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At least 901 records · Page 50Linked to original sources
PubMed2004

Orientation and interaction of oblique cylindrical inclusions embedded in a lipid monolayer: a theoretical model for viral fusion peptides.

We consider the elastic behavior of flat lipid monolayer embedding cylindrical inclusions oriented obliquely with respect to the monolayer plane. An oblique inclusion models a fusion peptide, a part of a specialized protein capable of inducing merger of biological membranes in the course of fundamental cellular processes. Although the crucial importance of the fusion peptides for membrane merger is well established, the molecular mechanism of their action remains unknown. This analysis is aimed at revealing mechanical deformations and stresses of lipid monolayers induced by the fusion peptides, which, potentially, can destabilize the monolayer structure and enhance membrane fusion. We calculate the deformation of a monolayer embedding a single oblique inclusion and subject to a lateral tension. We analyze the membrane-mediated interactions between two inclusions, taking into account bending of the monolayer and tilt of the hydrocarbon chains with respect to the surface normal. In contrast to a straightforward prediction that the oblique inclusions should induce tilt of the lipid chains, our analysis shows that the monolayer accommodates the oblique inclusion solely by bending. We find that the interaction between two inclusions varies nonmonotonically with the interinclusion distance and decays at large separations as square of the distance, similar to the electrostatic interaction between two electric dipoles in two dimensions. This long-range interaction is predicted to dominate the other interactions previously considered in the literature.

Computer Simulation↗
PubMed1999

A theoretical model of the evolution of virulence in sexually transmitted HIV/AIDS.

INTRODUCTION: The evolution of virulence in host-parasite relationships has been the subject of several publications. In the case of HIV virulence, some authors suggest that the evolution of HIV virulence correlates with the rate of acquisition of new sexual partners. In contrast some other authors argue that the level of HIV virulence is independent of the sexual activity of the host population. METHODS: Provide a mathematical model for the study of the potential influence of human sexual behaviour on the evolution of virulence of HIV is provided. RESULTS: The results indicated that, when the probability of acquisition of infection is a function both of the sexual activity and of the virulence level of HIV strains, the evolution of HIV virulence correlates positively with the rate of acquisition of new sexual partners. CONCLUSION: It is concluded that in the case of a host population with a low (high) rate of exchange of sexual partners the evolution of HIV virulence is such that the less (more) virulent strain prevails.

Acquired Immunodeficiency Syndrome↗
PubMedDate not supplied

A comprehensive theoretical model of psychological reaction to loss.

This article describes a multidimensional model which may be applicable to study three types of loss, interpersonal loss, loss of a limb or function, and environmental loss or loss of peace of mind. It is a preliminary attempt to collate relevant variables associated with the three types of loss into one comprehensive model which assumes that the dynamic processes associated with and encountered in one area of loss can be extrapolated for use in comprehending other areas of loss. A number of testable hypotheses can be derived for future validation and research.

Culture↗
PubMed1981

Biophysical approach. Theoretical models of deformability in blood flow.

The physical properties of red blood cell membranes pertinent to deformability are the membrane shear modulus, the bending stiffness and shear viscosity. In filtration tests the rate of deformation is sufficiently high that the viscous behaviour of the membrane and of cell contents predominate.

Biophysical Phenomena↗
PubMed1990

Theoretical models to explain the association of HLA-B27 with ankylosing spondylitis.

A critical examination of the five different models proposed to explain the "AS-B27" problem, has demonstrated, that two of these, the two-gene theory and the plasmid theory are no longer considered to provide satisfactory explanations of the phenomena. The remaining three theories are molecular mimicry, chemotaxis hypothesis and the receptor theory. It is difficult at this stage to decide which of these three models provides the best solution to the "AS-B27" problem, but a majority of research workers favour the receptor theory.

Animals↗
PubMed2001

Investigation of theoretical models and therapy activities: phonological difficulties.

A focus group of seven speech and language therapists (SLTs) who work with children with phonological difficulties was invited to share their views on their current ways of working. Qualitative data was generated through discussion of two video-taped recordings of children with phonological difficulties and a series of brainstorming tasks. The results indicate the use of an electric approach, a mixture of theories rather than a single one. Similarities and differences between SLTs are identified. Implications for the development of clinical guidelines and the investigation of effectiveness are discussed.

Child↗
PubMed1975

Relationship between ruminal ammonia and nonprotein nitrogen utilization by ruminants. II. Application of published evidence to the development of theoretical model for predicting nonprotein nitrogen utilization.

Results from published experiments dealing with several aspects of nitrogen utilization by ruminants were used to test the concept of zero utilization of nonprotein nitrogen under conditions where more ammonia (greater than 5 mg ammonia nitrogen/100 ml) is in the rumen than can be converted to microbial protein. Results from experiments where the flow of non-ammonia nitrogen to the abomasum of sheep was measured indicate that when urea was the source of supplemental nitrogen, a constant quantity of amino acids reached the abomasum for all rations ranging from 10 to 23% crude protein. From growth studies, addition of nonprotein nitrogen to low protein, high energy rations caused an improved rate of gain. Additions of nonprotein nitrogen to rations resulting in predicted ruminal ammonia concentrations greater than 5 mg ammonia nitrogen/100 ml rumen fluid were without benefit. From lactation studies, nonprotein nitrogen supplementation did not improve milk production if the ration contained more than 12.5% crude protein prior to supplementation or if the predicted ruminal ammonia concentration was greater than 4 mg ammonia nitrogen/100 ml rumen fluid. The importance of the amino acid requirement of the animal as well as the composition of the ration in designing and evaluating nitrogen supplementation studies is discussed.

Ammonia↗
PubMed1984

Origin and evolution of plateau waves. Experimental observations and a theoretical model.

Laboratory observations made in cats with fluid-percussion head injuries have suggested that plateau waves or Lundberg "A-waves" are not independent of systemic circulatory events. Four distinct phases in the evolution of the plateau wave have been identified, and each related to a circulatory change in a causal manner. The first phase is the premonitory drift phase where intracranial pressure (ICP) gradually increases prior to the plateau proper. This phase is caused by a slow gradual decline in systemic arterial blood pressure (SABP) which increases ICP by autoregulatory vasodilation and reduces cerebral perfusion pressure (CPP) to a range of 70 to 80 mm Hg. The second phase is the plateau phase initiated at a CPP of about 70 to 80 mm Hg, and is characterized by a rapid increase in ICP as CPP falls further to 40 to 50 mm Hg. The plateau lasts as long as the CPP remains stable and above ischemic levels. The third phase is the ischemic response, characterized by CPP being returned toward normal by increases in SABP in response to very low CPP's. The fourth phase is the resolution, characterized by a rapid decline in the ICP to baseline levels with stabilization of the SABP and CPP, and is best explained by autoregulatory vasoconstriction. Plateau waves appear to occur as the result of intact or mostly intact autoregulation responding to changes in CPP. The series of events that follow are best explained by what is known of normal autoregulation; the various properties of plateau waves are viewed and explained as the expected and logical consequences of an unstable CPP acting upon a generally intact cerebrovascular bed in the face of elevated ICP and decreased compliance.

Animals↗
PubMed1984

Radionuclide estimation of cerebrospinal fluid shunt flow. Evidence supporting an alternative theoretical model.

Flow of cerebrospinal fluid through a surgically implanted valve may be estimated by analyzing the disappearance curve resulting from the injection of a radiotracer into the valve. The standard method for estimating flow assumes an exponential disappearance of the tracer from the valve. This method models the valve as a single well-mixed compartment. Experimental evidence, showing that estimates of flow were dependent upon the site of injection, is at variance with this assumption. An alternative method of analyzing the disappearance curves, based on the area to height ratio (A/H) of the curves, was found to be more consistent with the experimental evidence and resulted in greater precision than the exponential method. It was concluded that optimal results are obtained using the A/H method with a fixed injection technique.

Cerebral Ventriculography↗
PubMed1984

Single-beam radiotherapy knife. A practical theoretical model.

A feasibility study reveals that a standard linear accelerator, with only slight modification, can function in a manner similar to Leksell's multiple cobalt radiation device. A high radiation dose can be delivered to a 1-cm or 0.5-cm volume target with insignificant radiation effect on the surrounding tissue.

Models, Biological↗