Heart rate as an indicator of activity, metabolic rate, food intake and digestion in pike, Esox lucius

For pike in the laboratory, the relationship between heart rate (f_H) and oxygen consumption ( VO₂ , mg h⁻¹ standardized to a 500-g fish weight) can be described by a significant linear regression valid for heart rates below 55 beats min⁻¹. Heart rate increases during activity and feeding, accurately reflecting fluctuations in metabolic rate; so it can be used as a measure of metabolic rate in the field, using heart beat telemetry techniques. Post-prandial heart rates are elevated, and closely correlated with the calorific value of the food taken in the preceding meal. Meal size can be estimated from the heart rate record with an error of less than 10%.     

Allometric estimation of metabolic rate from heart rate in penguins

Studies of the relationship between heart rate (f(H)) and rate of oxygen consumption (V(.) (O(2))), which are then used to predict field metabolic rate, frequently fail to incorporate body mass as a predictive variable. This is a potentially important omission in the study of animals whose body mass fluctuates substantially during their annual cycle. In an attempt further to improve estimates of field metabolic rate from f(H), we re-evaluated data on M(b), f(H) and V(.) (O(2)) from previous studies of macaroni penguins (Eudyptes chrysolophus) and king penguins (Aptenodytes patagonicus) and derived a new relationship to integrate these three quantities. This relationship is at least as accurate and precise as previously determined relationships. We applied this same principle to published data on 11 of the 20 recognised penguin taxa to derive a relationship to predict V(.) (O(2)) from f(H) and M(b) in penguins of any species. This result has interesting implications in terms of reducing the logistical burden in studies of field metabolic rate.     

The influence of metabolic rate on otolith increment width in Atlantic salmon parr, Salmo salar L.

The influence of metabolic rate on otolith accretion in Atlantic salmon parr was investigated by comparing oxygen consumption rate and increment width in fast and slow growing individuals. Increment width was found to be positively correlated to mean daily oxygen consumption in both fast growing (S1) and slow growing (S2) parr. The results support previous suggestions that a process related to metabolic rate, rather than somatic growth, governs the rate of otolith accretion.     

Relationships between heart rate and metabolic rate of pike: integration of existing data

The percentage contribution of heart rate ( f _H) to change in oxygen consumption ( V o₂) was examined in relation to body weight and across the metabolic scope of pike. Also the consequences of variability around the regression relating V o₂ and f _H for estimating V o₂ were considered. The percentage contribution of f _H was calculated using two equations, one that ignored and one that included an estimate for oxygen consumed by the gills and absorbed across the skin ( V o_2s). Using both equations the percentage contribution of f _H calculated using maximum and resting values for f _H and V o₂ decreased with weight of pike. The omission of V o_2s, resulted in erroneously high estimates of the percentage contribution of f _H for pike of any given weight. The omission of V o_2s resulted in erroneously high estimates of the percentage contribution of f _H over the region of the metabolic scope where f _H is related linearly to V o₂, whereas the equation that included V o_2s resulted in the expected value of 100%. Assuming zero experimental error and under normoxic conditions, the 95% confidence limits for single estimates of V o₂ from 30–60-min readings of heart rate are ±39% at a heart rate of 30 beats min ⁻¹. Averaged over longer periods the error decreases, and used over several days to estimate meal size the error is of the order of 1%.     

An allometric analysis of oxygen consumption rate and cardiovascular function in the cockroach, Blaberus discoidalis.

The interrelationship of metabolic rate and cardiovascular function has been well documented in vertebrates through allometric analyses. However, similar studies are lacking in insects. Unlike vertebrates, the cardiovascular system of insects does not play a significant role in oxygen transport. A comparison of the interrelationship in insects and vertebrates might provide insight into the nature of the connection between metabolic rate and the cardiovascular system. Oxygen consumption, heart rate and heart dimensions were measured in the nymphs of the cockroach Blaberus discoidalis over a mass range of 0.03-5 g. Oxygen consumption rate scaled with an exponent of 0.83. The relationship between heart rate and body mass scaled negatively, however, it did not appear to be linear. Using measurements of heart widths, abdominal length and heart rate, stroke volume and cardiac output were estimated. Cardiac output appeared to scale linearly with an exponent of 0.85, which was not significantly different from the exponent observed for the rate of oxygen consumption. Thus, the observed similarity between the exponents for oxygen consumption rate and cardiac output in vertebrates also appears to be present in insects.     

Assessing maternal effects on metabolic rate dynamics along early development in brown trout (Salmo trutta): an individual-based approach

Routine metabolic rate (oxygen consumption) of individual eggs and larvae of brown trout (Salmo trutta) originating from different families were monitored from fertilisation to the onset of emergence by means of flow through micro-respirometry. This measuring system revealed an accurate tool to measure oxygen consumption on small organisms at the individual level, and daily consumption proved to be very stable. The mass-specific metabolic rate remained low from fertilisation to hatching, and then increased quickly until the age of emergence. A Bayesian modelling approach was used to adequately infer maternal effects on metabolic rate dynamics all along the development period. Substantial differences were found between families, affecting average metabolic rate as well as intra-family variance. That is, offspring originating from different females may have different energetic needs at emergence from gravel. Moreover, between siblings, variability in metabolic rate is also under the influence of maternal effects. Implications of this metabolic rate variability are discussed with regard to life history strategies and early behaviours.     

Reduced metabolic rate and oxygen radicals production in stored insect sperm

Females of internally fertilizing species can significantly extend sperm lifespan and functionality during sperm storage. The mechanisms for such delayed cellular senescence remain unknown. Here, we apply current hypotheses of cellular senescence developed for diploid cells to sperm cells, and empirically test opposing predictions on the relationship between sperm metabolic rate and oxygen radical production in an insect model, the cricket Gryllus bimaculatus. Using time-resolved microfluorimetry, we found a negative correlation between metabolic rate (proportion of protein-bound NAD[P]H) and in situ intracellular oxygen radicals production in freshly ejaculated sperm. In contrast, sperm stored by females for periods of 1 h to 26 days showed a positive correlation between metabolic rate and oxygen radicals production. At the same time, stored sperm showed a 37 per cent reduced metabolic rate, and 42 per cent reduced reactive oxygen species (ROS) production, compared with freshly ejaculated sperm. Rank differences between males in ROS production and metabolic rate observed in ejaculated sperm did not predict rank differences in stored sperm. Our method of simultaneously measuring ROS production and metabolic rate of the same sample has the advantage of providing data that are independent of sperm density and any extracellular antioxidants that are proteins. Our method also excludes effects owing to accumulated hydrogen peroxide. Our results unify aspects of competing theories of cellular ageing and suggest that reducing metabolic rate may be an important means of extending stored sperm lifespan and functionality in crickets. Our data also provide a possible explanation for why traits of ejaculates sampled from the male may be rather poor predictors of paternity in sexual selection studies and likelihood of pregnancy in reproductive medicine.     

Behavioral inference of diving metabolic rate in free-ranging leatherback turtles

Good estimates of metabolic rate in free-ranging animals are essential for understanding behavior, distribution, and abundance. For the critically endangered leatherback turtle (Dermochelys coriacea), one of the world's largest reptiles, there has been a long-standing debate over whether this species demonstrates any metabolic endothermy. In short, do leatherbacks have a purely ectothermic reptilian metabolic rate or one that is elevated as a result of regional endothermy? Recent measurements have provided the first estimates of field metabolic rate (FMR) in leatherback turtles using doubly labeled water; however, the technique is prohibitively expensive and logistically difficult and produces estimates that are highly variable across individuals in this species. We therefore examined dive duration and depth data collected for nine free-swimming leatherback turtles over long periods (up to 431 d) to infer aerobic dive limits (ADLs) based on the asymptotic increase in maximum dive duration with depth. From this index of ADL and the known mass-specific oxygen storage capacity (To(2)) of leatherbacks, we inferred diving metabolic rate (DMR) as To2/ADL. We predicted that if leatherbacks conform to the purely ectothermic reptilian model of oxygen consumption, these inferred estimates of DMR should fall between predicted and measured values of reptilian resting and field metabolic rates, as well as being substantially lower than the FMR predicted for an endotherm of equivalent mass. Indeed, our behaviorally derived DMR estimates (mean=0.73+/-0.11 mL O(2) min(-1) kg(-1)) were 3.00+/-0.54 times the resting metabolic rate measured in unrestrained leatherbacks and 0.50+/-0.08 times the average FMR for a reptile of equivalent mass. These DMRs were also nearly one order of magnitude lower than the FMR predicted for an endotherm of equivalent mass. Thus, our findings lend support to the notion that diving leatherback turtles are indeed ectothermic and do not demonstrate elevated metabolic rates that might be expected due to regional endothermy. Their capacity to have a warm body core even in cold water therefore seems to derive from their large size, heat exchangers, thermal inertia, and insulating fat layers and not from an elevated metabolic rate.     

Assessment of cardiac output as a predictor of metabolic rate in rainbow trout

When water temperature was increased from 12 to 27°C at a rate of 2°C h⁻¹, oxygen consumption of rainbow trout Oncorhynchus mykiss was correlated strongly with both heart rate and blood oxygen extraction but the relationship with cardiac output was variable and weak. On the other hand, when water temperature was decreased from 21 to 12°C at a rate of 0·5°C h⁻¹, oxygen consumption was correlated with both heart rate and cardiac output but not with blood oxygen extraction. When fish were forced to swim increasingly faster, heart rate, cardiac output and blood oxygen extraction all correlated positively with oxygen consumption. For both cardiac output and heart rate, the slope of the regression line with oxygen consumption was elevated significantly more when the fish were forced to swim at increasingly higher swimming speeds than when water temperature was increased or decreased. The variation of the regression lines between cardiac output and oxygen consumption indicated that cardiac output presents few advantages over heart rate as a predictor of metabolic rate.     

On the relation between basal and maximum metabolic rate in mammals

Basal and maximum metabolic rates, measured by oxygen consumption, for 18 species of wild mammals have been obtained from a search of literature records. The mass exponent of the allometric regression equation for maximum metabolic rate is significantly higher than that for BMR (0.841 and 0.745, respectively; P less than 0.05) in the group of animals examined. No significant correlation between mass-independent basal and maximum metabolic rates has been found. These results do not support the 'aerobic capacity' model of the origin of endothermy.     

Feedback control of human metabolic work rate during robotics-assisted treadmill exercise

A novel approach is presented which suggests the use of human metabolic work rate to define and regulate exercise intensity during robotics-assisted treadmill training. The work describes the design and technical validation of the new method.A feedback structure is proposed which provides automatic regulation of metabolic work rate, in conjunction with an embedded feedback loop for volitional control of mechanical work rate. Human metabolic work rate was derived in real time from breath-by-breath measurements of oxygen uptake and carbon dioxide output.The results show that the feedback method provides close to nominal performance for square-wave and ramp reference tracking tasks and that good disturbance rejection properties are obtained. A collateral finding of this work is an estimate of 14.5% of the metabolic efficiency of robotics-assisted treadmill exercise.The use of feedback control of human metabolic work rate provides a direct measure of exercise intensity as perceived by the exercising human as it directly reflects the energy requirements of the working muscles. This complements previous approaches to guiding robotics-assisted treadmill training based on mechanical work rate, heart rate or oxygen uptake. The new approach based on metabolic work rate may have advantages in populations with compromised and widely varying exercise responses. This provides a new approach for driving and controlling active patient participation during robotics-assisted treadmill exercise.     

Involvement of basal metabolic rate in determination of type of cold tolerance

This study aimed to assess the relationship between basal metabolic rate (BMR) and metabolic heat production, and to clarify the involvement of BMR in determining the phenotype of cold tolerance. Measurements of BMR, maximum oxygen uptake, and cold exposure test were conducted on ten males. In the cold exposure test, rectal (T(rec)) and mean skin temperatures (T(ms)), oxygen uptake, and blood flow at forearm (BF(arm)) were measured during exposure to cold (10 degrees C) for 90 min. Significant correlations were observed between BMR and increasing rate of oxygen uptake, as well as between decreasing rate of BF(arm) and increasing rate of oxygen uptake at the end of cold exposure. These findings suggested that individuals with a lower BMR were required to increase their metabolic heat production during cold exposure, and that those with a higher BMR were able to moderate increased metabolic heat production during cold exposure because they were able to reduce heat loss. This study showed that BMR is an important factor in determining the phenotype of cold tolerance, and that individuals with a low BMR showed calorigenic-type cold adaptation, whereas subjects with a high BMR exhibited adiabatic-type cold adaptation by peripheral vasoconstriction.     

Metabolic rate does not scale with body mass in cultured mammalian cells

The allometric scaling of metabolic rate with organism body mass can be partially accounted for by differences in cellular metabolic rates. For example, hepatocytes isolated from horses consume almost 10-fold less oxygen per unit time as mouse hepatocytes [Porter and Brand, Am J Physiol Regul Integr Comp Physiol 269: R226-R228, 1995]. This could reflect a genetically programmed, species-specific, intrinsic metabolic rate set point, or simply the adaptation of individual cells to their particular in situ environment (i.e., within the organism). We studied cultured cell lines derived from 10 mammalian species with donor body masses ranging from 5 to 600,000 g to determine whether cells propagated in an identical environment (media) exhibited metabolic rate scaling. Neither metabolic rate nor the maximal activities of key enzymes of oxidative or anaerobic metabolism scaled significantly with donor body mass in cultured cells, indicating the absence of intrinsic, species-specific, cellular metabolic rate set points. Furthermore, we suggest that changes in the metabolic rates of isolated cells probably occur within 24 h and involve a reduction of cellular metabolism toward values observed in lower metabolic rate organisms. The rate of oxygen delivery has been proposed to limit cellular metabolic rates in larger organisms. To examine the effect of oxygen on steady-state cellular respiration rates, we grew cells under a variety of physiologically relevant oxygen regimens. Long-term exposure to higher medium oxygen levels increased respiration rates of all cells, consistent with the hypothesis that higher rates of oxygen delivery in smaller mammals might increase cellular metabolic rates.     

Functional coupling as a basic mechanism of feedback regulation of cardiac energy metabolism

In this review we analyze the concepts and the experimental data on the mechanisms of the regulation of energy metabolism in muscle cells. Muscular energetics is based on the force-length relationship, which in the whole heart is expressed as a Frank-Starling law, by which the alterations of left ventricle diastolic volume change linearly both the cardiac work and oxygen consumption. The second basic characteristics of the heart is the metabolic stability--almost constant levels of high energy phosphates, ATP and phosphocreatine, which are practically independent of the workload and the rate of oxygen consumption, in contrast to the fast-twitch skeletal muscle with no metabolic stability and rapid fatigue. Analysis of the literature shows that an increase in the rate of oxygen consumption by order of magnitude, due to Frank-Starling law, is observed without any significant changes in the intracellular calcium transients. Therefore, parallel activation of contraction and mitochondrial respiration by calcium ions may play only a minor role in regulation of respiration in the cells. The effective regulation of the respiration under the effect of Frank-Starling law and metabolic stability of the heart are explained by the mechanisms of functional coupling within supramolecular complexes in mitochondria, and at the subcellular level within the intracellular energetic units. Such a complex structural and functional organisation of heart energy metabolism can be described quantitatively by mathematical models.     

The rate of free radical production as a determinant of the rate of aging: evidence from the comparative approach

The relationship of oxidative stress with maximum life span (MLSP) in different vertebrate species is reviewed. In all animal groups the endogenous levels of enzymatic and non-enzymatic antioxidants in tissues negatively correlate with MLSP and the most longevous animals studied in each group, pigeon or man, show the minimum levels of antioxidants. A possible evolutionary reason for this is that longevous animals produce oxygen radicals at a low rate. This has been analysed at the place where more than 90% of oxygen is consumed in the cell, the mitochondria. All available work agrees that, across species, the longer the life span, the lower the rate of mitochondrial oxygen radical production. This is true even in animal groups that do not conform to the rate of living theory of aging, such as birds. Birds have low rates of mitochondrial oxygen radical production, frequently due to a low free radical leak in their respiratory chain. Possibly the low rate of mitochondrial oxygen radical production of longevous species can decrease oxidative damage at targets important for aging (like mitochondrial DNA) that are situated near the places of free radical generation. A low rate of free radical production can contribute to a low aging rate both in animals that conform to the rate of living (metabolic) theory of aging and in animals with exceptional longevities, like birds and primates. Available research indicates there are at least two main characteristics of longevous species: a high rate of DNA repair together with a low rate of free radical production near DNA. Simultaneous consideration of these two characteristics can explain part of the quantitative differences in longevity between animal species. Accepted: 12 December 1997     

Comparison of methods to quantify metabolic rate and its relationship with activity in larval lake sturgeon Acipenser fulvescens

Until recently most studies have focussed on method development for metabolic rate assessment in adult and/or juvenile fish with less focus on measurement of oxygen consumption (ṀO₂) during early life history stages, including fast-growing larval fish and even less focus on nonteleostean species. In the present study we evaluated measurement techniques for standard metabolic rate (SMR), maximum metabolic rate (MMR) and aerobic scope in an Acipenseriform, the lake sturgeon Acipenser fulvescens, throughout the first year of life. Standardized forced exercise protocols to assess MMR were conducted for 5 or 15 min before or after measurement of SMR. We used different levels of oxygen decline during the measurement period of MMR post forced exercise to understand the influence these may have on the calculation of MMR. Opercular rate and tail beat frequencies were recorded by video as measures of behaviours and compared to metabolic rate recorded over a 24 h period. Results indicate that calculated values for aerobic scope were lower in younger fish. Neither exercise sequence nor exercise duration influenced metabolic rate measurements in the younger fish, but exercise duration did affect measurement of MMR in older fish. Finally, there was no strong correlation between metabolic rate and the measured behaviours in the lake sturgeon at either age. Based on the results, we recommend that a minimum of 6 h of acclimation to the respirometry chamber should be given prior to measuring SMR, a chasing protocol to elicit MMR should ideally be performed at the end of experiment, a short chasing time should be avoided to minimize variation and assessment of MMR should balance measurement limitations of the probes along with when and for how long oxygen consumption is measured.     

The ultrasonic telemetry of cardiac rhythms of wild brown trout (Salmo trutta L.) as an indicator of bio-energetics and behaviour

A new type of miniature ultrasonic telemetry transmitter weighing less than 4 g underwater and usable on fish of less than 0.5 kg weight is described. The signal consisting of a sound pulse triggered by the QRS spike of the electrocardiogram is detectable at ranges to 400 m. Continuous records lasting 7 days were made of cardiac rhythms of brown trout. Mean heart rates were low indicating a metabolic rate of not more than 1.55 times the basal metabolic rate. Maximum heart rates were rare, occupying less than 0.5% of the time; there was no evidence of build up of oxygen debt due to periods of high activity. Following a settling down period after the attachment of the transmitter, the fish exhibited a diurnal rhythm with higher heart rates during the day. The transition from day to night heart rates and vice versa in one fish was shown to anticipate solar altitude change by half-hour. Missing heart beats (cardiac inhibitory reflexes) are discussed as indicators of a sensory input, but they are not reliable indicators of feeding activity. It is concluded that stamina is not important for normal day to day survival of adult trout and the influence of various ecological factors is discussed.     

Coupling of heart rate and locomotor activity in sole, Solea solea (L.), and bass, Dicentrarchus labrax (L.), in their natural environment by using ultrasonic telemetry

Field measurements of distance moved and heart rate in sole, Solea solea (L.) and bass, Dicentrarchus labrax (L.) using ultrasonic telemetry revealed two different strategies. Heart rate in the sole increases during activity, accurately reflecting fluctuation in metabolic rate and so can be used as a measure of metabolic rate in the field. In contrast, the relatively stable value of the heart rate in bass during the whole tracking period whatever the activity level suggests that in this species heart rate in the field cannot be associated with metabolic rate determination.     

Ventilation and metabolic rate of young rainbow trout (Salmo gairdneri) exposed to sublethal environmental pH.

Differences between ventilatory response and metabolic rates of young rainbow trout tested within the sublethal range of pH 6 to pH 9 were observed using a flowing water respirometer. The oxygen consumption was monitored at swimming speeds of 12 cm/sec and 24 cm/sec. The oxygen consumption rates at 24 cm/sec and pH 6 (423 mg/kg-hr) and pH 9 (367 mg/kg-hr) were considerably higher than those determined near neutrality (328 mg/kg-hr). Ventilation rate increased to either side of neutrality, but significantly fewer respiratory reversals, or "coughs," were observed at pH 6 and a greater number at pH 9 than occurred at pH 7 and 8 or in untested fish. The respiratory-cough response is shown to be pH-dependent in rainbow trout and may therefore not be as reliable an indication of pollutant-caused stress in studies where the experimental pH has not been specified or controlled.     

The maximum oxygen consumption and aerobic scope of birds and mammals: getting to the heart of the matter

Resting or basal metabolic rates, compared across a wide range of organisms, scale with respect to body mass as approximately the 0.75 power. This relationship has recently been linked to the fractal geometry of the appropriate transport system or, in the case of birds and mammals, the blood vascular system. However, the structural features of the blood vascular system should more closely reflect maximal aerobic metabolic rates rather than submaximal function. Thus, the maximal aerobic metabolic rates of birds and mammals should also scale as approximately the 0.75 power. A review of the literature on maximal oxygen consumption and factorial aerobic scope (maximum oxygen consumption divided by basal metabolic rate) suggests that body mass influences the capacity of the cardiovascular system to raise metabolic rates above those at rest. The results show that the maximum sustainable metabolic rates of both birds and mammals are similar and scale as approximately the 0.88 +/- 0.02 power of body mass (and aerobic scope as approximately the 0.15 +/- 0.05 power), when the measurements are standardized with respect to the differences in relative heart mass and haemoglobin concentration between species. The maximum heart beat frequency of birds and mammals is predicted to scale as the -0.12 +/- 0.02 power of body mass, while that at rest should scale as -0.27 +/- 0.04.