Endogenous triglyceride turnover in liver and plasma of the dog

Fatty liver disrupts glycerol metabolism in gluconeogenic and lipogenic pathways in humans

It is a challenge to assess metabolic dysregulation in fatty liver of human patients prior to clinical manifestations. Here, we recruited obese, but otherwise healthy, subjects to examine biochemical processes in the liver with simple triglyceride accumulation using stable isotopes and NMR analysis of metabolic products in blood. Intrahepatic triglycerides were measured using ¹H magnetic resonance spectroscopy, and volunteers received ²H₂O and [U-¹³C₃]glycerol orally, followed by a series of blood draws. NMR analysis of plasma triglycerides and glucose provided detailed information about metabolic pathways in patients with simple hepatic steatosis. Compared with subjects with low hepatic fat, patients with hepatic steatosis were characterized by the following: lower ¹³C enrichments in the glycerol backbones of triglycerides (i.e., TG-[¹³C]glycerol), higher [U-¹³C₃]glycerol metabolism through the tricarboxylic acid (TCA) cycle, delayed gluconeogenesis from [U-¹³C₃]glycerol, and less flexibility in adjusting supporting fluxes of glucose production upon an oral load of glycerol. In summary, simple hepatic steatosis was associated with enhanced [U-¹³C₃]glycerol metabolism through pathways that intersect the TCA cycle and delayed gluconeogenesis from glycerol.

Participation in a 1,000-mile race increases the oxidation of carbohydrate in Alaskan sled dogs

The Alaskan Husky has been specifically bred for endurance performance and is capable of extreme endurance performance. We examined sled dogs in the trained state at the beginning of the race season and after a 1,600-km race (Iditarod). Our hypothesis was that lipids would be the predominant substrate during submaximal exercise in long-distance racing sled dogs, and a 1,600-km race would increase the reliance on lipids during an exercise bout at the same absolute exercise intensity. The experiments were completed over three testing periods, which were completed in January of two different years before participation in a 1,600-km race, or in March shortly after completion of a 1,600-km race. After determination of H(13)CO3 (-) recovery, the dogs were tested with primed continuous infusions of [1,1,2,3,3-(2)H]glycerol, [3-(13)C]lactate, or [6,6-(2)H2]glucose. During exercise, respiratory exchange ratio was significantly higher in raced (0.92 ± 0.01) compared with nonraced (0.87 ± 0.01) dogs. During exercise, glucose rate of appearance was potentially sustained by a large glycerol rate of disappearance with an increase in lactate rates of oxidation after a 1,600-km race. Therefore, contrary to our hypothesis, the sled dogs were dependent on carbohydrate energy sources, a reliance that increased further after participation in a 1,600-km race.

A computer model of gluconeogenesis and lipid metabolism in the perfused liver

A mathematical model of the perfused rat liver was developed to predict intermediate metabolite concentrations and fluxes in response to changes in various substrate concentrations in the perfusion medium. The model simulates gluconeogenesis in the liver perfused separately with lactate and pyruvate and the combination of these substrates with fatty acids (oleate). The model consists of key reactions representing gluconeogenesis, glycolysis, fatty acid metabolism, tricarboxylic acid cycle, oxidative phosphorylation, and ketogenesis. Michaelis-Menten-type kinetic expressions, with control by ATP/ADP, are used for many of the reactions. For key regulated reactions (fructose-1,6-bisphosphatase, phosphofructokinase, pyruvate carboxylase, pyruvate dehydrogenase complex, and pyruvate kinase), rate expressions were developed that incorporate allosteric effectors, specific substrate relationships (e.g., cooperative binding), and/or phosphorylation/dephosphorylation using in vitro enzyme activity data and knowledge of the specific mechanisms. The model was independently validated by comparing model predictions with 10 sets of experimental data from 7 different published works, with no parameter adjustments. The simulations predict the same trends, in terms of stimulation of substrate uptake by fatty acid addition, as observed experimentally. In general, the major metabolic indicators calculated by the model are in good agreement with experimental results. For example, the simulated glucose/pyruvate mass yield is 43% compared with the average of 45% reported in the literature. The model accurately predicts the specific time constants of the glucose response (2.5-4 min) and the dynamic behavior of substrate and product fluxes. It is expected that this model will be a useful tool for analyzing the complex relationships between carbohydrate and fat metabolism.

A Distributed Model of Carbohydrate Transport and Metabolism in the Liver during Rest and High-Intensity Exercise

A model of reaction and transport in the liver was developed that describes the metabolite concentration and reaction flux dynamics separately within the tissue and blood domains. The blood domain contains equations for convection, axial dispersion, and transport to the surrounding tissue; and the tissue domain consists of reactions representing key carbohydrate metabolic pathways. The model includes the metabolic heterogeneity of the liver by incorporating spatial variation of key enzymatic maximal activities. Simulation results of the overnight fasted, resting state agree closely with experimental values of overall glucose uptake and lactate output by the liver. The incorporation of zonation of glycolytic and gluconeogenic enzyme activities causes the expected increase in glycolysis and decrease in gluconeogenesis along the sinusoid length from periportal to perivenous regions, while fluxes are nearly constant along the sinusoid length in the absence of enzyme zonation. These results confirm that transport limitations are not sufficient to account for the observed tissue heterogeneity of metabolic fluxes. Model results indicate that changes in arterial substrate concentrations and hepatic blood flow rate, which occur in the high-intensity exercise state, are not sufficient to shift the liver metabolism enough to account for the 5-fold increase in hepatic glucose production measured during exercise. Changes in maximal activities, whether caused by exercise-induced changes in insulin, glucagon, or other hormones are shown to be needed to achieve the expected glucose output. This model provides a framework for evaluating the relative importance to hepatic function of various phenomenological changes that occur during exercise. The model can also be used to assess the potential effect of metabolic heterogeneity on metabolism.

Measuring VLDL-triglyceride turnover in humans using ex vivo-prepared VLDL tracer

There has been more interest in VLDL-triglyceride (TG) kinetics during the last decade. Unfortunately, robust measurement methods are elaborate and not readily available. Here, we describe a method using unique, ex vivo labeling of the fatty acid moiety of VLDL-TG followed by intravenous bolus infusion in the same person. We found that plasma disappearance of ex vivo-labeled VLDL-TG was comparable to that of in vivo-labeled VLDL-TG and that turnover rates can be safely estimated from the log linear decay of VLDL-TG specific activity. We found minor labeling of the plasma FFA (oleate) pool, which was largely attributable to coinfusion of free [14C]triolein; VLDL-TG did not contribute substantially to the plasma FFA pool. The plasma decay curve of VLDL-TG was not affected by the presence of tracer in the FFA pool, provided that the data from 2 h after the VLDL tracer bolus infusion was used. The FFA contamination problem was circumvented by minor modification of the VLDL-TG tracer preparation. The approach we describe should expand the opportunity to study processes that cannot be assessed if the FFA precursor pool is labeled. This method for VLDL-TG tracer preparation can allow measurement of VLDL turnover, tissue uptake of VLDL-TG, and oxidation of VLDL-TG.

Labelled lipids distribution following [3H]glycerol injection to pregnant rabbits

1. The time dependent variation in specific activity of serum triglycerides and phospholipids has been studied following an intravenous pulse-labelling with 2-[3H]glycerol to pregnant rabbits. 2. The specific activity of triglycerides varied according to the gestation time and the maximal values of specific activity shifted to shorter times when gestation progressed to term. 3. The specific activity of serum phospholipids was higher in pregnant animals and increased with gestation time. 4. Concentrations and specific activities of both triglycerides and phospholipids were investigated in serum of pregnant and fetal rabbits at term. Concentrations of both lipid classes were notably higher in the fetal blood than in the maternal one. 5. The results were discussed on the basis of changes in hepatic biosynthesis of phosphatidic acid. Also, it was suggested a high drainage of either lipids or their degradation products by the feto-placental unit at the end of pregnancy. 6. Levels and specific activities of maternal and fetal lipids from liver and lung were also examined. The different metabolic roles of the main tissues involved in triglyceride and phospholipid metabolism were discussed.

Very low density lipoprotein metabolism in domestic pigs

Patterns of triacylglycerol (TG) turnover in plasma and liver, and the hepatic secretion of very density lipoprotein triacylglycerol (VLDL TG) into the circulation, have been studied in young Large White female pigs, using i.v. tracer [1,3-14C]- and [2-3H]glycerol. Serial measurements were made of plasma [14C]glycerol and [14C]glucose and of liver TG and plasma VLDL TG specific activities. In other studies VLDL TG obtained from a donor pig was reinjected into recipient animals to measure the early disappearance (dilution) of VLDL TG. Multicompartmental analysis revealed a mean rate of hepatic TG turnover somewhat slower than the rate of VLDL TG turnover, suggesting that almost all of the hepatic TG turnover was due to secretion of VLDL TG, and that the intestine probably contributed an appreciable part of the newly synthesized plasma VLDL TG. The t1/2 of reinjected VLDL TG, whether prepared by the most rapid possible techniques and reinjected immediately or stored for several days, was approximately 10 min. This was much faster than the t1/2 of the falling limb of the plasma VLDL TG curve seen after injection of labelled glycerol (t1/2 approximately 2 H). Thus, in these respects, the pig resembled all other species studied rather than human subjects as described by Farquhar et al.