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  • 1
    Electronic Resource
    Electronic Resource
    Calcium dependency of the AT1-receptor mediated effects in the rat portal vein: influence of calcium antagonists (1994)
    Springer
    Naunyn-Schmiedeberg's archives of pharmacology 349 (1994), S. 437-442 
    Details
    ISSN: 1432-1912
    Keywords: Ang II ; AT1-receptor ; Calcium ions ; Calcium antagonists ; Rat portal vein
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The calcium dependency of AT1-receptor mediated contractions was studied in isolated rat portal vein preparations. The spontaneous phasic contractile force of the rat portal vein was increased (ED50 = 1.76 mmol/l) and the frequency of contractions decreased by raising the extracellular calcium concentration. The Ang 11-induced rise in phasic contractile force (mediated by AT1-receptors, Zhang et al. 1993) proved most pronounced at 0.9 mmol/l of calcium chloride, but it was weaker at either lower or higher calcium concentrations. The maximal increases in the phasic contractile force induced by Ang II were 2.4±0.4, 14.8±0.9 and 5±0.5 mN at calcium concentrations of 0.5, 0.9 and 2.5 mmol/l, respectively. Calcium antagonists reduced at the lower and abolished at the higher concentrations (nifedipine 2×10−8 or 10−7 mol/l; verapamil 10−7 or 5 × 10−7 mol/l; diltiazem 3 × 10−7 or 10–6 mol/l) the spontaneous contractile force. All of these calcium antagonists caused a strong inhibition or suppression of the phasic contractions induced by Ang II.The rank order of potency was nifedipine 〉verapamil 〉 diltiazem. Ang II (10−6 mol/l) elicited a tonic contraction which was abolished by the AT1-receptor antagonist losartan 10-6 mol/l but not by the AT2-receptor antagonist PD 123177 (10–5 mol/l). Very high concentrations of nifedipine (10–6 mol/l), verapamil (5 × 10-6 mol/l) and diltiazem (5 × 10−6 mol/l) almost suppressed the tonic effect evoked by the activation of AT1-receptors. In a nominally Ca2+ “free”, EGTA-containing solution, a single supra-maximal concentration of Ang II (10−6 mol/l) caused a transient contraction, also mediated by AT1-receptors. This finding suggests the existence of Ang II-sensitive intracellular calcium stores in this preparation. The depletion of such stores proved complete after 4–6 min of perfusion in a Ca2+ “free”, EGTA-containing solution. In conclusion, various types of contractions (a transient contraction in a Ca2+-“free” medium, phasic and tonic contractions) induced by Ang II in the rat portal vein proved to be mediated by AT1-receptors. These contractions were clearly modified by changes in the availability of extra- and possibly intracellular calcium ions. The calcium movements elicited by stimulation of AT1-receptors in a calcium containing solution were inhibited by the three calcium antagonists investigated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00170892
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  • 2
    Electronic Resource
    Electronic Resource
    Inhibitory effect of dithiothreitol on angiotensin II-induced contractions mediated by AT1-receptors in rat portal vein and rabbit aorta (1994)
    Springer
    Naunyn-Schmiedeberg's archives of pharmacology 349 (1994), S. 538-542 
    Details
    ISSN: 1432-1912
    Keywords: AT1-receptors ; Angiotensin II ; Dithiothreitol ; Losartan ; Rat portal vein ; Rabbit aorta
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The disulfide-reducing agent dithiothreitol (DTT) has been shown to reduce angiotensin II (Ang II) subtype 1 receptor (AT,) binding sites in various tissues. Its effect on Ang II-induced contractions was studied in the rat portal vein and rabbit aorta. In the isolated rat portal vein, DTT shifted the concentration-response curve for Ang II to the right (DTT 0.5–3 mmol/l) and depressed the maximal response (DTT 1–3 mmol/l). DTT 5 mmol/l almost abolished the effect of Ang II. In the isolated rabbit aorta, the inhibitory effect of DTT was more pronounced and its pattern of effect was different,since DTT 0.3 and 0.5 mmol/l caused a progressive flattening of the concentration-response curve of Ang II. DTT (1 mmol/l) fully suppressed the effect of Ang II. A biphasic curve consisting of a high sensitivity component and a component of low sensitivity for Ang II was observed after pretreatment with DTT 1 mmol/l in the rat portal vein but not in the rabbit aorta. In the presence of DTT 1 mmol/l, the AT1-receptor antagonist losartan antagonized the high sensitivity response to Ang II in a competitive manner with a pA2 value very similar to that obtained in the absence of DTT, suggesting that this response to Ang II is mediated by those AT1-receptors which were not inactivated by DTT The biphasic curve may be explained by the occurrence of a single AT1-receptor subtype existing in two different states. Another possibility might be the involvement of two AT1-receptor subpopulations. It is concluded that disulfide bonds are critical for the functional role of AT1-receptors in Ang II-induced contractions in the rat portal vein and rabbit aorta.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00169144
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  • 3
    Electronic Resource
    Electronic Resource
    Characterization of the angiotensin II-receptor subtype in the longitudinal smooth muscle of the rat portal vein (1993)
    Springer
    Naunyn-Schmiedeberg's archives of pharmacology 347 (1993), S. 220-224 
    Details
    ISSN: 1432-1912
    Keywords: Angiotensin II-receptor ; Dithiothreitol ; Nonpeptide angiotensin II-receptor antagonists ; Rat portal vein
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The purpose of the present study was to identify the angiotensin II-receptor subtype involved in the enhancement of the amplitude of the phasic contractions by angiotensin II in the isolated rat portal vein preparation. At an extracellular Ca2+ concentration of 0.9 mmol/l and a K+ concentration of 4 mmol/l, angiotensin II induced concentration-dependent increases in the amplitude of the phasic contractions. The enhancement of phasic contraction amplitude caused by angiotensin II was not significantly altered by pretreatment of the rat portal vein with indomethacin 10−5 mol/l or nitro-L-arginine 10−4 mol/l, indicating that neither prostaglandins nor the endothelium derived-relaxing factor (NO) are involved. Losartan (DuP 753), a nonpeptide selective AT1-receptor antagonist, concentration-dependently shifted the concentration-response curve for the effect of angiotensin II on the amplitude of the contractions to the right, without reducing the maximal response (pA2 = 8.6, slope = 0.98), thus suggesting competitive antagonism at the level of AT1-receptors. By contrast, PD 123177, a nonpeptide selective AT2-receptor antagonist, even at 10−5 mol/l, caused no significant change of the phasic myogenic response to angiotensin II, indicating the absence of AT2-receptor involvement. Dithiothreitol, a disulfide-reducing agent which is known to inactivate AT1-receptors in various tissues, markedly inhibited (3 mmol/l) or even abolished (5 mmol/l) the contractile response of the rat portal vein to angiotensin II, supporting the conclusion that these receptors can be classified as AT1-receptors. In conclusion, the receptor subtype mediating the angiotensin II-induced potentiation of the spontaneous phasic contractions in the rat portal vein appears to belong to the AT1-receptor subtype.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00169271
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  • 4
    Electronic Resource
    Electronic Resource
    A six-node finite element for plate bending (1985)
    Chichester [u.a.] : Wiley-Blackwell
    International Journal for Numerical Methods in Engineering 21 (1985), S. 131-143 
    Details
    ISSN: 0029-5981
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A six-node plate bending element has been developed by employing mixed formulation based on a modified Hellinger-Reissner principle and the Reissner-Mindlin plate bending theory. The numerical result indicates that, among the types of assumed independent transverse shear strains considered, a combination of 2α version with either 5α version or 6α version is free of spurious kinematic modes and leads to accurate and reliable solutions even for very thin plates.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/nme.1620210111
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  • 5
    Electronic Resource
    Electronic Resource
    Post-buckling analysis of shallow shells by the field-boundary-element method (1988)
    Chichester [u.a.] : Wiley-Blackwell
    International Journal for Numerical Methods in Engineering 26 (1988), S. 571-587 
    Details
    ISSN: 0029-5981
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The non-linear field-boundary-element technique is applied to the analysis of snap-through phenomena in thin shallow shells. The equilibrium path is traced by using the arc-length method and the solution strategy is discussed in detail. The results show that, as compared to the approaches based on the popular symmetric-variational Galerkin finite element formulation, the current approach based on an unsymmetric variational Petrov-Galerkin field-boundary-element formulation gives a faster convergence while using fewer degrees of freedom. The illustrative numerical examples deal with post-buckling responses of several shallow shells with different geometries.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/nme.1620260305
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  • 6
    Electronic Resource
    Electronic Resource
    A semi-analytic method for dynamic response analysis based on Gurtin's variational principle (1995)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 11 (1995), S. 297-306 
    Details
    ISSN: 1069-8299
    Keywords: semi-analytic method ; dynamic response ; convolution ; variational principle ; series ; finite element method ; Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In the paper a semi-analytic approach for solving dynamic response problems is developed which is based on Gurtin's convolution-type variational principle. A finite element discretization in the space domain and a series representation in the time domain are considered. This approach overcomes the shortcomings of existing methods yet utilizes their advantages for solving dynamic response problems. The example of a beam shows that this new approach is a very effective method in obtaining solutions for dynamic response problems. The paper also concentrates on utilizing time domain series for various boundary conditions, so that solutions calculated by this approach have a very high accuracy and efficiency.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640110403
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  • 7
    Electronic Resource
    Electronic Resource
    A hybrid finite element method for heterogeneous materials with randomly dispersed rigid inclusions (1995)
    Chichester [u.a.] : Wiley-Blackwell
    International Journal for Numerical Methods in Engineering 38 (1995), S. 1635-1653 
    Details
    ISSN: 0029-5981
    Keywords: hybrid method ; heterogeneous materials ; random distribution ; Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A hybrid finite element approach is proposed for the mechanical response of two-dimensional heterogeneous materials with linearly elastic matrix and randomly dispersed rigid circular inclusions of arbitrary sizes. In conventional finite element methods, many elements must be used to represent one inclusion. In this work, each inclusion is embedded inside a polygonal element and only one element is required to represent one inclusion.In numerically approximating stress and displacement distributions around the inclusion, classical elasticity solutions for a multiply-connected region are employed. A modified hybrid functional is used as the basis of the element formulation where the displacement boundary conditions of the element are automatically considered in a variational sense.The accuracy and efficiency of the proposed method are demonstrated by two boundary value problems. In one example, the results based on the proposed method with only 64 hybrid elements (450 degrees of freedom) are shown to be almost identical to those based on the traditional method with 2928 conventional elements (5526 degrees of freedom).
    Additional Material: 19 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/nme.1620381004
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  • 8
    Electronic Resource
    Electronic Resource
    Problems related to application of eigenstrains in a finite element analysis (1994)
    Chichester [u.a.] : Wiley-Blackwell
    International Journal for Numerical Methods in Engineering 37 (1994), S. 3185-3193 
    Details
    ISSN: 0029-5981
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Accorsi has recently proposed a general method for modelling microstructural material discontinuities in a finite element analysis. In this work, it is shown that his method can be significantly simplified. In particular, the transformation strain (eigenstrain) plays no role in his application. Numerical results based on the simplified equations with 8-node and 9-node isoparametric elements are presented for the same examples as in his papers. The accuracy of the results is examined in comparison with the solution obtained from a complex mesh model.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/nme.1620371811
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  • 9
    Electronic Resource
    Electronic Resource
    Numerical modelling of crack propagation using multi-variable and breakable finite elements (1990)
    New York, NY [u.a.] : Wiley-Blackwell
    Communications in Applied Numerical Methods 6 (1990), S. 215-222 
    Details
    ISSN: 0748-8025
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In this study, the advantage of the multi-variable finite-element model based on the generalized virtual work principle is described. The model has been applied to the elasto-plastic fracture analysis with finite deformations. Using the multi-variable finite-element model with a‘breakable finite-element’ concept, the authors proposed an algorithm which can more accurately simulate the crack growth. Numerical examples illustrate that the algorithm presented can be successfully used to predict whether the crack growth is stable or unstable. The rupture effective strain is considered as a fracture criterion, and some comparisons between various criteria are also provided.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1630060308
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  • 10
    Electronic Resource
    Electronic Resource
    Probabilistic approach for transport of contaminants through porous mediaPaper presented at the ASME Winter Annual Meeting, December 1987. (1989)
    Chichester : Wiley-Blackwell
    International Journal for Numerical Methods in Fluids 9 (1989), S. 1431-1451 
    Details
    ISSN: 0271-2091
    Keywords: Probabilistic diffusion-convection equation ; Gaussian field approach ; Random walk method ; Fluctuation Peclet number ; Porous media ; Two-phase system ; Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: The channels formed between individual particles in porous media have variable dimensions and orientations. The porosity, permeability and its anisotropy exhibit random spatial distributions. The probabilistic approach can effectively describe the transport of contaminants through porous media and is analysed in this paper. Numerical results are obtained by considering (I) random dispersion coefficients without and with spatial structure, (II) random time distribution of concentration at the inlet boundary, (III) random velocity distribution in the flow field without and (IV) with variable dispersion coefficient, (V) non-linearity of the governing equation and (VI) anisotropy of the dispersion coefficient. Two methods are used for probabilistic predictions: (1) Gaussian field approach in conjunction with Monte Carlo method and (2) random walk method. The input random parameters are assumed to have normal and log-normal distributions according to available experimental data. The probability distribution functions of the contaminant concentration at different locations within the flow domain are calculated and compared with the input distributions as a function of the mean and fluctuation Peclet numbers. The one-dimensional case is analysed in detail and the illustrative numerical predictions are compared with analytical and experimental results. The extension to a two-dimensional domain is discussed in the last part of this paper.
    Additional Material: 16 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/fld.1650091202
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