Is trabecular bone permeability governed by molecular ordering-induced fluid viscosity gain? Arguments from re-evaluation of experimental data in the framework of homogenization theory.

It is generally agreed on that trabecular bone permeability, a physiologically important quantity, is governed by the material׳s (vascular or intertrabecular) porosity as well as by the viscosity of the pore-filling fluids. Still, there is less agreement on how these two key factors govern bone permeability. In order to shed more light onto this somewhat open issue, we here develop a random homogenization scheme for upscaling Poiseuille flow in the vascular porosity, up to Darcy-type permeability of the overall porous medium "trabecular bone". The underlying representative volume element of the macroscopic bone material contains two types of phases: a spherical, impermeable extracellular bone matrix phase interacts with interpenetrating cylindrical pore channel phases that are oriented in all different space directions. This type of interaction is modeled by means of a self-consistent homogenization scheme. While the permeability of the bone matrix equals to zero, the permeability of the pore phase is found through expressing the classical Hagen-Poiseuille law for laminar flow in the format of a "micro-Darcy law". The upscaling scheme contains pore size and porosity as geometrical input variables; however, they can be related to each other, based on well-known relations between porosity and specific bone surface. As two key results, validated through comprehensive experimental data, it appears (i) that the famous Kozeny-Carman constant (which relates bone permeability to the cube of the porosity, the square of the specific surface, as well as to the bone fluid viscosity) needs to be replaced by an again porosity-dependent rational function, and (ii) that the overall bone permeability is strongly affected by the pore fluid viscosity, which, in case of polarized fluids, is strongly increased due to the presence of electrically charged pore walls.

A multiscale mechanobiological model of bone remodelling predicts site-specific bone loss in the femur during osteoporosis and mechanical disuse.

We propose a multiscale mechanobiological model of bone remodelling to investigate the site-specific evolution of bone volume fraction across the midshaft of a femur. The model includes hormonal regulation and biochemical coupling of bone cell populations, the influence of the microstructure on bone turnover rate, and mechanical adaptation of the tissue. Both microscopic and tissue-scale stress/strain states of the tissue are calculated from macroscopic loads by a combination of beam theory and micromechanical homogenisation. This model is applied to simulate the spatio-temporal evolution of a human midshaft femur scan subjected to two deregulating circumstances: (i) osteoporosis and (ii) mechanical disuse. Both simulated deregulations led to endocortical bone loss, cortical wall thinning and expansion of the medullary cavity, in accordance with experimental findings. Our model suggests that these observations are attributable to a large extent to the influence of the microstructure on bone turnover rate. Mechanical adaptation is found to help preserve intracortical bone matrix near the periosteum. Moreover, it leads to non-uniform cortical wall thickness due to the asymmetry of macroscopic loads introduced by the bending moment. The effect of mechanical adaptation near the endosteum can be greatly affected by whether the mechanical stimulus includes stress concentration effects or not.

Calculation of isotope effects from first principles.

Various means of calculating the effect of changing the mass of a given atom upon a chemical process are reviewed. Of particular interest is the deuterium isotope effect comparing the normal protium nucleus with its heavier deuterium congener. The replacement of the bridging protium in a neutral hydrogen bond such as the water dimer by a deuterium strengthens the interaction by a small amount via effects upon the vibrational energy. In an ionic H-bond such as the protonated water dimer, on the other hand, the reverse trend is observed in that replacement of the bridging protium by dimer weakens the interaction. In addition to the stability of a given complex, the rate at which a proton transfers from one group to another is likewise affected by deuterium substitution, viz. kinetic isotope effects (KIEs). The KIE is enlarged as the temperature drops, particularly so if the calculation of KIE includes proton tunneling. The KIE is also sensitive to any angular distortions or stretches present in the H-bond of interest. KIEs can be computed either by the standard transition state theory which is derived via only two points on the potential energy surface, or by more complete formalisms which take account of larger swaths of the surface. While more time intensive, the latter can also be applied to provide insights important in interpretation of experimental data.

The genetics of phenotypic plasticity. IX. Genetic architecture, temperature, and sex differences in Drosophila melanogaster.

We examined the genetic architecture of plasticity of thorax and wing length in response to temperature in Drosophila melanogaster. Reaction norms as a function of growth temperature were analyzed in 20 isofemale lines in a natural population collected from Grande Ferrade near Bordeaux (southern France) in two different years. We found evidence for a complex genetic architecture underlying the reaction norms and differences between males and females. Reaction norms were negative quadratics. Genetic correlations were moderately high between traits within environments. Among characteristic values, the magnitudes of genetic correlations varied among traits and sexes. We hypothesized that genetic correlations among environments would decrease as temperatures became more different. This expectation was upheld for only one trait, female thorax length. For males for both traits, the correlations were large for both very similar and very different temperatures. These correlations may constrain the evolution of the shape of the reaction norms. Whether the extent of independence implies specific regulatory genes or only a specific allelic regulation of trait genes can not be decided from our results.

Quantum chemical studies of proton transport through biomembranes.

A proposed mechanism of proton transport through biological membranes was investigated with the aid of ab initio molecular orbital methods. It requires the existence within a transmembrane protein of a hydrogen-bonded chain of residues. The full transport process was broken down into a number of simpler steps, each of which involves the transfer of a proton from one residue to the next along the chain. The hydroxyl-containing residues were modeled by water molecules. Linear hydrogen-bonded systems H+ (H2O)n, n = 2, 3, 4, 5 were studied using the 4-31G basis set. The dimer was shown to furnish an excellent model for study of proton transfer in larger systems. Deformations of the hydrogen bond lead to large increases in the energy barrier to proton transfer. Certain properties of the electron density distribution show a close correspondence with transfer energetics; consequently, they may have some predictive use. The transition state to double proton transfer in the trimer and pentamer involve partial transfer of only one proton. Implications of the above observations upon the proposed mechanism were discussed.

Size and fecundity hierarchies in an herbaceous perennial.

Inequalities in size in populations have potentially important effects on fitness but have rarely been examined in natural populations. I measured size (number of culms) and fecundity (number of spikelets) in five populations of the grass Danthonia spicata from 1981 to 1985. The populations were in sites in a Pinus-Quercus-Populus forest in northern lower Michigan, USA comprising a secondary succession sequence. The sites had been burned in 1980, 1954, 1948, 1936, and 1911, respectively. Mean sizes and fecundities and the amount of hierarchy in size and fecundity, measured by the Gini coefficient, were compared between the YOUNG population, 1980 burn site, and the OLD populations, 1954, 1948, 1936, and 1911 burn sites. I found large differences in mean size and fecundity between the YOUNG and OLD populations with much larger individuals in the YOUNG population. No differences in size hierarchies were found in either the first year of measurement or after five years. The fecundity hierarchies showed no significant difference among the populations in the first year but after five years the YOUNG population showed a significant decrease in amount of inequality. The longterm patterns of size and fecundity hierarchies differed because fecundity was a cumulative trait while size was not. Size inequalities may not always be a good measure of fecundity inequalities. Short-term measures of inequality in perennials may not be a good indicator of long-term values. In contrast to greenhouse studies, habitat light levels did not affect size hierarchies although they did affect fecundity hierarchies.

Ab initio investigation of interactions between models of local anesthetics and receptor: complexes involving amine, phosphate, amide, Na+, K+, Ca2+, and Cl-.

Ab initio molecular orbital methods are used to study the interactions between models of local anesthetic molecules and the putative receptors within the nerve membrane: phospholipids and lipoproteins. The tertiary amine terminus of local anesthetics was modeled by ionized and un-ionized trimethylamine, while phosphate monoanion and formamide emulated the appropriate portions of the receptor. The protonated amine forms a very strong complex with the phosphate anion in which the charge is transferred to the phosphate. While somewhat weaker than this, the complex involving the amine (in either its ionized or un-ionized state) and peptide is considerably stronger than interpeptide H bonds, suggesting the anesthetic can disrupt the normal H-bond patterns in a protein. On the other hand, such interactions must compete with the rather tight binding of the anesthetic with the Na+, K+, Ca2+ and Cl- ions present in vivo.

Ab initio investigation of interactions between models of membrane-active compounds and polar groups of membranes: complexes involving amine, ether, amide, phosphate, and carboxylate.

Ab initio (MINI-1) molecular orbital calculations were performed on model systems to investigate the hydrogen bonds and proton transfer between antiarrhythmics and polar groups of the cell membrane. Methylamine cation, dimethyl ether, and N-methylacetamide served as models of associative sites for the antiarrhythmics mexiletine and tocainide. Formate and phosphate anions, the methylamine cation, and formamide were chosen as models for the membrane polar groups. Protonated methylamine forms a very strong complex with the formate and phosphate anions. However, the formate COO- group is a better proton acceptor than the phosphate PO4- group. The effect of specific hydration on the proton potential functions was investigated in the HCOO- ... +HNH2CH3 and H2PO4- ... +HNH2CH3 systems. The proton potential functions, calculated at the equilibrium distances RO ... N, with a single minimum were found. The ab initio calculations at the longer RO ... N = 0.275 nm distance indicate double-minimum potentials. The increasing hydration stabilizes a second minimum corresponding to the charged O- ... +HN structures. The complexes involving the amide and ether groups of tocainide and mexiletine and the protonated primary amine group of the membrane are considerably weaker. The weakest hydrogen bonds are formed by the amine group of the drug (in its neutral and ionized state) with the peptide group.

Angular dependence of the interaction energy between the N lone pair of amines and a proton: relevance to drug-receptor systems.

Ab initio molecular orbital calculations with a 4-31G basis set have been performed to study the angular dependence of the interaction energy between a lone electron pair of nitrogen and a proton. In this study ammonia and trimethylamine were used as models of biologically active amines. A proton was used as a model of an electrophilic site at the receptor. Results obtained confirm previous indications that the energy required to bend the proton from the lone pair direction decreases markedly as the two species are further separated from one another. Implications regarding the interactions of drugs and hormones at specific receptors are discussed.

New insights in the clastic binding hypothesis for opiate-receptor interactions II: Proton-transfer mechanism.

Ab initio (4-31G) molecular orbital calculations were performed on model systems to investigate the proton-transfer version of the clastic binding hypothesis for opiate-receptor interactions. Ammonia was chosen as the model for the nitrogen-containing portion of the opiate molecule, while ammonia and water were chosen as models for the proton acceptor at the receptor. The equilibrium position of a proton situated between the two molecules is found to be determined primarily by the orientation of the proton-donor molecule with some influence also from the other molecule. Misalignments of the lone pairs can significantly alter equilibrium populations when the proton affinities of the two molecules are similar.

[Standards, Options and Recommendations (SOR) for the use of appetite stimulants in oncology. Work group. Federation of the French Cancer Centres (FNCLCC)]. / Standards, Options et Recommandations (SOR) pour l'utilisation des médicaments orexigènes en cancérologie.

CONTEXT: The "Standards, Options and Recommendations" (SOR) project, started in 1993, is a collaboration between the Federation of the French Cancer Centres (FNCLCC), the 20 French Cancer Centres and specialists from French Public Universities, General Hospitals and Private Clinics. The main objective is the development of clinical practice guidelines to improve the quality of health care and outcome for cancer patients. The methodology is based on literature review and critical appraisal by a multidisciplinary group of experts, with feedback from specialists in cancer care delivery. OBJECTIVES: To develop clinical practice guidelines according to the definitions of the Standards, Options and Recommendations project for the use of appetite stimulants in oncology. METHODS: Data were identified by searching Medline and the personal reference lists of members of the expert groups. Once the guidelines were defined, the document was submitted for review to 55 independent reviewers, and to the medical committees of the 20 French Cancer Centres. RESULTS: The main recommendations for the use of appetite stimulants in oncology are: 1) Corticosteroids and the synthetic progestogens (megestrol acetate and medroxyprogesterone acetate) are appetite stimulants. 2) They can be useful in managing anorexia and weight loss in cancer patients, especially in the palliative setting, despite the potential side-effects of these agents. 3) The most effective way of using these drugs is not known. Inclusion in clinical trials is recommended. 4) Cyproheptadine, metoclopramide, nandrolone and pentoxif line should not be used outside prospective clinical trials. 5) Hydrazine sulfate should not be used.

Mathematical modeling of postmenopausal osteoporosis and its treatment by the anti-catabolic drug denosumab.

Denosumab, a fully human monoclonal antibody, has been approved for the treatment of postmenopausal osteoporosis. The therapeutic effect of denosumab rests on its ability to inhibit osteoclast differentiation. Here, we present a computational approach on the basis of coupling a pharmacokinetics model of denosumab with a pharmacodynamics model for quantifying the effect of denosumab on bone remodeling. The pharmacodynamics model comprises an integrated systems biology-continuum micromechanics approach, including a bone cell population model, considering the governing biochemical factors of bone remodeling (including the action of denosumab), and a multiscale micromechanics-based bone mechanics model, for implementing the mechanobiology of bone remodeling in our model. Numerical studies of postmenopausal osteoporosis show that denosumab suppresses osteoclast differentiation, thus strongly curtailing bone resorption. Simulation results also suggest that denosumab may trigger a short-term bone volume gain, which is, however, followed by constant or decreasing bone volume. This evolution is accompanied by a dramatic decrease of the bone turnover rate by more than one order of magnitude. The latter proposes dominant occurrence of secondary mineralization (which is not anymore impeded through cellular activity), leading to higher mineral concentration per bone volume. This explains the overall higher bone mineral density observed in denosumab-related clinical studies.

Effect of intermolecular orientation upon proton transfer within a polarizable medium.

Ab initio calculations are used to investigate the proton transfer process in bacteriorhodopsin. HN = CH2 serves as a small prototype of the Schiff base while HCOO- models its carboxylate-containing counterion and HO- the hydroxyl group of water of tyrosine, leading to the HCOO-..H+..NHCH2 and HO-..H+..NHCH2 complexes. In isolation, both complexes prefer a neutral pair configuration wherein the central proton is associated with the anion. However, the Schiff base may be protonated in the former complex, producing the HCOO-..+HNHCH2 ion pair, when there is a high degree of dielectric coupling with an external polarizable medium. Within a range of intermediate level coupling, the equilibrium position of the proton (on either the carboxylate or Schiff base) can be switched by suitable changes in the intermolecular angle. pK shift resulting from a 60 degrees reorientation are calculated to be some 5-12 pK U within the coupling range where proton transfers are possible. The energy barrier to proton transfer reinforces the ability of changes in angle and dielectric coupling to induce a proton transfer.

Influence of intramolecular hydrogen bonding on the electronic structure of oxymorphone.

Approximate ab initio molecular orbital methods are used to examine the structural and electronic properties of oxymorphone. The most stable conformation of the molecule is found to include an intramolecular hydrogen bond between the C-14 hydroxyl group and the nitrogen atom in agreement with available experimental data. The total molecular electron density is transformed to a set of localized molecular orbitals, one of which corresponds to the lone electron pair on nitrogen. The hydrogen bond is shown to produce substantial bending and stretching of the lone pair when compared to its shape when such hydrogen bonding is precluded.

Molecular orbital studies of enzyme activity: catalytic mechanism of serine proteinases.

The catalytic activity of the serine proteinases is studied using molecular orbital methods on a model of the enzyme-substrate complex. A mechanism is employed in which Ser-195, upon donating a proton to the His-57-Asp-102 dyad, attacks the substrate to form the tetrahedral intermediate. As His-57 then donates a proton to the leaving group, the intermediate decomposes to the acyl enzyme. An analogous process takes place during deacylation, as a water molecule takes the place of Ser-195 as the nucleophile. The motility of the histidine is found to be an important factor in both steps. An attempt is made to include the effects of those atoms not explicitly included in the calculations and to compare the reaction rate of the proposed mechanism with that of the uncatalyzed hydrolysis. This mechanism is found to be in good agreement with structural and kinetic data.

Modification of pK values caused by change in H-bond geometry.

The competition between various groups for a proton is studied by ab initio molecular orbital methods. It is found that reorientations of the two groups involved in a H-bond can reverse the equilibrium position of the proton shared between them. Specifically, the carbonyl and hydroxyl groups were modeled by H2CO and HOH. In the H-bond between these two groups, association of the proton with the carbonyl (H2COH...OH2)+ is favored over the hydroxyl (H2CO...HOH2)+ when the latter group is situated along a lone pair of the carbonyl oxygen. However, displacement of the water to the C = O axis between the two carbonyl lone pairs reverses the situation and (H2CO...HOH2)+ is more stable. A similar reversal of stability is observed in the H-bond involving a Schiff base (modeled by CH2NH) and amine (NH3). In one arrangement where the lone pairs of the two groups point toward one another, the proton prefers the Schiff base to the amine--i.e., (H2CHNH...NH3)+ is more stable than (H2CHN...HNH3)+. On the other hand, rotation of the lone pair of the amine away from the Schiff base nitrogen results in proton transfer across to the amine. These shifts in stability correspond to reversal of relative pK of the groups involved. A fundamental principle emerging from the calculations is that ion-dipole electrostatic interactions favor transfer of a proton to the group that is positioned as closely as possible to the negative end of the dipole moment vector of the other. The ideas developed here suggest a number of means by which conformational changes may be utilized to shift protons from residue to residue within a protein molecule such as an enzyme or bacteriorhodopsin.