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Biomedical subjects

J Arends

Publications and source records attributed to J Arends.

At least 307 records · Page 17Linked to original sources
PubMed1977

The temperature dependency of 45Ca diffusion in bovine enamel.

The diffusion of 45Ca ions in sound bovine enamel at pH 7.0 was studied as a function of temperature in the range 20 degrees-40 degrees C. The experimental data show that the diffusion is governed by two processes with activation energies of about 0.7 and 1.3 eV respectively. These values are considerably larger than the activation energy of calcium ion diffusion in water (0.2 eV). The diffusion process with the activation energy of 1.3 eV is dependent upon the calcium concentration in solution. A model based on the complex pore structure of enamel and on interactions of diffusing ions with the enamel constituents is discussed.

Animals↗
PubMed1977

Diffusion of 45Ca in bovine enamel.

The penetration of 45Ca ions into whole bovine enamel from an aqueous solution was studied as a function of the Ca content in the solution. A sectioning technique was employed to measure the penetration of 45Ca into the enamel. Probably only Ca ions occupying surface positions in the hydroxyapatite crystallites participate in the exchange process at the solution-enamel interface. The fraction of Ca ions participating in the exchange was 2% of the total number of Ca ions. Two different diffusion processes occur; firstly, a process governed by a diffusion coefficient, D1 ranging from about 5 x 10(-14) to 2 x 10(-12) cm2/s and strongly dependent on the calcium concentration in the solutions; and secondly a process with a concentration-independent diffusion coefficient, D2, with a value of 2.8 x 10(-12) cm2/s. It is assumed that the 2 diffusion types correspond to intra- and interprismatic transport, respectively. A model based on the pore structure of dental enamel and on surface charges on hydroxyapatite crystallites is discussed.

Animals↗
PubMed1977

Permissive effect of glucose on the glucagon-induced accumulation of cAMP in isolated rat pancreatic islets.

Glucose stimulation increased the cAMP content of collagenase-isolated rat pancreatic islets fourfold above baseline values. The elevation was transient, lasting about 5 min, and was dose-dependent. Insulin release continued at a constant rate throughout the incubation. Glucagon, in the absence of glucose, increased cAMP for about 1 min but only slightly, and had no effect on insulin release. In the presence of glucose, however, glucagon enhanced islet cAMP content 15-fold and increased the release of insulin. Glucagon was most effective at high glucose concentrations (16.6 and 25 mM). This indicates that glucagon is critically dependent on the presence of glucose in order to increase the islet cAMP content and to stimulate insulin release. The inability of glucagon to generate sufficient cAMP in the absence of glucose might be one of the reasons why the hormone is a potentiator rather than an initiator of insulin release.

Animals↗
PubMed1977

Onset and reversibility of changes in secretory function and composition of isolated rat pancreatic islets following long-term administrationof high or low tolbutamide doses.

Chronic administration of a high tolbutamide dose to rats induces islet hypertrophy associated with a decreased insulin content per islet and with a diminished insulin release in response to a glucose or leucine stimulus. These changes are reversible after discontinuation of tolbutamide. Chronic administration of a low tolbutamide dose (effective on islet size, on insulin content per islet, or on leucine-induced insulin release is normal in the presence of glucagon (5 mug/ml) or theophylline (5 mM). Since islet hypertrophy occurs following administration of high tolbutamide doses only and is associated with hypofunction rather than with hyperfunction, it seems hardly conceivable that the therapeutic principle of tolbutamide is based on a beta-cytotrophic effect. B-cell hypofunction seems to be due to at least three factors: the decrease in the insulin content per islet, an impairement in secretory signal recognition, and an interference with the process of signal transmission.

Animals↗
PubMed1977

Polymer adhesion to enamel. The influence of viscosity and penetration.

For a four-component acrylate adhesive system the influence of viscosity and penetration on adhesion to enamel was investigated. While low viscous monomer adhesives yield both inter- and intraprismatic penetration in enamel after polymerization, highly viscous adhesives result in the former type only. This difference in penetration of the adhesive in enamel is reflected quantitatively in the tensile strength values. Scanning electron microscopic measurements on the tag formation under various circumstances have been carried out. The SEM experiments show, e.g. that at polymerization times less than 1-5-2 min (a clinical necessity) polymer-tags were usually fractured from the bulk material due to stresses generated.

Acrylic Resins↗
PubMed1977

Mechanism of enamel dissolution and its prevention.

In this paper the mechanism of dental enamel dissolution is discussed. Special attention is given to the anisotropic solubility behaviour (hollow tube formation) of demineralized enamel crystallites. The combined evidence strongly indicates that this peculiar dissolution behaviour is caused by dislocations. The dislocations--linear lattice defects--are present in the crystallite center and are the cause of two active dissolution sites in each crystallite. Initial as well as later stages in the caries process are discussed. Experimental evidence is presented which indicates that the active sites can be inactivated by agents like EHDP and MFP. After inactivation, acid penetration is either prevented or strongly retarded.

Apatites↗
PubMed1977

The enamel substrate-characteristics of the enamel surface.

The outer enamel surface is of considerable interest in dentistry as it is the substrate for the acquired pellicle; the pellicle itself is the plaque carrier. In this paper the interface between outer enamel and pellicle is discussed with special emphasis on the outer enamel from a structural and chemical point of view. The outer enamel layer (less than 1 micron from the anatomical surface) is most likely a non-stoĭchiometric apatite. The chemical composition is obviously strongly different from the composition of bulk enamel. A model for the structure and atomic composition of surface enamel is presented. The available experimental evidence indicates that at an atomic scale, enamel crystallites have mainly phosphate groups (HPO42-groups) on the solid surface; the liquid adhering Stern layer contains most likely large amounts of calcium ions.

Dental Enamel↗
PubMed1976

Direct bonding of orthodontic brackets.

The direct adhesive bonding of orthodontic brackets to enamel was investigated. For a number of resin systems, all based on a bisphenol-GMA type of resin, commercially available or synthesized, the adhesive bonding to the enamel as well as to the bracket was studied. The maximum average bond strength to enamel was 121 Kg. per square centimeter; the maximum adhesion to the bracket material was 53 Kg. per square centimeter. Consequently, the attachment of the adhesive to the bracket material is the bottleneck of the direct-bonding procedure. Special attention has been given to preconditioning procedures to enhance the bonding between adhesive and bracket. The best results were obtained when the bracket base was roughened by means of a small amount of resin powder. Because all adhesives studied show large standard deviations of the bond strength, criteria of reliability are discussed. They show that future approaches should aim not only for higher average bonds strengths but also for a smaller standard deviation. The setting process, one of the factors influencing reliability, has been considered in some detail. Results of different mixing ratios as well as the influence of the amount of catalyst are presented.

Animals↗