Timothy M. Healy

Thermal Performance Curves, Phenotypic Plasticity, and the Time Scales of Temperature Exposure

Thermal performance curves (TPCs) describe the effects of temperature on biological rate processes. Here, we use examples from our work on common killifish (Fundulus heteroclitus) to illustrate some important conceptual issues relating to TPCs in the context of using these curves to predict the responses of organisms to climate change. Phenotypic plasticity has the capacity to alter the shape and position of the TPCs for acute exposures, but these changes can be obscured when rate processes are measured only following chronic exposures. For example, the acute TPC for mitochondrial respiration in killifish is exponential in shape, but this shape changes with acclimation. If respiration rate is measured only at the acclimation temperature, the TPC is linear, concealing the underlying mechanistic complexity at an acute time scale. These issues are particularly problematic when attempting to use TPCs to predict the responses of organisms to temperature change in natural environments. Many TPCs are generated using laboratory exposures to constant temperatures, but temperature fluctuates in the natural environment, and the mechanisms influencing performance at acute and chronic time scales, and the responses of the performance traits at these time scales may be quite different. Unfortunately, our current understanding of the mechanisms underlying the responses of organisms to temperature change is incomplete, particularly with respect to integrating from processes occurring at the level of single proteins up to whole-organism functions across different time scales, which is a challenge for the development of strongly grounded mechanistic models of responses to global climate change.

Rapid evolution of cold tolerance in stickleback

Climate change is predicted to lead to increased average temperatures and greater intensity and frequency of high and low temperature extremes, but the evolutionary consequences for biological communities are not well understood. Studies of adaptive evolution of temperature tolerance have typically involved correlative analyses of natural populations or artificial selection experiments in the laboratory. Field experiments are required to provide estimates of the timing and strength of natural selection, enhance understanding of the genetics of adaptation and yield insights into the mechanisms driving evolutionary change. Here, we report the experimental evolution of cold tolerance in natural populations of threespine stickleback fish (Gasterosteus aculeatus). We show that freshwater sticklebacks are able to tolerate lower minimum temperatures than marine sticklebacks and that this difference is heritable. We transplanted marine sticklebacks to freshwater ponds and measured the rate of evolution after three generations in this environment. Cold tolerance evolved at a rate of 0.63 haldanes to a value 2.5°C lower than that of the ancestral population, matching values found in wild freshwater populations. Our results suggest that cold tolerance is under strong selection and that marine sticklebacks carry sufficient genetic variation to adapt to changes in temperature over remarkably short time scales.

Thermal Acclimation Is Not Necessary to Maintain a Wide Thermal Breadth of Aerobic Scope in the Common Killifish (Fundulus heteroclitus)

Loss of aerobic scope at high and low temperatures is a physiological mechanism proposed to limit the thermal performance and tolerance of organisms, a theory known as oxygen- and capacity-limited thermal tolerance (OCLTT). Eurythermal organisms maintain aerobic scope over wide ranges of temperatures, but it is unknown whether acclimation is necessary to maintain this breadth. The objective of this study was to examine changes in aerobic scope in Fundulus heteroclitus, a eurythermal fish, after acclimation and acute exposure to temperatures from 5° to 33°C. The range of temperatures over which aerobic scope was nonzero was similar in acclimated and acutely exposed fish, suggesting that acclimation has modest effects on the thermal breadth of aerobic scope. However, in acclimated fish, there was a clear optimum temperature range for aerobic scope between 25° and 30°C, whereas aerobic scope was relatively constant across the entire temperature range with acute temperature exposure. Therefore, the primary effect of acclimation was to increase aerobic scope between 25° and 30°C, which paradoxically resulted in a narrower temperature range of optimal performance in acclimated fish compared to acutely exposed fish. There was only weak evidence for correlations between the thermal optimum of aerobic scope and the thermal optimum of measures of performance (specific growth rate and gonadosomatic index), and indicators of anaerobic metabolism (lactate accumulation and lactate dehydrogenase activity) only increased at high temperatures. Together these data fit many, but not all, of the predictions made by OCLTT.

Application of an adaptive control grid interpolation technique to morphological vascular reconstruction

The problem of interslice magnetic resonance (MR) image reconstruction arises in a broad range of medical applications. In such cases, there is a need to approximate information present in the original subject that is not reflected in contiguously acquired MR images because of hardware sampling limitations. In the context of vascular morphology reconstruction, this information is required in order for subsequent visualization and computational analysis of blood vessels to be most effective. Toward that end we have developed a method of vascular morphology reconstruction based on adaptive control grid interpolation (ACGI) to function as a precursor to visualization and computational analysis. ACGI has previously been implemented in addressing various problems including video coding and tracking. This paper focuses on the novel application of the technique to medical image processing. ACGI combines features of optical flow-based and block-based motion estimation algorithms to enhance insufficiently dense MR data sets accurately with a minimal degree of computational complexity. The resulting enhanced data sets describe vascular geometries. These reconstructions can then be used as visualization tools and in conjunction with computational fluid dynamics (CFD) simulations to offer the pressure and velocity information necessary to quantify power loss. The proposed ACGI methodology is envisioned ultimately to play a role in surgical planning aimed at producing optimal vascular configurations for successful surgical outcomes.

Numerical Simulation of Flow in Mechanical Heart Valves: Grid Resolution and the Assumption of Flow Symmetry

A numerical method is developed for simulating unsteady, 3-D, laminar flow through a bileaflet mechanical heart valve with the leaflets fixed. The method employs a dual-time-stepping artificial-compressibility approach together with overset (Chimera) grids and is second-order accurate in space and time. Calculations are carried out for the full 3-D valve geometry under steady inflow conditions on meshes with a total number of nodes ranging from 4 x 10(5) to 1.6 x 10(6). The computed results show that downstream of the leaflets the flow is dominated by two pairs of counter-rotating vortices, which originate on either side of the central orifice in the aortic sinus and rotate such that the common flow of each pair is directed away from the aortic wall. These vortices intensify with Reynolds number, and at a Reynolds number of approximately 1200 their complex interaction leads to the onset of unsteady flow and the break of symmetry with respect to both geometric planes of symmetry. Our results show the highly 3-D structure of the flow; question the validity of computationally expedient assumptions of flow symmetry; and demonstrate the need for highly resolved, fully 3-D simulations if computational fluid dynamics is to accurately predict the flow in prosthetic mechanical heart valves.

Responses to temperature and hypoxia as interacting stressors in fish: implications for adaptation to environmental change.

Anthropogenic environmental change is exposing animals to changes in a complex array of interacting stressors and is already having important effects on the distribution and abundance of species. However, despite extensive examination of the effects of stressors in isolation, knowledge of the effects of stressors in combination is limited. This lack of information makes predicting the responses of organisms to anthropogenic environmental change challenging. Here, we focus on the effects of temperature and hypoxia as interacting stressors in fishes. A review of the available evidence suggests that temperature and hypoxia act synergistically such that small shifts in one stressor could result in large effects on organismal performance when a fish is exposed to the 2 stressors in combination. Although these stressors pose substantial challenges for fish, there also is substantial intraspecific variation in tolerance to these stressors that could act as the raw material for the evolution of improved tolerance. However, the potential for adaptive change is, in part, dependent on the nature of the correlations among traits associated with tolerance. For example, negative genetic correlations (or trade-offs) between tolerances to temperature and hypoxia could limit the potential for adaptation to the combined stressors, while positive genetic correlations might be of benefit. The limited data currently available suggest that tolerances to hypoxia and to high-temperature may be positively correlated in some species of fish, suggesting the possibility for adaptive evolution in these traits in response to anthropogenic environmental change.

Heat shock response of killifish (Fundulus heteroclitus): candidate gene and heterologous microarray approaches

Northern and southern subspecies of the Atlantic killifish, Fundulus heteroclitus, differ in maximal thermal tolerance. To determine whether these subspecies also differ in their heat shock response (HSR), we exposed 20°C acclimated killifish to a 2 h heat shock at 34°C and examined gene expression in fish from both subspecies during heat shock and recovery using real-time quantitative PCR and a heterologous cDNA microarray designed for salmonid fishes. The heat shock proteins Hsp70-1, hsp27, and hsp30 were upregulated to a greater extent in the high temperature-tolerant southern subspecies than in the less tolerant northern subspecies, whereas hsp70-2 (which showed the largest upregulation of all the heat shock proteins) in both gill and muscle and hsp90α in muscle was upregulated to a greater extent in northern than in southern fish. These data demonstrate that differences in the HSR between subspecies cannot be due to changes in a single global regulator but must occur via gene-specific mechanisms. They also suggest that the role, if any, of hsps in establishing thermal tolerance is complex and varies from gene to gene. Heterologous microarray hybridization provided interpretable gene expression signatures, detecting differential regulation of genes known to be involved in the heat shock response in other species. Under control conditions, a variety of genes were differentially expressed in muscle between subspecies that suggest differences in muscle fiber type and could relate to previously observed differences between subspecies in the thermal sensitivity of swimming performance and metabolism.

In vivo flow dynamics of the total cavopulmonary connection from three-dimensional multislice magnetic resonance imaging ☆

BACKGROUND The total cavopulmonary connection (TCPC) design continues to be refined on the basis of flow analysis at the connection site. These refinements are of importance for myocardial energy conservation in the univentricular supported circulation. In vivo magnetic resonance phase contrast imaging provides semiquantitative flow visualization information. The purpose of this study was to understand the in vivo TCPC flow characteristics obtained by magnetic resonance phase contrast imaging and compare the results with our previous in vitro TCPC flow experiments in an effort to further refine TCPC surgical design. METHODS Twelve patients with TCPC underwent sedated three-dimensional, multislice magnetic resonance phase contrast imaging. Seven patients had intraatrial lateral tunnel TCPC and 5 had extracardiac TCPC. RESULTS In all patients in both groups a disordered flow pattern was observed in the inferior caval portion of the TCPC. Flow at the TCPC site appeared to be determined by connection geometry, being streamlined at the superior vena cava-pulmonary junction when the superior vena cava was offset and flared toward the left pulmonary artery. Without caval offset, intense swirling and dominance of superior vena caval flow was observed. In TCPC with bilateral superior vena cavae, the flow patterns observed included secondary vortices, a central stagnation point, and influx of the superior vena cava flow into the inferior caval conduit. A comparative analysis of in vivo flow and our previous in vitro flow data from glass model prototypes of TCPC demonstrated significant similarities in flow disturbances. Three-dimensional magnetic resonance phase contrast imaging in multiple coronal planes enabled a comprehensive semiquantitative flow analysis. The data are presented in traditional instantaneous images and in animated format for interactive display of the flow dynamics. CONCLUSIONS Flow in the inferior caval portion of the TCPC is disordered, and the TCPC geometry determines flow characteristics.

Fluid Mechanic Assessment of the Total Cavopulmonary Connection using Magnetic Resonance Phase Velocity Mapping and Digital Particle Image Velocimetry

AbstractThe total cavopulmonary connection (TCPC) is currently the most promising modification of the Fontan surgical repair for single ventricle congenital heart disease. The TCPC involves a surgical connection of the superior and inferior vena cavae directly to the left and right pulmonary arteries, bypassing the right heart. In the univentricular system, the ventricle experiences a workload which may be reduced by optimizing the cavae-to-pulmonary anastomosis. The hypothesis of this study was that the energetic efficiency of the connection is a consequence of the fluid dynamics which develop as a function of connection geometry. Magnetic resonance phase velocity mapping (MRPVM) and digital particle image velocimetry (DPIV) were used to evaluate the flow patterns in vitro in three prototype glass models of the TCPC: flared zero offset, flared 14 mm offset, and straight 21 mm offset. The flow field velocity along the symmetry plane of each model was chosen to elucidate the fluid mechanics of the connection as a function of the connection geometry and pulmonary artery flow split. The steady flow experiments were conducted at a physiologic cardiac output (4 L/min) over three left/right pulmonary flow splits (70/30, 50/50, and 30/70) while keeping the superior/inferior vena cavae flow ratio constant at 40/60. MRPVM, a noninvasive clinical technique for measuring flow field velocities, was compared to DPIV, an established in vitro fluid mechanic technique. A comparison between the results from both techniques showed agreement of large scale flow features, despite some discrepancies in the detailed flow fields. The absence of caval offset in the flared zero offset model resulted in significant caval flow collision at the connection site. In contrast, offsetting the cavae reduced the flow interaction and caused a vortex-like low velocity region between the caval inlets as well as flow disturbance in the pulmonary artery with the least total flow. A positive correlation was also found between the direct caval flow collision and increased power losses. MRPVM was able to elucidate these important fluid flow features, which may be important in future modifications in TCPC surgical designs. Using MRPVM, two- and three-directional velocity fields in the TCPC could be quantified. Because of this, MRPVM has the potential to provide accurate velocity information clinically and, thus, to become the in vivo tool for TCPC patient physiological/functional assessment.

Noninvasive fluid dynamic power loss assessments for total cavopulmonary connections using the viscous dissipation function: a feasibility study.

The total cavopulmonary connection (TCPC) has shown great promise as an effective palliation for single-ventricle congenital heart defects. However, because the procedure results in complete bypass of the right-heart, fluid dynamic power losses may play a vital role in postoperative patient success. Past research has focused on determining power losses using control volume methods. Such methods are not directly applicable clinically without highly invasive pressure measurements. This work proposes the use of the viscous dissipation function as a tool for velocity gradient based estimation of fluid dynamic power loss. To validate this technique, numerical simulations were conducted in a model of the TCPC incorporating a 13.34 mm (one caval diameter) caval offset and a steady cardiac output of 2 L x min(-1). Inlet flow through the superior vena cava was 40 percent of the cardiac output, while outflow through the right pulmonary artery (RPA) was varied between 30 and 70 percent, simulating different blood flow distributions to the lungs. Power losses were determined using control volume and dissipation function techniques applied to the numerical data. Differences between losses computed using these techniques ranged between 3.2 and 9.9 percent over the range of RPA outflows studied. These losses were also compared with experimental measurements front a previous study. Computed power losses slightly exceeded experimental results due to different inlet flow conditions. Although additional experimental study is necessary to establish the clinical applicability of the dissipation function, it is believed that this method, in conjunction with velocity gradient information derived from imaging modalities such as magnetic resonance imaging, can provide a noninvasive means of assessing power losses within the TCPC in vivo.