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At least 19 recordsLinked to original sources
PubMed1977

Permeability, phase-boundary potential and conductance in a cholinergic channel without constant field.

A potassium-selective, chemically excitable channel, whose characteristics cannot be accurately described by constant-field theory, is studied by a new approach based on diffusion theory but with no need for the classical assumptions of constant field, homogeneous membrane, and equal phase-boundary potentials at both interfaces. Permeability is defined, free of these constraints, and the Goldman coefficient is demonstrated to be a special case useful only when the constraints apply. Permeability can be evaluated directly from current-voltage data, and it is found not to be a parameter in this channel, but rather a function of both the voltage and the concentration of the permeant ion. However, it becomes concentration-independent when the membrane voltage is equal to the sum of the phase-boundary potentials. That sum can therefore be determined from these data, and it is -65 mV in this channel. The permeability at that potential is a channel parameter, and equal to 8.66 X 10(-6) cm/s for this channel. A constant field is shown not to exist in this channel and the Goldman coefficient not to be a parameter but a function of potential and concentration. Although errors introduced into this coefficient by nonconstant field and unequal surface potentials partially cancel each other, the coefficient is nevertheless not a correct measure of permeability.

Animals↗
PubMed1977

Permeability and phase-boundary potentials determined from conductance in a transmitter-activated potassium channel in Aplysia californica in the absence of a constant field.

A potassium-selective, chemically excitable channel, whose characteristics cannot be accurately described by "constant field" theory, is studied using a new approach based on diffusion theory but having no need for the classical assumptions of constant field, homogeneous membrane, and equal phase-boundary potentials at both interfaces. Permeability is defined, free of these constraints, and the Goldman coefficient is demonstrated to be a special case useful only when the constraints apply. Permeability can be evaluated directly from chord conductance, and it is found not to be a parameter in this channel, but rather a function of both the voltage and the concentration of the permeant ion. However, it becomes concentration-independent when the membrane voltage is equal to the sum of the phase-boundary potentials. That sum can therefore be determined from these data, and it is -65 mV in this channel. The permeability at that potential is a channel parameter, and equal to 8.77 (10)-6 cm/sec for this channel. A constant field is shown not to exist in this channel, and the Goldman coefficient not to be a parameter but a function of potential and concentration. Although errors introduced into this coefficient by nonconstant field and unequal surface potentials partially cancel each other, the coefficient is nevertheless not a correct measure of permeability.

Animals↗
PubMed1977

Advances in the study of diffusion of innovation in health care organizations.

Federal government programs of the 1960s to rapidly diffuse technologies have been displaced on the '70s by efforts to constrain costly technological growth. As a guide to action, the understanding of reasons for adoption of innovation is essential; but the utility of available diffusion theory is limited by its focus on the speed of diffusion rather than any reasons for its adoption by organizations. In a practical sense, more is known about the administrator as decision maker than about those increasing situatiions in which physicians play a more central part. Until coherent, empirically grounded theories of organizational innovation are available, large-scale "tests" are premature and wasteful.

Creativity↗
PubMed1977

Application of a convective diffusion model to membrane transport.

Studies were carried out on the permeation rate of butambed through a dimethicone membrane. Under conditions of "aqueous diffusion layer control", the permeation rate was accurately described by a mathematical model based on convective diffusion theory. In accordance with model, the rate of permeation from a saturated donor phase was shown to be equal to the rate of dissolution from a pure solid.

Aminobenzoates↗
PubMed1978

Potassium contracture of smooth muscle in the presence of dextran.

The apparent diffusion coefficient of Ca ions in the extracellular space of guinea pig taenia coli was estimated from model experiments to be 3.2 x 10(-6) cm2 sec -1, while the relationship between Ca concentration in the medium and tonic tension in 40 mM K-contracture was measured in the same muscle both in the presence and absence of dextran 10. On the basis of these experiments and certain assumptions, the time course of tension decline by Ca withdrawal during K-contracture was calculated. Under all dextran concentrations tested (0--15%) the calculated time course of tension decline was in good agreement with one actually observed except in a short period of time immediately after Ca withdrawal. The results suggested that Ca distribution in the extracellular space during the loss of Ca is in good agreement with the diffusion theory in cylinder, and that each muscle fiber shows its tension without delay in response to the change in Ca concentration in the vicinity of the fiber according to the tonic tension-Ca relationship mentioned above. The discrepancy between calculation and observation was partially explained on an experimental basis.

Animals↗
PubMed1977

Turbulence measurement by the pulse luminescence method using a nitrogen pulse laser.

In order to visualize and measure with ease the velocity distribution and diffusion of turbulent flow, the pulse luminescence method was investigated. Turbulence intensity was obtained from the turbulent diffusion patterns by Taylor's diffusion theory. Apparatus was developed for easier measurement. A nitrogen pulse laser was used for instantaneous, high-power excitation. With the use of a night vision scope a bright image was recorded by a TV camera and video tape recorder. The optimum concentration of LC-G1A luminescent particles was about 0-05% wt for the measurement. High fidelity of the particles as an indicator of the fluid velocity was confirmed. It was demonstrated that the turbulence intensity could be visualized and measured quantitatively by the pulse luminescence method.

Lasers↗
PubMed1977

Mutation-selection balance with stochastic selection.

Diffusion theory has been used to analyze a model of mutation-selection balance in which the selection process is assumed to be stochastic in time. The limiting outcome of the mutation-stochastic selection process is determined qualitatively by the geometric mean fitnesses of the genotypes, and the conditions for fixation or polymorphism are similar to those that determine the outcome of the mutation-selection process when selection is constant. However, in the case of a completely recessive allele, detailed numerical study of the polymorphism associated with stochastic selection has shown that the average allele frequency maintained is greater than the equilibrium frequency expected when selection is constant, even when the geometric mean fitness of the recessive homozygotes is identical in the stochastic and deterministic models. Thus, allele frequencies in natural populations that are too high to be plausibly explained by a balance between mutation and constant selection can be accounted for if selection is stochastic.

Alleles↗
PubMed2026

What influenced the use of embedded genetic expertise by non-genetic clinicians: a qualitative study using the diffusion of innovations theory and theoretical domains framework.

BACKGROUND: While employing change agents is a widely used strategy to support implementation of evidence-based practices in healthcare, the perspectives of the target users of this intervention are under-explored. Within a genomic care setting in Australia, we examined non-genetic clinicians' views of and experience with embedded genetic counsellors employed as external change agents to support the adoption of genomics in clinical care. METHODS: We conducted qualitative interviews with 16 non-genetic healthcare professionals involved in different models of genomic care, all of which employed genetic counsellors as external change agents to provide embedded genetic expertise and support in non-genetic specialty clinics. The Diffusion of Innovations (DOI) theory and the Theoretical Domains Framework (TDF) were drawn upon to guide the study design and data analysis to explore the enablers/barriers to non-genetic clinicians' use of genetic expertise and the wider implementation context of the change agent approach. RESULTS: The TDF domain 'Belief about Consequences' was a positive influence on non-genetic specialists' view of the embedded change agents, when they believed that the change agents positively impacted patient outcomes and the clinicians' knowledge and practice of using genomics in routine care. Meanwhile, major barriers were related to 'Environmental Context & Resources,' e.g. time constraints and logistic hurdles of involving change agents. Perceived attributes related to 'complexity' and 'observability' of genetic expertise were critical factors influencing the decision to put the support to actual use, impacting the diffusion of the genomic innovation. CONCLUSIONS: Our study offers insights into behavioural influences and contextual conditions shaping clinicians' decisions to interact with genetic counsellor change agents and incorporate (or not) their expertise into their routine care. Such understandings can inform future design and implementation of interventions that use change agents to support the adoption of innovations, such as genomics, effectively and sustainably.

Journal Article↗
PubMed1977

Solid state theory of competitive diffusion of associated Na+ and K+ in cells by free cation and vacancy (hole) mechanisms, with application to nerve.

If, as recent evidence indicates, most cell potassium is associated with macromolecular fixed charge, then diffusion of potassium ions in cells might occur by (1) diffusion of the small fraction of free potassium in cell water (analogous to electrons in the conduction band of a semiconductor) or by (2) diffusion of vacancies on association sites (analogous to holes in a semiconductor). Derivations of the Fick first law of diffusion predict that partial substitution of sodium for potassium in the cell produces opposite effects on the effective diffusion constant of potassium for those mechanisms. Application of that substitution to nerve data suggests that rubidium ions diffuse by a free cation result when the nerve is clamped at its resting potential, but by a vacancy mechanism when the nerve is clamped at zero voltage.

Cations, Monovalent↗
PubMed1976

Mobility measurement by analysis of fluorescence photobleaching recovery kinetics.

Fluorescence photobleaching recovery (FPR) denotes a method for measuring two-dimensional lateral mobility of fluorescent particles, for example, the motion of fluorescently labeled molecules in approximately 10 mum2 regions of a single cell surface. A small spot on the fluorescent surface is photobleached by a brief exposure to an intense focused laser beam, and the subsequent recovery of the fluorescence is monitored by the same, but attenuated, laser beam. Recovery occurs by replenishment of intact fluorophore in the bleached spot by lateral transport from the surrounding surface. We present the theoretical basis and some practical guidelines for simple, rigorous analysis of FPR experiments. Information obtainable from FPR experiments includes: (a) identification of transport process type, i.e. the admixture of random diffusion and uniform directed flow; (b) determination of the absolute mobility coefficient, i.e. the diffusion constant and/or flow velocity; and (c) the fraction of total fluorophore which is mobile. To illustrate the experimental method and to verify the theory for diffusion, we describe some model experiments on aqueous solutions of rhodamine 6G.

Biological Transport↗