Anthony Papadopoulos

Spaceflight-induced alterations in cerebral artery vasoconstrictor, mechanical, and structural properties: implications for elevated cerebral perfusion and intracranial pressure

Evidence indicates that cerebral blood flow is both increased and diminished in astronauts on return to Earth. Data from ground‐based animal models simulating the effects of microgravity have shown that decrements in cerebral perfusion are associated with enhanced vasoconstriction and structural remodeling of cerebral arteries. Based on these results, the purpose of this study was to test the hypothesis that 13 d of spaceflight [Space Transportation System (STS)‐135 shuttle mission] enhances myogenic vasoconstriction, increases medial wall thickness, and elicits no change in the mechanical properties of mouse cerebral arteries. Basilar and posterior communicating arteries (PCAs) were isolated from 9‐wk‐old female C57BL/6 mice for in vitro vascular and mechanical testing. Contrary to that hypothesized, myogenic vasoconstrictor responses were lower and vascular distensibility greater in arteries from spaceflight group (SF) mice (n=7) relative to ground‐based control group (GC) mice (n=12). Basilar artery maximal diameter was greater in SF mice (SF: 236±9 μm and GC: 215±5 μm) with no difference in medial wall thickness (SF: 12.4±1.6 μm; GC: 12.2±1.2 μm). Stiffness of the PCA, as characterized via nanoindentation, was lower in SF mice (SF: 3.4±0.3 N/m; GC: 5.4±0.8 N/m). Collectively, spaceflight‐induced reductions in myogenic vasoconstriction and stiffness and increases in maximal diameter of cerebral arteries signify that elevations in brain blood flow may occur during spaceflight. Such changes in cerebral vascular control of perfusion could contribute to increases in intracranial pressure and an associated impairment of visual acuity in astronauts during spaceflight.—Taylor, C. R., Hanna, M., Behnke, B. J., Stabley, J. N., McCullough, D. J., Davis III, R. T., Ghosh, P., Papadopoulos, A., Muller‐Delp, J. M., Delp, M. D. Spaceflight‐induced alterations in cerebral artery vasoconstrictor, mechanical, and structural properties: implications for elevated cerebral perfusion and intracranial pressure. FASEB J. 27, 2282–2292 (2013). www.fasebj.org

Evolution with Stochastic Fitness and Stochastic Migration

Background Migration between local populations plays an important role in evolution - influencing local adaptation, speciation, extinction, and the maintenance of genetic variation. Like other evolutionary mechanisms, migration is a stochastic process, involving both random and deterministic elements. Many models of evolution have incorporated migration, but these have all been based on simplifying assumptions, such as low migration rate, weak selection, or large population size. We thus have no truly general and exact mathematical description of evolution that incorporates migration. Methodology/Principal Findings We derive an exact equation for directional evolution, essentially a stochastic Price equation with migration, that encompasses all processes, both deterministic and stochastic, contributing to directional change in an open population. Using this result, we show that increasing the variance in migration rates reduces the impact of migration relative to selection. This means that models that treat migration as a single parameter tend to be biassed - overestimating the relative impact of immigration. We further show that selection and migration interact in complex ways, one result being that a strategy for which fitness is negatively correlated with migration rates (high fitness when migration is low) will tend to increase in frequency, even if it has lower mean fitness than do other strategies. Finally, we derive an equation for the effective migration rate, which allows some of the complex stochastic processes that we identify to be incorporated into models with a single migration parameter. Conclusions/Significance As has previously been shown with selection, the role of migration in evolution is determined by the entire distributions of immigration and emigration rates, not just by the mean values. The interactions of stochastic migration with stochastic selection produce evolutionary processes that are invisible to deterministic evolutionary theory.

Stochastic Processes Driving Directional Evolution

Evolution is a stochastic process, resulting from a combination of deterministic and random factors. We present results from a general theory of directional evolution that reveals how random variation in fitness, heritability, and migration influence directional evolution. First, we show how random variation in fitness produces a directional trend toward phenotypes with minimal variation in fitness. Furthermore, we demonstrate that stochastic variation in population growth rate amplifies the expected change due to directional selection in small populations. Second, we show that the evolutionary impacts of migration depend on the entire distribution of migration rates such that increasing the variance in migration rates reduces the impact of migration relative to selection. This means that changing the variance in migration rates, holding the mean constant, can substantially change the potential for local adaptation. Finally, we show that covariation between stochastic selection and stochastic heritability can drive directional evolutionary change, and that this can substantially alter the outcome of evolution in variable environments.

Hydrodynamics-Based Functional Forms of Activity Metabolism: A Case for the Power-Law Polynomial Function in Animal Swimming Energetics

The first-degree power-law polynomial function is frequently used to describe activity metabolism for steady swimming animals. This function has been used in hydrodynamics-based metabolic studies to evaluate important parameters of energetic costs, such as the standard metabolic rate and the drag power indices. In theory, however, the power-law polynomial function of any degree greater than one can be used to describe activity metabolism for steady swimming animals. In fact, activity metabolism has been described by the conventional exponential function and the cubic polynomial function, although only the power-law polynomial function models drag power since it conforms to hydrodynamic laws. Consequently, the first-degree power-law polynomial function yields incorrect parameter values of energetic costs if activity metabolism is governed by the power-law polynomial function of any degree greater than one. This issue is important in bioenergetics because correct comparisons of energetic costs among different steady swimming animals cannot be made unless the degree of the power-law polynomial function derives from activity metabolism. In other words, a hydrodynamics-based functional form of activity metabolism is a power-law polynomial function of any degree greater than or equal to one. Therefore, the degree of the power-law polynomial function should be treated as a parameter, not as a constant. This new treatment not only conforms to hydrodynamic laws, but also ensures correct comparisons of energetic costs among different steady swimming animals. Furthermore, the exponential power-law function, which is a new hydrodynamics-based functional form of activity metabolism, is a special case of the power-law polynomial function. Hence, the link between the hydrodynamics of steady swimming and the exponential-based metabolic model is defined.

On the Hydrodynamics-Based Power-Law Function and Its Application in Fish Swimming Energetics

Abstract The standard metabolic rate and metabolic transport differential represent parameters of energetic costs and are derived from either the conventional exponential function or the hydrodynamics-based power-law function (HPF). These two models are used independently to describe the relationship between total metabolic rate and sustained swimming speed for fish. Inappropriate estimates of the standard metabolic rate and inaccurate estimates of the metabolic transport differential have been reported in the literature owing to a lack of practical guidance in methods, especially in regard to the estimation, interpretation, and use of the parameters of the HPF. Hence, the field of fish swimming energetics could benefit from clarification and standardization of parameter estimation and use. The distinction between the parameters in the two models is addressed to ensure appropriate use of the HPF in evaluating the standard metabolic rate and metabolic transport differential. The analytical derivation of th...

Method to Screen for Abnormal Deformation at the Interface between Foot and Functional Foot Orthosis

Introduction The weight-bearing foot assumes the reference plantar morphology while resting on a functional foot orthosis that is supported by a flat surface. The weight-bearing foot, however, can assume a different plantar morphology from the reference while resting on the orthosis that is inside a shoe in which the shank is a curved surface. If there is a difference in the plantar morphology, then the plantar deformation is abnormal because a functional foot orthosis should not deform along the shank of a shoe. Materials and Methods This technical note describes a procedure to screen for abnormal deformation of the weight-bearing foot at the orthosis-foot interface. Casting the weight-bearing foot at the orthosis-foot interface on a flat surface and inside a shoe is performed to capture the reference and the in-shoe plantar morphology, respectively. Both the reference and the in-shoe plantar surface contour are then superimposed via Bookstein two-point registration to screen for abnormal deformation. Conclusions The procedure described in this note should be implemented to determine the suitability of footwear with a functional foot orthosis in refractory cases of functional foot disorders.

Stochastic Ontogenetic Allometry: The Statistical Dynamics of Relative Growth

Background In the absence of stochasticity, allometric growth throughout ontogeny is axiomatically described by the logarithm-transformed power-law model, , where and are the logarithmic sizes of two traits at any given time t. Realistically, however, stochasticity is an inherent property of ontogenetic allometry. Due to the inherent stochasticity in both and , the ontogenetic allometry coefficients, and k, can vary with t and have intricate temporal distributions that are governed by the central and mixed moments of the random ontogenetic growth functions, and . Unfortunately, there is no probabilistic model for analyzing these informative ontogenetic statistical moments. Methodology/Principal Findings This study treats and as correlated stochastic processes to formulate the exact probabilistic version of each of the ontogenetic allometry coefficients. In particular, the statistical dynamics of relative growth is addressed by analyzing the allometric growth factors that affect the temporal distribution of the probabilistic version of the relative growth rate, , where is the expected value of the ratio of stochastic to stochastic , and and are the numerator and the denominator of , respectively. These allometric growth factors, which provide important insight into ontogenetic allometry but appear only when stochasticity is introduced, describe the central and mixed moments of and as differentiable real-valued functions of t. Conclusions/Significance Failure to account for the inherent stochasticity in both and leads not only to the miscalculation of k, but also to the omission of all of the informative ontogenetic statistical moments that affect the size of traits and the timing and rate of development of traits. Furthermore, even though the stochastic process and the stochastic process are linearly related, k can vary with t.