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J Dijkstra

Publications and source records attributed to J Dijkstra.

At least 37 records · Page 2Linked to original sources
PubMed2001

A mechanistic model of whole-tract digestion and methanogenesis in the lactating dairy cow: model development, evaluation, and application.

Dietary intervention to reduce methane emissions from lactating dairy cattle is both environmentally and nutritionally desirable due to the importance of methane as a causative agent in global warming and as a significant loss of feed energy. Reliable prediction systems for methane production over a range of dietary inputs could be used to develop novel dietary regimes for the limitation of feed energy loss to methane. This investigation builds on previous attempts at modeling methanogenesis and involves the development of a dynamic mechanistic model of wholerumen function. The model incorporates modifications to certain ruminal fermentation parameters and the addition of a postruminal digestive element. Regression analysis showed good agreement between observed and predicted results for experimental data taken from the literature (r2 = 0.76, root mean square prediction error = 15.4%). Evaluation of model predictions for experimental observations from five calorimetry studies (67 observations) with lactating dairy cows at the Centre for Dairy Research, in Reading, U.K., shows an underprediction (2.1 MJ/d) of methane production (r2 = 0.46, root mean square prediction error = 12.4%). Application of the model to develop diets for minimizing methanogenesis indicated a need to limit the ratio of lipogenic to glucogenic VFA in the rumen and hindgut. This may be achieved by replacing soluble sugars in the concentrate with starch or substituting corn silage for grass silage. On a herd basis, the model predicted that increasing dietary energy intake per cow can minimize the annual loss of feed energy through methane production. The mechanistic model is a valuable tool for predicting methane emissions from dairy cows.

Animal Feed↗
PubMed2000

Estimating the extent of degradation of ruminant feeds from a description of their gas production profiles observed in vitro: comparison of models.

An evaluation of general models that describe gas production profiles is presented. The models are derived from first principles by considering a simple three-pool scheme and permit the extent of ruminal degradation to be calculated, as described in the companion paper. The models evaluated were the generalized Mitscherlich, simple Mitscherlich, generalized Michaelis-Menten, simple Michaelis-Menten, Gompertz, and logistic. Five sets of gas production data consisting of 216 curves, obtained using a wide range of feeds (including straw, hay, silage, grain and various byproducts), were analysed to study the performance of these gas production models. Application of the non-sigmoidal models (simple Mitscherlich and Michaelis-Menten) to the data resulted in convergence problems and these models were found to be inadequate in many cases. Based on results of a pairwise comparison between models (variance ratio test), ranking of residual mean squares, lack-of-fit test, and of analyses of residuals, the generalized Mitscherlich and the generalized Michaelis-Menten models seemed particularly suited because of their flexibility to encompass sigmoidal and non-sigmoidal shapes of gas production profiles, whether symmetrical or not.

Animal Feed↗
PubMed2000

Estimating the extent of degradation of ruminant feeds from a description of their gas production profiles observed in vitro: derivation of models and other mathematical considerations.

Equations to describe gas production profiles, obtained using manual or automated systems for in vitro fermentation of ruminant feeds, were derived from first principles by considering a simple three-pool scheme. The pools represented were the potentially degradable and undegradable feed fractions, and accumulated gases. The equations derived and investigated mathematically were the generalized Mitscherlich, generalized Michaelis-Menten, Gompertz, and logistic. They were obtained by allowing the fractional rate of degradation to vary with time. The equations permit the extent of ruminal degradation (hence the supply of microbial protein to the duodenum) to be evaluated, thus linking the gas production technique to animal production.

Animal Feed↗
PubMed2000

Multicenter evaluation of epidemiological typing of methicillin-resistant Staphylococcus aureus strains by repetitive-element PCR analysis. The European Study Group on Epidemiological Markers of the ESCMID.

Rapid and efficient epidemiologic typing systems would be useful to monitor transmission of methicillin-resistant Staphylococcus aureus (MRSA) at both local and interregional levels. To evaluate the intralaboratory performance and interlaboratory reproducibility of three recently developed repeat-element PCR (rep-PCR) methods for the typing of MRSA, 50 MRSA strains characterized by pulsed-field gel electrophoresis (PFGE) (SmaI) analysis and epidemiological data were blindly typed by inter-IS256, 16S-23S ribosomal DNA (rDNA), and MP3 PCR in 12 laboratories in eight countries using standard reagents and protocols. Performance of typing was defined by reproducibility (R), discriminatory power (D), and agreement with PFGE analysis. Interlaboratory reproducibility of pattern and type classification was assessed visually and using gel analysis software. Each typing method showed a different performance level in each center. In the center performing best with each method, inter-IS256 PCR typing achieved R = 100% and D = 100%; 16S-23S rDNA PCR, R = 100% and D = 82%; and MP3 PCR, R = 80% and D = 83%. Concordance between rep-PCR type and PFGE type ranged by center: 70 to 90% for inter-IS256 PCR, 44 to 57% for 16S-23S rDNA PCR, and 53 to 54% for MP3 PCR analysis. In conclusion, the performance of inter-IS256 PCR typing was similar to that of PFGE analysis in some but not all centers, whereas other rep-PCR protocols showed lower discrimination and intralaboratory reproducibility. None of these assays, however, was sufficiently reproducible for interlaboratory exchange of data.

Bacterial Typing Techniques↗
PubMed2000

A generalized Michaelis-Menten equation for the analysis of growth.

The functional form W = (W0Kc + Wf t(c)) /(Kc + t(c)), where W is body size at age t, W0 and Wf are the zero- and infinite-time values of W, respectively, and K and c are constants, is derived. This new generalized Michaelis-Menten-type equation provides a flexible model for animal growth capable of describing sigmoidal and diminishing returns behavior. The parameters of the nonlinear model are open to biological interpretation and can be used to calculate reliable estimates of growth traits, such as maximum or average postnatal growth rates. To evaluate the new model, the derived equation and standard growth functions such as the Gompertz and Richards were used to fit 83 growth data sets of different animal species (fish, mice, hamsters, rats, guinea pigs, rabbits, cats, dogs, broilers, turkeys, sheep, goats, pigs, horses, and cattle) with a large range in body size. A comparative study was carried out based on mathematical, statistical, and biological characteristics of the models. The statistical goodness-of-fit achieved with the new model was similar to that of Richards, and both were slightly superior to the Gompertz. The new model differed from the others with respect to some of the estimated growth traits, but there were highly significant correlation coefficients between estimates obtained with the different models, and the ranking of animals based on growth parameters computed with the new function agreed with the rankings computed by the other models. Therefore, the new model, with its variable inflection point, was able to adequately describe growth in a wide variety of animals, to fit a range of data showing sigmoidal growth patterns, and to provide satisfactory estimates of traits for quantifying the growth characteristics of each type of animal.

Animals↗
PubMed2000

Induction of aggregation and fusion of cholesterol-containing membrane vesicles by an anti-cholesterol monoclonal antibody.

A monoclonal IgM antibody that reacts with cholesterol was able to aggregate small and large unilamellar lipid vesicles. Vesicles aggregated by the antibody could be dispersed by trypsin digestion. Inclusion of unsaturated phosphatidylethanolamine in the vesicle formulation lowered the relative amount of cholesterol necessary for aggregation, and prevented disaggregation by trypsin treatment. Fluorimetric assays indicated that membrane mixing occurred in aggregates resistant to trypsinization, but the vesicles did not mix or leak their aqueous contents. Analysis of the kinetics of lipid-mixing showed an increase in the aggregation and fusion rate constants with increasing antibody concentrations, indicating that the antibody reaction promotes both processes. An apparent inactivation process whose rate increased with antibody dose has been considered. We conclude that the simultaneous binding of antibodies to more than one vesicle at densities that allow the contact of membrane surfaces, induces first aggregation followed by hemifusion, and with excess of antibody also results in inactivation of the latter process.

Antibodies, Monoclonal↗
PubMed1999

An isotope dilution model for partitioning leucine uptake by the liver of the lactating dairy cow.

An isotope dilution model for partitioning leucine uptake by the liver of the lactating dairy cow is constructed and solved in the steady state. If assumptions ae made, model solution permits calculation of the rate of leucine uptake from portal and hepatic arterial blood supply, leucine export into the hepatic vein, leucine oxidation and transamination, and synthesis and degradation of hepatic constitutive and export proteins. The model requires the measurement of plasma flow rate through the liver in combination with leucine concentrations and plateau isotopic enrichments in arterial, portal and hepatic plasma during a constant infusion of [1-13C]leucine tracer. The model can be applied to other amino acids with similar metabolic fates and will provide a means for assessing the impact of hepatic metabolism on amino acid availability to peripheral tissues. This is of particular importance when considering the dairy cow and the requirements of the mammary gland for milk protein synthesis.

Animals↗
PubMed1999

Quantitative measurements in IVUS images.

IntraVascular UltraSound (IVUS) is a catheter-based technique which provides real-time high resolution tomographic images of both the lumen and arterial wall of a coronary segment, this in contrast to X-ray arteriography that provides a shadow image (luminogram) of the entire lumen. Nowadays the lumen and vessel parameters are measured manually, which is very time consuming and suffers from high inter- and intra-obser variability. With the continuing improvement in IVUS imaging, it is now feasible to develop and clinically apply automated methods of three-dimensional quantitative analysis of the coronary vessel morphology in an objective and reproducible way with automated contour detection techniques (QCU). Quantification, in 2D and 3D, as well as volume rendering for visualization of the IVUS images requires segmentation of the images (contour detection). The 3D contour detection system described in this article is based on the combination of contour detection in the transversal and sagital view. This article provides some of the basic principles of IVUS, the IVUS image quantification, the three-dimensional reconstruction and the contour detection and quantification in three-dimensional IVUS images.

Coronary Disease↗
PubMed1999

Correcting the calculation of extent of degradation to account for particulate matter loss at zero time when applying the polyester bag method.

The established calculation of ruminal extent of degradation using the polyester bag method overestimates extent. The wash fraction, at least in part, is subject to losses from the rumen due to passage. Four formulae are proposed to minimize this risk of overestimation. Four options are considered: 1) passage losses for particulate matter escaping from the bag at zero time are according to the particulate fractional passage rate, 2) the liquid rate, and 3) the average of the two, and additionally 4) there is no instantly degradable fraction. The established and proposed formulae were examined using polyester bag data for six forages: grass silage, fresh ryegrass, fresh white clover, alfalfa hay, mixed grasses hay, and hay from a permanent mountain meadow. The established formula gave appreciably higher estimates of extent of degradation in all cases. Overestimation was at least 6.9, 4.9, 2.9, 2.1, 2.4, and 4.5%, respectively, for the six forages.

Animal Feed↗
PubMed1999

Comparison of mathematical models to describe disappearance curves obtained using the polyester bag technique for incubating feeds in the rumen.

Different nonlinear models were evaluated as candidates to describe ruminal degradation kinetics of forages from data obtained by the incubation of the feeds in the rumen using polyester bags. Nine models were used: segmented model with three straight lines (Mod0); simple Mitscherlich or exponential (Mod1); inverse polynomial (Mod2); compartmental model with two exponential terms (Mod3); generalized Mitscherlich (Mod4); generalized Michaelis-Menten (Mod5); logistic (Mod6); Gompertz (Mod7); and generalized Von Bertalanffy (Mod8). All these models can be represented in the general form D = W + S0 x phi(t), where D is in situ disappearance at incubation time t, W and S0 are positive scalars, and phi is a positive monotonically increasing function unique to each of the models studied. Based on first principles, a general formula for calculating the extent of degradation of feeds in the rumen has been derived that is applicable to all the models. The disappearance curves of different feed components (DM, N, and NDF) of 87 Mediterranean forages (i.e., a total of 261 curves) were fitted to all the models. A comparative study was carried out based on the mathematical, statistical, and biological characteristics of the models. Flexible models that can accommodate both diminishing returns and sigmoidal behavior were more appropriate in describing the curves. A discrete-lag parameter was introduced into Mod0, Mod1, and Mod2 to describe the initial stage of the disappearance curve, and this parameter considerably improved the fit of experimental data. Based on statistical criteria, models Mod1, Mod4, Mod5, and Mod8 were better than the others for most statistical tests and disappearance curves, but differences among these four models were not consistent. The estimates of degradation parameters to quantify the rate (half-life, fractional degradation rate), and extent (undegradable fraction, effective degradability) of ruminal degradation of feeds were also used as a means to discriminate between models, although in most cases all of the models gave similar values of the degradation parameters. In particular, when the extent of degradation was calculated for each forage and feed component, differences between the estimates obtained with the different models were of little nutritional significance for the animal.

Animal Feed↗
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↗