STUDIES ON THE DIFFUSION EFFECT UPON IONIC DISTRIBUTION : II. EXPERIMENTS ON IONIC ACCUMULATION

The aim of this paper is to present confirmation of the theory of the diffusion effect. By diffusion effect is understood a redistribution of the ions in a system where a continuous diffusion of one electrolyte is going on, which results in a steady state showing accumulation and impoverishment of certain ions. Some typical experiments are recorded and discussed. The results are found in satisfactory agreement with the theory: this can be demonstrated in several ways. The importance of considering activity changes is pointed out. Water migration and the diffusion membrane probably have no appreciable influence upon the diffusion effect under the conditions of these experiments. The diffusion effect may have biological analogies.     

Directional Selection and Variation in Finite Populations

Predictions are made of the equilibrium genetic variances and responses in a metric trait under the joint effects of directional selection, mutation and linkage in a finite population. The "infinitesimal model" is analyzed as the limiting case of many mutants of very small effect, otherwise Monte Carlo simulation is used. If the effects of mutant genes on the trait are symmetrically distributed and they are unlinked, the variance of mutant effects is not an important parameter. If the distribution is skewed, unless effects or the population size is small, the proportion of mutants that have increasing effect is the critical parameter. With linkage the distribution of genotypic values in the population becomes skewed downward and the equilibrium genetic variance and response are smaller as disequilibrium becomes important. Linkage effects are greater when the mutational variance is contributed by many genes of small effect than few of large effect, and are greater when the majority of mutants increase rather than decrease the trait because genes that are of large effect or are deleterious do not segregate for long. The most likely conditions for "Muller's ratchet" are investigated.     

Scaled marginal models for multiple continuous outcomes

In studies that involve multivariate outcomes it is often of interest to test for a common exposure effect. For example, our research is motivated by a study of neurocognitive performance in a cohort of HIV-infected women. The goal is to determine whether highly active antiretroviral therapy affects different aspects of neurocognitive functioning to the same degree and if so, to test for the treatment effect using a more powerful one-degree-of-freedom global test. Since multivariate continuous outcomes are likely to be measured on different scales, such a common exposure effect has not been well defined. We propose the use of a scaled marginal model for testing and estimating this global effect when the outcomes are all continuous. A key feature of the model is that the effect of exposure is represented by a common effect size and hence has a well-understood, practical interpretation. Estimating equations are proposed to estimate the regression coefficients and the outcome-specific scale parameters, where the correct specification of the within-subject correlation is not required. These estimating equations can be solved by repeatedly calling standard generalized estimating equations software such as SAS PROC GENMOD. To test whether the assumption of a common exposure effect is reasonable, we propose the use of an estimating-equation-based score-type test. We study the asymptotic efficiency loss of the proposed estimators, and show that they generally have high efficiency compared to the maximum likelihood estimators. The proposed method is applied to the HIV data.     

A new nonparametric approach for baseline covariate adjustment for two-group comparative studies

SUMMARY: We consider two-armed clinical trials in which the response and/or the covariates are observed on either a binary, ordinal, or continuous scale. A new general nonparametric (NP) approach for covariate adjustment is presented using the notion of a relative effect to describe treatment effects. The relative effect is defined by the probability of observing a higher response in the experimental than in the control arm. The notion is invariant under monotone transformations of the data and is therefore especially suitable for ordinal data. For a normal or binary distributed response the relative effect is the transformed effect size or the difference of response probability, respectively. An unbiased and consistent NP estimator for the relative effect is presented. Further, we suggest a NP procedure for correcting the relative effect for covariate imbalance and random covariate imbalance, yielding a consistent estimator for the adjusted relative effect. Asymptotic theory has been developed to derive test statistics and confidence intervals. The test statistic is based on the joint behavior of the estimated relative effect for the response and the covariates. It is shown that the test statistic can be used to evaluate the treatment effect in the presence of (random) covariate imbalance. Approximations for small sample sizes are considered as well. The sampling behavior of the estimator of the adjusted relative effect is examined. We also compare the probability of a type I error and the power of our approach to standard covariate adjustment methods by means of a simulation study. Finally, our approach is illustrated on three studies involving ordinal responses and covariates.     

On tests of equal effect per fraction in microcolony assays of survival after fractionated irradiations

H. D. Thames, Jr. and H. R. Withers [Br. J. Radiol. 53, 1071-1077 (1980)] propose a test of an equal effect per fraction in microcolony assays after fractionated radiation, in which the total effect is measured by counting microcolonies derived from surviving cells in a tissue. The factors considered to influence the cytocidal effect per fraction are incomplete repair, repopulation, and synchrony. The statistics used in the method are criticized and conditions are given under which the test should not be used. An alternative method of testing for an equal effect per fraction is proposed. The pros and cons of each test are discussed and compared using some mouse jejunal crypt cell survival data.     

Influence of chemical modification on enzyme inactivation kinetics and stability

Enzymes are placed in different categories depending on the effect of chemical modification on their inactivation kinetics and residual activity. This is done using a series-type mechanism involving degraded but stable enzyme states. The major distinction in the three basic categories is the effect of modification on residual activity. Each category is further sub-divided depending on the effect of modification on the values of the deactivation rate constants. The classification provides for a framework for comparison of a wide variety of enzyme deactivation data. Structure-function relations are provided wherever possible.     

Effet de la caféine sur le developpement des tumeurs melaniques chez la drosophile

Caffeine, added to the nutrient medium of strain cl tu^48a larvae which can develop spontaneous melanotic tumours, produces a double effect: a toxic effect and an inhibitory effect on tumorigenesis.The toxic effect manifests itself especially if young larvae are treated, and is observed as an increase in their mortality.The effect of caffeine on tumorigenesis is only seen if it is administered during the 2nd and 3rd day of the larval life. Outside of this period, or if caffeine is only applied during a part of the period, it exerts no influence or tumorigenesis. The effect on tumorigenesis consists of a decrease in the frequency of tumourous individuals. In individuals who have developed at least one tumour,caffeine does not modify the probability of the development of other tumours. Its action thus, is at the individual scale and not at that of the tumour.The time during which larvae are sensitive to caffeine indicates that the effect must intervene before the appearance of the first tumour cells.     

On the influence of polyvalent ligands on membrane curvature

The suggestion is made that a polyvalent ligand attached to a membrane will induce a curvature in that membrane which is concave toward the side to which that ligand is bound. This article presents a semiquantitative thermodynamic analysis predicting this effect for a simple system. The criterion for equilibrium of the ligand membrane complex is stated and for the simple system this is calculated for an arbitrary set of parameters. The influence of changes in these parameters is discussed. The energies calculated for this effect are of the order of 0·1 kcal/mol suggesting that for observable effects on curvature an array of such ligands will be needed. Some real systems in which this effect may be playing a role are discussed.     

An investigation into the use and limitations of different spatial integration schemes and finite element software in head impact analyses

To understand the mechanopathogenesis of brain lesions, finite element (FE) head models are used. There is a broad range of material properties, contact interfaces and integration schemes used for the different parts in current FE head models. The effect of material behaviour and contact definitions on a head impact analysis is reported in the literature, whereas the effect of FE integration schemes is a rather unexplored domain. This paper starts with the development of a simplified head model to which small adaptations are made in the integration scheme to obtain multiple analyses that are compared using an accident reconstruction. The performed study highlighted potential hazards of different integration schemes and the significant effect they have on the simulated mechanical responses of the head. Based on a comparison between FE softwares using an impact test and patch test, it was seen that also the software could have an effect on the FE analysis results.     

Testing homogeneity in Weibull-regression models

In survival studies with families or geographical units it may be of interest testing whether such groups are homogeneous for given explanatory variables. In this paper we consider score type tests for group homogeneity based on a mixing model in which the group effect is modelled as a random variable. As opposed to hazard-based frailty models, this model presents survival times that conditioned on the random effect, has an accelerated failure time representation. The test statistics requires only estimation of the conventional regression model without the random effect and does not require specifying the distribution of the random effect. The tests are derived for a Weibull regression model and in the uncensored situation, a closed form is obtained for the test statistic. A simulation study is used for comparing the power of the tests. The proposed tests are applied to real data sets with censored data.     

CERTAIN EFFECTS OF SALTS ON THE PENETRATION OF BRILLIANT CRESYL BLUE INTO NITELLA

The effect of various substances on living cells may be advantageously studied by exposing them to such substances and observing their subsequent behavior in solutions of a basic dye, brilliant cresyl blue. The rate of penetration of the basic dye, brilliant cresyl blue, is decreased when cells are exposed to salts with monovalent cations before they are placed in the dye solution (made up with borate buffer mixture). This inhibiting effect is assumed to be due to the effect of the salts on the protoplasm. This effect is not readily reversible when cells are transferred to distilled water, but it is removed by salts with bivalent or trivalent cations. In some cases it disappears in dye made up with phosphate buffer mixture, or with borate buffer mixture at the pH value in which the borax predominates, and in the case of NaCl it disappears in dye containing NaCl. No inhibiting effect is seen when cells are exposed to NaCl solution containing MgCl₂ before they are placed in the dye solution. The rate of penetration of dye is not decreased when cells are previously exposed to salts with bivalent and trivalent cations. The rate is slightly increased when cells are placed in the dye solution containing a salt with monovalent cation and probably with bivalent or trivalent cations. In the case of the bivalent and trivalent salts the increase is so slight that it may be negligible.     

Effect of chloramphenicol on the growth and viability of exponentially growing mouse leukemic cells (L5178Y)

Two effects of chloramphenicol on mouse leukemic cells (L5178Y) are described. The drug induces a prolongation of the cell cycle (reversible effect). The degree of prolongation is directly proportional to the concentration of the drug. The effect is observed only in the presence of chloramphenicol and cells return to normal cell-growth kinetics when the drug is removed from the culture medium. Chloramphenicol also kills a portion of the cell population immediately (irreversible effect). Data are presented which suggest that chloramphenicol is toxic to cells in the G2 phase of the cell cycle.     

Effect of the opiate antagonist naloxone on the electrical activity of identified neurons in the edible snail

The opiate antagonist naloxone modifies the electric activity of some identified neurons of the Helix lucorum which have not been preliminary exposed to the effect of exogenous opioids. Some neurons are excited while others are inhibited by naloxone, and in both cases the reaction may have both a short and long latent period. The reactions depend on naloxone dose and become less expressed or are blocked when naloxone is administered together with the agonists of opiate receptor (morphine, D-Ala2, D-Leu5-enkephalin, bremazocine and beta-endorphin). Opioids alone do not produce any effect on neurons. The effect of naloxone on neurons is assumed to be a result of the elimination by the opiate antagonist of the tonic effect of endogenous opioids by their replacing on opiate receptors which are constantly stimulated by endogenous ligands.     

The QTN program and the alleles that matter for evolution: all that's gold does not glitter

The search for the alleles that matter, the quantitative trait nucleotides (QTNs) that underlie heritable variation within populations and divergence among them, is a popular pursuit. But what is the question to which QTNs are the answer? Although their pursuit is often invoked as a means of addressing the molecular basis of phenotypic evolution or of estimating the roles of evolutionary forces, the QTNs that are accessible to experimentalists, QTNs of relatively large effect, may be uninformative about these issues if large‐effect variants are unrepresentative of the alleles that matter. Although 20th century evolutionary biology generally viewed large‐effect variants as atypical, the field has recently undergone a quiet realignment toward a view of readily discoverable large‐effect alleles as the primary molecular substrates for evolution. I argue that neither theory nor data justify this realignment. Models and experimental findings covering broad swaths of evolutionary phenomena suggest that evolution often acts via large numbers of small‐effect polygenes, individually undetectable. Moreover, these small‐effect variants are different in kind, at the molecular level, from the large‐effect alleles accessible to experimentalists. Although discoverable QTNs address some fundamental evolutionary questions, they are essentially misleading about many others.     

The Light Requirement for Different Steps in the Development of Chloroplasts in Excised Wheat Roots

For the formation of chloroplasts in excised wheat roots blue light is necessary, but red light enhances the blue light effect. Intensity-response relationships and action spectra have been determined for the specific blue light effect as well as for the effect of red light when given before or after blue light. The longest wave length giving a blue light effect is about 500 nm. The spectrum shows a peak at about 450 nm, and a considerable effect extends down to 368 nm, the shortest wave length tested. The photoreceptor mechanism for the “blue reaction” is thought to be related to that of certain phototropisms and chloroplast movements, and to that involved in carotenoid synthesis in certain fungi. The active pigment is thought to be either a carotenoid or a flavoprotein, probably the latter. The effect of red light shows similar action spectra whether the red light is given before or after the blue, and they are consistent with protochlorophyll absorption. However, much less light is needed to produce a detectable response when the red light is given before the blue light than after. Because of the similarity of the spectra phytochrome may also be involved in the response to red light when given before blue. Far-red reversal has not been tested. The results are discussed in relation to the development of chloroplasts in leaves.     

The inotropic memory of amphibian myocardium. I.-Identification of two stimultaneous mechanisms and statement of a model.

(1) The effects of previous contractions on the actual contractile strength is studied in strips of toad ventricle. The inotropic effect is quantified by superposing a conditioning contraction to a rhythm of definite frequency, its measure being the difference in strength between a rhythm contraction in the presence and in the absence of the conditioning one. (2) The inotropic effect is studied as a function of the interval between the actual and conditioning contractions. Depressed sections of the curve, associated to shortened action potentials, are detected and excluded. (3) The inotropic effect is always positive at low frequencies, but at higher frequencies and long intervals it becomes negative. (4)In high calcium concentration the inotropic effect is always negative and does not depend on frequency. Morever, the joint effect of two previous contractions is equal to the sum of the individual effects of each one. (5) The results are interpreted in terms of two independent elementary processes, one of which potentiates whereaas the other inhibits the strength of contraction. The former disappears in high calcium. Assuming some simple properties for these processes a mathematical expression has been achieved. This expression describes the inotropic effect of any sequence of contractions as a function of intervals involved.     

Current Issues in Statistics and Models for Ecotoxicological Risk Assessment

A review is given on statistical and modelling issues in ecotoxicology. The issues discussed are: 1. How to estimate an (almost) no effect concentration chemicals in the laboratory. 2. Combining single-species acceptable effect levels to an acceptable effect level for a multi-species ecosystem. 3. The combined effect of exposure to several chemicals. 4. Bioavailability in the natural environment and food-web models. Most current procedures in setting standards allow the environmental concentration to be above the acceptable effect concentration for a small fraction of the species. It is shown that a considerable part of the fraction of the affected species will suffer a severe effect.     

The use of isotope effects to determine enzyme mechanisms

Isotope effects are one of the most powerful kinetic tools for determining enzyme mechanisms. There are three methods of measurement. First, one can compare reciprocal plots with labeled and unlabeled substrates. The ratio of the slopes is the isotope effect on V/K, and the ratio of the vertical intercepts is the isotope effect on V(max). This is the only way to determine V(max) isotope effects, but is limited to isotope effects of 5% or greater. The second method is internal competition, where the labeled and unlabeled substrates are present at the same time and the change in their ratio in residual substrate or in product is used to calculate an isotope effect, which is that on V/K of the labeled reactant. This is the method used for tritium or (14)C, or with the natural abundances of (13)C, (15)N, or (18)O. The third method involves perturbations from equilibrium when a labeled substrate and corresponding unlabeled product are present at chemical equilibrium. This also gives just an isotope effect on V/K for the labeled reactant. The chemistry is typically not fully rate limiting, so that the isotope effect on V/K is given by: (x)(V/K)=((x)k+c(f)+c(r)(x)K(eq))/(1+c(f)+c(r)) where x defines the isotope (D, T, 13, 15, 18 for deuterium, tritium, (13)C, (15)N, or (18)O), and (x)(V/K), (x)k, and (x)K(eq) are the observed isotope effect, the intrinsic one on the chemical step, and the isotope effect on the equilibrium constant, respectively. The constants c(f) and c(r) are commitments in forward and reverse directions, and are the ratio of the rate constant for the chemical reaction and the net rate constant for release from the enzyme of the varied substrate (direct comparison) or labeled substrate (internal competition and equilibrium perturbation) for c(f), or the first product released or the one involved in the perturbation for c(r). The intrinsic isotope effect, (x)k, can be estimated by comparing deuterium and tritium isotope effects on V/K, or by comparing the deuterium isotope effect with (13)C ones with deuterated and undeuterated substrates. Adding a secondary deuterium isotope effect and its effect on the (13)C one can give an exact solution for all intrinsic isotope effects and commitments. The effect of deuteration on a (13)C isotope effect allows one to tell if the two isotope effects are on the same or different steps. Applications of these methods to several enzyme systems will be presented.     

Dose-response and thresholds in mutagenicity studies: a statistical testing approach

The analysis of dose-response relationships is an important objective in toxicology, and one in which both modelling and testing approaches are used. One particular question is whether a threshold exists at low doses. The concept of a pragmatic threshold is used, i.e. low doses with biologically unimportant effects are assumed to be threshold doses. "Biologically unimportant" means, in statistical terms, a lower effect than the effect of the negative control, or at least a just-tolerable margin delta higher than the effect of the negative control. Therefore, threshold doses can be tested in terms of a one-sided hypothesis of equivalence. A new approach is proposed, assuming, at the least, that the low dose is a threshold dose, and the highest dose is superior to the negative control. By analogy to the k-fold rule commonly used in mutagenicity studies, tests on ratio-to-control are used. The a priori definition of the threshold margin is inherently needed. A further approach proposes the analysis of dose-response relationships by means of order-restricted inference (the so-called trend test). A modification of a multiple-contrast test is used, in which only those contrasts are included that are sensitive for no effects at low doses. A further modification treats the complicated, but real, problem of simultaneous existence of a threshold, a monotonic increase, and a downturn effect at high dose(s). A parametric procedure is considered, together with an extension for proportions. The important problem of a priori sample size definition is discussed. The approaches are demonstrated by means of examples based on real data.