Search PubMed⌕ Search

Biomedical subjects

D C Clark

Publications and source records attributed to D C Clark.

At least 37 records · Page 2Linked to original sources
PubMed1997

The influence of membrane fluidity, TNF receptor binding, cAMP production and GTPase activity on macrophage cytokine production in rats fed a variety of fat diets.

The effects of different dietary fats on peritoneal macrophage plasma membrane fluidity, intracellular cyclic AMP (cAMP) production, GTP hydrolysis and TNF binding and TNF-induced IL1 and IL6 production was investigated. After a four week period, fluidity, as determined by both fluorescence recovery after photobleaching (FRAP) and anisotropy was lowest and highest in animals fed corn and fish oil respectively. After eight weeks feeding, lateral membrane movements were decreased substantially in fish, olive and coconut oil fed dietary groups, whereas an increase in the corn oil fed group was observed, no effect was observed in macrophages from the butter fed group. However, an increase in the packing was observed in macrophages from all dietary groups except in the olive oil fed group. GTPase values for the coconut oil and butter groups were higher than in any other dietary group. After receiving the diet for 8 weeks these differences between the groups were no longer apparent. Exposure of macrophages to TNF had no effect on the rate of GTP hydrolysis. A major enhancement of cAMP production became apparent between weeks 4 and 8 of dietary treatment. After 4 weeks on the diet, values were significantly higher from cells of animals fed corn and olive oils than from animals fed fish oil. After 8 weeks, while there was a general enhancement of production, further differences became apparent. Feeding corn and coconut oils resulted in the highest values and olive oil and chow in the lowest. It is proposed that fats rich in n-3 fatty acids (fish oils) alter membrane fluidity, decrease TNF binding affinity, GTPase activity and cAMP production which appears not to modify cytokine production after short term dietary supplementation. However, after long term feeding it appears that increases in the sensitivity of the TNF receptors plays a major role in modifying cytokine production.

Animals↗
PubMed1997

The influence of various fluoride exposures on the prevalence of esthetic problems resulting from dental fluorosis.

OBJECTIVES: This study sought to determine the prevalence of esthetic problems due to dental fluorosis, and determine the relationship of different fluoride exposure histories to the occurrence of these problems. METHODS: In 1993-94 2,715 children in grades 2 and 3 and 3,297 adolescents in grades 8 and 9 were examined by four dentists. Questionnaires detailing exposures to various fluoride technologies were collected from 3,022 of these study participants. Esthetic ratings of the participants' maxillary anterior teeth were made by the examiners, the participants themselves, and their parents using questionnaires designed for this purpose. RESULTS: Data indicate that 46 percent of the participants had dental fluorosis. Only 40 percent had fluorosis on anterior maxillary teeth. The prevalence of esthetic problems ranged from about 1 percent to 4 percent, depending on how an esthetic problem was defined. Esthetic problems as defined by the participant were more prevalent for the "over 11" age group. Logistic regression results demonstrated significant associations between several of the classifications of esthetic problems and the use of fluoride supplements and dentifrices, and exposure to fluoridated water during the third year of life. CONCLUSIONS: Results suggest that the prevalence of esthetic problems is low in the communities surveyed, and that exposure to any number of fluoride technologies in the third year of life can increase a child's risk for this problem.

Adolescent↗
PubMed1997

The relationship between the number of sound, decayed, and filled permanent tooth surfaces and the number of sealed surfaces in children and adolescents.

OBJECTIVES: The purpose of this investigation was to determine the prevalence of sound, decayed, filled, and sealed permanent tooth surfaces for children and adolescents from three fluoridated communities in British Columbia and to develop a predictive model of sealant use that included the dental caries status of tooth surfaces. METHODS: Children in grades 2 and 3 (n = 2,715) and adolescents in grades 8 and 9 (3,317) were surveyed to determine the prevalence of sound, decayed, filled, and sealed permanent tooth surfaces. The DMFS index, modified to include incipient (D1) and cavitated (D2) lesions, was used to measure the status of all tooth surfaces. The prevalence of D1D2MF and sealed tooth surfaces was estimated by age group and tooth type. Logistic regression models were developed to analyze the association between a number of independent variables and sealant use. RESULTS: About 60 percent of surveyed students had one or more sealants present, with a mean of 3.2 sealants per subject. Of all pit and fissure surfaces on permanent first molars, 23.4 percent and 20.1 percent were sealed for the 11 years and younger and 12 years and older age groups, respectively. For these same two age groups, the percentages of pit and fissure surfaces that were decayed and filled were 6.7 percent and 19.7 percent, respectively. For both age groups combined, 10.8 percent and 23.7 percent of all pit and fissure surfaces on second molars and premolars, respectively, were sealed. The prevalence of decayed and filled pit and fissure surfaces was 5.6 percent and 1.8 percent, respectively, for second molars and premolars. Premolars were being sealed at a rate of 13 to 1 compared to the number of decayed and filled surfaces. Logistic regression failed to identify meaningful predictors of sealant use. CONCLUSIONS: Results from this study found an increase in the prevalence of sealants in the three communities surveyed, yet failed to identify criteria used by dentists or auxiliaries when making decisions about sealant placement. Professional education in the appropriate use of sealants may be necessary.

Adolescent↗
PubMed1997

Lateral mobility of plasma membrane lipids in bull spermatozoa: heterogeneity between surface domains and rigidification following cell death.

Compartmentalization of surface membrane antigens into discrete regions or domains is a characteristic feature of differentiated cells. In mammalian spermatozoa at least 5 surface domains are known, implying the presence of barriers or boundaries within the plasma membrane. Using the technique of fluorescence recovery after photobleaching (FRAP) to measure diffusibility of fluorescent lipid analogues 1,1'-dihexadecyl-3,3,3'3'-tetramethylindocarbocyanine (DiIC[16]) and 5-(N-octa-decanoyl) aminofluorescein (ODAF), we have investigated lipid topology and dynamics in the plasma membrane of ejaculated bull spermatozoa. Contrary to reports in the literature, we have found that DiIC(16) stains only dead or damaged spermatozoa whereas ODAF intercalates into the plasma membrane of both live and dead cells, each type showing a distinctive staining pattern. FRAP analysis with ODAF revealed that diffusion coefficients on live spermatozoa are significantly faster on the acrosome and postacrosome (29.3x10(-9) cm2/second) than on the midpiece and principal piece (11.8x10(-9) cm2/second). Recovery (R) is >90% in all domains. ODAF diffusion also shows regionalized temperature-sensitivity with a 4-fold increase over the sperm head and a 1.8-fold increase on the tail between 20 degrees C and 37 degrees C. Remarkably, dead or permeabilized spermatozoa rapidly develop a large immobile phase (R<25%) over the whole plasma membrane. This rigidification is temperature insensitive and irreversible suggesting major changes in the physical state of membrane lipids. It is concluded that lipid diffusion in the plasma membrane of live bull spermatozoa is rapid and varies significantly between surface domains. Following permeabilization or cell death, however, a large immobile phase develops indicating substantial changes in membrane lipid disposition.

Animals↗
PubMed1997

An outcomes assessment of a hybrid-PBL course in treatment planning.

As part of an ongoing process of curriculum development, a new course was developed at the University of British Columbia (UBC) to address problems such as lack of student enjoyment of the learning process, poor student preparedness for clinical treatment planning, and underdeveloped critical thinking skills in students beginning their clinical experience. The new course utilized a hybrid problem-based learning (PBL) format that provided students with an overview in lecture format while encouraging active learning in small group tutorials and seminars. Half the second-year class was randomly selected to participate in the new course, while the other half received the standard lectures on the subject. An outcomes assessment examined whether the aforementioned problems had been addressed in the new course. Course participants completed a post-course evaluation, and all students completed a self-assessment of their preparedness and progress in treatment planning and diagnostic. Clinical instructors, who were blind to the identity of the new course participants, were asked to independently assess each student using the same criteria. Results indicate that students who participated in the hybrid-PBL course enjoyed the learning process and later rated themselves as being better prepared and improving more in the areas of treatment planning and diagnostic records than their counterparts, although the latter results were not routinely statistically significant (p < 0.1). Instructor evaluations supported these differences between the two groups.

British Columbia↗
PubMed1996

Interaction of nisin with planar lipid bilayers monitored by fluorescence recovery after photobleaching.

Nisin, a prominent member of the lantibiotic family of antimicrobial agents, has wide application as a food preservative despite poor understanding of its mode of action. Fluorescence recovery after photobleaching has been used with planar lipid bilayers as a model membrane system to examine how nisin might interact with the surface of bacterial cells. Nisin associates with planar lipid bilayers in the absence of an applied membrane potential causing an array of effects consistent with adsorption of nisin onto the membrane surface which involves inhibition of the lateral diffusion and fluorescence of the lipid probe N-(7--1,2,3-benzoxadiazol-4-yl) phosphatidylethanolamine (NBD-PE) and a reduction of the capacitance of the bilayer. Nisin adsorption is dependent on phospholipid composition. In the presence of dioleoylphosphatidylcholine (PC): cardiolipin (CL) 4:1, the rate of lateral mobility of phospholipid is reduced to 61% of the control level which decreases to a value of 46% when CL is replaced by 1-palmitoyl-2-oleoylphosphatidylserine (PS). These effects on bilayer parameters are transient, and with time the values return to near original levels. High electrical conductivity is observed on application of a voltage ramp suggesting that insertion into the membrane follows surface association. Results have been interpreted in terms of a model in which nisin initially binds to the surface of the membrane causing a modulation of bilayer properties.

Anti-Bacterial Agents↗
PubMed1996

Lateral diffusion in planar lipid bilayers: a fluorescence recovery after photobleaching investigation of its modulation by lipid composition, cholesterol, or alamethicin content and divalent cations.

In spite of the fact that planar lipid bilayers are still the best-suited artificial membrane system for the study of reconstituted ion channels and receptors, data dealing with their physical characterization, especially as regards dynamics, are scanty. A combined electrical and optical chamber was designed and allowed fluorescence recovery after photobleaching recovery curves to be recorded from stable virtually solvent-free bilayers. D, the lateral diffusion coefficient of N-(7-nitrobenzoyl-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn- glycero-3-phosphoethanolamine, was found to be relatively insensitive to the phospholipid composition (headgroup, chain unsaturation, etc.), whereas inclusion of 33-50% cholesterol in the membrane reduced D by a factor of 2. Divalent cations significantly reduced D of negatively charged bilayers. These results compare well with data gathered on other model and natural systems. In addition, the incorporation of the voltage-dependent pore-former alamethicin did slightly reduce lipid lateral mobility. This study demonstrates the feasibility of such experiments with planar bilayers, which are amenable to physical constraints, and thus offers new opportunities for systematic studies of structure-function relationships in membrane-associating molecules.

Alamethicin↗
PubMed1996

Molecular mobility in the monolayers of foam films stabilized by porcine lung surfactant.

Certain physical properties of a range of foam film types that are believed to exist in vivo in the lung have been investigated. The contribution of different lung surfactant components found in porcine lung surfactant to molecular surface diffusion in the plane of foam films has been investigated for the first time. The influence of the type and thickness of black foam films, temperature, electrolyte concentration, and extract composition on surface diffusion has been studied using the fluorescence recovery after photobleaching technique. Fluorescent phospholipid probe molecules in foam films stabilized by porcine lung surfactant samples or their hydrophobic extracts consisting of surfactant lipids and hydrophobic lung surfactant proteins, SP-B and SP-C, exhibited more rapid diffusion than observed in films of its principal lipid component alone, L-alpha-phosphatidylcholine dipalmitoyl. This effect appears to be due to contributions from minor lipid components present in the total surfactant lipid extracts. The minor lipid components influence the surface diffusion in foam films both by their negative charge and by lowering the phase transition temperature of lung surfactant samples. In contrast, the presence of high concentrations of the hydrophillic surfactant protein A (SP-A) and non-lung-surfactant proteins in the sample reduced the diffusion coefficient (D) of the lipid analog in the adsorbed layer of the films. Hysteresis behavior of D was observed during temperature cycling, with the cooling curve lying above the heating curve. However, in cases where some surface molecular aggregation and surface heterogeneity were observed during cooling, the films became more rigid and molecules at the interfaces became immobilized. The thickness, size, capillary pressure, configuration, and composition of foam films of lung surfactant prepared in vitro support their investigation as realistic structural analogs of the surface films that exist in vivo in the lung. Compared to other models currently in use, foam films provide new opportunities for studying the properties and function of physiologically important alveolar surface films.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1995

Genetic aspects of communication during male-male competition in the Madagascar hissing cockroach: honest signalling of size.

Male Madagascar hissing cockroaches, Gromphadorhina portentosa, engage in agonistic contests with other males and produce audible sounds or 'hisses' during these interactions. Hisses are used to maintain, rather than to establish, social relationships among males. The agonistic hisses of males are variable and could be used as signals to communicate size or competitive ability of an individual. In this study we examined how size influences male-male competition, as well as the genetic variation and covariation of male body size and components of the agonistic hiss. We found that male size affected the outcome of agonistic interactions between pairs of males: a male that dominated in a pair was significantly larger than the male that was subordinate. However, we found no differences in the hisses produced by dominant and subordinate males after controlling for male weight. We estimated heritabilities, evolvability and genetic correlations for male size and characteristics of the hiss from a full-sib analysis of brothers. The patterns of heritabilities and evolvabilities were very similar. The heritabilities of both male weight and duration of the hiss were significantly greater than zero. There was a significant positive genetic correlation between duration of the agonistic hiss and male weight, and a significant negative genetic correlation between hiss duration and the beginning dominant frequency. There was also a positive phenotypic correlation and a negative environmental correlation between male weight and hiss duration. Thus, hiss duration can signal the present influence of the environment on male size, whereas information from hiss duration and beginning dominant frequency can signal the male's ability to transmit genetic influence for size.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Effects of lifelong consumption of fluoridated water or use of fluoride supplements on dental caries prevalence.

This study was undertaken to determine the prevalence of dental caries in children ages 6-14, from fluoridated and non-fluoridated areas in British Columbia, Canada, and the effects of receiving certain fluoride preventive procedures during childhood. Children from two communities were surveyed using a modified Decayed, Missing and Filled Tooth Surface Index (D1D2MFS). Questionnaires on the use of various fluoride preventive practices and residence histories during childhood were collected. Completed questionnaires were returned and exams were performed on 1131 children. Crude caries prevalence scores for the different fluoride exposure groups were tested for differences in dental age and the level of educational attainment of parents and/or guardians. No significant group differences were found. The 110 children with lifelong exposure only to fluoridated water had 35%, or 0.88 (S.D. = 2.91), fewer decayed or filled tooth surfaces per child (P < 0.07) than children with no reported exposure to systemic fluorides. For the 122 children who had taken fluoride supplements for 4 yr or more, 0.67 fewer decayed and filled tooth surfaces (26% reduction) were observed per child when compared to children with no exposure to fluoridated water or supplements. For children who used fluoride supplements for less than 4 yr, no significant benefits were observed. Approximately 75% of the caries prevalence for control and fluoride-exposed groups was on pit and fissured surfaces. Reductions by surface type showed savings on both smooth and pit and fissured surfaces.

Adolescent↗