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Results for “Influentials”

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At least 127 records · Page 7Linked to original sources

PubMed1984

Effect of cement pressure and bone strength on polymethylmethacrylate fixation.

The effect of the quality of the bone and of the cement pressurization magnitude and duration on the fixation achieved with polymethylmethacrylate (PMMA) bone cement is studied in vitro. Seventy-one cement-bone interface specimens, prepared under various conditions of pressurization of low-viscosity bone cement, are tested in tension. The load at failure and the maximum cement penetration are measured to assess the fixation achieved, and the quality of the bone is assessed by determining the compressive strength of each of the bone specimens. Statistical analysis of the data indicates that the pressure magnitude is the most influential of the factors considered in the cement penetration behavior and in the development of failure load capacity. The duration of the pressure does not appear to be a significant factor. The cement penetration is a decreasing function of the bone strength, reflecting a decrease in the porosity and an increase in the area fraction. Although not directly measured in these tests, these latter bone properties are indirectly measured by the bone compressive strength. The effect of increasing bone strength on the failure load is nonlinear. The development of adequate failure load capacity is the result of a balance between the cement penetration allowed by the porosity of the bone and the inherent strength of the cancellous bone itself. Weak bone, although adequately penetrated by cement, cannot provide strong fixation. Stronger, denser bone limits cement penetration, but pressurization enhances development of failure load capacity through more complete infusion and interlocking of the cement in the available pore space. The strength of the fixation achievable for any bone is limited by the intrinsic strength of the bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical Phenomena↗

PubMed1984

Laser workshops in neurological surgery.

With the recent increased popularity of laser use in neurosurgery, there arises the question of the value and necessity of a "hands-on" laser workshop in preparing the surgeon for clinical use of the instrument. To find an answer, a questionnaire was recently circulated to those who had participated in such a workshop. Participants were asked whether they were actually using the laser clinically since attending the workshop and, if so, the type of laser they were using. Whether the course was influential in their decision to use a laser and whether they felt they could have used the instrument without such a course was also asked. In addition, the surgeons were asked how long it took them to become comfortable using the laser. The types of procedures, the percentage of operative cases and the mode (freehand or microlinked) in which the laser was used was also investigated. Finally, absolute indications for laser use in neurosurgery and methods to improve laser education were sought. The answers were varied and interesting, but it is our conclusion that a "hands-on" workshop is both a valuable and necessary means of preparing a neurosurgeon for clinical use of a laser, and participation in such a course should be encouraged.

Clinical Competence↗

PubMed1980

Perceived choice, social interaction, and dimensions of moral of residents in a home for the aged.

In a study of 50 residents in a home for the aged, the relationships of health at relocation, choice in relocation, income, perceived choice within an institutional environment, and social interaction to four indices of morale were examined in hierarchical multiple regression designs. The theoretical concept of decisional control (i.e., perceived choice) guided the investigation. Significant multivariate effects were found for all dimensions of morale: agitation, attitude toward own aging, lonely dissatisfaction, and a combined morale index. Choice within the institution and social interaction were the major contributors to these significant multivariate relationships, but income was also influential in analysis of the attitudinal and combined morale indices for a subsample. Retrospective measures of health at relocation and choice in relocation were unstable in their contributions to overall multivariate relationships. Relationships of age, recency of loss of a significant other, and length of residency to morale were explored and found not to be significant. Results are discussed against the background of theoretical expectations, and avenues for future research are identified.

Aged↗

PubMed1986

Factors influencing career success in nursing.

The purpose of this descriptive study was to examine educational preparation, personal characteristics, and significant others in the career success of women in nursing. The sample was a random selection of 194 doctorally prepared female nurses who reported some degree of career success. The factor, personal characteristics, was ranked as the most important in facilitating career success, educational preparations was ranked second, and significant others was ranked third. Among significant others ranked as influential were teachers, peer/colleagues, and supervisors.

Achievement↗

PubMed1989

Influences of the social network on choice and duration of breast-feeding in mothers of preterm infants.

A prospective analytic study was conducted to examine the influences of the social network on the choice and duration of breast-feeding among 125 mothers of preterm infants. The Influence of Specific Referents (ISR) Scale was used to measure mothers' perceptions of the wishes of seven social referents regarding feeding choice and to assess mothers' motivation to comply with their referents' wishes. Referents included family members, friends, and health care professionals. Mothers choosing to breast-feed reported greater influence from referents than those choosing to formula-feed. Among the 88 mothers who initiated breast-feeding or expression, the number of supports reported by mothers was the most influential factor on duration of lactation. Women with no source of support were six times more likely to cease lactation than women with six sources of support.

Breast Feeding↗

PubMedDate not supplied

Assessing the adequacy of the logistic regression model for matched case-control studies.

We apply results for the detection of outliers in logistic regression to the analysis of matched pairs data and illustrate the techniques with data from a matched pairs study of the relationship between iron-binding proteins and mortality in the Solomon Islands. Although the study includes 90 matched pairs, the conclusions change substantially with deletion of one or two pairs. Thus, even with reasonably large data sets, single observations may have a great impact on the results of an analysis. The identification of influential observations should constitute part of the analysis of matched case-control data.

Clinical Trials as Topic↗

PubMed1991

Validation techniques for logistic regression models.

This paper presents a comprehensive approach to the validation of logistic prediction models. It reviews measures of overall goodness-of-fit, and indices of calibration and refinement. Using a model-based approach developed by Cox, we adapt logistic regression diagnostic techniques for use in model validation. This allows identification of problematic predictor variables in the prediction model as well as influential observations in the validation data that adversely affect the fit of the model. In appropriate situations, recommendations are made for correction of models that provide poor fit.

Benzothiadiazines↗

PubMed1990

Effect of acute and daily cocaine treatment on extracellular dopamine in the nucleus accumbens.

The behavioral stimulant effect of peripheral cocaine injection into rats is augmented following daily administration. In vivo dialysis in the nucleus accumbens of conscious rats was used to determine if the increased behavioral response following daily cocaine administration is associated with an increase in extracellular dopamine concentration. Acute injection of cocaine (15 mg/kg, ip) produced an elevation in extracellular dopamine concentration in the nucleus accumbens. Following daily pretreatment with cocaine (15 mg/kg, ip X 4 days), a subsequent acute injection of cocaine (15 mg/kg, ip) significantly elevated the extracellular dopamine levels compared to that produced by a single acute injection. Although the levels of extracellular dopamine metabolites was significantly lowered by both acute cocaine and daily cocaine, no difference between these two groups of animals was measured. The increase in extracellular dopamine following a single acute injection of cocaine was not correlated to the motor stimulant response. However, after daily pretreatment with cocaine the motor stimulant response to acute cocaine was positively correlated with the increased extracellular concentration of dopamine in the nucleus accumbens. These data demonstrate that enhanced dopamine release into the nucleus accumbens may mediate the behavioral sensitization produced by daily injections of cocaine, but that other neural systems are influential in mediating the acute motor stimulant effect of cocaine.

3,4-Dihydroxyphenylacetic Acid↗

PubMed1986

Caffeine effects on cyclic AMP levels in the mouse embryonic limb and palate in vitro.

Caffeine is a teratogen that causes limb and palate malformations in rodents. Since the ability to raise cyclic nucleotide levels is a known biological action of caffeine, cyclic AMP levels were measured in CD-1 mouse embryonic forelimb from whole embryo culture and embryonic limb and palate cells grown in primary culture following treatment with various concentrations of caffeine (0, 1, 3, or 10 mM). In forelimb buds from whole embryo culture, a dose-dependent response was observed. Caffeine at 1 mM concentration stimulated cyclic AMP levels to 151% of control value at 60 min. Even greater stimulation of cyclic AMP occurred at higher caffeine concentrations. A dose-dependent response was seen in both limb and palate cell culture. In limb cell culture, all caffeine concentrations significantly stimulated cyclic AMP after 10 min compared to control. In palate cell culture, there was a twofold increase in cyclic AMP at the 1-mM caffeine concentration. At higher caffeine concentrations, cyclic AMP was significantly increased after 60 min. In addition, stimulation of cyclic AMP in cultured limb and palate cells by isoproterenol, a beta-adrenergic agonist, was used as a positive control. Isoproterenol stimulated a 2.5-fold greater response in the palate cells than in the limb bud cells at isoproterenol levels of 10(-5) or 10(-4) M. The increase of cyclic AMP may be influential in the process of abnormal limb or palate development.

Animals↗

PubMed1991

Transplacental distribution of weak acids in mice: accumulation in compartments of high pH.

The transplacental distribution of three weak acids was measured in mice on gestational day 11. The ratio of drug concentration in maternal, embryonic, and extraembryonic compartments compared to maternal plasma followed a consistent hierarchy such that amniotic fluid greater than or equal to exocoelomic fluid greater than embryo greater than embryo plasma greater than maternal skeletal muscle. This distribution pattern correlates well with pH in these compartments and suggests that pH gradients between compartments are an influential factor in determining weak acid drug disposition during pregnancy.

Acids↗

PubMed1987

Social drinking as a health and psychosocial risk factor. Anstie's limit revisited.

This chapter reviews empirical evidence dealing with the risk of using beverage alcohol in quantities generally considered to be within the range of moderate or social drinking. It begins with a discussion of terms and concepts typically employed to define the limits of moderate and social drinking. The historical debate over moderate drinking is considered next, using Francis Anstie's daily limit of 1.5 oz of absolute ethanol as an example of an influential and enduring approach to this issue. The scientific evidence relevant to the association between moderate drinking and damage is reviewed in sections dealing with the risks of acute ingestion of alcohol and the hazards of chronic drinking. In a final section, a multidimensional model of risk is developed to serve as a basis for early intervention and prevention planning. It is concluded that both social and moderate drinking entail risk of health hazards and psychosocial consequences. These risks are not always a direct function of the amount of ethanol consumed but rather reflect complex interactions among a host of antecedent and mediating variables. When these are taken into account, a better estimate of the relative risk of different consequences of drinking can be made.

Alcohol Drinking↗

PubMed1985

Significance of esophageal manometry and long-term pH monitoring for the evaluation of gastroesophageal reflux in infancy and childhood.

From examinations and clinical observations of 40 infants and a varying number of children from 2 to 11 years who all had characteristic symptoms of gastroesophageal reflux (GER), an attempt was made to establish the diagnostic contribution of esophageal manometry and long-term pH monitoring. A correlation between decreased resting pressure (less than or equal to 12 mmHg), or a sphincter insufficiency, and the degree of reflux symptoms was completely absent in infancy and increased only very slightly in young children. The most influential pH monitoring parameters were reflux during sleep, beyond 2 h postprandial, and during crying. These correlated most with the symptoms in infants. On the other hand, clinical developments were independent of the extent of the pH monitoring findings.

Child↗

PubMed1987

Chronic cardiac reactions. III. Factors involved in the development of structural dilatation.

The significance of various factors for the development of structural dilatation in the chronically pressure-loaded and failing heart were evaluated. The investigations were performed on male rats with renal (Goldblatt II) and spontaneous (Aoki-Okamoto) hypertension at different stages of haemodynamic overload. Two groups of SHR were submitted to intermittent feeding (SHR IF); one group received additionally the beta-blocking agent atenolol (50 mg/kg b.w.; SHR IF + beta Bl.). Haemodynamic measurements were carried out under open chest conditions. Myosin isoenzyme pattern, hydroxyproline concentration and circulating blood volume were determined. Transformation to slower myocardium per se, induced by IF, did not lead to significant change in ventricular configuration. After additional blockade of beta-adrenergic receptors there were indications of unfavourable development of left ventricular configuration. Inhibition of hypertrophic mass increase due to curtailed adrenergic stimulation could be an influential factor in the development of dilatation. Further investigations, however, are required to establish the relationship between the adrenergic system, on the one hand, and degree of hypertrophy as well as structural dilatation of the ventricle, on the other hand. The established marked increase in hydroxyproline concentration of the dilated ventricle of SHR in congestive failure is consistent with the assumption of a causal link between the degree of fibrosis and structural dilatation. Observations on rats with aorto-caval shunt and Goldblatt II rats with eccentric hypertrophy and corresponding increase in filling potential or circulating blood volume indicate a correlation between the latter and ventricular size. Thus, we assume that curtailed protein synthesis, fibrosis and regulatory processes related to water and electrolyte balance, but not myocardial transformation per se, play a role in the development of structural dilatation. The relative contribution of each factor, however, may depend on the experimental model that is used.

Animals↗

PubMed1992

The regulation of subcortical dopamine systems by the prefrontal cortex: interactions of central dopamine systems and the pathogenesis of schizophrenia.

A recent hypothesis of the pathogenesis of schizophrenia posits a developmentally-specific dysfunction of the dopaminergic innervation of the prefrontal cortex (PFC; Weinberger, 1987; Berman and Weinberger, 1990). It has been difficult to reconcile this hypothesis with the observation that all clinically effective antipsychotic drugs used for the treatment of schizophrenia block dopamine D2 receptors (see Deutch et al., 1991a). A resolution between the suggestion of functional dopamine (DA) "depletion" in the PFC and enhanced subcortical DA function was offered by studies of Carter, Pycock, and associates (Carter and Pycock, 1980; Pycock et al., 1980a, b). These investigators reported that depletion of DA in the rat PFC enhanced DA utilization in subcortical sites such as the nucleus accumbens septi (NAS) and striatum. Thus, a functional deficit in DA neurotransmission in the PFC would increase subcortical DA turnover, and the D2 receptor blockade induced by antipsychotic drugs would counteract the increase in dopaminergic tone in subcortical sites. This hypothesis has been particularly influential because it incorporates both an explanation for negative symptoms, which are thought to reflect cortical dysfunction (a derangement in DA transmission in the PFC), and the efficacy of antipsychotic drugs in the treatment of positive symptoms (arising from increases in subcortical DA tone). As attractive as this hypothesis has been, the physiological underpinnings that subserve such system interactions have remained elusive. Pycock, Carter, and colleagues (Carter and Pycock, 1980; Pycock et al., 1980a, b) reported that 6-hydroxydopamine (6-OHDA) lesions of the PFC increase DA levels and DA turnover in the striatum; certain aspects of their findings have been confirmed (Martin-Iversen et al., 1986; Leccese and Lyness, 1987; Haroutounian et al., 1988). However, other groups have been unable to confirm either the biochemical or behavioral findings of Pycock and associates (Joyce et al., 1983; Oades et al., 1986; Deutch et al., 1990). Moreover, Pycock and colleagues did not observe consistent effects of PFC DA deafferentation on various indices of subcortical DA function (Carter and Pycock, 1980; Pycock et al., 1980a, b). In light of the importance that such DA system interactions may have in the pathogenesis of schizophrenia, we have reinvestigated the effects of cortical DA lesions on subcortical DA function.

Cerebral Cortex↗

PubMed1990

Cellular interactions in metastasis.

The metastatic cascade is a sequence of events that must be completed for metastases to be established. The realization that tumors are heterogeneous, consisting of many different subpopulations differing in many characteristics, and the belief that there are selective events in the metastatic process have led several laboratories to isolate and characterize variants with both high and low metastatic potential. Typically, the highly metastatic variants have been able to form distant metastases when implanted into the subcutis. Such lines have been popular for studies of metastatic mechanisms and anti-metastatic therapy, but they may be atypical examples, and thus not the best experimental models. Recent studies indicate that normal tissue influences metastasis such that many tumors metastasize only if placed in the orthotopic site. Furthermore, some cells that do not metastasize individually are able to do so in conjunction with other variant subpopulations. Thus, mixtures of tumor cells in the tissue of origin can express a more malignant character. We review possible mechanisms for such influential interactions, as well as the role of cellular interactions in generating heterogeneity and stabilizing tumor characteristics.

Animals↗

PubMed1991

Limited regions of the alpha 2-domain alpha-helix control anti-A2 allorecognition: an analysis using a panel of A2 mutants.

The regions of the HLA-A2 molecule controlling anti-A2 alloreactivity were explored using naturally occurring allelic variants of HLA-A, and a panel of transfectants expressing the products of A2.1 genes that had been mutated at multiple positions encoding residues in the alpha 2 domain alpha-helix. As a means of detecting distant conformational effects, these altered A2.1 molecules were also examined serologically. Amino acid substitutions at the carboxy-terminal end of the alpha 2 domain alpha-helix led to diminished staining with the monoclonal antibody (mAb) MA2.1. The epitope for this antibody has previously been mapped to the alpha 1 domain alpha-helix (residues 62-65). This suggests that interdomain contacts may cause conformational alteration, and that mutants can have distant, as well as local effects. Of the 24 positions where substitutions were made, only six led to loss of the anti-A2 alloresponse by the three clones and three lines that were tested. In addition, the mutations that altered the MA2.1 epitope, located on the alpha 1 domain alpha-helix, did not inhibit allorecognition. This suggests that a limited number of regions on the A2.1 molecule are responsible for allodeterminant expression. The most influential substitutions were those at positions 152, 154, 162, and 166. It is notable that three of these are predicted to be T-cell receptor (Tcr)-contacting residues, and one (152) to contribute to peptide binding. These results suggest that the specificity of alloreactive T cells is determined by exposed polymorphisms, directly contacted by the Tcr, and by concealed polymorphisms which influence peptide binding.

Cell Line↗

PubMed1992

Mercury concentrations in seabirds from colonies in the northeast Atlantic.

Total mercury concentrations were determined in samples of body feathers from a range of common seabird species breeding at Låtrabjarg, northwest Iceland, St. Kilda, Foula and the Firth of Forth, Scotland and Bleiksøy, Syltefjord, and Hornøy, Norway. Seabirds from Låtrabjarg generally exhibited the highest mercury concentrations, with a trend of decreasing mercury concentrations in a southwest to northeast direction in seabirds at the other colonies; seabirds at Hornøy were generally found to have the lowest mercury concentrations. Some species at the Firth of Forth exhibited relatively elevated mercury concentrations compared to those at Foula and Norwegian sites. Inter-colony differences in diet were thought to be relatively small for most species and unlikely to account for the range of mercury concentrations measured in the seabirds (Låtrabjarg: lowest arithmetic mean mercury concentration in common guillemots Uria aalge, 1.6 micrograms/g, s.d. = 0.6, n = 45; highest arithmetic mean mercury concentration in kittiwakes Rissa tridactyla, 5.5 micrograms/g, s.d. = 1.7, n = 36). The oceanic transport of mercury, together with the effects of anthropogenic inputs of mercury to the northeast Atlantic, and the removal of mercury from the water column via biological activity are discussed as influential factors determining the observed patterns of mercury concentration in seabirds.

Animals↗

PubMed1985

Processing vibratory stimuli in isolated frog muscle spindle.

Experiments were performed on isolated frog muscle spindle receptors to study the particular transducer and encoder mechanisms involved in the signal transfer of high frequency sinusoids (vibration). In order to systematically investigate the signal transfer over the entire dynamic range of the receptor, vibration stimuli were applied to the intrafusal muscle bundle at different prestretch levels, so that the isolated receptor potential or the afferent impulse train were recorded at different operating points. The vibration-induced receptor potential displayed severe distortion, because the depolarization during stretch rose steeply, whereas the repolarization transient during release of stretch declined more slowly. The positive peak velocity values of the depolarization transient increased with increasing stimulus frequency, although the ac-component of the receptor potential decreased. The negative peak velocity values of the repolarization transient remained constant throughout the frequency range. The amplitude of the receptor potential grew larger when vibration of constant amplitude was applied at increasing levels of prestretch, revealing another non-linearity of the transducer. These two types of non-linearity were influential in determining the afferent discharge pattern. Each fast depolarization transient facilitated the generation of a single action potential, which therefore could be firmly phase-locked to a small segment of the vibratory movement. Due to its short rise-time, the depolarization transient tended to prevent multiple firing during one stimulus cycle. The prolonged depolarizing afterpotential of the evoked action potential operated in the same direction. Increasing prestretch greatly enhanced the responsiveness of the spindle to vibration. Thus, under appropriate conditions, the afferent discharge was driven in 1:1 synchrony with the vibration. An analysis is given of the after-effects of repetitive activity at the receptor site. The progressive decline of the mean membrane voltage during long lasting stimulation and the "post-tetanic" hyperpolarization ("off-effect") on termination of the vibration suggest the action of an electrogenic pumping mechanism. As a consequence, the afferent impulse train possessed a complex structure segmented into several transient and steady states, which differed in impulse rate, phase response, and in the degree of phase-locking.

Action Potentials↗