PubMed HealthSearch

SEARCH · PubMed Health

Results for “Structural equation modeling”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources

Conformational requirements at the prostaglandin cyclooxygenase receptor site: a template for designing non-steroidal anti-inflammatory drugs.

When space-filling models of the peroxy radical precursor of PGG were compared with models of 2(S)-(3-chloro-4-cyclohexylphenyl) propionic acid and other acidic cyclooxygenase inhibitors several common structural features were revealed. This led us to propose a template for designing non-steroidal anti-inflammatory drugs (NSAID's) based on the conformation of the peroxy radical immediately prior to its cyclisation to PGG. The template can be equated with a complementary cyclooxygenase receptor site.

Anti-Inflammatory Agents

Electrotonic properties of neurons: steady-state compartmental model.

1. If a neuron is represented by a network of resistively coupled isopotential regions, the passive flow of current in its dendritic structure and soma is described by a matrix differential equation. The matrix elements are defined in terms of membrane resistances and capacitances and of coupling resistances between adjoining regions. 2. A uniform cylidrical dendrite can be represented by a chain of identical regions. In this case, a closed-form mathematical expression is derived for the voltage attenuation factor of the dendrite at steady state in terms of the ratio of membrane resistance to coupling resistance. A numerical method is given to determine the coupling resistances, which in turn yield a specified attenuation factor. Related expressions are given for a dendrite coupled to a soma. Formulas are also derived for the input resistance in these configurations. 3. For more complicated neuronal structures, matrix manipulations are described which yield values for input resistances in all regions, attenuation factors between all pairs of regions, and values of applied voltages necessary to attain specified steady-state potentials. 4. Dynamic solutions to the differential equation provide voltage transients (PSPs). Comparison of the shape paramenters of these transients with those of experimental or cable-theoretical PSPs establishes the number of regions necessary to achieve a given degree of approximation to the transients predicted by cable theory.

Dendrites

Toward a general theory of fertility: a causal modeling approach.

A general theory of fertility is derived hypothesizing that the demand for children is primarily an outcome of social psychological processes within the family, subject to certain socioeconomic constraints. Two broad social psychological processes are posited as determinants of fertility. The first suggests that the attitudes or tastes of family members influence the demand for children. The second maintains that the nature of the husband-wife interaction (in terms of power, conflict, decision making, and marital satisfaction) determines family size. Socioeconomic variables, in the form of the normative social structure and social stratification, and economic constraints, such as income and price, are hypothesized to influence fertility through their impact on social psychological processes within the family. The overall theory is tested on two independent samples--one in Ankara, Turkey, the second in Mexico City, Mexico--using a structural equation methodology.

Attitude

Deposition of particles in model airways.

The deposition of monodisperse aerosols in models of single lung airway bifurcations was studied. The particles, generated by spinning disk, were in the size range of 4.2--8.8 microns (density 1.3 X 10(3) kg . m-3). The model bifurcations were symmetrical, of physiological size, and based on established morphological data. The effects on percentage deposition (P) of flow rate (expressed as Reynolds' number. Re), particle diameter (d), tube diameter (D), and bifurcation angle (2 theta) were studied and a generalized expression for deposition based on these variables was obtained, viz., P = 11.04 X 10(5) Re (d/D)2.5sin3 theta. The applicability of this equation is discussed.

Aerosols

Gene regulatory network structure informs the distribution of perturbation effects.

Gene regulatory networks (GRNs) govern many core developmental and biological processes underlying human complex traits. Even with broad-scale efforts to characterize the effects of molecular perturbations and interpret gene coexpression, it remains challenging to infer the architecture of gene regulation in a precise and efficient manner. Key properties of GRNs, like hierarchical structure, modular organization, and sparsity, provide both challenges and opportunities for this objective. Here, we seek to better understand properties of GRNs using a new approach to simulate their structure and model their function. We produce realistic network structures with a novel generating algorithm based on insights from small-world network theory, and we model gene expression regulation using stochastic differential equations formulated to accommodate modeling molecular perturbations. With these tools, we systematically describe the effects of gene knockouts within and across GRNs, finding a subset of networks that recapitulate features of a recent genome-scale perturbation study. With deeper analysis of these exemplar networks, we consider future avenues to map the architecture of gene expression regulation using data from cells in perturbed and unperturbed states, finding that while perturbation data are critical to discover specific regulatory interactions, data from unperturbed cells may be sufficient to reveal regulatory programs.

Gene Regulatory Networks

[Spike transmission in statistical neuronal ensembles. Induced epileptic focus in a model of hippocampal field CA3].

In a spatially heterogeneous model the transition to supercritical phase was investigated as to the parameter characterizing the activation level of the pyramidal cells related to one another assuming the nonuniformity radius to be R0 = const. This is a transition from spontaneous activity to epileptoid bursts. Before the onset of epileptoid bursts the region of stochastic nonequilibrium of solutions is developed likely to produce pathologic dynamic patterns. With further increase of the activation parameter the system comes to epileptoid state. This synchronized firing of pyramidal cells is accompanied with phases of inhibition. A decrease in the nonuniformity radius leads to the formation of an epileptic focus. It is a dissipative structure. The evolution of it is not further followed, since the transport equations do not include its dynamics.

Hippocampus

[Model of physiological thermoregulation].

Temperature dependence of energy transformations in the cell being considered as the basis of thermoregulatory reactions in any organism, the mathematical model of energy obtaining and utilization of some formal system which can be considered as a cell prototype is composed. The mode is realized in the form of differential equations which describe the energy balance in the dynamic system and the energy exchange between the system and the environment. The steady state of the system structural elements is interpreted as a "hypothetic" aim of the regulation. Temperature "set interval" and "non-thermal" factors role in the formation of the last is based on the equations solutions which correspond to quasi-stationary states of the system. The model can be used as an element of mathematical description of organism thermoregulation. A scheme of possible synthesis of organism of thermoregulation model is considered.

Adenosine Triphosphate

Structure-activity relationships in glucocorticoids.

Meaningful answers to the question of the relationship between glucocorticoid structure and activity have emerged. Structural change has predictable effects on susceptibility to the action of metabolizing enzymes, on receptor affinity, and on intrinsic activity. These effects are, in principle, amenable to mathematical modeling techniques. The fascinating possibility of being able to calculate receptor affinity directly from chemical structure has already been realized through the development of an equation [19] that allows the calculation of receptor binding of any glucocorticoid from structural parameters. Utilizing knowledge of the free energy contributions of the substituents and the hydrophobicity and A-ring conformation of the steroids, receptor affinity for a large number of compounds could be described in terms of four parameters. A general relationship was derived relating the equilibrium dissociation constant to a surface area term, a polar interaction term, and A-ring tilt term, and a size limitation function for the 9 alpha-substituent. The excellent correlation obtained suggests that these four factors are the major determinants of glucocorticoid receptor interactions. It is clear that the use of a mathematical relationship that defines the strength of steroid-receptor interaction is a valuable tool for investigating structure-activity relationships. This would be especially true in the design of steroid drugs. The use of a linear free-energy equation is superior to the assumption of substituent additivity in predicting binding affinities. This type of relationship will be useful in the preparation of steroids for use in affinity labeling studies and should be adaptable to other binding systems in which it is desirable to obtain synthetic analogs for more potent activity or specificity.

Animals

A simulation study of oscillating glycolysis: a comparison between a model and experiments.

Glycolysis is the best known biochemical oscillator and suitable for the exploration of the basic features of biological rhythms on a biochemical and mathematical level. Because of our detailed knowledge of its component structure (enzyme kinetics, metabolite pattern) glycolysis can be described by a set of coupled nonlinear differential equations of first order with respect to time, whose individual terms consist of enzyme velocities assuming a steady state hypothesis for the enzymatic forms. Due to the feedback control of PFK by the adenylates the system is able to oscillate. The outcomes of a simplified model, containing only the basic elements of glycolysis, show good agreements with experimental results.

Adenine Nucleotides

A toxicity estimation model.

A statistical model has been developed for estimation of acute toxicity. The model, currently operational for rat oral LD50, permits the estimation of rat oral LD50 for untested chemical compounds. Only the chemical structure, partition coefficient, and molecular weight for a compound are needed for estimation purposes. The chemical structure is partitioned into substructural fragments using the CIDS fragment keys. A regression model is developed on the basis of 425 compounds. A test of the regression equation with 100 compounds not used in its design shows that 56 percent of the compounds are predicted with less than 0.4 log unit deviation between extimated and measured LD50. This toxicity estimation model can be readily adapted to other species and to other measures of toxicity by the use of suitable design data bases. The model also identifies the contribution to toxicity of the fragments and physical characteristics. The use of this model can materially reduce the amount of toxicological testing for new compounds. It also permits the ranking of potentially toxic compounds to allow the most likely candidates to be tested. The method may also prove applicable to the determination of optimum dosages for new drugs.

Animals

Quantitative structure-activity relationships. 2. A mixed approach, based on Hansch and Free-Wilson Analysis.

Based on the theoretical and numerical equivalence of Hansch's linear multiple regression model and the modified Free-Wilson model a mixed approach is developed. The mixed approach is a combination of both models which makes use of the advantages of each model and widens the applicability of Hansch and Free-Wilson analysis. The Free-Wilson approach now is applicable also in the case of parabolic dependence of biological activity on a particular physical property, e.g., log P or pi. A rational explanation is given for the use of dummy variables in Hansch equations and the derivation of Hansch correlations for de novo group contributions obtained from Free-Wilson analysis. Some examples illustrate the mixed approach and demonstrate its usefulness to establish biologically meaningful structure-activity relationships.

Animals

Structure-activity studies on hallucinogenic amphetamines using molecular connectivity.

A series of ring-substituted hallucinogenic amphetamines has been analyzed using molecular connectivity. A correlating equation has been found between potency and connectivity terms. The equation permits an interpretation of SAR. The equation is capable of predicting potency for amphetamines not in the list and mescalines and tryptamines.

Amphetamines

Action of heparin on mammalian nuclei. II. Cell-cycle-specific changes in chromatin organization correlate temporally with histone H1 phosphorylation.

The interaction of the polyanion heparin with the inner histones of chromatin has been used to detect changes in chromatin organization associated with cell-cycle traverse. Synchronized populations of Chinese hamster cells were obtained either in early G1 or near the G1/S boundary. The rate of interaction of heparin with chromatin-associated inner histones was measured using nuclei isolated from synchronized cell populations in different phases of the cell cycle. A G1-specific decrease in rate of interaction of heparin with inner histones was observed and found to be independent of the presence of hydroxyurea during traverse of G1. A further decrease in heparin-inner histone interaction occurred in late S and G2. These changes correlate temporally with the interphase phosphorylation(s) of histone H1. This correlation is discussed within the framework of current models of higher order chromatin structure (i.e. organization above the nucleosome level). Analysis of the cooperativity of interaction of heparin with inner histones was performed using the kinetic analog of the Hill equation. This analysis suggests that the organization of inner histones on chromatin does not undergo large variations during the cell cycle.

Cell Cycle

Cell shape as an indicator of volume reabsorption in proximal nephron.

If the complex shape of cells and intercellular channels in the renal proximal tubule is determined in part by the forces of large transepithelial water flow, the cell and channel shapes might serve as indicators of the type and magnitude of the forces required for water flow and the routes of that flow. We review here the known morphologic and functional data from the convoluted and straight portions of the rabbit proximal tubule and test the hypothesis of structure-function correlation in that tissue by means of a mass balance equation. If the lateral cell walls are sufficiently deformable to communicate small transmembrane differences in hydrostatic pressure, the resulting phenomenological model suggests an important new role for peritubular serum proteins and can be used to compute reasonable values for cell wall hydraulic conductivity, intercellular protein diffusion constant, and a channel fluid osmolality not more than 1% greater than that of luminal fluid. We conclude that quantitative morphologic studies may serve as a powerful means for evaluating and understanding transport phenomenons in the nephron.

Animals

Formation of topographic maps and columnar microstructures in nerve fields.

Topographic connections are found in many parts of the vertebrate nervous systems, known for example as retinotopy. The self-organizing ability of Hebb type modifiable synapses plays an important role in forming, at least in refining, the topographic connections. We present a mathematical analysis of a revised version of the Willshaw-Malsburg model of topographic formation, solving the equations of synaptic self-organization coupled with the field equation of neural excitations. The equilibrium solutions are obtained and their stability is studied. It is proved that two cases exist depending on parameters. In one case, the smooth topographic organization is obtained as a stable equilibrium of the equations. In the other case, this solution becomes unstable, and instead the topographic organization with columnar microstructures appears. This might explain the columnar structures in the cerebrum. The theory is confirmed by computer simulated experiments.

Computers

Representation of Nonepistatic selection models and analysis of multilocus Hardy-Weinberg Equilibrium configurations.

The paper develops conditions for the existence and the stability of central equilibria emanating from selection recombination interaction with generalized nonepistatic selection forms operating in multilocus multiallele systems. The selection structure admits a natural representation as simple sums of Kronecker products based on a common set of marginal selection components. A flexible parametrization of the recombination process is introduced leading to a canonical derivation of the transformation equations connecting gamete frequency states over successive generations. Conditions for the existence and stability of multilocus Hardy-Weinberg (H.W.) type equilibria are elaborated for the classical nonepistatic models (multiplicative and additive viability effects across loci) as well as for generalized nonepistatic selection expressions. It is established that the range of recombination distributions maintaining a stable H.W. polymorphic equilibrium is confined to loose linkage in the pure multiplicative case, but is not restricted in the additive model. In the bisexual case we ascertain for the generalized nonepistatic model the stability conditions of a common H.W polymorphism.

Alleles

The [18F]fluorodeoxyglucose method for the measurement of local cerebral glucose utilization in man.

A method has been developed to measure local glucose consumption in the various structures of the brain in man with three-dimensional resolution. [18F]-2-deoxy-2-fluoro-D-glucose is used as a tracer for the exchange of glucose between plasma and brain and its phosphorylation by hexokinase in the tissue. A mathematical model and derived operational equation are used which enable local cerebral glucose consumption to be calculated in terms of the following measurable variables. An intravenous bolus of [18F]-2-deoxy-2-fluoro-D-glucose is given and the arterial specific activity monitored for a predetermined period of from 30 to 120 minutes. Starting at 30 minutes, the activity in a series of sections through the brain is determined with three-dimensional resolution by an emission tomographic scanner. The method was used to measure local cerebral glucose consumption in two normal volunteers. The values in gray matter structures range from 5.79 mg/100 g per minute in the cerebellar cortex to 10.27 in the visual cortex, whereas, in white matter structures, the values range from 3.64 mg/100 g per minute in the corpus callosum to 4.22 in the occipital lobe. Average values for gray matter, white matter, and whole brain metabolic rates, calculated as a weighted average based on the approximate volume of each structure, are 8.05, 3.80, and 5.90 mg/100 g per minute, respectively. The value of 5.9 mg/100 g per minute compares favorably with values previously reported.

Adult