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PubMed1993

[Life conditions and health behavior of socially disadvantaged women: implications of an action-theoretic model of social structure for health research].

To operationalize social status epidemiology of social inequalities in health uses uncompounded indices, as school and job education, income and occupational status. Social epidemiology describes social inequalities from a more comprehensive perspective, including indicators of social status as well as subjective attitudes and behavior. Examples of a study on smoking in women indicate a broader framework of the relation between social indicators and healthy lifestyles. Action oriented theory of social inequalities substantiate restrictions of this epidemiological approach. Social analysis should focus on lifestyles and behavioral resources to reach overall life targets. An example of a qualitative study on health behavior in women prove the impact of this sociological theory for public health research. Health behavior of women from the lower SES is developed and improved in life history. It can be described as the capability to reach life targets regarding to health and social conditions.

Adult↗
PubMed2000

[Stent implantation as initial coronary interventional therapy? A theoretical model on clinical and economical consequences of in-stent restenosis].

The reduction of acute complications and late restenosis compared to conventional PTCA has led to a rapid increase in stent implantation as initial treatment for coronary stenosis. As a result, in-stent restenosis has become an important clinical and economical problem, especially the diffuse form, which is much more likely to reappear. In order to compare the consequences of initial stenting and initial angioplasty, we developed an analytic model, considering the differences between diffuse and focal in-stent restenosis. The simulation based on the optimized therapeutic proceeding following an elective 1-vessel revascularization of a 60-year-old patient, dealing with probabilities for acute complications and late restenosis taken from the literature and in-hospital costs obtained from 200 elective interventions. In the stent group 71.0% of patients were free of any target lesion-related event, compared to 60.2% in the PTCA group. Catheter reintervention was necessary for 32.1% of the patients initially treated with angioplasty and for 17.6% of the initially stented patients, whereas 7.7% of the stent patients had to undergo elective bypass surgery as final treatment compared to 2.8% in the PTCA arm. Long-term medical costs for initial stenting (6,237 Euros) were 14% higher than for conventional PTCA (5,345 Euros). Taking also into consideration the indirect costs (loss of productivity) for a collective with an employment rate of 50%, the difference between stent implantation (9,067 Euros) and angioplasty (8,581 Euros) is smaller. Initial treatment of coronary stenosis by stent implantation decreases the rate of repeat revascularization compared to initial PTCA, but there is a greater likelihood that elective bypass surgery will become necessary. This difference in following treatment is related to the occurrence of diffuse in-stent restenosis. When calculating the long-term costs stenting still appeared to be more expensive than PTCAA because the savings in following costs can not compensate for the higher primary in-hospital costs. An empirical study which collects cost data in different hospitals as well as in the outpatient setting over 1 year is necessary to confirm this preliminary result.

Angioplasty, Balloon, Coronary↗
PubMed1995

A theoretical model of circulatory interstitial fluid flow and species transport within porous cortical bone.

A three-dimensional model of interstitial fluid flow and passive species transport within mineralized regions surrounding cross-cortical vessel canals is developed. In contrast to earlier studies, the present model applies to circulatory, non-stress-induced interstitial flow in porous cortical bone. Based on previous experimental observations, the canals are modeled as line sources that pass at an oblique angle through the cortex. Cross-cortical interstitial flow from the endosteal surface to the periosteal surface is also taken into account. It is found that model transport characteristics are qualitatively consistent with reported observations. In addition, parametric studies reveal the following: (1) Solute contact with the matrix is maximized when the ratio of canal radius to cortex thickness (R) is near physiological R values. (2) Solute-matrix contact falls to low levels when R falls below the physiological range. (3) Solute-matrix contact is maximized when the cross-cortical velocity is approximately an order of magnitude smaller than the canal outflow velocity. The first and second findings suggest that within porous bone physiological ranges of R promote near optimal species contact with the mineralized matrix. The third finding suggests that relatively impermeable layers of bone within the cortex can effectively promote solute-matrix contact by limiting cross-cortical flow. Finally, the model suggests that intra-canal resorption associated with reduced external loading may effectively compensate for reduced stress-induced interstitial flow by enhancing circulatory interstitial flow and species transport.

Animals↗
PubMed1992

Kinetic coupling between protein folding and prolyl isomerization. I. Theoretical models.

Kinetic models were developed to describe the influence of prolyl peptide bond isomerization on the kinetics of reversible protein folding for cases in which structural intermediates do not occur. In the simulations, the number of prolyl residues and the relative rates of folding and isomerization were varied. The experimentally observed rate constants were found to be identical with the intrinsic rate constants of folding and isomerization only when folding remains much faster than prolyl isomerization throughout the transition region. When the rate of folding becomes similar to or lower than the rate of isomerization, the observed kinetic parameters are complex functions of all microscopic rate constants. In particular, the observed folding rates in the transition region decrease with the number of prolyl residues. Pseudo two-state kinetics with single folding and unfolding reactions are observed in several cases, although the apparent folding rates depend strongly on prolyl isomerization reactions in the unfolded chain. This virtual simplicity can easily lead to misinterpretation of kinetic data. Additional phases can be resolved when refolding is started from the fast-folding species (UF). The coupling between folding and prolyl peptide bond isomerization also modifies the dependence on denaturant concentration of the apparent rate constants of folding. We suggest several tests to detect and characterize the contributions of folding and isomerization steps to the observed folding kinetics.

Kinetics↗
PubMed1993

Problems, pitfalls, perspectives and potentials of quantitative theoretical models for cellular radiation action.

The problem of quantitative mathematical models in cellular radiation biology is discussed in a general way. It is emphasized that there are a number of stages, starting from the spatial pattern of energy deposition and ending with repair/misrepair processes which all need to be incorporated. Since different types of radiation commonly yield very similar dose-response curves a model which is only valid for one special case cannot claim general applicability. Interaction experiments with ultraviolet and ionizing radiation are discussed in this context. Also the role of different experimental systems (microorganisms versus mammalian cells) has to be taken into account. A number of current model approaches are discussed within this context, and it is shown that most of them do not satisfy the criterion of universal applicability and can therefore not claim to give a 'true' picture of biological reality. Shouldered survival curves are taken as an example to illustrate these points in a more specific way.

Animals↗
PubMed1996

Theoretical model of HZE particle fragmentation by hydrogen targets.

The fragmenting of high energy, heavy ions (HZE particles) by hydrogen targets is an important, physical process in several areas of space radiation research. In this work quantum mechanical optical model methods for estimating cross sections for HZE particle fragmentation by hydrogen targets are presented. The cross sections are calculated using a modified abrasion-ablation collision formalism adapted from a nucleus-nucleus collision model. Elemental and isotopic production cross sections are estimated and compared with report measurements for the breakup of neon, sulphur, and iron, nuclei at incident energies between 400 and 910 MeV/nucleon. Good agreement between theory and experiment is obtained.

Cosmic Radiation↗
PubMed2006

Reduction of nitric oxide in bacterial nitric oxide reductase--a theoretical model study.

The mechanism of the nitric oxide reduction in a bacterial nitric oxide reductase (NOR) has been investigated in two model systems of the heme-b(3)-Fe(B) active site using density functional theory (B3LYP). A model with an octahedral coordination of the non-heme Fe(B) consisting of three histidines, one glutamate and one water molecule gave an energetically feasible reaction mechanism. A tetrahedral coordination of the non-heme iron, corresponding to the one of Cu(B) in cytochrome oxidase, gave several very high barriers which makes this type of coordination unlikely. The first nitric oxide coordinates to heme b(3) and is partly reduced to a more nitroxyl anion character, which activates it toward an attack from the second NO. The product in this reaction step is a hyponitrite dianion coordinating in between the two irons. Cleaving an NO bond in this intermediate forms an Fe(B) (IV)O and nitrous oxide, and this is the rate determining step in the reaction mechanism. In the model with an octahedral coordination of Fe(B) the intrinsic barrier of this step is 16.3 kcal/mol, which is in good agreement with the experimental value of 15.9 kcal/mol. However, the total barrier is 21.3 kcal/mol, mainly due to the endergonic reduction of heme b(3) taken from experimental reduction potentials. After nitrous oxide has left the active site the ferrylic Fe(B) will form a mu-oxo bridge to heme b(3) in a reaction step exergonic by 45.3 kcal/mol. The formation of a quite stable mu-oxo bridge between heme b(3) and Fe(B) is in agreement with this intermediate being the experimentally observed resting state in oxidized NOR. The formation of a ferrylic non-heme Fe(B) in the proposed reaction mechanism could be one reason for having an iron as the non-heme metal ion in NOR instead of a Cu as in cytochrome oxidase.

Bacteria↗
PubMed2004

Theoretical modeling of photo-induced wave propagation in liquid-crystalline Langmuir monolayers.

A phenomenological model of wave propagation in photo-excited liquid-crystalline Langmuir monolayers is constructed. The spontaneous splay deformation of the liquid-crystalline order and the anisotropy of photo-excitation of molecules are taken into account in this model. Numerical simulations of the model well reproduce qualitative features of the wave propagation phenomenon observed in recent experiments. A linear stability analysis of the model equations reveals that an interplay between the spontaneous splay deformation and the anisotropy of the photo-excitation can lead to the wave propagation.

Crystallization↗
PubMed2004

Oxidative stress: a theoretical model or a biological reality?

Although oxidative stress has been extensively studied the last fifteen years, many physicians and biologists are still sceptical concerning its interest in biology and medicine. This is probably due, in part, to the fact that this subject is a matter of biophysics, and the first studies reported were written using a physical language that inspired these people used to a more concrete problematic very little. Another problem is the difficulty to detect the species mediating oxidative stress, and to determine their role in biological processes. This review is aimed at presenting oxidative stress, as well as reactive oxygen species and free radicals--the molecules that mediate it--in a clear form able to convince all researchers involved in life sciences that these short-lived intermediates are indissociable from any aerobic organism. Moreover, if reactive oxygen species and free radicals are undoubtedly involved in many pathologies, they have physiological functions too.

Humans↗
PubMed2005

Theoretical model of the interactions between Ca2+, calmodulin and myosin light chain kinase.

Active Ca2+/calmodulin (CaM)-dependent myosin light chain kinase (MLCK) plays an important role in the process of MLC phosphorylation and consecutive smooth muscle contraction. Here, we propose a mathematical model of a detailed kinetic scheme describing interactions among Ca2+, CaM and MLCK and taking into account eight different aggregates. The main model result is the prediction of the Ca2+ dependent active form of MLCK, which is in the model taken as proportional to the concentration of Ca4CaM.MLCK complex. Wegscheider's condition is additionally applied as a constraint enabling the prediction of some parameter values that have not yet been obtained by experiments.

Calcium↗
PubMed2006

Recreating an artificial biological pacemaker: insights from a theoretical model.

BACKGROUND: Normal cardiac rhythm is critically dependent on the sinoatrial (SA) node, the natural biological pacemaker. Although recent studies have focused on the development of "artificial" biological pacemakers using gene transfer, less is known about the functional consequences of such interventions. OBJECTIVE: The purpose of this study was to investigate the electrophysiological consequences of two approaches used to create a biological pacemaker: overexpression of the hyperpolarization-activated cyclic nucleotide gated channel (HCN "pacemaker" channels) and suppression of the inward-rectifier potassium current, I(K1). METHODS: We used a linear multicellular Luo-Rudy (LRd) AP model consisting of 130 ventricular cells connected by resistive gap junctions. To induce automaticity, I(K1) current was reduced or I(f) (HCN) current was introduced in endocardial and midmyocardial (M) cells. RESULTS: Similar to the previously published results for a single LRd model, myocyte I(K1) suppression induced automaticity in the fiber. While introduction of I(f) also resulted in automaticity, the main differences between I(K1) suppression and I(f) expression were (1) a relatively more gradual phase 4 depolarization with HCN expression, (2) stabilization of cycle lengths during I(K1) suppression, but not during HCN expression, and (3) responsiveness to beta-adrenergic stimulation during HCN expression, but not during I(K1) suppression. Upon further investigation, we found that cycle length instability during HCN expression was primarily due to a gradual reduction of intracellular potassium ([K(+)](i)) from its baseline value of 142 mM to 120 mM in 600 beats and subsequent alteration of potassium-dependent ionic currents. A twofold increase in HCN expression also led to a similar behavior. We attribute this decrease in [K(+)](i) to a large I(K1) during phase 4 depolarization. When intracellular [K(+)](i) loss was minimized, cycle lengths stabilized during HCN expression. CONCLUSIONS: Our results help to further understand the electrophysiologic consequences as well as some of the challenges associated with the creation of biological pacemakers using HCN and I(K1) gene transfer strategies.

Action Potentials↗