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Results for “Models, Theoretical”

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At least 433 records · Page 24Linked to original sources

PubMed1987

The effect of mercuric salts on the electro-rotation of yeast cells and comparison with a theoretical model.

The rotational spectrum of yeast cells changed after pre-treatment of the cells with HgCl2 or Hg(NO3)2 and became indistinguishable from that of ultrasonically produced cell walls. The spectrum of the affected cells contained a peak which could only be explained by attributing a conductivity to the cell walls that was higher than that of the medium. Theoretical models of the rotational response are fully in accord with the experimental spectra. It is shown that the rotation method is capable of measuring even the low cell wall conductivity of yeast cells (which was found to be 33 microS/cm at 10 microS/cm medium conductivity). Knowledge of the spectra allowed a field frequency to be selected at which untreated cells showed no rotation, but at which cells affected by treatment with Hg(II) identified themselves by rotating in the same direction as the field. Calculation of the percentage of cells showing this co-field rotation gave an index (termed the co-field rotation value) of the proportion of the cells that were affected. Using this technique, effects of 25 nmol/l Hg(II) could be demonstrated. In media of low conductivity (10 microS/cm) the change in the rotational spectrum was usually 'all-or-none', whereas at 200 microS/cm a graded Hg(II)-mediated change became apparent. The co-field rotation method showed that the action of small quantities of Hg(II) was still increasing after 3 h of incubation and paralleled the Hg(II)-induced K+ release. A rapid reduction of the effects of Hg(II) was seen when 3-30 mM K+ (or Na+) or when 1 mM Ca2+ were present in the incubation medium, or as the pH was increased. At high incubation cell concentrations the toxic effect of Hg(II) was reduced, apparently due to binding by the cells.

Calcium↗

PubMed1997

Theoretical models for drug delivery to solid tumors.

The effectiveness of anti-cancer drug therapies is often limited by the difficulty of achieving drug delivery throughout solid tumors. Mathematical models permit an analysis of the factors leading to inadequate drug delivery to tumors and can suggest strategies for improving delivery. An overview is given of key factors that influence drug delivery and the extent to which they have been incorporated into existing theoretical models. These factors include spatial gradients of drug concentration and other variables within tumors and other parts of the body, and the relative magnitudes of the time scales involved in drug transport, tumor cell kinetics, and host toxicity. Models for both systemic and regional delivery methods are considered, including intravenous, intraarterial, intraperitoneal, intrathecal, and intratumoral delivery. Strategies for improving delivery are discussed, including use of two-step therapies, hyperthermia, liposome encapsulation, and magnetic targeting. Until now, modeling has mainly developed in separate subfields of tumor growth and cell kill kinetics, compartmental modeling of the body, spatially distributed models for single tissues, radiation dose calculations, tumor oxygenation, tumor blood flow, and cellular pharmacokinetics. In the future, models that integrate these subfields should be developed.

Animals↗

PubMed2001

Heme-based sensors: theoretical modeling of heme-ligand-protein interactions.

Heme proteins are uniquely adapted to bind the important diatomic molecules O(2), NO and CO. An increasing number of heme proteins are being discovered that sense these molecules and thereby regulate a variety of biochemical responses. The interactions of diatomic molecules with heme, and with the surrounding protein, are therefore of great interest. Recent theoretical modeling, using density functional theory, captures many features of these interactions, as exemplified by the well-characterized heme protein myoglobin. Important details, however, especially the mutual influence of the bound diatomic molecule and the proximal ligand, remain to be clarified.

Biosensing Techniques↗

PubMed2006

An ESS for the height of a plant population, or an optimal height for an individual?--Rethinking game-theoretic models for plant height.

Plants only interact with neighbors over restricted distances, so local conditions are of great significance for plants. It is therefore important to consider spatial structure and neighborhood effects if we are to understand plants' strategies. We constructed a spatially-explicit, game theory model to explore optimal height growth at the individual-level. In the model, there is no ESS for height growth at the population level, because there is an "instantaneous" optimal height growth strategy for the individual plant that changes depending on the local light environment. The optimal strategy is plasticity in response to local conditions. Game-theoretic models for plant phenotypic traits should move from "mean-field approximations" towards explicit modeling of local interactions.

Ecosystem↗

PubMed2005

A mathematical model of Doxorubicin treatment efficacy for non-Hodgkin's lymphoma: investigation of the current protocol through theoretical modelling results.

Doxorubicin treatment outcomes for non-Hodgkin's lymphomas (NHL) are mathematically modelled and computationally analyzed. The NHL model includes a tumor structure incorporating mature and immature vessels, vascular structural adaptation and NHL cell-cycle kinetics in addition to Doxorubicin pharmacokinetics (PK) and pharmacodynamics (PD). Simulations provide qualitative estimations of the effect of Doxorubicin on high-grade (HG), intermediate-grade (IG) and low-grade (LG) NHL. Simulation results imply that if the interval between successive drug applications is prolonged beyond a certain point, treatment will be inefficient due to effects caused by heterogeneous blood flow in the system.

Cell Cycle↗

PubMed2003

A theoretical model of the role of brain stem nuclei in alcohol-mediated arrhythmogenesis in older adults.

Uncertainty about the mechanism of alcohol-mediated arrhythmogenesis and the effect of alcohol use on arrhythmic risk among older adults is an increasing concern in light of population aging and recent reports that moderate alcohol consumption may protect older adults against coronary artery disease. In this review, a theoretical model of the role of brain stem nuclei in alcohol-mediated arrhythmogenesis in older adults is developed. The model is based on the hypothesis that the effects of alcohol on central autonomic pathways of cardiac control may alter the threshold for alcohol-mediated arrhythmogenesis among older adults. Findings from multiple lines of research including cellular, electrophysiological, epidemiological, experimental, and clinical studies in human, animal, and in vitro models were synthesized in developing the model. Suggestions for future research on the topic of alcohol-mediated arrhythmogenesis in older adults are offered.

Aging↗

PubMed2026

Structure-informed theoretical modeling defines principles governing avidity in bivalent protein interactions.

In signaling cascades, signaling proteins often encode multiple domains or motifs, which presents the possibility for avidity -- where multivalent binding drastically increases interaction strength and duration. However, predicting and validating multivalent interactions that interact with avidity is a challenge. Here, we integrate mechanistic modeling, structure-based analysis, and experimental approaches as a framework for defining the conditions under which avidity plays a role. We explore the tandem SH2 domain family of interactions with bisphosphorylated partners as a multivalent archetype, which encompasses key secondary messengers in tyrosine kinase signaling networks. Theoretical modeling suggests that maximum avidity occurs with closely spaced tyrosine phosphorylation sites combined with moderate monovalent affinities - exactly around the innate range of SH2 domain affinity - or with phosphorylation sites separated by sufficiently flexible linkers. Surprisingly, despite sequence diversity, structure-based analysis showed relatively conserved three-dimensional spacing between SH2 domains across all tandem SH2 families, which we corroborate experimentally, suggesting evolutionary optimization for avidity interactions. The combination of structure-based analysis of domain spacing with available monovalent experimental data appears, along with iterative experimental refinement of biophysical parameters, can identify high affinity interactions of tandem SH2 domain recruitment to the EGFR C-terminal tail. Using these principles, we extended bivalent predictions into the full phosphoproteome space and structural parameterization of other partners of SH2 domain binding, providing resources and methods for more rapid expansion of bivalent analysis. These approaches lay the groundwork for larger utility in multivalent prediction and testing to help better understand protein interactions that drive cell signaling.

BLI↗

PubMed2003

Spherical aberration after laser in situ keratomileusis and photorefractive keratectomy. Clinical results and theoretical models of etiology.

PURPOSE: To assess changes in corneal asphericity after laser refractive surgery and mathematically model possible causes of the changes. SETTING: Cornea and Laser Eye Institute, Hersh Vision Group, Teaneck, New Jersey, USA. METHODS: The corneal topography (EyeSys 2000) of 20 eyes was measured before and after laser in situ keratomileusis, laser-assisted subepithelial keratectomy, and photorefractive keratectomy for myopia. All preoperative and postoperative maps were analyzed using the CTView 4.0, a computer software program for determining quantitative corneal spherical aberration. To define possible mechanisms of asphericity change, 2 mathematical models of corneal ablation were constructed and theoretical postoperative corneal asphericities were determined over a range of corrections from -12.0 to +6.0 diopters. Model 1 assumes homogeneous beam fluence over the ablation zone, and model 2 accounts for a theoretical ablation rate drop off peripherally as a result of the angle of incidence of the laser beam on the cornea. Postoperative clinical corneal spherical aberration was compared to the theoretically predicted asphericity values. RESULTS: After excimer laser procedures, all corneas had positive asphericity within the ablation zone, generally changing from a prolate to an oblate optical contour. The mean asphericity (Q) was -0.17 +/- 0.14 (SD) preoperatively and +0.92 +/- 0.70 postoperatively. The mean change in spherical aberration was +1.09 +/- 0.67 of positive asphericity; the range of asphericity change was +0.40 to +2.73 in the direction of a more oblate corneal profile. A trend toward greater change in asphericity and more oblateness was observed among eyes receiving higher correction. A mathematical model taking into account theoretical beam fluence changes across the ablation zone was highly predictive of the actual postoperative asphericity measurements. CONCLUSIONS: The cornea within the ablation zone becomes more oblate after laser refractive surgery. A mathematical model of the change in asphericity, which accounts for the angle of incidence of the laser beam across the ablation area, predicted this change in spherical aberration. If the model is correct, possible changes in laser algorithms, delivering more ablation to the peripheral optical zone, may better retain the native corneal prolate conformation. Moreover, wavefront-guided ablations may have to consider the effects of fluence variability across the optical zone to fully correct spherical as well as other aberrations.

Adult↗

PubMed2003

A theoretical model of axon guidance by the Robo code.

After crossing the midline, different populations of commissural axons in Drosophila target specific longitudinal pathways at different distances from the midline. It has recently been shown that this choice of lateral position is governed by the particular combination of Robo receptors expressed by these axons, presumably in response to a gradient of Slit released by the midline. Here we propose a simple theoretical model of this combinatorial coding scheme. The principal results of the model are that purely quantitative rather than qualitative differences between the different Robo receptors are sufficient to account for the effects observed following removal or ectopic expression of specific Robo receptors, and that the steepness of the Slit gradient in vivo must exceed a certain minimum for the results observed experimentally to be consistent.

Animals↗

PubMed1992

Opposite effects of enantiomers of clofibric acid derivative on rat skeletal muscle chloride conductance: antagonism studies and theoretical modeling of two different receptor site interactions.

The R-(+) enantiomer of 2-(p-chlorophenoxy) propionic acid (CPP) produces a biphasic effect on chloride channel conductance (GCl) of rat skeletal muscle, increasing GCl at low concentrations and decreasing it at concentrations greater than 10 microM; on the other hand, the S-(-) isomer mostly blocks GCl in a concentration-dependent manner. To explain the peculiar behavior of these compounds, a theoretical model based on the presence of two opposing receptor populations controlling chloride channel conductance has been used to fit the experimental data of the concentration-response curves of both S-(-) and R-(+) CPP. An analysis performed by means of the algebraic summation of two logistic terms suggests a reasonable merit of the proposed model and explains the resultant effect of each optical form as follows: S-(-) acts as a full agonist on an inhibitory sites, whereas R-(+) acts as a full agonist at both the inhibitory and excitatory sites. Antagonism studies appear to be consistent with the proposed model. Dose-response curves in which the block of GCl by the S-(-) isomer was evaluated in the presence of the R-(+) isomer (3-10 microM) clearly showed an antagonistic interaction between the two enantiomers, with an increase in the S-(-) concentration for half-maximal block. The antagonism was overcome by high concentrations of S-(-), and this might be consistent with the hypothesis that the block of GCl is modulated by an inhibitory site at which the two enantiomers compete.

2-Methyl-4-chlorophenoxyacetic Acid↗

PubMed1996

A theoretical model for nursing systems outcomes research.

Nursing research on patient and administrative outcomes has typically examined the relationships between selected structural characteristics and outcomes, without taking into account the organization's context. In contrast, health services research has focused on the relationships between the organization's context and outcomes, most often mortality, without taking into account structural characteristics. Although widely used, both approaches develop fragmented knowledge. This article describes a comprehensive theoretical model that takes into account the relationships among the organization's context, its structure, and both patient and nursing administrative outcomes.

Humans↗

PubMed2005

Theoretically modeling microarray with the chemical equilibrium and thermodynamics.

To date, the idea that microarray may shed the light on cellular processes by identifying groups of genes that appear to be co-expressed seems to remain a dream. This is partly because that there are some blank (meaning the knowledge is unavailable) or even erroneous areas in the fundamental theory in this field. This paper attempts to present the digest of microarray hybridization system with chemical thermodynamics, theoretically clarifying some misunderstandings and looking for answers to some critical questions around this technology, such as the mechanisms and conditions of quantitative measuring by hybridization reaction, the reasons of inconsistency of the data and the analysis results and the solutions, how to analyze the data, etc. A theoretical model for the next generation of microarray is proposed. We believe that this model is universal, laying the foundation for microarray technology from array design through the data analysis.

Computer Simulation↗

PubMed2006

Examining theoretic models of adherence for validity in resource-limited settings. A heuristic approach.

Mortality and morbidity for HIV-infected patients in resource-limited settings have declined markedly with increased access to combination antiretroviral therapy. One explanation for this is the high rates of adherence that patients in these settings have achieved. To help patients sustain early successes, detailed understanding of adherence processes in these locales is needed. This understanding is best arrived at using socioculturally valid theoretic models as a guide. This article introduces a heuristic schema for examining the validity of conceptual models of adherence in resource-limited settings. The schema consists of 4 analytic questions to be asked of a model being considered for use in a sociocultural context other than the one in which it was developed. To demonstrate use of the schema, an illustrative validation exercise in which the 4 questions are applied to the information-motivation-behavioral skills model of behavioral change is carried out. Examples for the illustrative exercise come from preliminary qualitative research in HIV/AIDS treatment settings in Mbarara, Uganda and Jos, Nigeria.

Anti-HIV Agents↗

PubMed2005

A simple theoretical model explains dynein's response to load.

Recent experiment showed that cytoplasmic dynein 1, a molecular motor responsible for cargo transport in cells, functions as a gear in response to external load. In the presence of vanishing or small external load, dynein walks with 24- or 32-nm steps, whereas at high external load, its step size is reduced to 8 nm. A simple model is proposed to account for this property of dynein. The model assumes that the chemical energy of ATP hydrolysis is used through a loose coupling between the chemical reaction and the translocation of dynein along microtubule. This loose chemomechanical coupling is represented by the loosely coupled motions of dynein along two different reaction coordinates. The first reaction coordinate is tightly coupled to the chemical reaction and describes the protein conformational changes that control the chemical processes, including ATP binding and hydrolysis, and ADP-Pi release. The second coordinate describes the translocation of dynein along microtubule, which is directly subject to the influence of the external load. The model is used to explain the experimental data on the external force dependence of the dynein step size as well as the ATP concentration dependence of the stall force. A number of predictions, such as the external force dependence of speed of translocation, ATP hydrolysis rate, and dynein step sizes, are made based on this theoretical model. This model provides a simple understanding on how a variable chemomechanical coupling ratio can be achieved and used to optimize the biological function of dynein.

Adenosine Diphosphate↗

PubMed1994

A theoretical model of independence for nursing home elders.

Dependence-oriented care is very much a part of nursing home care in the United States. Yet independence is a salient part of American culture and is desired by nursing home elders. Such a dichotomy sets the stage for a loss of competence, a loss of control, and a subsequent loss of well-being in nursing home elders. Potential reasons for dependence-oriented care are identified in this article, and a theoretical model of independence for frail elders is presented as a guide for professional caregivers. Findings from previous research studies related to the model suggest perceived situational expectancy of control of valued reinforcements as a mediator of the relationship between behavioral competence and psychological well-being. Findings from the literature propose the subjective environment as a mediator of the relationship between behavioral competence and perceived situational expectancy of control of valued reinforcements. Using the model, there is the potential to understand more fully the process of independence in nursing home elders and to guide professional caregivers through this process as care is delivered.

Aged↗

PubMed1998

Shapes of membrane permeability-lipophilicity curves: extension of theoretical models with an aqueous pore pathway.

The objective of this study was to rationalize the shape of membrane permeability-lipophilicity curves, when considering, in addition to the usual transcellular route, a parallel diffusion pathway through aqueous pores as present in biological membranes. The theoretical influence of different pH in donor and acceptor compartment and the molecular weight on the permeability curves was studied. We combined and extended two previously proposed absorption models, namely one describing diffusion through a simple membrane (two stagnant aqueous and two organic layers in series, no pores) as the sum of the two distribution steps at both membrane interfaces, and a second theoretical model considering the sum of different diffusional resistances through stagnant layers and membrane, respectively. Under certain conditions the equivalence of the two-step distribution model and the diffusional resistance model can be demonstrated. Incorporation of an aqueous diffusion pathway leads to an extended two-step distribution model. This theoretical membrane permeation model will permit a more physicochemical-based interpretation of permeation data and shows that combined log D values and molecular weight are important determinants for membrane transport processes through, e.g. Caco-2 monolayers and the mucosal GI membranes. We have demonstrated that the well-known sigmoidal permeability-lipophilicity relationship should be considered as a molecular weight-dependent set of sigmoidal relationships.

Algorithms↗

PubMedDate not supplied

A theoretical model of support network conflict experienced by new mothers.

Women who have had their first child often receive help from persons comprising their support network. Although this help is well-intentioned, studies show that it may be associated with conflict between the new mother and others within the support network. Identification of factors influencing support network conflict is important so that nursing interventions can be implemented to reduce or avoid conflict. This article explores the topic of support network conflict and proposes a theoretical model of factors influencing support network conflict.

Conflict, Psychological↗

PubMedDate not supplied

From limbo to legitimacy: a theoretical model of the transition to the primary care nurse practitioner role.

The initial transitional year of professional practice is thought to provide the critical foundation on which new professionals build their expertise. The purpose of this study was to describe the experiences of new nurse practitioner graduates during their first year of primary care practice. Thirty-five persons were interviewed alone or in focus groups at approximately 1, 6, and 12 months after graduation. Grounded theory methodology guided the data collection and analysis. A theoretical model was constructed that represents the transition to the primary care nurse practitioner role. This model consists of a process called From Limbo to Legitimacy, which encompasses four major categories: Laying the Foundation, Launching, Meeting the Challenge, and Broadening the Perspective. Each category contains a set of subcategories that detail the multiple aspects of the experience. This model highlights both the distress and the accomplishments of the initial year of advanced practice.

Adult↗