Glyceroneogenesis and the source of glycerol for hepatic triacylglycerol synthesis in humans.

Glyceroneogenesis, i.e. the synthesis of the glycerol moiety of triacylglycerol from pyruvate, has been suggested to be quantitatively important in both the liver and adipose tissue during fasting. However, the actual contribution of glyceroneogenesis to triacylglycerol synthesis has not been quantified in vivo in human studies. In the present study we have measured the contribution of glycerol and pyruvate to in vivo synthesis of hepatic triacylglycerol in nonpregnant and pregnant women after an overnight fast. Five nonpregnant women were administered [(13)C(3)]glycerol tracer as prime constant rate infusion, and the appearance of tracer in plasma glucose and triacylglycerol was quantified using gas chromatography-mass spectrometry. The contribution of pyruvate to hepatic triacylglycerol was quantified in nonpregnant and pregnant women using the deuterium labeling of body water method. The appearance of [(2)H] in hydrogens on C(1) and C(3) of triacylglycerol was measured following periodate oxidation of the glycerol isolated from hydrolyzed triacylglycerol. After a 16-h fast, approximately 6.1% of the plasma triacylglycerol pool was derived from plasma glycerol, whereas 10 to 60% was derived from pyruvate in nonpregnant women and pregnant women early in gestation. Our data suggest that glyceroneogenesis from pyruvate is quantitatively a major contributor to plasma triacylglycerol synthesis and may be important for the regulation of very low density lipoprotein triacylglycerol production. Our data also suggest that 3-glycerol phosphate is in rapid equilibrium with the triosephosphate pool, resulting in rapid labeling of the triose pool by the administered tracer glycerol. Because the rate of flux of triosephosphate to glucose during fasting far exceeds that to triacylglycerol, more glycerol ends up in glucose than in triacylglycerol. Alternatively, there may be two distinct pools of 3-glycerol phosphate in the liver, one involved in generating triosephosphate from glycerol and the other involved in glyceride-glycerol synthesis.

Quantification of gluconeogenesis in cirrhosis: response to glucagon.

BACKGROUND & AIMS: Accelerated starvation and early recruitment of alternate fuels in cirrhosis have been attributed to reduced availability of hepatic glycogen. The aim of this study was to measure gluconeogenesis (as a marker of protein oxidation) in relation to total glucose production and glucagon-stimulated glycogenolysis. METHODS: Glucose and urea production, gluconeogenesis, and glycogenolysis were calculated using stable isotope methods before and during glucagon infusion (3 ng. kg-1. min-1) in 5 cirrhotic patients and 5 matched controls before and after glycogen repletion. RESULTS: In the basal state, cirrhotic patients had a normal rate of glucose production, but the contribution of gluconeogenesis was increased (74.3% +/- 4.1% vs. 55. 6% +/- 12.1%; P < 0.005). Glycogen repletion normalized the rate of gluconeogenesis. The glycemic response to glucagon (3 ng. kg-1. min-1) was blunted in cirrhotic patients because of a lower rate of glycogenolysis (0.63 +/- 0.23 vs. 1.22 +/- 0.23 mg. kg-1. min-1; P < 0.01) and was not affected by glycogen repletion. Despite increased gluconeogenesis, the simultaneously measured rate of urea synthesis was lower in cirrhotic patients (3.11 +/- 1.02 vs. 5.0 +/- 1.0 mg/kg; P < 0.05). CONCLUSIONS: These data show that in cirrhosis, glucose production is sustained by an increased rate of gluconeogenesis. The hepatic resistance to glucagon action is not caused by reduced glycogen stores.

Glucose turnover and gluconeogenesis in human pregnancy.

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

Modification of the theophylline radioimmunoassay.

The routine utilization of a commercially available radioimmunoassay (RIA) for theophylline (GammaDab), although reliable, is currently prohibited by the high cost of the reagents. In an effort to reduce these costs we have diluted the [125I]theophylline tracer and theophylline antiserum reagents by one-half, contrary to the manufacturer's recommendations. We have demonstrated that an excellent correlation exists (r = 0.968) between our modified RIA method and a conventional high-performance liquid chromatographic technique, despite reagent dilution. Accordingly, our reagent costs have been reduced by half. We conclude that the GammaDab kit reagents can be diluted twofold and still provide an accurate determination of serum theophylline. We must also emphasize that any further alteration(s) of this theophylline RIA procedure would require a thorough evaluation before its routine use could be substantiated.