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J Dijkstra

Publications and source records attributed to J Dijkstra.

At least 19 recordsLinked to original sources
PubMed1998

Application of a mechanistic model to study competitive inhibition of amino acid uptake by the lactating bovine mammary gland.

A mathematical model is used to describe uptake by a countertransport system and subsequent flow of three amino acids (AA), Phe, Val, and Met, from arterial blood to milk protein in the mammary gland of a lactating cow. The model suggests that total uptake of all AA is higher than net uptake and that a large proportion of the incoming AA is released from the cell directly back to blood. The model is used to predict which of the three AA is limiting the rate of milk protein synthesis and the response to increased arterial concentration of the first-limiting AA. Simulations are performed to predict possible outcomes of several experimental protocols to AA infusion, which might be used to test in vivo the responsiveness of the bovine mammary gland to an altered arterial concentration of AA. Of the three AA considered, arterial Met concentration appears to be first-limiting. The infusion profile that gives the greatest response in milk protein synthesis rate alters the arterial profile of AA such that it is identical to that of proteins originating in the mammary gland. Model construction can be simplified by acknowledging normal biological constraints.

Amino Acids↗
PubMed1998

Different mathematical approaches to estimating microbial protein supply in ruminants.

Many of the amino acids that are available for absorption in ruminants are derived from microbial protein that has been synthesized in the reticulorumen. This paper focuses on the prediction of the microbial protein supply and evaluates different approaches to represent mathematically the process of microbial protein synthesis. In current protein evaluation systems for ruminants, the microbial protein supply is predicted using empirical equations that relate microbial protein production to the amounts of ruminally available energy and nitrogen. In contrast, mechanistic models of rumen function endeavor to describe quantitatively the microbial protein production that is based on underlying identifiable processes. A brief description is presented of two culture techniques used to examine microbial ecosystems, namely, batch culture and chemostat culture. The mathematical equations describing these cultures are helpful in understanding key parameters of microbial production for inclusion in models, including specific growth rate, growth yield, and substrate affinity. The availability of carbohydrates is a primary determinant of microbial protein production in the rumen, and the adequacy of mathematical representations of this relationship in empirical and mechanistic models is assessed. The representation of substrate utilization for nongrowth processes and the relationship between microbial protein production and the availability of various nitrogen sources are discussed. A variable part of the synthesized microbial protein does not reach the duodenum but is degraded in the rumen, and its representation is examined. The prediction of microbial protein supply should be based on a sound representation of the underlying mechanisms, including the interactions among microbes and between microbial activity and substrate degradation.

Animals↗
PubMed1997

Causes of inaccurate prediction of volatile fatty acids by simulation models of rumen function in lactating cows.

Extant mechanistic models of rumen functions are unable to predict the molar proportion of volatile fatty acids (VFA) accurately. In order to make these models useful in investigating theories on nutrient flows that go beyond the rumen, the representations adopted need to be improved. This theoretical study was directed at identifying what parts of a rumen model may be responsible for the inaccurate VFA prediction. For five distinct parts of a model, not involving the description of the microbiological metabolism, the hypothesis was tested that their inappropriate description can be a probable cause of inaccurate VFA prediction. These five parts were: (1) the input functions of feed ingestion; (2) the representation of insoluble, degradable substrate of roughage and concentrate origin; (3) the kinetics of VFA absorption from the rumen; (4) the VFA coefficients that describe the stoichiometry of the conversion of fermented substrate into VFA; and (5) the representation of the rumen particle dynamics. Every hypothesis was tested by modifying the model description and simulating it to steady state. Observations required were derived from digestion trials with lactating dairy cows. Simulation results demonstrated that the predicted molar proportion of rumen VFA concentrations is particularly influenced by VFA absorption kinetics and VFA coefficients. Although the description of particle dynamics also had a large influence with certain choices of its parameterization, it is probably a less important cause of inaccurate predication when rumen feed degradation (apparent from rumen outflow) is predicted well. In conclusion, to obtain improved predictions of the molar proportions of rumen VFA, further work is required on the representation of VFA absorption kinetics and of VFA coefficients of fermentation stoichiometry.

Animals↗
PubMed1997

Description of a model integrating protein and energy metabolism in preruminant calves.

This paper describes the development of a mechanistic model integrating protein and energy metabolism in preruminant calves of 80-240 kg live weight. The objectives of the model are to gain insight into the partitioning of nutrients in the body of growing calves and to provide a tool for the development of feeding strategies for calves in this weight range. The model simulates the partitioning of nutrients from ingestion through intermediary metabolism to growth, consisting of accretions of protein, fat, ash and water. The model contains 10 state variables, comprising fatty acids, glucose, acetyl-CoA and amino acids as metabolite pools, and fat, ash and protein in muscle, hide, bone and viscera as body constituent pools. Turnover of protein and fat is represented. The model also includes a routine to check possible dietary amino acid imbalance and can be used to predict amino acid requirements on a theoretical basis. The model is based on two experiments, specifically designed for this purpose. Simulations of protein and fat accretion rates over a wide range of nutrient input suggest that the model is sound. In can be used as a research tool and for the development of feeding strategies for preruminant calves.

Animals↗
PubMed1997

Evaluation of a model integrating protein and energy metabolism in preruminant calves.

In a companion paper, a mechanistic model is described, integrating protein and energy metabolism in preruminant calves of 80-240 kg live weight. The model simulates the partitioning of nutrients from ingestion through intermediary metabolism to growth, consisting of accretions of protein, fat, ash and water. The model also includes a routine to check possible dietary amino acid imbalance and can be used to predict amino acid requirements. This paper describes a sensitivity and behavioral analysis of the model, as well as tests against independent data. Increasing the carbohydrate:fat ratio at equal gross energy intakes leads to higher simulated protein- and lower simulated fat-deposition rates. Simulation of two experiments, not used for the development of the model, showed that rates of gain of live weight, protein and fat were predicted satisfactorily. The representation of protein turnover enables the investigation of the quantitative importance of hide, bone and visceral protein in protein and energy metabolism. The model is highly sensitive to 25% changes in kinetic parameters describing muscle protein synthesis and amino acid oxidation. Comparing simulated with experimentally derived amino acid requirements shows agreement for most amino acids for calves of approximately 90 kg live weight. For calves of approximately 230 kg live weight, however, lower requirements for lysine and for methionine+cystine are suggested by the model. More attention has to be paid to the inevitable oxidative losses of amino acids. It is concluded that the model provides a useful tool for the development of feeding strategies for preruminant calves in this weight range.

Amino Acids↗
PubMed1997

A model to describe growth patterns of the mammary gland during pregnancy and lactation.

Extensive proliferation and death of cells in the mammary gland occur during pregnancy and lactation. In this study, a mechanistic model was developed that yielded a single equation to describe the pattern of mammary growth of mammals throughout pregnancy and lactation. The model contains a single pool, which is the cell population of the mammary gland; one influx, representing cell proliferation; and one efflux, representing cell death. The parameters of the equation lend themselves to direct physiological interpretation. The model fitted data on mammary gland DNA adequately and can be related to current knowledge on factors and inhibitors of mammary gland growth. A unique definition of the parameters of the model can be difficult because of the high degree of variation among animals, an improper number of observations, or timing, as indicated by analyses of simulated data. The model can also be applied to the study of the entire lactation curve. The widely applied gamma equation and the equation that was developed in this study were compared using weekly production data from dairy cows. The new model performed well, particularly when a sharp peak in milk production occurred. The model has the advantage of providing, for the first time, a simple biological description of the lactation curve that can be used to discriminate changes in lactational performance that are associated with experimental treatments.

Animals↗
PubMed1996

Interaction of anti-cholesterol antibodies with human lipoproteins.

Inoculation of mice with cholesterol-rich liposomes containing the adjuvant monophosphoryl lipid A results in the production of antiserum containing IgM Ab to cholesterol. The specificity of the Ab was to cholesterol and structurally similar sterols containing a 3 beta-hydroxyl group. Anti-cholesterol binding activity was significantly diminished if the 3 beta-hydroxyl was altered by either epimerization, substitution, oxidation, or esterification. A similar specificity for 3 beta-hydroxy-sterols was observed for an anti-cholesterol IgM mAb. Both hyperimmune serum and the mAb reacted with intact human very-low-/intermediate-density lipoprotein (VLDL/IDL) and low-density lipoproteins (LDL), but not high-density lipoproteins (HDL), in an ELISA, but could react with total lipid extracts containing cholesterol that were prepared from all three lipoprotein classes. Functionally, immune serum or the mAb aggregated and induced a fusion-like reaction with VLDL/IDL and LDL at low temperatures: these aggregates result in spherical structures visible with light microscopy. Similarly, binding of anti-cholesterol A to small cholesterol-rich liposomes resulted in the appearance of vesicular structures with approximately 20- to 200-fold increased diameters. These data demonstrate that the anti-cholesterol Ab recognize unesterified cholesterol in VLDL/IDL and LDL; high-density lipoprotein cholesterol in the intact lipoprotein, however, appears to be protected from reaction with these Ab.

Animals↗
PubMed1996

Activation of murine lymphocytes by lipopolysaccharide incorporated in fusogenic, reconstituted influenza virus envelopes (virosomes).

We have studied the in vitro activation of murine lymphocytes with LPS incorporated in the membranes of both phospholipid vesicles (liposomes) and vesicles composed of fusogenic, reconstituted influenza virus envelopes (virosomes). The incorporation of Salmonella minnesota rough-LPS in liposomes reduced the potency of LPS to stimulate splenocyte proliferation and cell surface kappa-light chain expression on 70 Z/3 pre-B cells by over 100-fold. Salmonella minnesota rough-LPS inserted into virosomes was at least 10-fold more potent than free LPS, both when prebound virosomes were allowed to be taken up by the cells at neutral pH and when the virosomes were fused into the plasma membrane by low pH treatment. Inactivation of the virosomes by low pH pretreatment reduced the potency of the virosomal LPS to the level of liposome-incorporated LPS. The association of the various LPS forms with the cells was quantitated using radio-iodinated LPS. Correcting for uptake, virosomal LPS remained 2- to 10-fold more potent than free LPS in stimulating B lymphocytes and at least 100-fold more active than liposomal LPS or fusion-inactivated virosomes. After low pH-induced fusion with the plasma membrane, the majority (80%) of the prebound virosomes had fused with the cells, compared with about 8% after neutral uptake. From these results we conclude that LPS inserted into the plasma or endosomal membranes efficiently activates murine lymphocytes. The fusion data suggest that the incorporation into endosomal membranes might be a more effective stimulus.

Animals↗
PubMed1996

A simple but flexible growth function.

The functional form W(t) = Wf - (Wf - Wo) exp[-k infinite (t - T) + 2c(root of t - root of T)] where Wf, Wo, k infinite, c and T are constants, is derived as a growth equation and evaluated using commonly applied growth functions such as the Gompertz, logistic, monomolecular and Richards. Further evaluation is made with reference to sets of observations on growth in a number of animal species ranging from mice to horses. The new function provides a flexible growth equation capable of describing sigmoidal and diminishing returns behaviour. It appears adept at describing sigmoidal patterns exhibiting faster early growth and a fairly low but variable point of inflexion, and can therefore be perceived as a generalised Gompertz equation. The function also has the ability to describe a wide range of hyperbolic shapes when there is no point of inflexion. The analysis described suggests that this simple equation is a worthwhile addition to the corpus of growth functions.

Animals↗
PubMed1995

Analysis of anticholesterol antibodies using hydrophobic membranes.

An analytical immunoblotting procedure and a serological enzyme-linked immunosorbent assay (ELISA) for the characterization of antibodies to cholesterol are described. Hydrophobic membranes consisting of polyvinylidene fluoride (PVDF) are used to immobilize cholesterol for immunodetection by anti-sterol antibodies. To determine whether antibodies to cholesterol were induced after immunization with liposomal cholesterol, we separated total lipid extracts of very-low density lipoproteins by thin layer chromatography (TLC) on silica gel plates and transferred the separated lipid classes to PVDF membranes using isopropanol to facilitate passive diffusion. Lipid transfer was confirmed by exposure of membranes to iodine vapors or by staining of cholesterol with filipin complex. Serum from immunized mice reacted with cholesterol, whereas pre-immune serum or serum from mice injected with control liposomes did not bind. To determine the amount of anti-cholesterol activity in serum, we coated microtiter plates consisting of PVDF membrane wells with cholesterol. The PVDF membrane-based ELISA was found to be more reproducible and four-fold more sensitive than the conventional ELISA on polystyrene plates. These techniques may be useful in the analysis of anti-sterol antibodies and antibodies to other hydrophobic antigens.

Antibodies↗
PubMed1995

Liposomal vaccines.

Liposomes have been used therapeutically to deliver drugs to certain anatomical sites. The use of liposomes to deliver antigens, although not a new concept, has received less attention. At least two vaccines of nearly identical liposome base composition to our vaccines have been tested in humans. A malaria vaccine study showed that the liposomal preparation is quite safe: reaction profiles of volunteers receiving the vaccine demonstrated little reactivity and virtually no pyrogenicity (14). The concentration of MPLA in the vaccine was substantially higher (nearly 50,000 times) than the pyrogenic dose of free lipid A. The same vaccine, but different antigen (gp120, an HIV protein), was tested in volunteers and had the same lack of toxicity (27). In both studies, antibodies and cytotoxic cells specific for the respective antigens were produced. We have several subunit vaccines under development for infectious diseases (gram negative sepsis, fungal infections, protozoan infections), metabolic disorders (hypercholesterolemia, diabetic retinopathy, macular degeneration), and neoplastic diseases (multi-drug resistant cancer, primary and metastatic tumors, and angiogenic hyperproliferative disorders). In each case, one or more antigens were identified that might be useful in immunologic control of biologic proliferation (i.e., pathogen or tumor growth, rise in serum cholesterol, growth of blood vessels). We anticipate that at least one of these vaccines will be ready for testing in humans in the next calendar year.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1995

Simulation of the effects of diet on the contribution of rumen protozoa to degradation of fibre in the rumen.

A previously described mathematical model, that stimulates the metabolic activities of rumen bacteria and protozoa, was used to examine the contribution of protozoa to neutral-detergent fibre (NDF) degradation in the rumen of cattle. Comparisons between predicted and experimentally observed NDF degradation showed general agreement. Further simulations were performed with diets containing variable proportions of concentrate (between 0 and 1 kg/kg diet DM) and at intake levels ranging between 5.3 and 21.0 kg DM/d. The simulated protozoal contribution to NDF degradation was 17-21% at the lowest intake level. Except for the all-concentrate diets, raising the feed intake level reduced this contribution to 5-13% at the highest intake level. The changes in contribution of protozoa to NDF degradation were related to variations in the fibrolytic bacteria: protozoa value and the NDF-degrading activities of protozoa predicted by the model. In simulations where dietary NDF levels were reduced and starch and sugar levels were increased independently, protozoal contribution to NDF degradation generally increased. These differences were reflected also in the generally increased protozoal contribution to NDF degradation predicted in response to a decreased roughage:concentrate value. The contribution of protozoa also generally declined in response to added N. These changes in predicted protozoal contribution to NDF degradation resulting from dietary variations provided possible explanations for the differences in rumen NDF degradation observed when animals are defaunated.

Animal Feed↗
PubMed1995

Idiopathic inflammatory bowel disease: endoscopic-radiologic correlation.

PURPOSE: To assess the comparative and adjunctive diagnostic value of double-contrast barium enema examination (DCBE) and colonoscopy in cases of idiopathic inflammatory bowel disease. MATERIALS AND METHODS: Radiographic and endoscopic findings were correlated, with use of a standardized evaluation sheet, in a prospective, blinded study in 99 patients with previously well-established Crohn disease of the colon (CD [ n = 51]) or ulcerative colitis (UC [n = 48]). RESULTS: Overall, substantial to almost perfect agreement was found in more than 80% of scores with respect to the items of the standardized evaluation sheet in the colon segments depicted with both imaging modalities. Almost perfect agreement (kappa > 60%) was found for two essential diagnostic and differential diagnostic features: aphthoid erosions and fine granularity. CONCLUSION: DCBE and colonoscopy are complementary imaging modalities for optimal detection of all mucosal and structural colon lesions, except inflammatory lesions without distortion of the mucosal relief and small numbers of inflamed superficial erosions and ulcers.

Adolescent↗
PubMed1994

Distribution and localization of bean common mosaic virus and bean black root virus in stems of doubly infected bean plants.

Plants of bean (Phaseolus vulgaris) inoculated first on one primary leaf with strain NY15 of bean common mosaic virus, as inducer, and after three days, on the opposite leaf, with the strain NL3 of bean black root virus, as challenger, did not show systemic necrosis characteristic of the latter strain. This interference phenomenon was studied by determining the amount, distribution and localization of both strains in the part of stem between primary leaves and first trifoliolate leaf in both challenge-inoculated and singly inoculated (control) plants. In dot-blot immunoassay, NL3 was detected seven days after its inoculation as challenger, whereas in control plants its presence was established on day four. Immunostained thick sections revealed a large accumulation of NL3 antigen on day eight in both phloem and cambium, but not yet in the xylem and cortex, contrasted with the controls. In immunogold-silver stained semi-thin sections, most of the NL3 label was present in the companion cells and other phloem parenchyma cells, while in the control plants this virus was also present in xylem vessels and xylem parenchyma cells. Inducer strain NY15 was abundantly present in practically all the cells, including xylem vessels, from day two after challenge inoculation onwards. It is concluded that inducer strain NY15 hampers transport of NL3 to, and its spread in, the stem and prevents the latter strain from exerting its deleterious influence on the water conducting elements.

Fabaceae↗
PubMed1994

Cowpea aphid borne mosaic virus-Morocco and South African Passiflora virus are strains of the same potyvirus.

High performance liquid chromatography (HPLC) profiles of tryptic peptides and partial amino acid sequence analysis have been employed to establish the taxonomic status of the Moroccan isolate of cowpea aphid-borne mosaic virus (CABMV). Some previous reports have suggested CABMV to be very closely related to blackeye cowpea mosaic virus (B1CMV) while other reports have concluded that this relationship is distant. In this report a tryptic digest of the coat protein of CABMV-Morocco was compared with those of the coat proteins of B1CMV-Type, B1CMV-W, the mild mottle strain of peanut stripe virus (PStV-MM) and the NY15 strain of bean common mosaic virus (BCMV-NY15), all of which are now recognised as strains of BCMV. The comparisons also included the NL-3 strain of bean necrosis mosaic virus (BNMV-NL3), which had previously been classified as a strain of BCMV. The HPLC peptide profiles indicated that CABMV-Morocco was distinct from BCMV and BNMV. Amino acid sequence analysis of peptides accounting for more than half of the coat protein confirmed that CABMV-Morocco was not a strain of BNMV or BCMV but was a distinct member of the BCMV subset of viruses that previously has been shown to include BCMV, BNMV, soybean mosaic virus, zucchini yellow mosaic virus, passionfruit woodiness virus and South African Passiflora virus (SAPV). Comparison of the partial sequence data with these and other published sequences revealed that the coat protein of CABMV-Morocco is very similar to that of SAPV suggesting that they are strains of the same virus. Since CABMV was described over 25 years earlier than SAPV, the name CABMV should take precedence and SAPV should be renamed CABMV-SAP, the South African Passiflora strain of CABMV.

Amino Acid Sequence↗