Showing papers in "The Journal of Experimental Biology in 2009"

Rotational accelerations stabilize leading edge vortices on revolving fly wings

Abstract: The aerodynamic performance of hovering insects is largely explained by the presence of a stably attached leading edge vortex (LEV) on top of their wings. Although LEVs have been visualized on real, physically modeled, and simulated insects, the physical mechanisms responsible for their stability are poorly understood. To gain fundamental insight into LEV stability on flapping fly wings we expressed the Navier–Stokes equations in a rotating frame of reference attached to the wing's surface. Using these equations we show that LEV dynamics on flapping wings are governed by three terms: angular, centripetal and Coriolis acceleration. Our analysis for hovering conditions shows that angular acceleration is proportional to the inverse of dimensionless stroke amplitude, whereas Coriolis and centripetal acceleration are proportional to the inverse of the Rossby number. Using a dynamically scaled robot model of a flapping fruit fly wing to systematically vary these dimensionless numbers, we determined which of the three accelerations mediate LEV stability. Our force measurements and flow visualizations indicate that the LEV is stabilized by the `quasi-steady' centripetal and Coriolis accelerations that are present at low Rossby number and result from the propeller-like sweep of the wing. In contrast, the unsteady angular acceleration that results from the back and forth motion of a flapping wing does not appear to play a role in the stable attachment of the LEV. Angular acceleration is, however, critical for LEV integrity as we found it can mediate LEV spiral bursting, a high Reynolds number effect. Our analysis and experiments further suggest that the mechanism responsible for LEV stability is not dependent on Reynolds number, at least over the range most relevant for insect flight (100

A new paradigm for ammonia excretion in aquatic animals: role of Rhesus (Rh) glycoproteins.

Abstract: Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells, whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H(+) excretion and Na(+) uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na(+) uptake and acid excretion. In this model, Rhag facilitates NH(3) flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical "Na(+)/NH (+)(4) exchange complex" consisting of several membrane transporters (Rhcg, V-type H(+)-ATPase, Na(+)/H(+) exchanger NHE-2 and/or NHE-3, Na(+) channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase (CA-2) and basolateral Na(+)/HCO (-)(3) cotransporter (NBC-1) and Na(+)/K(+)-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH(3) by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H(+) ions created by the catalysed or non-catalysed hydration of expired metabolic CO(2). Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO(2) transporters, their responses to feeding, and their roles in early life stages prior to the full development of gills.

Transcriptomic response of sea urchin larvae Strongylocentrotus purpuratus to CO2-driven seawater acidification.

Abstract: Ocean acidification from the uptake of anthropogenic CO(2) is expected to have deleterious consequences for many calcifying marine animals. Forecasting the vulnerability of these marine organisms to climate change is linked to an understanding of whether species possess the physiological capacity to compensate for the potentially adverse effects of ocean acidification. We carried out a microarray-based transcriptomic analysis of the physiological response of larvae of a calcifying marine invertebrate, the purple sea urchin, Strongylocentrotus purpuratus, to CO(2)-driven seawater acidification. In lab-based cultures, larvae were raised under conditions approximating current ocean pH conditions (pH 8.01) and at projected, more acidic pH conditions (pH 7.96 and 7.88) in seawater aerated with CO(2) gas. Targeting expression of approximately 1000 genes involved in several biological processes, this study captured changes in gene expression patterns that characterize the transcriptomic response to CO(2)-driven seawater acidification of developing sea urchin larvae. In response to both elevated CO(2) scenarios, larvae underwent broad scale decreases in gene expression in four major cellular processes: biomineralization, cellular stress response, metabolism and apoptosis. This study underscores that physiological processes beyond calcification are impacted greatly, suggesting that overall physiological capacity and not just a singular focus on biomineralization processes is essential for forecasting the impact of future CO(2) conditions on marine organisms. Conducted on targeted and vulnerable species, genomics-based studies, such as the one highlighted here, have the potential to identify potential ;weak links' in physiological function that may ultimately determine an organism's capacity to tolerate future ocean conditions.

Ammonia and urea transporters in gills of fish and aquatic crustaceans

Abstract: The diversity of mechanisms of ammonia and urea excretion by the gills and other epithelia of aquatic organisms, especially fish and crustaceans, has been studied for decades. Although the decades-old dogma of ;aquatic species excrete ammonia' still explains nitrogenous waste excretion for many species, it is clear that there are many mechanistic variations on this theme. Even within species that are ammonoteles, the process is not purely ;passive', often relying on the energizing effects of proton and sodium-potassium ATPases. Within the ammonoteles, Rh (Rhesus) proteins are beginning to emerge as vital ammonia conduits. Many fishes are also known to be capable of substantial synthesis and excretion of urea as a nitrogenous waste. In such species, members of the UT family of urea transporters have been identified as important players in urea transport across the gills. This review attempts to draw together recent information to update the mechanisms of ammonia and urea transport by the gills of aquatic species. Furthermore, we point out several potentially fruitful avenues for further research.

Environment, antecedents and climate change: lessons from the study of temperature physiology and river migration of salmonids

Abstract: Animal distributions are shaped by the environment and antecedents. Here I show how the temperature dependence of aerobic scope (the difference between maximum and minimum rates of oxygen uptake) is a useful tool to examine the fundamental temperature niches of salmonids and perhaps other fishes. Although the concept of aerobic scope has been recognized for over half a century, only recently has sufficient evidence accumulated to provide a mechanistic explanation for the optimal temperature of salmonids. Evidence suggests that heart rate is the primary driver in supplying more oxygen to tissues as demand increases exponentially with temperature. By contrast, capacity functions (i.e. cardiac stroke volume, tissue oxygen extraction and haemoglobin concentration) are exploited only secondarily if at all, with increasing temperature, and then perhaps only at a temperature nearing that which is lethal to resting fish. Ultimately, however, heart rate apparently becomes a weak partner for the cardiorespiratory oxygen cascade when temperature increases above the optimum for aerobic scope. Thus, the upper limit for heart rate may emerge as a valuable, but simple predictor of optimal temperature in active animals, opening the possibility of using biotelemetry of heart rate in field situations to explore properly the full interplay of environmental factors on aerobic scope. An example of an ecological application of these physiological discoveries is provided using the upriver migration of adult sockeye salmon, which have a remarkable fidelity to their spawning areas and appear to have an optimum temperature for aerobic scope that corresponds to the river temperatures experienced by their antecedents. Unfortunately, there is evidence that this potential adaptation is incompatible with the rapid increase in river temperature presently experienced by salmon as a result of climate change. By limiting aerobic scope, river temperatures in excess of the optimum for aerobic scope directly impact upriver spawning migration and hence lifetime fecundity. Thus, use of aerobic scope holds promise for scientists who wish to make predictions on how climate change may influence animal distributions.

Influence of flexibility on the aerodynamic performance of a hovering wing.

Abstract: In the present study, a computational investigation was carried out to understand the influence of flexibility on the aerodynamic performance of a hovering wing. A flexible, two-dimensional, two-link model moving within a viscous fluid was considered. The Navier-Stokes equations governing the fluid dynamics were solved together with the equations governing the structural dynamics by using a strongly coupled fluid-structure interaction scheme. Harmonic kinematics was used to prescribe the motions of one of the links, thus effectively reducing the wing to a single degree-of-freedom oscillator. The wing's flexibility was characterized by the ratio of the flapping frequency to the natural frequency of the structure. Apart from the rigid case, different values of this frequency ratio (only in the range of 1/2 to 1/6) were considered at the Reynolds numbers of 75, 250 and 1000. It was found that flexibility can enhance aerodynamic performance and that the best performance is realized when the wing is excited by a non-linear resonance at 1/3 of the natural frequency. Specifically, at Reynolds numbers of 75, 250 and 1000, the aerodynamic performance that is characterized by the ratio of lift coefficient to drag coefficient is respectively increased by 28%, 23% and 21% when compared with the corresponding ratios of a rigid wing driven with the same kinematics. For all Reynolds numbers, the lift generated per unit driving power is also enhanced in a similar manner. The wake capture mechanism is enhanced, due to a stronger flow around the wing at stroke reversal, resulting from a stronger end of stroke vortex at the trailing edge. The present study provides some clues about how flexibility affects the aerodynamic performance in low Reynolds number flapping flight. In addition, it points to the importance of considering non-linear resonances for enhancing aerodynamic performance.

What is the function of neuroglobin

Abstract: For a long time, haemoglobin and myoglobin had been assumed to represent the only globin types of vertebrates. In 2000, however, we discovered a third globin type by mining the genome sequence data. Based on a preferential expression in the nervous system, this globin is referred to as neuroglobin. Despite nine years of research, its function is still uncertain and a number of hypotheses have been put forward. Neuroglobin enhances cell viability under hypoxia and under various types of oxidative stress in transgenic systems, but does not appear to be strongly upregulated in response to stress. A close phylogenetic relationship with invertebrate nerve globins and its positive correlation with the oxidative metabolism and mitochondria suggest a role in O(2) supply. In vitro studies and cell culture experiments imply that neuroglobin may detoxify reactive oxygen or nitric oxide. Still other studies propose neuroglobin as being part of a signalling chain that transmits the redox state of the cell or that inhibits apoptosis. Although some functions are more probable than others, we conclude that it is still too early to definitively decide what may be the physiological role(s) of neuroglobin in vertebrates. Nevertheless, there is no doubt that neuroglobin has an essential, conserved function and is beneficial to neurons.

Distinct freshwater and seawater isoforms of Na+/K+-ATPase in gill chloride cells of Atlantic salmon.

Abstract: Gill Na(+)/K(+)-ATPase (NKA) in teleost fishes is involved in ion regulation in both freshwater and seawater. We have developed and validated rabbit polyclonal antibodies specific to the NKA alpha1a and alpha1b protein isoforms of Atlantic salmon (Salmo salar Linnaeus), and used western blots and immunohistochemistry to characterize their size, abundance and localization. The relative molecular mass of NKA alpha1a is slightly less than that for NKA beta1b. The abundance of gill NKA alpha1a was high in freshwater and became nearly undetectable after seawater acclimation. NKA alpha1b was present in small amounts in freshwater and increased 13-fold after seawater acclimation. Both NKA isoforms were detected only in chloride cells. NKA alpha1a was located in both filamental and lamellar chloride cells in freshwater, whereas in seawater it was present only as a faint background in filamental chloride cells. In freshwater, NKA alpha1b was found in a small number of filamental chloride cells, and after seawater acclimation it was found in all chloride cells on the filament and lamellae. Double simultaneous immunofluorescence indicated that NKA alpha1a and alpha1b are located in different chloride cells in freshwater. In many chloride cells in seawater, NKA alpha1b was present in greater amounts in the subapical region than elsewhere in the cell. The combined patterns in abundance and immunolocalization of these two isoforms can explain the salinity-related changes in total NKA and chloride cell abundance. The results indicate that there is a freshwater and a seawater isoform of NKA alpha-subunit in the gills of Atlantic salmon and that they are present in distinct chloride cells.

Inhibitors of V-ATPases: old and new players.

Abstract: V-ATPases constitute a ubiquitous family of heteromultimeric, proton translocating proteins. According to their localization in a multitude of eukaryotic endomembranes and plasma membranes, they energize many different transport processes. Currently, a handful of specific inhibitors of the V-ATPase are known, which represent valuable tools for the characterization of transport processes on the level of tissues, single cells or even purified proteins. The understanding of how these inhibitors function may provide a basis to develop new drugs for the benefit of patients suffering from diseases such as osteoporosis or cancer. For this purpose, it appears absolutely essential to determine the exact inhibitor binding site in a target protein on the one side and to uncover the crucial structural elements of an inhibitor on the other side. However, even for some of the most popular and long known V-ATPase inhibitors, such as bafilomycin or concanamycin, the authentic structures of their binding sites are elusive. The aim of this review is to summarize the recent advances for the old players in the inhibition game, the plecomacrolides bafilomycin and concanamycin, and to introduce some of the new players, the macrolacton archazolid, the benzolactone enamides salicylihalamide, lobatamide, apicularen, oximidine and cruentaren, and the indolyls.

Control and function of arm swing in human walking and running

Abstract: We investigated the control and function of arm swing in human walking and running to test the hypothesis that the arms act as passive mass dampers powered by movement of the lower body, rather than being actively driven by the shoulder muscles. We measured locomotor cost, deltoid muscle activity and kinematics in 10 healthy adult subjects while walking and running on a treadmill in three experimental conditions: control; no arms (arms folded across the chest); and arm weights (weights worn at the elbow). Decreasing and increasing the moment of inertia of the upper body in no arms and arm weights conditions, respectively, had corresponding effects on head yaw and on the phase differences between shoulder and pelvis rotation, consistent with the view of arms as mass dampers. Angular acceleration of the shoulders and arm increased with torsion of the trunk and shoulder, respectively, but angular acceleration of the shoulders was not inversely related to angular acceleration of the pelvis or arm. Restricting arm swing in no arms trials had no effect on locomotor cost. Anterior and posterior portions of the deltoid contracted simultaneously rather than firing alternately to drive the arm. These results support a passive arm swing hypothesis for upper body movement during human walking and running, in which the trunk and shoulders act primarily as elastic linkages between the pelvis, shoulder girdle and arms, the arms act as passive mass dampers which reduce torso and head rotation, and upper body movement is primarily powered by lower body movement.

From cells to coastlines: how can we use physiology to forecast the impacts of climate change?

Abstract: The interdisciplinary fields of conservation physiology, macrophysiology, and mechanistic ecological forecasting have recently emerged as means of integrating detailed physiological responses to the broader questions of ecological and evolutionary responses to global climate change. Bridging the gap between large-scale records of weather and climate (as measured by remote sensing platforms, buoys and ground-based weather stations) and the physical world as experienced by organisms (niche-level measurements) requires a mechanistic understanding of how ;environmental signals' (parameters such as air, surface and water temperature, food availability, water flow) are translated into signals at the scale of the organism or cell (e.g. body temperature, food capture, hydrodynamic force, aerobic capacity). Predicting the impacts of how changing environments affect populations and ecosystems further mandates an understanding of how organisms ;filter' these signals via their physiological response (e.g. whether they respond to high or low frequencies, whether there is a time lag in response, etc.) and must be placed within the context of adult movement and the dispersal of larvae and gametes. Recent studies have shown that patterns of physiological stress in nature are far more complex in space and time than previously assumed and challenge the long-held paradigm that patterns of biogeographic distribution can be based on simple environmental gradients. An integrative, systems-based approach can provide an understanding of the roles of environmental and physiological variability in driving ecological responses and can offer considerable insight and predictive capacity to researchers, resource managers and policy makers involved in planning for the current and future effects of climate change.

Molecular physiology and genetics of Na+-independent SLC4 anion exchangers.

Abstract: Plasmalemmal Cl–/HCO3– exchangers are encoded by the SLC4 and SLC26 gene superfamilies, and function to regulate intracellular pH, [Cl–] and cell volume. The Cl–/HCO3– exchangers of polarized epithelial cells also contribute to transepithelial secretion and reabsorption of acid–base equivalents and Cl–. This review focuses on Na+-independent electroneutral Cl–/HCO3– exchangers of the SLC4 family. Human SLC4A1/AE1 mutations cause the familial erythroid disorders of spherocytic anemia, stomatocytic anemia and ovalocytosis. A largely discrete set of AE1 mutations causes familial distal renal tubular acidosis. The Slc4a2/Ae2–/– mouse dies before weaning with achlorhydria and osteopetrosis. A hypomorphic Ae2–/– mouse survives to exhibit male infertility with defective spermatogenesis and a syndrome resembling primary biliary cirrhosis. A human SLC4A3/AE3 polymorphism is associated with seizure disorder, and the Ae3–/– mouse has increased seizure susceptibility. The transport mechanism of mammalian SLC4/AE polypeptides is that of electroneutral Cl–/anion exchange, but trout erythroid Ae1 also mediates Cl– conductance. Erythroid Ae1 may mediate the DIDS-sensitive Cl– conductance of mammalian erythrocytes, and, with a single missense mutation, can mediate electrogenic SO42–/Cl– exchange. AE1 trafficking in polarized cells is regulated by phosphorylation and by interaction with other proteins. AE2 exhibits isoform-specific patterns of acute inhibition by acidic intracellular pH and independently by acidic extracellular pH. In contrast, AE2 is activated by hypertonicity and, in a pH-independent manner, by ammonium and by hypertonicity. A growing body of structure–function and interaction data, together with emerging information about physiological function and structure, is advancing our understanding of SLC4 anion exchangers.

Carbonic anhydrase and acid–base regulation in fish

Abstract: Carbonic anhydrase (CA) is the zinc metalloenzyme that catalyses the reversible reactions of CO(2) with water. CA plays a crucial role in systemic acid-base regulation in fish by providing acid-base equivalents for exchange with the environment. Unlike air-breathing vertebrates, which frequently utilize alterations of breathing (respiratory compensation) to regulate acid-base status, acid-base balance in fish relies almost entirely upon the direct exchange of acid-base equivalents with the environment (metabolic compensation). The gill is the critical site of metabolic compensation, with the kidney playing a supporting role. At the gill, cytosolic CA catalyses the hydration of CO(2) to H(+) and HCO(3)(-) for export to the water. In the kidney, cytosolic and membrane-bound CA isoforms have been implicated in HCO(3)(-) reabsorption and urine acidification. In this review, the CA isoforms that have been identified to date in fish will be discussed together with their tissue localizations and roles in systemic acid-base regulation.

The dual roles of red blood cells in tissue oxygen delivery: oxygen carriers and regulators of local blood flow.

Abstract: SUMMARY Vertebrate red blood cells (RBCs) seem to serve tissue oxygen delivery in two distinct ways. Firstly, RBCs enable the adequate transport of O 2 between respiratory surfaces and metabolizing tissues by means of their high intracellular concentration of hemoglobin (Hb), appropriate allosteric interactions between Hb ligand-binding sites, and an adjustable intracellular chemical environment that allows fine-tuning of Hb O 2 affinity. Secondly, RBCs may sense tissue O 2 requirements via their degree of deoxygenation when they travel through the microcirculation and release vasodilatory compounds that enhance blood flow in hypoxic tissues. This latter function could be important in matching tissue O 2 delivery with local O 2 demand. Three main mechanisms by which RBCs can regulate their own distribution in the microcirculation have been proposed. These are: (1) deoxygenation-dependent release of ATP from RBCs, which stimulates production of nitric oxide (NO) and other vasodilators in the endothelium; (2) release of vasoactive NO from S -nitroso-Hb upon deoxygenation; and (3) reduction of naturally occurring nitrite to vasoactive NO by deoxygenated Hb. This Commentary inspects all three hypotheses with regard to their mechanisms, experimental evidence in their support and details that remain unresolved. The prime focus is on human/mammalian models, where most evidence for a role of erythrocyte ATP and NO release in blood flow regulation have accumulated. Information from other vertebrate groups is integrated in the analysis and used to discuss the evolutionary origin and general relevance of each hypothesis.

Wetting properties on nanostructured surfaces of cicada wings

Abstract: This study has investigated the wettability of forewings of 15 species of cicadas, with distinctly different wetting properties related to their nanostructures. The wing surfaces exhibited hydrophilic or weak to strong hydrophobic properties with contact angles ranging from 76.8 deg. to 146.0 deg. The nanostructures (protrusions), observed using environmental scanning electron microscopy (ESEM), were classified into four types according to the patterning, diameter (82–148 nm), spacing (44–117 nm) and height (159–446 nm). Surface analysis by X-ray photoelectron spectroscopy (XPS) showed significant differences in wing membrane chemistry. Thus, wetting properties at the macroscopic scale were dependent on slight differences in nanoscale architecture and composition of the wax layer. This investigation offers insights into the diversity of nanostructuring and how subtle small-scale changes may facilitate large changes in wettability.

Built for speed: musculoskeletal structure and sprinting ability

Abstract: The musculoskeletal structure of the foot and ankle has the potential to influence human sprinting performance in complex ways. A large Achilles' tendon moment arm improves the mechanical advantage of the triceps surae but also produces larger shortening velocity during rapid plantarflexion, which detracts from the force-generating capacity of the plantarflexors. The lever arm of the ground reaction force that resists the muscular plantarflexor moment during propulsive push-off is constrained in part by the skeletal structure of the foot. In this study, we measured the plantarflexion moment arms of the Achilles' tendon, lateral gastrocnemius fascicle lengths and pennation angles, and anthropometric characteristics of the foot and lower leg in collegiate sprinters and height-matched non-sprinters. The Achilles' tendon moment arms of the sprinters were 25% smaller on average in sprinters than in non-sprinters (P<0.001) whereas the sprinters' fascicles were 11% longer on average (P=0.024). The ratio of fascicle length to moment arm was 50% larger in sprinters (P<0.001). Sprinters were found to have longer toes (P=0.032) and shorter lower legs (P=0.026) than non sprinters. A simple computer simulation of the sprint push-off demonstrated that shorter plantarflexor moment arms and longer toes, like those measured in sprinters, permit greater generation of forward impulse. Simulated propulsion was enhanced in both cases by increasing the ;gear ratio' of the foot, thus maintaining plantarflexor fibre length and reducing peak fibre shortening velocity. Longer toes especially prolonged the time of contact, giving greater time for forward acceleration by propulsive ground reaction force.

Ion uptake and acid secretion in zebrafish (Danio rerio).

Abstract: Transepithelial transport is one of the major processes involved in the mechanism of homeostasis of body fluids in vertebrates including fish. The current models of ion regulation in fish gill ionocytes have been proposed mainly based on studies in traditional model species like salmon, trout, tilapia, eel and killifish, but the mechanisms are still being debated due to the lack of convincing molecular physiological evidence. Taking advantage of plentiful genetic databases for zebrafish, we studied the molecular/cellular mechanisms of ion regulation in fish skin/gills. In our recently proposed model, there are at least three subtypes of ionocytes in zebrafish skin/gills: Na(+)-K(+)-ATPase-rich (NaR), Na(+)-Cl(-) cotransporter (NCC) and H(+)-ATPase-rich (HR) cells. Specific isoforms of transporters and enzymes have been identified as being expressed by these ionocytes: zECaC, zPMCA2 and zNCX1b by NaR cells; zNCC gill form by NCC cells; and zH(+)-ATPase, zNHE3b, zCA2-like a and zCA15a by HR cells. Serial molecular physiological experiments demonstrated the distinct roles of these ionocytes in the transport of various ions: HR, NaR and NCC cells are respectively responsible for acid secretion/Na(+) uptake, Ca(2+) uptake and Cl(-) uptake. The expression, regulation and function of transporters in HR and NaR cells are much better understood than those in NCC cells. The basolateral transport pathways in HR and NCC cells are still unclear, and the driving forces for the operations of apical NHE and NCC are another unresolved issue. Studies on zebrafish skin/gill ionocytes are providing new insights into fish ion-regulatory mechanisms, but the zebrafish model cannot simply be applied to other species because of species differences and a lack of sufficient molecular physiological evidence in other species.

Biofluiddynamic scaling of flapping, spinning and translating fins and wings

Abstract: Organisms that swim or fly with fins or wings physically interact with the surrounding water and air. The interactions are governed by the morphology and kinematics of the locomotory system that form boundary conditions to the Navier–Stokes (NS) equations. These equations represent Newton's law of motion for the fluid surrounding the organism. Several dimensionless numbers, such as the Reynolds number and Strouhal number, measure the influence of morphology and kinematics on the fluid dynamics of swimming and flight. There exists, however, no coherent theoretical framework that shows how such dimensionless numbers of organisms are linked to the NS equation. Here we present an integrated approach to scale the biological fluid dynamics of a wing that flaps, spins or translates. Both the morphology and kinematics of the locomotory system are coupled to the NS equation through which we find dimensionless numbers that represent rotational accelerations in the flow due to wing kinematics and morphology. The three corresponding dimensionless numbers are (1) the angular acceleration number, (2) the centripetal acceleration number, and (3) the Rossby number, which measures Coriolis acceleration. These dimensionless numbers consist of length scale ratios, which facilitate their geometric interpretation. This approach gives fundamental insight into the physical mechanisms that explain the differences in performance among flapping, spinning and translating wings. Although we derived this new framework for the special case of a model fly wing, the method is general enough to make it applicable to other organisms that fly or swim using wings or fins.

Powered ankle exoskeletons reveal the metabolic cost of plantar flexor mechanical work during walking with longer steps at constant step frequency

Abstract: We examined the metabolic cost of plantar flexor muscle-tendon mechanical work during human walking. Nine healthy subjects walked at constant step frequency on a motorized treadmill at speeds corresponding to 80% (1.00 m s(-1)), 100% (1.25 m s(-1)), 120% (1.50 m s(-1)) and 140% (1.75 m s(-1)) of their preferred step length (L(*)) at 1.25 m s(-1). In each condition subjects donned robotic ankle exoskeletons on both legs. The exoskeletons were powered by artificial pneumatic muscles and controlled using soleus electromyography (i.e. proportional myoelectric control). We measured subjects' metabolic energy expenditure and exoskeleton mechanics during both unpowered and powered walking to test the hypothesis that ankle plantarflexion requires more net metabolic power (W kg(-1)) at longer step lengths for a constant step frequency (i.e. preferred at 1.25 m s(-1)). As step length increased from 0.8 L(*) to 1.4 L(*), exoskeletons delivered approximately 25% more average positive mechanical power (P=0.01; +0.20+/-0.02 W kg(-1) to +0.25+/-0.02 W kg(-1), respectively). The exoskeletons reduced net metabolic power by more at longer step lengths (P=0.002; -0.21+/-0.06 W kg(-1) at 0.8 L(*) and -0.70+/-0.12 W kg(-1) at 1.4 L(*)). For every 1 J of exoskeleton positive mechanical work subjects saved 0.72 J of metabolic energy ('apparent efficiency'=1.39) at 0.8 L(*) and 2.6 J of metabolic energy ('apparent efficiency'=0.38) at 1.4 L(*). Declining ankle muscle-tendon ;apparent efficiency' suggests an increase in ankle plantar flexor muscle work relative to Achilles' tendon elastic energy recoil during walking with longer steps. However, previously stored elastic energy in Achilles' tendon still probably contributes up to 34% of ankle muscle-tendon positive work even at the longest step lengths we tested. Across the range of step lengths we studied, the human ankle muscle-tendon system performed 34-40% of the total lower-limb positive mechanical work but accounted for only 7-26% of the net metabolic cost of walking.

Do mitochondrial properties explain intraspecific variation in thermal tolerance

Abstract: As global temperatures rise, there is a growing need to understand the physiological mechanisms that determine an organism's thermal niche. Here, we test the hypothesis that increases in mitochondrial capacity with cold acclimation and adaptation are associated with decreases in thermal tolerance using two subspecies of killifish (Fundulus heteroclitus) that differ in thermal niche. We assessed whole-organism metabolic rate, mitochondrial amount and mitochondrial function in killifish acclimated to several temperatures. Mitochondrial enzyme activities and mRNA levels were greater in fish from the northern subspecies, particularly in cold-acclimated fish, suggesting that the putatively cold-adapted northern subspecies has a greater capacity for increases in mitochondrial amount in response to cold acclimation. When tested at the fish's acclimation temperature, maximum ADP-stimulated (State III) rates of mitochondrial oxygen consumption in vitro were greater in cold-acclimated northern fish than in southern fish but did not differ between subspecies at higher acclimation temperatures. Whole-organism metabolic rate was greater in fish of the northern subspecies at all acclimation temperatures. Cold acclimation also changed the response of mitochondrial respiration to acute temperature challenge. Mitochondrial oxygen consumption was greater in cold-acclimated northern fish than in southern fish at low test temperatures, but the opposite was true at high test temperatures. These differences were reflected in whole-organism oxygen consumption. Our data indicate that the plasticity of mitochondrial function and amount differs between killifish subspecies, with the less high-temperature tolerant, and putatively cold adapted, northern subspecies having greater ability to increase mitochondrial capacity in the cold. However, there were few differences in mitochondrial properties between subspecies at warm acclimation temperatures, despite differences in both whole-organism oxygen consumption and thermal tolerance at these temperatures.

Tracking stress: localisation, deposition and stability of corticosterone in feathers

Abstract: How animals cope with stressors is an important determinant of their well being and fitness Understanding what environmental perturbations are perceived as stressors, and quantifying how they are responded to, how often they occur and the negative consequences of exposure to glucocorticoids, has been problematic and limited to short-term physiological measures By contrast, the quantification of corticosterone (CORT) in feathers represents a long-term, integrated measure of hypothalamic-pituitary-adrenal activity In the present study, we show that by understanding how the hormone is deposited in feathers, in combination with specific sampling protocols, one can identify localised patterns of CORT deposition that reveal different temporal patterns of a bird's response to stressors CORT in feathers appears to be stable over time, is resistant to heat exposure and is useful in determining both the overall exposure of the bird to the hormone over days or weeks, as well as identifying discrete, punctuated, stressful events Variation in feather CORT can also be examined among individuals of a population at one point in time, as well as over years by using museum specimens The ability to track stress over time allows for new questions to be asked about the health and ecology of birds and their environment

The physiology of long-distance migration: extending the limits of endurance metabolism.

Abstract: SUMMARY Long-distance migrants have evolved specific adaptations that make their athletic records possible. Unique mechanisms explaining their amazing capacity for endurance exercise have now been uncovered, particularly with respect to energy storage, mobilization, transport and utilization. Birds are champions of migration because flying offers a key compromise: it allows more rapid movement than swimming, but has a lower cost of transport than running. High efficiency for muscle contraction, pointed wings, low wingloading, travelling in V-formations, storing fuel as energy-dense lipids and atrophy of non-essential organs are some of their strategies to decrease the cost of transport. The ability to process lipids rapidly also emerges as a crucial component of the migrant phenotype. High lipid fluxes are made possible by lipoprotein shuttles and fatty acid binding proteins (FABPs) that accelerate lipid transport and by upgrading the metabolic machinery for lipolysis and lipid oxidation. Preparation for long flights can include natural doping on n-3 polyunsaturated fatty acids (n-3 PUFAs) from unique invertebrate diets. Muscle performance is improved by restructuring membrane phospholipids and by activating key genes of lipid metabolism through peroxisome proliferator-activated receptors (PPARs). The physiological secret to long migrations does not depend on a single `magic9 adaptation but on the integration of multiple adjustments in morphology, biomechanics, behavior, nutrition and metabolism. Research on the physiology of migrants improves the fundamental knowledge of exercise biology, but it also has important implications for wildlife conservation, treating obesity and improving the performance of human athletes.

Walking, running and the evolution of short toes in humans

Abstract: The phalangeal portion of the forefoot is extremely short relative to body mass in humans. This derived pedal proportion is thought to have evolved in the context of committed bipedalism, but the benefits of shorter toes for walking and/or running have not been tested previously. Here, we propose a biomechanical model of toe function in bipedal locomotion that suggests that shorter pedal phalanges improve locomotor performance by decreasing digital flexor force production and mechanical work, which might ultimately reduce the metabolic cost of flexor force production during bipedal locomotion. We tested this model using kinematic, force and plantar pressure data collected from a human sample representing normal variation in toe length (N=25). The effect of toe length on peak digital flexor forces, impulses and work outputs was evaluated during barefoot walking and running using partial correlations and multiple regression analysis, controlling for the effects of body mass, whole-foot and phalangeal contact times and toe-out angle. Our results suggest that there is no significant increase in digital flexor output associated with longer toes in walking. In running, however, multiple regression analyses based on the sample suggest that increasing average relative toe length by as little as 20% doubles peak digital flexor impulses and mechanical work, probably also increasing the metabolic cost of generating these forces. The increased mechanical cost associated with long toes in running suggests that modern human forefoot proportions might have been selected for in the context of the evolution of endurance running.

Flexible clap and fling in tiny insect flight

Abstract: Of the insects that have been filmed in flight, those that are 1 mm in length or less often clap their wings together at the end of each upstroke and fling them apart at the beginning of each downstroke. This ;clap and fling' motion is thought to augment the lift forces generated during flight. What has not been highlighted in previous work is that very large forces are required to clap the wings together and to fling the wings apart at the low Reynolds numbers relevant to these tiny insects. In this paper, we use the immersed boundary method to simulate clap and fling in rigid and flexible wings. We find that the drag forces generated during fling with rigid wings can be up to 10 times larger than what would be produced without the effects of wing-wing interaction. As the horizontal components of the forces generated during the end of the upstroke and beginning of the downstroke cancel as a result of the motion of the two wings, these forces cannot be used to generate thrust. As a result, clap and fling appears to be rather inefficient for the smallest flying insects. We also add flexibility to the wings and find that the maximum drag force generated during the fling can be reduced by about 50%. In some instances, the net lift forces generated are also improved relative to the rigid wing case.

Embryonic temperature affects muscle fibre recruitment in adult zebrafish: genome-wide changes in gene and microRNA expression associated with the transition from hyperplastic to hypertrophic growth phenotypes.

Abstract: We investigated the effects of embryonic temperature (ET) treatments (22, 26 and 31 degrees C) on the life-time recruitment of fast myotomal muscle fibres in zebrafish Danio rerio L. reared at 26/27 degrees C from hatching. Fast muscle fibres were produced until 25 mm total length (TL) at 22 degrees C ET, 28 mm TL at 26 degrees C ET and 23 mm TL at 31 degrees C ET. The final fibre number (FFN) showed an optimum at 26 degrees C ET (3600) and was 19% and 14% higher than for the 22 degrees C ET (3000) and 31 degrees C ET (3100) treatments, respectively. Further growth to the maximum TL of approximately 48 mm only involved fibre hypertrophy. Microarray experiments were used to determine global changes in microRNA (miRNA) and mRNA expression associated with the transition from the hyperplasic myotube-producing phenotype (M(+), 10-12 mm TL) to the hypertrophic growth phenotype (M(-), 28-31 mm TL) in fish reared at 26-27 degrees C over the whole life-cycle. The expression of miRNAs and mRNAs obtained from microarray experiments was validated by northern blotting and real-time qPCR in independent samples of fish with the M(+) and M(-) phenotype. Fourteen down-regulated and 15 up-regulated miRNAs were identified in the M(-) phenotype together with 34 down-regulated and 30 up-regulated mRNAs (>2-fold; P<0.05). The two most abundant categories of down-regulated genes in the M(-) phenotype encoded contractile proteins (23.5%) and sarcomeric structural/cytoskeletal proteins (14.7%). In contrast, the most highly represented up-regulated transcripts in the M(-) phenotype were energy metabolism (26.7%) and immune-related (20.0%) genes. The latter were mostly involved in cell-cell interactions and cytokine pathways and included beta-2-microglobulin precursor (b2m), an orthologue of complement component 4, invariant chain-like protein 1 (iclp), CD9 antigen-like (cd9l), and tyrosine kinase, non-receptor (tnk2). Five myosin heavy chain genes that were down-regulated in the M(-) phenotype formed part of a tandem repeat on chromosome 5 and were shown by in situ hybridisation to be specifically expressed in nascent myofibres. Seven up-regulated miRNAs in the M(-) phenotype showed reciprocal expression with seven mRNA targets identified in miRBase Targets version 5 (http://microrna.sanger.ac.uk/targets/v5/), including asporin (aspn) which was the target for four miRNAs. Eleven down-regulated miRNAs in the M(-) phenotype had predicted targets for seven up-regulated genes, including dre-miR-181c which had five predicted mRNA targets. These results provide evidence that miRNAs play a role in regulating the transition from the M(+) to the M(-) phenotype and identify some of the genes and regulatory interactions involved.

Aquaporins: translating bench research to human disease.

Abstract: There is considerable potential for translating knowledge of aquaporin structure, function and physiology to the clinic. One area is in aquaporin-based diagnostics. The discovery of AQP4 autoantibodies as a marker of the neuromyelitis optica form of multiple sclerosis has allowed precise diagnosis of this disease. Other aquaporin-based diagnostics are possible. Another area is in aquaporin-based genetics. Genetic diseases caused by loss-of-function mutations in aquaporins include nephrogenic diabetes insipidus and cataracts, and functionally significant aquaporin polymorphisms are beginning to be explored. Perhaps of greatest translational potential is aquaporin-based therapeutics. Information largely from aquaporin knockout mice has implicated key roles of aquaporin-facilitated water transport in transepithelial fluid transport (urinary concentrating, gland fluid secretion), water movement into and out of the brain, cell migration (angiogenesis, tumor metastasis, wound healing) and neural function (sensory signaling, seizures). A subset of aquaporins that transport both water and glycerol, the `aquaglyceroporins', regulate glycerol content in epidermal, fat and other tissues, and are involved in skin hydration, cell proliferation, carcinogenesis and fat metabolism. Aquaporin-based modulator drugs are predicted to be of broad potential utility in the treatment of edematous states, cancer, obesity, wound healing, epilepsy and glaucoma. These exciting possibilities and their associated challenges are reviewed.

Visual control of flight speed in Drosophila melanogaster.

Abstract: Flight control in insects depends on self-induced image motion (optic flow), which the visual system must process to generate appropriate corrective steering maneuvers. Classic experiments in tethered insects applied rigorous system identification techniques for the analysis of turning reactions in the presence of rotating pattern stimuli delivered in open-loop. However, the functional relevance of these measurements for visual free-flight control remains equivocal due to the largely unknown effects of the highly constrained experimental conditions. To perform a systems analysis of the visual flight speed response under free-flight conditions, we implemented a `one-parameter open-loop' paradigm using `TrackFly' in a wind tunnel equipped with real-time tracking and virtual reality display technology. Upwind flying flies were stimulated with sine gratings of varying temporal and spatial frequencies, and the resulting speed responses were measured from the resulting flight speed reactions. To control flight speed, the visual system of the fruit fly extracts linear pattern velocity robustly over a broad range of spatio–temporal frequencies. The speed signal is used for a proportional control of flight speed within locomotor limits. The extraction of pattern velocity over a broad spatio–temporal frequency range may require more sophisticated motion processing mechanisms than those identified in flies so far. In Drosophila, the neuromotor pathways underlying flight speed control may be suitably explored by applying advanced genetic techniques, for which our data can serve as a baseline. Finally, the high-level control principles identified in the fly can be meaningfully transferred into a robotic context, such as for the robust and efficient control of autonomous flying micro air vehicles.

Redirection of center-of-mass velocity during the step-to-step transition of human walking

Abstract: Simple dynamic walking models based on the inverted pendulum predict that the human body's center of mass (COM) moves along an arc during each step, with substantial work performed to redirect the COM velocity in the step-to-step transition between arcs. But humans do not keep the stance leg perfectly straight and need not redirect their COM velocity precisely as predicted. We therefore tested a pendulum-based model against a wide range of human walking data. We examined COM velocity and work data from normal human subjects (N=10) walking at 24 combinations of speed (0.75 to 2.0 m s(-1)) and step length. These were compared against model predictions for the angular redirection of COM velocity and the work performed on the COM during redirection. We found that the COM is redirected through angular changes increasing approximately linearly with step length (R(2)=0.68), with COM work increasing with the squared product of walking speed and step length (R(2)=0.82), roughly in accordance with a simple dynamic walking model. This model cannot, however, predict the duration of COM redirection, which we quantified with two empirical measures, one based on angular COM redirection and the other on work. Both indicate that the step-to-step transition begins before and ends after double support and lasts about twice as long - approximately 20-27% of a stride. Although a rigid leg model can predict trends in COM velocity and work, the non-rigid human leg performs the step-to-step transition over a duration considerably exceeding that of double support.

Epithelial ultrastructure and cellular mechanisms of acid and base transport in the Drosophila midgut.

Abstract: SUMMARY There is a resurgence of interest in the Drosophila midgut on account of its potential value in understanding the structure, development and function of digestive organs and related epithelia. The recent identification of regenerative or stem cells in the adult gut of Drosophila has opened up new avenues for understanding development and turnover of cells in insect and mammalian gastrointestinal tracts. Conversely, the physiology of the Drosophila gut is less well understood as it is a difficult epithelial preparation to study under controlled conditions. Recent progress in microperfusion of individual segments of the Drosophila midgut, in both larval and adult forms, has enabled ultrastructural and electrophysiological study and preliminary characterization of cellular transport processes in the epithelium. As larvae are more active feeders, the transport rates are higher than in adults. The larval midgut has at least three segments: an anterior neutral zone, a short and narrow acid-secreting middle segment and a long and wider posterior segment (which is the best studied) that secretes base (probably HCO 3 – ) into the lumen. The posterior midgut has a lumen-negative transepithelial potential (35–45 mV) and a high resistance (800–1400 Ω.cm 2 ) that correlates with little or no lateral intercellular volume. The primary transport system driving base secretion into the lumen appears to be a bafilomycin-A 1 -sensitive, electrogenic H + V-ATPase located on the basal membrane, which extrudes acid into the haemolymph, as inferred from the extracellular pH gradients detected adjacent to the basal membrane. The adult midgut is also segmented (as inferred from longitudinal gradients of pH dye-indicators in the lumen) into anterior, middle and posterior regions. The anterior segment is probably absorptive. The middle midgut secretes acid (pH<4.0), a process dependent on a carbonic-anhydrase-catalysed H + pool. Cells of the middle segment are alternately absorptive (apically amplified by ≈9-fold, basally amplified by >90-fold) and secretory (apically amplified by >90-fold and basally by ≈10-fold). Posterior segment cells have an extensively dilated basal extracellular labyrinth, with a volume larger than that of anterior segment cells, indicating more fluid reabsorption in the posterior segment. The luminal pH of anterior and posterior adult midgut is 7–9. These findings in the larval and adult midgut open up the possibility of determining the role of plasma membrane transporters and channels involved in driving not only H + fluxes but also secondary fluxes of other solutes and water in Drosophila .

Estimating energy expenditure of animals using the accelerometry technique: activity, inactivity and comparison with the heart-rate technique

Abstract: SUMMARY Several methods have been used to estimate the energy expenditure of free-ranging animals. A relatively new technique uses measures of dynamic body acceleration as a calibrated proxy for energy expenditure and has proved an excellent predictor of energy expenditure in active animals. However, some animals can spend much of their time inactive and still expend energy at varying rates for a range of physiological processes. We tested the utility of dynamic body acceleration to estimate energy expenditure during a range of active (locomotion, eating) and inactive (digesting, thermoregulating) behaviours exhibited by domestic chickens. We also compared this technique with the more established heart-rate method for estimating energy expenditure. During activity, the error of estimation using body acceleration was very similar to that from the heart-rate method. Importantly, our results also showed that body acceleration can be used to estimate energy expenditure when birds are inactive. While the errors surrounding these estimates were greater than those during activity, and those made using the heart-rate method, they were less than those made using interspecific allometric equations. We highlight the importance of selecting a methodology that is appropriate for the life-history of the subject animal. We suggest that, to achieve the greatest possible accuracy and precision when estimating energy expenditure in free-ranging animals, the two techniques should be combined, and both heart rate ( f H ) and dynamic body acceleration could be included as covariates in predictive models. Alternatively, measures of acceleration can be used to ascertain which behaviour is being exhibited at each moment and hence which predictive model should be applied.