urea levels were measured daily. In a second group of subjects, muscle biopsies for determination of free amino acid concentrations were taken in the overnight fasted state and after three days of fasting. During the period with a balanced diet, urea production fell initially and stabilized after two to three days at a level of 146 +/- 15 mmol/24 h. During the period of fasting, serum urea increased from 3.0 +/- 0.4 to a maximum value of 6.2 +/- 0.7 mmol/L and urea production rose markedly, to a peak of 293 +/- 16 mmol/24 h. Ammonia excretion was 24 +/- 2 mmol/24 h before and 71 +/- 13 mmol/24 h after three days of fasting. 3-Methylhistidine excretion was stable before fasting and then rose from 154 +/- 17 to 198 +/- 17 mumol/24 h. 1-Methylhistidine excretion was unchanged during fasting. Blood glucose levels were stable at 4.8 +/- 0.2 mmol/L before fasting and then fell to 3.7 +/- 0.3 mmol/L. Intracellular concentrations of amino acids in skeletal muscle decreased markedly during fasting; after three days of fasting the glutamine concentration had fallen by 34%.(ABSTRACT TRUNCATED AT 250 WORDS)

Utilization of intravenously administered N-acetyl-L-glutamine in humans

L-glutamine is too unstable for inclusion in solutions for parenteral nutrition, but its acetylated analogue, N-acetyl-L-glutamine is not. The purpose of this three-part study was to investigate the utilization of intravenously (IV) administered acetylglutamine in humans. In study 1, nine healthy postabsorptive subjects were given 9.4 g acetylglutamine IV during four hours. In study 2, five healthy subjects were studied on two occasions following an overnight fast. They were given 9.4 g of acetylglutamine or an equivalent amount of glutamine as part of a total parenteral nutrition (TPN) regimen during 7.2 hours. A control group of five subjects was given the same TPN regimen, but without acetylglutamine or glutamine. The nutrient solution included glucose, amino acids, and a fat emulsion, supplying 9.4 g nitrogen and 6,300 kJ in a total volume of 1.8 L. In study 3, four patients were studied the day after major surgery. They were given the same TPN regimen as in study 2, containing 9.4 g acetylglutamine, during 7.2 hours. Plasma concentrations and urinary excretion of acetylglutamine and glutamine were measured in all three studies, and so were splanchnic and renal exchange of acetylglutamine and glutamine in study 1. In study 1, the plasma concentration of glutamine rose from 594 +/- 28 mumol/L to 728 +/- 26 mumol/L (P less than .001), whereas plasma levels of acetylglutamine exceeded 1,000 mumol/L in all subjects at the end of infusion. The eight-hour urinary excretion of acetylglutamine and glutamine corresponded to 18% of the infused amount of acetylglutamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathways of hepatic glycogen synthesis in humans.

To estimate the relative contribution of the indirect pathway to liver glycogen formation in humans, different experimental approaches have been used. Although the estimates vary, it appears that in overnight fasted humans approximately 50% of liver glycogen is derived from the direct pathway following a glucose load and that the direct pathway increases to approximately 70% after the first meal of the day. While the source of the gluconeogenic substrates is still unknown, the animal data suggest that the liver itself may be an important source.