Biomedical subjects

Enrico L Rezende

Publications and source records attributed to Enrico L Rezende.

13 recordsLinked to original sources

PubMed2006

Baseline and stress-induced plasma corticosterone concentrations of mice selectively bred for high voluntary wheel running.

The hypothalamic-pituitary-adrenal (HPA) axis is important in regulating energy metabolism and in mediating responses to stressors, including increasing energy availability during physical exercise. In addition, glucocorticoids act directly on the central nervous system and influence behavior, including locomotor activity. To explore potential changes in the HPA axis as animals evolve higher voluntary activity levels, we characterized plasma corticosterone (CORT) concentrations and adrenal mass in four replicate lines of house mice that had been selectively bred for high voluntary wheel running (HR lines) for 34 generations and in four nonselected control (C) lines. We determined CORT concentrations under baseline conditions and immediately after exposure to a novel stressor (40 min of physical restraint) in mice that were housed without access to wheels. Resting daytime CORT concentrations were approximately twice as high in HR as in C mice for both sexes. Physical restraint increased CORT to similar concentrations in HR and C mice; consequently, the proportional response to restraint was smaller in HR than in C animals. Adrenal mass did not significantly differ between HR and C mice. Females had significantly higher baseline and postrestraint CORT concentrations and significantly larger adrenal glands than males in both HR and C lines. Replicate lines showed significant variation in body mass, length, baseline CORT concentrations, and postrestraint CORT concentrations in one or both sexes. Among lines, both body mass and length were significantly negatively correlated with baseline CORT concentrations, suggesting that CORT suppresses growth. Our results suggest that selection for increased locomotor activity has caused correlated changes in the HPA axis, resulting in higher baseline CORT concentrations and, possibly, reduced stress responsiveness and a lower growth rate.

Adrenal Glands↗

PubMed2006

Maximal oxygen consumption in relation to subordinate traits in lines of house mice selectively bred for high voluntary wheel running.

We studied relations between maximal O2 consumption (VO2 max) during forced exercise and subordinate traits associated with blood O2 transport and cellular respiration in four lines of mice selectively bred for high voluntary wheel running (S lines) and their four nonselected control (C) lines. Previously, we reported VO2 max of 59 females at three Po2 (hypoxia = 14% O2, normoxia = 21%, hyperoxia = 30%). Here, we test the hypothesis that variation in VO2 max can be explained, in part, by hemoglobin concentration and Po2 necessary to obtain 50% O2 saturation of Hb (an estimate of Hb affinity for O2) of the blood as well as citrate synthase activity and myoglobin concentration of ventricles and gastrocnemius muscle. Statistical analyses controlled for body mass, compared S and C lines, and also considered effects of the mini-muscle phenotype (present only in S lines and resulting from a Mendelian recessive allele), which reduces hindlimb muscle mass while increasing muscle mass-specific aerobic capacity. Although S lines had higher VO2 max than C, subordinate traits showed no statistical differences when the presence of the mini-muscle phenotype was controlled. However, subordinate traits did account for some of the individual variation in VO2 max. Ventricle size was a positive predictor of VO2 max at all three Po2. Blood Hb concentration was a positive predictor of VO2 max in S lines but a negative predictor in C lines, indicating that the physiological underpinnings of VO2 max have been altered by selective breeding. Mice with the mini-muscle phenotype had enlarged ventricles, with higher mass-specific citrate synthase activity and myoglobin concentration, which may account for their higher VO2 max in hypoxia.

Animals↗

PubMed2006

Maximum aerobic performance in lines of Mus selected for high wheel-running activity: effects of selection, oxygen availability and the mini-muscle phenotype.

We compared maximum aerobic capacity during forced exercise (VO2max) in hypoxia (PO2=14% O2), normoxia (21%) and hyperoxia (30%) of lines of house mice selectively bred for high voluntary wheel running (S lines) with their four unselected control (C) lines. We also tested for pleiotropic effects of the ;mighty mini-muscle' allele, a Mendelian recessive that causes a 50% reduction in hind limb muscle but a doubling of mass-specific aerobic enzyme activity, among other pleiotropic effects. VO2max of female mice was measured during forced exercise on a motorized treadmill enclosed in a metabolic chamber that allowed altered PO2. Individual variation in VO2max was highly repeatable within each PO2, and values were also significantly correlated across PO2. Analysis of covariance showed that S mice had higher body-mass-adjusted VO2max than C at all PO2, ranging from +10.7% in hypoxia to +20.8% in hyperoxia. VO2max of S lines increased practically linearly with PO2, whereas that of C lines plateaued from normoxia to hyperoxia, and respiratory exchange ratio (=CO2 production/VO2max) was lower for S lines. These results suggest that the physiological underpinnings of VO2max differ between the S and C lines. Apparently, at least in S lines, peripheral tissues may sustain higher rates of oxidative metabolism if central organs provide more O2. Although the existence of central limitations in S lines cannot be excluded based solely on the present data, we have previously reported that both S and C lines can attain considerably higher VO2max during cold exposure in a He-O2 atmosphere, suggesting that limitations on VO2max depend on interactions between the central and peripheral organs involved. In addition, mini-muscle individuals had higher VO2max than did those with normal muscles, suggesting that the former might have higher hypoxia tolerance. This would imply that the mini-muscle phenotype could be a good model to test how exercise performance and hypoxia tolerance could evolve in a correlated fashion, as previous researchers have suggested.

Analysis of Variance↗

PubMed2005

Effects of size, sex, and voluntary running speeds on costs of locomotion in lines of laboratory mice selectively bred for high wheel-running activity.

Selective breeding for over 35 generations has led to four replicate (S) lines of laboratory house mice (Mus domesticus) that run voluntarily on wheels about 170% more than four random-bred control (C) lines. We tested whether S lines have evolved higher running performance by increasing running economy (i.e., decreasing energy spent per unit of distance) as a correlated response to selection, using a recently developed method that allows for nearly continuous measurements of oxygen consumption (VO2) and running speed in freely behaving animals. We estimated slope (incremental cost of transport [COT]) and intercept for regressions of power (the dependent variable, VO2/min) on speed for 49 males and 47 females, as well as their maximum VO2 and speeds during wheel running, under conditions mimicking those that these lines face during the selection protocol. For comparison, we also measured COT and maximum aerobic capacity (VO2max) during forced exercise on a motorized treadmill. As in previous studies, the increased wheel running of S lines was mainly attributable to increased average speed, with males also showing a tendency for increased time spent running. On a whole-animal basis, combined analysis of males and females indicated that COT during voluntary wheel running was significantly lower in the S lines (one-tailed P=0.015). However, mice from S lines are significantly smaller and attain higher maximum speeds on the wheels; with either body mass or maximum speed (or both) entered as a covariate, the statistical significance of the difference in COT is lost (one-tailed P> or =0.2). Thus, both body size and behavior are key components of the reduction in COT. Several statistically significant sex differences were observed, including lower COT and higher resting metabolic rate in females. In addition, maximum voluntary running speeds were negatively correlated with COT in females but not in males. Moreover, males (but not females) from the S lines exhibited significantly higher treadmill VO2max as compared to those from C lines. The sex-specific responses to selection may in part be consequences of sex differences in body mass and running style. Our results highlight how differences in size and running speed can account for lower COT in S lines and suggest that lower COT may have coadapted in response to selection for higher running distances in these lines.

Animals↗

PubMed2005

Contractile abilities of normal and "mini" triceps surae muscles from mice (Mus domesticus) selectively bred for high voluntary wheel running.

As reported previously, artificial selection of house mice caused a 2.7-fold increase in voluntary wheel running of four replicate selected lines compared with four random-bred control lines. Two of the selected lines developed a high incidence of a small-muscle phenotype ("mini muscles") in the plantar flexor group of the hindlimb, which apparently results from a simple Mendelian recessive allele. At generations 36-38, we measured wheel running and key contractile characteristics of soleus and medial gastrocnemius muscles from normal and mini muscles in mice from these selected lines. Mice with mini muscles ran faster and a greater distance per day than normal individuals but not longer. As expected, in mini-muscle mice the medial and lateral gastrocnemius muscles were approximately 54 and 45% the mass of normal muscles, respectively, but the plantaris muscles were not different in mass and soleus muscles were actually 30% larger. In spite of the increased mass, contractile characteristics of the soleus were unchanged in any notable way between mini and normal mice. However, medial gastrocnemius muscles in mini mice were changed markedly toward a slower phenotype, having slower twitches; demonstrated a more curved force-velocity relationship; produced about half the mass-specific isotonic power, 20-50% of the mass-specific cyclic work and power (only 10-25% the absolute power if the loss in mass is considered); and fatigued at about half the rate of normal muscles. These changes would promote increased, aerobically supported running activity but may compromise activities that require high power, such as sprinting.

Animal Husbandry↗

PubMed2005

Maximal metabolic rates during voluntary exercise, forced exercise, and cold exposure in house mice selectively bred for high wheel-running.

Selective breeding for high wheel-running activity has generated four lines of laboratory house mice (S lines) that run about 170% more than their control counterparts (C lines) on a daily basis, mostly because they run faster. We tested whether maximum aerobic metabolic rates (V(O2max)) have evolved in concert with wheel-running, using 48 females from generation 35. Voluntary activity and metabolic rates were measured on days 5+6 of wheel access (mimicking conditions during selection), using wheels enclosed in metabolic chambers. Following this, V(O2max) was measured twice on a motorized treadmill and twice during cold-exposure in a heliox atmosphere (HeO2). Almost all measurements, except heliox V(O2max), were significantly repeatable. After accounting for differences in body mass (S < C) and variation in age at testing, S and C did not differ in V(O2max) during forced exercise or in heliox, nor in maximal running speeds on the treadmill. However, running speeds and V(O2max) during voluntary exercise were significantly higher in S lines. Nevertheless, S mice never voluntarily achieved the V(O2max) elicited during their forced treadmill trials, suggesting that aerobic capacity per se is not limiting the evolution of even higher wheel-running speeds in these lines. Our results support the hypothesis that S mice have genetically higher motivation for wheel-running and they demonstrate that behavior can sometimes evolve independently of performance capacities. We also discuss the possible importance of domestication as a confounding factor to extrapolate results from this animal model to natural populations.

Age Factors↗

PubMed2005

Phylogenetic approaches in comparative physiology.

Over the past two decades, comparative biological analyses have undergone profound changes with the incorporation of rigorous evolutionary perspectives and phylogenetic information. This change followed in large part from the realization that traditional methods of statistical analysis tacitly assumed independence of all observations, when in fact biological groups such as species are differentially related to each other according to their evolutionary history. New phylogenetically based analytical methods were then rapidly developed, incorporated into ;the comparative method', and applied to many physiological, biochemical, morphological and behavioral investigations. We now review the rationale for including phylogenetic information in comparative studies and briefly discuss three methods for doing this (independent contrasts, generalized least-squares models, and Monte Carlo computer simulations). We discuss when and how to use phylogenetic information in comparative studies and provide several examples in which it has been helpful, or even crucial, to a comparative analysis. We also consider some difficulties with phylogenetically based statistical methods, and of comparative approaches in general, both practical and theoretical. It is our personal opinion that the incorporation of phylogeny information into comparative studies has been highly beneficial, not only because it can improve the reliability of statistical inferences, but also because it continually emphasizes the potential importance of past evolutionary history in determining current form and function.

Computer Simulation↗

PubMed2004

Climatic adaptation and the evolution of basal and maximum rates of metabolism in rodents.

Metabolic rate is a key aspect of organismal biology and the identification of selective factors that have led to species differences is a major goal of evolutionary physiology. We tested whether environmental characteristics and/or diet were significant predictors of interspecific variation in rodent metabolic rates. Mass-specific basal metabolic rates (BMR) and maximum metabolic rates (MMR, measured during cold exposure in a He-O2 atmosphere) were compiled from the literature. Maximum (Tmax) and minimum (Tmin) annual mean temperatures, latitude, altitude, and precipitation were obtained from field stations close to the capture sites reported for each population (N = 57). Diet and all continuous-valued traits showed statistically significant phylogenetic signal, with the exception of mass-corrected MMR and altitude. Therefore, results of phylogenetic analyses are emphasized. Body mass was not correlated with absolute latitude, but was positively correlated with precipitation in analyses with phylogenetically independent contrasts. Conventional multiple regressions that included body mass indicated that Tmax (best), Tmin, latitude, and diet were significant additional predictors of BMR. However, phylogenetic analyses indicated that latitude was the only significant predictor of mass-adjusted BMR (positive partial regression coefficient, one-tailed P = 0.0465). Conventional analyses indicated that Tmax, Tmin (best), and altitude explained significant amounts of the variation in mass-adjusted MMR. With body mass and Tmin in the model, no additional variables were significant predictors. Phylogenetic contrasts yielded similar results. Both conventional and phylogenetic analyses indicated a highly significant positive correlation between residual BMR and MMR (as has also been reported for birds), which is consistent with a key assumption of the aerobic capacity model for the evolution of vertebrate energetics (assuming that MMR and exercise-induced maximal oxygen consumption are positively functionally related). Our results support the hypothesis that variation in environmental factors leads to variation in the selective regime for metabolic rates of rodents. However, the causes of a positive association between BMR and latitude remain obscure. Moreover, an important area for future research will be experiments in all taxa are raised under common conditions to allow definitive tests of climatic adaptation in endotherm metabolic rates and to elucidate the extent of adaptive phenotypic plasticity.

Animals↗

PubMed2004

Cold-acclimation in Peromyscus: temporal effects and individual variation in maximum metabolism and ventilatory traits.

Thermal acclimation in small endotherms provides an excellent model for the study of physiological plasticity, as energy requirements can be easily manipulated and the results are relevant for natural conditions. Nevertheless, how physiology changes throughout acclimation, and how individuals vary in their response to acclimation, remain poorly understood. Here we describe a high temporal-resolution study of cold acclimation in the deer mouse Peromyscus maniculatus. The experimental design was based on repeated measures at short intervals throughout cold acclimation, with controls (maintained at constant temperature) for measurement artifacts. We monitored body mass, maximum metabolic rate in cold exposure and ventilatory traits (respiratory frequency, tidal and minute volume and oxygen extraction) for 3 weeks at 23 degrees C. Then, half of the individuals were held for 7 weeks at 5 degrees C. Body mass was differently affected by cold acclimation depending on sex. Maximal metabolism ((O(2)max)) increased significantly during the first week of cold acclimation, "overshot" after 5 weeks and dropped to a plateau about 34% above control values at week 7. Similarly, ventilatory traits increased during cold acclimation, though responses were different in their kinetics and magnitude. Body mass, maximum metabolism, and most ventilatory traits were repeatable after 7 weeks in control and cold-acclimated animals. However, repeatability tended to be lower in the cold-acclimated group, especially while animals were still acclimating. Our results show that acclimation effects may be under- and/or overestimated, depending on when trials are performed, and that different traits respond differently, and at different rates, to acclimation. Hence, future studies should be designed to ensure that animals have attained steady-state values in acclimation experiments.

Acclimatization↗

PubMed2004

Voluntary running in deer mice: speed, distance, energy costs and temperature effects.

The energetics of terrestrial locomotion are of considerable interest to ecologists and physiologists, but nearly all of our current knowledge comes from animals undergoing forced exercise. To explore patterns of energy use and behavior during voluntary exercise, we developed methods allowing nearly continuous measurements of metabolic rates in freely behaving small mammals, with high temporal resolution over periods of several days. We used this approach to examine relationships between ambient temperature (Ta), locomotor behavior and energy costs in the deer mouse, a small mammal that routinely encounters a large range of temperatures in its natural habitat. We tested for individual consistency in running behavior and metabolic traits, and determined how locomotor costs vary with speed and Ta. Because of the importance of thermoregulatory costs in small mammals, we checked for substitution of exercise heat for thermostatic heat production at Ta below the thermal neutral zone and determined the fraction of the daily energy budget comprising exercise costs. Locomotor behavior was highly variable among individuals but had high repeatability, at least over short intervals. We found few temperature-related changes in speed or distance run, but Ta strongly affected energy costs. Partial substitution of exercise heat for thermogenic heat occurred at low Ta. This reduced energy expenditure during low-temperature running by 23-37%, but running costs comprised a fairly minor fraction of the energy budget, so the daily energy savings via substitution were much smaller. Deer mice did not adjust running speed to maximize metabolic economy, as they seldom used the high speeds that provide the lowest cost of transport. The highest voluntary speeds (4-5 km h(-1)) were almost always below the predicted maximal aerobic speed, and were much less than the species' maximal sprint speed. Maximum voluntarily attained rates of oxygen consumption (VO2) were highest at low Ta, but rarely approached maximal VO2 during forced treadmill exercise. Mean respiratory exchange ratios coincident with maximal voluntary VO2 increased slightly as Ta declined, but were always below 1.0 (another indication that metabolic rate was less than the aerobic maximum). Individuals with high running performance (cumulative distance and running time) had high resting metabolism, which suggests a cost of having high capacity or propensity for activity.

Animals↗

PubMed2003

Age and aerobic performance in deer mice.

Age impacts the phenotype of all multicellular animals, but lifetime changes in physiological traits are poorly understood for all but a few species. Here, we describe a cross-sectional study of age effects on body composition, aerobic performance and ventilation in deer mice Peromyscus maniculatus. This species lives considerably longer in captivity (in excess of 5 years) than most laboratory rodents, and the adaptational biology of its aerobic physiology is well studied. Our deer mice grew throughout life, and, as is typical for mammals, their basal metabolic rate (BMR) and maximal oxygen consumption in exercise ((VO(2)max)) and thermogenesis ((VO(2)sum)) increased as power functions of mass. Age did not affect BMR, but we found abrupt decreases in growth rate, (VO(2)max) and (VO(2)sum) at approximately 485 days of age, and the mass-adjusted maximal aerobic performance of old mice (5 years of age) was 20% ((VO(2)max)) to 35% ((VO(2)sum)) less than that of young animals. Breathing frequency (f) and oxygen extraction (E(O(2))) also declined with age but did not change abruptly. However, there were no consistent age-related changes in tidal volume (V(T)) or minute volume ((Vmin)) after accounting for the effects of mass and (VO(2)sum). Age influenced several aspects of body composition (lean and fat mass). However, these changes were insufficient to explain the age-related declines in aerobic performance, suggesting that mass-specific oxidative capacity of lean tissue decreased with age. The performance changes we found could engender substantial reductions in the mobility and thermal tolerances of old deer mice. However, very few wild mice are likely to survive to ages where substantial performance decreases occur, so these declines are probably not subjected to strong selection in natural populations.

Acclimatization↗

PubMed2002

Passerines versus nonpasserines: so far, no statistical differences in the scaling of avian energetics.

We analyzed and compared the scaling of both basal and maximal thermogenic metabolic rates in passerine and nonpasserine birds using conventional and phylogenetic methods. In spite of the presumed adaptive importance of both metabolic traits, few studies concerning both their relationships and their ecological and evolutionary constraints have been conducted. We found no statistical differences in the scaling of maximal metabolic rate between passerines and nonpasserines; hence, we suggest the use of a single allometric regression for this trait in birds. In addition, basal and maximal metabolic rates were indeed correlated after removing the effects of body mass and phylogeny. The apparent generality of this correlation within both birds and mammals reinforces the need for general ecological and physiological explanations for the evolution of endothermy.

Animals↗

PubMedDate not supplied

Dynamic thermal balance in the leaf-eared mouse: the interplay among ambient temperature, body size, and behavior.

Endotherms maintain constant body temperature through physiological and behavioral adjustments. Behavioral thermoregulation is an important factor influencing energy balance. We exposed the leaf-eared mouse, Phyllotis darwini, to temperatures corresponding to its natural thermal range and studied two forms of behavioral thermoregulation: diminishing surface to volume ratio by huddling and heat dissipation by increasing physical contact with the substrate (flattening). We predicted that at low ambient temperatures (T(a)) huddling would be used as a heat conservation mechanism and at high T(a) flattening would be used for heat loss. We simultaneously measured oxygen consumption (VO2) and flattening, in response to three independent factors: huddling, T(a), and body mass. Each experiment was a 6-h VO2 trial where five virgin females were measured at constant T(a). We performed this protocol for two body mass groups, small (ca. 40 g) and large (ca. 70 g), in a metabolic chamber. Treatments were groups with and without the ability to huddle at five different T(a), ranging from 5 degrees to 35 degrees C. A significant interaction between all three factors was found. Huddling and flattening were used as strategies for conserving or dissipating heat, respectively, and the shift between both strategies occurred at the lower limit of thermoneutrality. At T(a) below thermoneutrality, huddling was a more effective way of reducing metabolic requirements and was more efficient (H(E)) in small individuals than large individuals. So, by huddling, small individuals save more energy. At high T(a), flattening appeared to be an equally useful mechanism for heat loss, for both large and small animals.

Analysis of Variance↗