A multicompartmental model of cholesterol metabolism in rats

A complete model of the cholesterol system was developed in the rat. It synthesizes several partial models previously developed under different isotopic conditions. It contains 16 compartments and 40 parameters. It specifically describes the intestine (mucosa and contents), liver and plasma cholesterol subsystems. The model was validated by the similarity between the simulated and experimental values in all the compartments during the 48 hours following the single introduction of an isotopic label in two different parts of the system (single ingestion of 14C-cholesterol, intravenous injection of red cells containing 3H-cholesterol). The similarity between the simulations and the experimental values was also observed up to 4 months following labelled cholesterol ingestion as well as under other different isotopic conditions for shorter periods. The parameter values identified and the resulting fluxes agree with the already published data.     

A multicompartmental model of fluid-phase endocytosis in rabbit liver parenchymal cells.

Fluid-phase endocytosis was studied in isolated rabbit liver parenchymal cells by using 125I-poly(vinylpyrrolidone) (PVP) as a marker. First, uptake of 125I-PVP by cells was determined. Also, cells were loaded with 125I-PVP for 20, 60 and 120 min, and release of marker was monitored for 120-220 min. Then we used the Simulation, Analysis and Modeling (SAAM) computer program and the technique of model-based compartmental analysis to develop a mechanistic model for fluid-phase endocytosis in these cells. To fit all data simultaneously, a model with three cellular compartments and one extracellular compartment was required. The three kinetically distinct cellular compartments are interpreted to represent (1) early endosomes, (2) a prelysosomal compartment equivalent to the compartment for uncoupling of receptor and ligand (CURL) and/or multivesicular bodies (MVB), and (3) lysosomes. The model predicts that approx. 80% of the internalized 125I-PVP was recycled to the medium from the early-endosome compartment. The apparent first-order rate constant for this recycling was 0.094 min-1, thus indicating that an average 125I-PVP molecule is recycled in 11 min. The model also predicts that recycling to the medium occurs from all three intracellular compartments. From the prelysosomal compartment, 40% of the 125I-PVP molecules are predicted to recycle to the medium and 60% are transferred to the lysosomal compartment. The average time for recycling from the prelysosomal compartment to the medium was estimated to be 66 min. For 125I-PVP in the lysosomal compartment, 0.3%/min was transferred back to the medium. These results, and the model developed to interpret the data, predict that there is extensive recycling of material endocytosed by fluid-phase endocytosis to the extracellular environment in rabbit liver parenchymal cells.     

Compartmental modeling of whole-body vitamin A kinetics in unsupplemented and vitamin A-retinoic acid-supplemented neonatal rats

Little is known about the contribution of different tissues to whole-body vitamin A (VA) kinetics in neonates. Here, we have used model-based compartmental analysis of tissue tracer kinetic data from unsupplemented (control) and VA-retinoic acid (VARA)-supplemented neonatal rats to determine VA kinetics in specific tissues under control and supplemented conditions. First, compartmental models for retinol kinetics were developed for individual tissues, and then an integrated compartmental model incorporating all tissues was developed for both groups. The models predicted that 52% of chylomicron (CM) retinyl ester was cleared by liver in control pups versus 22% in VARA-treated pups, whereas about 51% of VA was predicted to be extrahepatic in 4- to 6-day-old unsupplemented neonatal rats. VARA increased CM retinyl ester uptake by lung, carcass, and intestine; decreased the release into plasma of retinol that had been cleared by liver and lung as CM retinyl esters; stimulated the uptake of retinol from plasma holo-retinol binding protein into carcass; and decreased the retinol turnover out of the liver. Overall, neonatal VA trafficking differed from that previously described for adult animals, with a larger contribution of extrahepatic tissues to CM clearance, especially after VA supplementation, and a significant amount of VA distributed in extrahepatic tissues.     

Iron and zinc status in rats with diet-induced marginal deficiency of vitamin A and/or copper

The hypothesis was tested that there are interactions of marginal copper and vitamin A deficiency regarding iron and zinc status. Copper restriction (1 vs 5 mg Cu/kg diet) significantly lowered copper concentrations in plasma and tissues of rats and reduced blood hemoglobin, hematocrit, and iron concentrations in tibia and femur, but raised iron concentrations in liver. Vitamin A restriction (0 vs 4000 IU vitamin A/kg diet) reduced plasma retinol concentrations and induced a fall of blood hemoglobin and hematocrit. Neither copper nor vitamin A restriction for up to 42 d affected feed intake and body wt gain. There were no interrelated effects of vitamin A and copper deficiency on iron status. Copper deficiency slightly depressed liver, spleen, and kidney zinc concentrations. Vitamin A deficiency lowered zinc concentrations in heart, but only when the diets were deficient in copper.     

Retinol kinetics in unsupplemented and vitamin A-retinoic acid supplemented neonatal rats: a preliminary model

Vitamin A (VA) metabolism in neonates is virtually uncharacterized. Our objective was to develop a compartmental model of VA metabolism in unsupplemented and VA-supplemented neonatal rats. On postnatal day 4, pups (n = 3/time) received 11,12-[³H]retinol orally, in either oil (control) or VA combined with retinoic acid (VARA) [VA (∼6 mg/kg body weight) + 10% retinoic acid]. Plasma and tissues were collected at 14 time points up to 14 days after dose administration. VARA supplementation rapidly, but transiently, increased total retinol mass in plasma, liver, and lung. It decreased the peak fraction of the dose in plasma. A multi-compartmental model developed to fit plasma [³H]retinol data predicted more extensive recycling of retinol between plasma and tissues in neonates compared with that reported in adults (144 vs. 12–13 times). In VARA pups, the recycling number for retinol between plasma and tissues (100 times) and the time that retinol spent in plasma were both lower compared with controls; VARA also stimulated the uptake of plasma VA into extravascular tissues. A VARA perturbation model indicated that the effect of VARA in stimulating VA uptake into tissues in neonates is both dramatic and transient.     

Utilization of vitamin A in rats with inflammation

Reductions in the concentration of retinol (vitamin A) in serum, lung and kidney were observed in rats subjected to inflammation-inducing treatments (turpentine oil injection of thermal injury). At the same time, the liver showed an almost normal vitamin A content. Feeding of retinol to vitamin A-depleted rats with inflammation revealed that intestinal absorption of retinol was still active in the inflamed state, and the livers of these rats showed good incorporation of retinol. The livers of normal and vitamin A-depleted rats subjected to the inflammatory treatments showed a normal RBP content (retinol-binding protein) and hepatic release of holo-RBP into the serum was not impaired functionally. These results suggest the possibility that the decreases of vitamin A in the lung, serum and kidney may be due primarily to enhanced local consumption of vitamin A related to the inflammation, rather than to a reduced supply of vitamin A from the liver or to decreased intestinal absorption. In bovine serum albumin (BSA)-sensitized rats produced by direct intubation of BSA into the lungs, the level of vitamin A in the lung decreased prior to that in the liver or serum, supporting the hypothesis that the decrease in vitamin A in the inflamed lungs of these rats may be due mainly to the consumption of vitamin A in the lung in response to inflammation.     

A marginal model of tolerated theft

Using marginal analysis to represent Blurton Jones's concept of tolerated theft, I show how equilibrium resource transfers among individuals might be affected by foraging behavior, resource qualities, and number of participants. The model applies to hominids and other species that exchange or share food or other resources. Among the results: Tolerated theft enhances the value to be derived from resources, packets intermediate in size are most likely to be subjected to tolerated theft, packet division is more likely to be unequal than equal, division is a function of group size, and tolerated theft is most likely in small groups. The model also suggests that among reciprocators the widest possible exchange or sharing is in the self-interest of the individual procuring the resource. In general, evolutionary cost-benefit accounting should track marginal changes in the value (fitness or utility) of resources. Marginal valuation is conceptually primary and may produce results that differ from direct measures of quantity.     

Modelling of fluid-phase endocytosis kinetics in the amoebae of the cellular slime mouldDictyostelium discoideum. A multicompartmental approach

Fluid-phase endocytosis (pinocytosis) kinetics were studied inDictyostelium discoideum amoebae from the axenic strain Ax-2 that exhibits high rates of fluid-phase endocytosis when cultured in liquid nutrient media. Fluorescein-labelled dextran (FITC-dextran) was used as a marker in continuous uptake- and in pulse-chase exocytosis experiments. In the latter case, efflux of the marker was monitored on cells loaded for short periods of time and resuspended in marker-free medium. A multicompartmental model was developed which describes satisfactorily fluid-phase endocytosis kinetics. In particular, it accounts correctly for the extended latency period before exocytosis in pulse-chase experiments and it suggests the existence of some sorts of maturation stages in the pathway.     

Modification of hepatic vitamin E stores in vivo. II. Alterations in plasma and liver vitamin E content by 1,2-dibromoethane

Previous studies with methyl ethyl ketone peroxide (MEKP), a radical generator, showed depletion of plasma vitamin E and liver glutathione (GSH) levels prior to a decrease of liver vitamin E levels. Since hepatic pools of this vitamin may serve to maintain circulating levels of vitamin E under conditions of oxidative challenge, we have evaluated the similarity of response after treatment with 1,2-dibromoethane (DBE), a compound that is not known to generate oxyradicals or to induce lipid peroxidation in vivo. Treatment of normal rats with DBE caused a depletion in hepatic vitamin E levels 1 day after treatment; however, in contrast to our prior findings with MEKP this depletion after DBE treatment was observed in tandem with elevations in the plasma content of vitamin E. Liver vitamin E depletion was neither dependent upon a sustained liver GSH depletion nor upon hepatocellular death. Mobilization and export of hepatic vitamin E did not result in an immediate whole body redistribution of this vitamin in that pulmonary and renal levels of vitamin E remained normal under conditions of liver vitamin E depletion. Moreover, the stimulus that resulted in exportation of liver vitamin E was maintained by daily treatments with DBE. DBE caused a substantial elevation above control values in liver GSH content and these elevations were also maintained by daily DBE treatments. In experiments to assess the influence of prandial replacement of vitamin E on the extent of depletion in response to DBE treatment, rats were fed a vitamin E-deficient diet for 2 days prior to treatment. This short pulse of a vitamin E-deficient diet delayed (to 2 days) both the elevation in liver GSH content and the depletion of liver vitamin E and hastened (to 1 day) the elevation in plasma vitamin E concentration. These observations suggest the presence of at least two pools of liver vitamin E and that one of these pools, which comprises at least 30% of the total hepatic vitamin E content, is able to be mobilized and exported in response to chemical challenge. The stimulus that resulted in liver vitamin E exportation in response to DBE treatment seems to result from wholly intrahepatic processes and may not be a direct response to lipid peroxidation. Moreover, the similarity between the time-course and the extent of hepatic vitamin E depletion observed after treatment with either MEKP or DBE suggests a similarity in physiochemical processes that function to mobilize hepatic vitamin E stores.