Oxygen transport and cardiovascular responses to exercise in the yellowfin tuna Thunnus albacares

Cardiorespiratory performance and blood chemistry during swimming and recovery in three populations of elite swimmers: Adult sockeye salmon

Every year, millions of adult sockeye salmon (Oncorhynchus nerka) perform an arduous, once-in-a-lifetime migration up the Fraser River (BC, Canada) to return to their natal stream to spawn. The changes in heart rate, stroke volume, and arterio-venous oxygen extraction (i.e., factors determining rates of oxygen delivery to the tissues by the cardiovascular system) have never been directly and simultaneously measured along with whole animal oxygen uptake in a maximally swimming fish. Here, such measurements were made using three sockeye salmon populations (Early Stuart, Chilko and Quesnel), which each performed two consecutive critical swimming speed (Ucrit) challenges to provide a comprehensive quantification of cardiovascular physiology, oxygen status and blood chemistry associated with swimming and recovery. Swim performance, oxygen uptake, cardiac output, heart rate and stroke volume did not significantly vary at rest, during swimming or during recovery between populations or sexes. Despite incomplete metabolic recovery between swim challenges, all fish repeated their swim performance and similar quantitative changes in the cardiorespiratory variables were observed for each swim challenge. The high maximum cardiorespiratory performance and excellent repeat swim performance are clearly beneficial in allowing the salmon to maintain steady ground speeds and reach the distant spawning grounds in a timely manner.

Heart rate responses to temperature in free-swimming Pacific bluefin tuna (Thunnus orientalis)

The bluefin tuna heart remains at ambient water temperature (Ta) but must supply blood to warm regions of the body served by countercurrent vascular heat exchangers. Despite this unusual physiology, inherent difficulties have precluded an understanding of the cardiovascular responses to Ta in free-swimming bluefin tunas. We measured the heart rate (fH) responses of two captive Pacific bluefin tunas (Thunnus orientalis; 9.7 and 13.3 kg) over a cumulative period of 40 d. Routine fH during fasting in the holding tank at a Ta of 20°C was 45.1±8.0 and 40.7±6.5 beats min-1 for Tuna 1 and Tuna 2, respectively. fH decreased in each fish with Q10 of 2.6 (Tuna 1) and 3.1 (Tuna 2) as Ta in the tank was slowly decreased to 15°C (~0.4oC h-1), despite a gradual increase in swimming speed. The same thermal challenge during digestion revealed similar thermal dependence of fH and indicated that the rate of visceral cooling is not buffered by the heat increment of feeding. Acutely decreasing Ta from 20 to 10°C while Tuna 1 swam in a tunnel respirometer caused a progressive increase in tail beat-frequency and oxygen consumption rate (Mo2). fH of this fish decreased with Q10 of 2.7 as Ta decreased between 20 and 15°C, while further cooling to 10°C saw a general plateau in fH around 35 beats min-1 with Q10 of 1.3. A discussion of the relationships between fH, Mo2 , and haemoglobin-oxygen binding sheds further light on how bluefin cardiorespiratory systems function in a changing thermal environment.

Effects of nitrite exposure on functional haemoglobin levels, bimodal respiration, and swimming performance in the facultative air-breathing fish Pangasianodon hypophthalmus

In this study we investigated nitrite (NO₂⁻) effects in striped catfish, a facultative air-breather. Fish were exposed to 0, 0.4, and 0.9 mM nitrite for 0, 1, 2, 4, and 7 days, and levels of functional haemoglobin, methaemoglobin (metHb) and nitrosyl haemoglobin (HbNO) were assessed using spectral deconvolution. Plasma concentrations of nitrite, nitrate, chloride, potassium, and sodium were also measured. Partitioning of oxygen consumption was determined to reveal whether elevated metHb (causing functional hypoxia) induced air-breathing. The effects of nitrite on maximum oxygen uptake (MO(2max)) and critical swimming speed (U(crit)) were also assessed. Striped catfish was highly tolerant to nitrite exposure, as reflected by a 96 h LC₅₀ of 1.65 mM and a moderate nitrite uptake into the blood. Plasma levels of nitrite reached a maximum after 1 day of exposure, and then decreased, never exceeding ambient levels. MetHb, HbNO and nitrate (a nitrite detoxification product) also peaked after 1 day and then decreased. Only high levels of nitrite and metHb caused reductions in MO(2max) and U(crit). The response of striped catfish contrasts with that seen in most other fish species and discloses efficient mechanisms of combating nitrite threats. Furthermore, even though striped catfish is an efficient air-breather, this species has the ability to sustain aerobic scope and swimming performance without air-breathing, even when faced with nitrite-induced reductions in blood oxygen carrying capacity. Our study is the first to confirm that high levels of nitrite and metHb reduce MO(2max) and thereby aerobic scope, while more moderate elevations fail to do so. Further studies are needed to elucidate the mechanisms underlying the low nitrite accumulation in striped catfish.

Temperature sensitivity of cardiac function in pelagic fishes with different vertical mobilities: yellowfin tuna (Thunnus albacares), bigeye tuna (Thunnus obesus), mahimahi (Coryphaena hippurus), and swordfish (Xiphias gladius)

We measured the temperature sensitivity, adrenergic sensitivity, and dependence on sarcoplasmic reticulum (SR) Ca(2+) of ventricular muscle from pelagic fishes with different vertical mobility patterns: bigeye tuna (Thunnus obesus), yellowfin tuna (Thunnus albacares), and mahimahi (Coryphaena hippurus) and a single specimen from swordfish (Xiphias gladius). Ventricular muscle from the bigeye tuna and mahimahi exhibited a biphasic response to an acute decrease in temperature (from 26 degrees to 7 degrees C); twitch force and kinetic parameters initially increased and then declined. The magnitude of this response was larger in the bigeye tuna than in the mahimahi. Under steady state conditions at 26 degrees C, inhibition of SR Ca(2+) release and reuptake with ryanodine and thapsigargin decreased twitch force and kinetic parameters, respectively, in the bigeye tuna only. However, the initial inotropy associated with decreasing temperature was abolished by SR inhibition in both the bigeye tuna and the mahimahi. Application of adrenaline completely reversed the effects of ryanodine and thapsigargin, but this effect was diminished at cold temperatures. In the yellowfin tuna, temperature and SR inhibition had minor effects on twitch force and kinetics, while adrenaline significantly increased these parameters. Limited data suggest that swordfish ventricular muscle responds to acute temperature reduction, SR inhibition, and adrenergic stimulation in a manner similar to that of bigeye tuna ventricular muscle. In aggregate, our results show that the temperature sensitivity, SR dependence, and adrenergic sensitivity of pelagic fish hearts are species specific and that these differences reflect species-specific vertical mobility patterns.

Moving with the beat: heart rate and visceral temperature of free-swimming and feeding bluefin tuna

Owing to the inherent difficulties of studying bluefin tuna, nothing is known of the cardiovascular function of free-swimming fish. Here, we surgically implanted newly designed data loggers into the visceral cavity of juvenile southern bluefin tuna (Thunnus maccoyii) to measure changes in the heart rate (fH) and visceral temperature (TV) during a two-week feeding regime in sea pens at Port Lincoln, Australia. Fish ranged in body mass from 10 to 21 kg, and water temperature remained at 18-19 degrees C. Pre-feeding fH typically ranged from 20 to 50 beats min(-1). Each feeding bout (meal sizes 2-7% of tuna body mass) was characterized by increased levels of activity and fH (up to 130 beats min(-1)), and a decrease in TV from approximately 20 to 18 degrees C as cold sardines were consumed. The feeding bout was promptly followed by a rapid increase in TV, which signified the beginning of the heat increment of feeding (HIF). The time interval between meal consumption and the completion of HIF ranged from 10 to 24 hours and was strongly correlated with ration size. Although fH generally decreased after its peak during the feeding bout, it remained elevated during the digestive period and returned to routine levels on a similar, but slightly earlier, temporal scale to TV. These data imply a large contribution of fH to the increase in circulatory oxygen transport that is required for digestion. Furthermore, these data oppose the contention that maximum fH is exceptional in bluefin tuna compared with other fishes, and so it is likely that enhanced cardiac stroke volume and blood oxygen carrying capacity are the principal factors allowing superior rates of circulatory oxygen transport in tuna.

Fish cardiovascular physiology and disease

Fish patients with cardiovascular disorders present a challenge in terms of diagnostic evaluation and therapeutic options. Veterinarians can approach these cases in fish using methods similar to those employed for other companion animals. Clinicians who evaluate and treat fish in private, aquarium, zoologic, or aquaculture settings need to rely on sound clinical judgment after thorough historical and physical evaluation. Pharmacokinetic data and treatments specific to cardiovascular disease in fish are limited; thus, drug types and dosages used in fish are largely empiric. Fish cardiovascular anatomy, physiology, diagnostic evaluation, monitoring, common diseases, cardiac pathologic conditions, formulary options, and comprehensive references are presented with the goal of providing fish veterinarians with clinically relevant tools.

Effects of soft-water acclimation on the physiology, swimming performance, and cardiac parameters of the rainbow trout, Oncorhynchus mykiss

Rainbow trout acclimated to soft water were submitted to an incremental velocity trial, and exhibited a 14% decrease in critical swimming speed (U(crit) * 1.37 +/- 0.055 vs. 1.54 +/- 0.044 m s(-1)) compared to fish kept in hard water. After a standardized swimming protocol, soft-water-acclimated fish had higher blood lactate concentrations (6.5 +/- 0.66 and 6.0 +/- 0.64 mmol L(-1) (soft water) vs. 5.0 +/- 0.46 and 3.9 +/- 0.32 mmol L(-1) (hard water)), revealing a greater use of anaerobic metabolism for the same exercise. Cardiovascular parameters were investigated while fish were swimming at increasing water velocities, revealing that soft-water-acclimated fish had lower increases in heart rate (105% vs. 118% of pre-exercise values), due to higher heart rates observed during acclimation and during the first 10 min of the swimming trial. This was also reflected by the plateau in heart rate and stroke volume observed during the swimming protocol, which can be attributed to increased cardiovascular function in response to soft-water acclimation. These results are in accord with previously reported increases in blood-to-water diffusion distance, due to proliferation of chloride cells at the gills in response to soft-water conditions, and underscore the costs and limitations of soft-water acclimation.

Changes in cardiac output during swimming and aquatic hypoxia in the air-breathing Pacific tarpon

Pacific tarpon (Megalops cyprinoides) use a modified gas bladder as an air-breathing organ (ABO). We examined changes in cardiac output (V(b)) associated with increases in air-breathing that accompany exercise and aquatic hypoxia. Juvenile (0.49 kg) and adult (1.21 kg) tarpon were allowed to recover in a swim flume at 27 degrees C after being instrumented with a Doppler flow probe around the ventral aorta to monitor V(b) and with a fibre-optic oxygen sensor in the ABO to monitor air-breathing frequency. Under normoxic conditions and in both juveniles and adults, routine air-breathing frequency was 0.03 breaths min(-1) and V(b) was about 15 mL min(-1) kg(-1). Normoxic exercise (swimming at about 1.1 body lengths s(-1)) increased air-breathing frequency by 8-fold in both groups (reaching 0.23 breaths min(-1)) and increased V(b) by 3-fold for juveniles and 2-fold for adults. Hypoxic exposure (2 kPa O2) at rest increased air-breathing frequency 19-fold (to around 0.53 breaths min(-1)) in both groups, and while V(b) again increased 3-fold in resting juvenile fish, V(b) was unchanged in resting adult fish. Exercise in hypoxia increased air-breathing frequency 35-fold (to 0.95 breaths min(-1)) in comparison with resting normoxic fish. While juvenile fish increased V(b) nearly 2-fold with exercise in hypoxia, adult fish maintained the same V(b) irrespective of exercise state and became agitated in comparison. These results imply that air-breathing during exercise and hypoxia can benefit oxygen delivery, but to differing degrees in juvenile and adult tarpon. We discuss this difference in the context of myocardial oxygen supply.

Elevated Ca2+ ATPase (SERCA2) activity in tuna hearts: comparative aspects of temperature dependence

Tunas have an extraordinary physiology including elevated metabolic rates and high cardiac performance. In some species, retention of metabolic heat warms the slow oxidative swimming muscles and visceral tissues. In all tunas, the heart functions at ambient temperature. Enhanced rates of calcium transport in tuna myocytes are associated with increased expression of proteins involved in the contraction-relaxation cycle. The cardiac SR Ca2+-ATPase (SERCA2) plays a major role during cardiac excitation-contraction (E-C) coupling. Measurements of oxalate-supported Ca2+-uptake in atrial SR vesicles isolated from four species of tunas indicate that bluefin have at least two fold higher Ca2+-uptake than all other tunas examined between 5 and 30 degrees C. The highest atrial Ca2+-uptake was measured in bluefin tuna at 30 degrees C (23.32+/-1.58 nmol Ca2+/mg/min). Differences among tunas in the temperature dependency of Ca2+-uptake were similar for ATP hydrolysis. Western blot analysis revealed a significant increase in SERCA2 content associated with higher Ca2+ uptake rates in the atrial tissues of bluefin tuna and similar RyR expression across species. We propose that the expression of EC coupling proteins in cardiac myocytes, and the higher rates of SERCA2 activity are an important evolutionary step for the maintenance of higher heart rates and endothermy in bluefin tuna.

Cardiorespiratory physiology and swimming energetics of a high-energy-demand teleost, the yellowtail kingfish (Seriola lalandi)

This study utilizes a swimming respirometer to investigate the effects of exercise and temperature on cardiorespiratory function of an active teleost,the yellowtail kingfish (Seriola lalandi). The standard aerobic metabolic rate (SMR) of S. lalandi (mean body mass 2.1 kg) ranges from 1.55 mg min-1 kg-1 at 20°C to 3.31 mg min-1 kg-1 at 25°C. This 2.1-fold increase in SMR with temperature is associated with a 1.5-fold increase in heart rate from 77 to 117 beats min-1, while cardiac stroke volume remains constant at 0.38 ml beat-1 kg-1 and the difference in oxygen content between arterial and mixed venous blood[(CaO2-Cv̄O2)]increases marginally from 0.06 to 0.08 mg ml-1. During maximal aerobic exercise (2.3 BL s-1) at both temperatures,however, increases in cardiac output are limited to about 1.3-fold, and increases in oxygen consumption rates (up to 10.93 mg min-1kg-1 at 20°C and 13.32 mg min-1 kg-1 at 25°C) are mediated primarily through augmentation of(CaO2-Cv̄O2)to 0.29 mg ml-1 at 20°C and 0.25 mg ml-1 at 25°C. It seems, therefore, that the heart of S. lalandi routinely works close to its maximum capacity at a given temperature, and changes in aerobic metabolism due to exercise are greatly reliant on high blood oxygen-carrying capacity and(CaO2-Cv̄O2). Gross aerobic cost of transport (GCOT) is 0.06 mg kg-1BL-1 at 20°C and 0.09 mg kg-1BL-1 at 25°C at the optimal swimming velocities(U) of 1.2 BL s-1opt and 1.7 BL s-1, respectively. These values are comparable with those reported for salmon and tuna, implying that the interspecific diversity in locomotor mode (e.g. subcarangiform, carangiform and thunniform) is not concomitant with similar diversity in swimming efficiency. A low GCOT is maintained as swimming velocity increases above Uopt,which may partly result from energy savings associated with the progressive transition from opercular ventilation to ram ventilation.

Necrophysiological determination of blood pressure in fishes

Bony fishes have an elastic chamber between the heart and aorta, the bulbus arteriosus, which has unique mechanical properties. On inflation, the isolated bulbus is initially very stiff but soon becomes extremely compliant yielding a steady (plateau) pressure upon further inflation, which appears to be similar in any given species. Here we show that the plateau pressure correlates with mean blood pressure determined in vivo. Consequently, inflation of the bulbus can be used to determine blood pressure in the living animal from recordings made after it is dead.

Tuna comparative physiology

Thunniform swimming, the capacity to conserve metabolic heat in red muscle and other body regions (regional endothermy), an elevated metabolic rate and other physiological rate functions, and a frequency-modulated cardiac output distinguish tunas from most other fishes. These specializations support continuous, relatively fast swimming by tunas and minimize thermal barriers to habitat exploitation, permitting niche expansion into high latitudes and to ocean depths heretofore regarded as beyond their range.

Effects of temperature, epinephrine and Ca2+ on the hearts of yellowfin tuna (Thunnus albacares)

Tuna are endothermic fish with high metabolic rates, cardiac outputs and aerobic capacities. While tuna warm their skeletal muscle, viscera, brain and eyes, their hearts remain near ambient temperature, raising the possibility that cardiac performance may limit their thermal niches. We used an in situ perfused heart preparation to investigate the effects of acute temperature change and the effects of epinephrine and extracellular Ca2+ on cardiac function in yellowfin tuna (Thunnus albacares). Heart rate showed a strong temperature-dependence, ranging from 20 beats min-1 at 10 °C to 109 beats min-1 at 25 °C. Maximal stroke volume showed an inverse temperature-dependence,ranging from 1.4 ml kg-1 at 15 °C to 0.9 ml kg-1 at 25 °C. Maximal cardiac outputs were 27 ml kg-1 min-1at 10 °C and 98 ml kg-1 min-1 at 25 °C. There were no significant effects of perfusate epinephrine concentrations between 1 and 100 nmoll-1 at 20 °C. Increasing extracellular Ca2+ concentration from 1.84 to 7.36 mmoll-1 at 20°C produced significant increases in maximal stroke volume, cardiac output and myocardial power output. These data demonstrate that changes in heart rate and stroke volume are involved in maintaining cardiac output during temperature changes in tuna and support the hypothesis that cardiac performance may limit the thermal niches of yellowfin tuna.

Differences in pH between interstitial fluid and arterial blood in water-breathing and air-breathing vertebrates

Most cells are bathed by interstitial fluid, but extracellular pH measurements are mostly for arterial plasma. Whole-body mean pH differences between the two fluids have been estimated in terms of a simple model. This relates to the diffusive exchange of carbon dioxide and oxygen and utilizes literature data, for 22 vertebrate species, on arterial and mixed-venous tensions of both gases. Uncertainties arise because the carbon dioxide reaction in blood may sometimes be in disequilibrium and because carbon dioxide diffusion is facilitated to unknown degrees in the presence of buffers. Nevertheless, the model suggests that the pH difference should tend to vary inversely with arterial carbon dioxide tension. In some species, this may aid interstitial pH homeostasis, but a clearer implication is that the difference should be generally greater in water breathers than in air breathers. It has previously been found that arterial pH in water-breathing teleosts also tends to be higher than in air-breathing tetrapods (when allowance is made for temperature and plasma sodium concentration) and to a comparable extent. Thus, mean interstitial pH may be more nearly similar in the two groups than is arterial pH. Direct measurements of interstitial pH do not yet suffice to test the model.

Review: Analysis of the evolutionary convergence for high performance swimming in lamnid sharks and tunas

Elasmobranchs and bony fishes have evolved independently for more than 400 million years. However, two Recent groups, the lamnid sharks (Family Lamnidae) and tunas (Family Scombridae), display remarkable similarities in features related to swimming performance. Traits separating these two groups from other fishes include a higher degree of body streamlining, a shift in the position of the aerobic, red, locomotor muscle that powers sustained swimming to a more anterior location in the body and nearer to the vertebral column, the capacity to conserve metabolic heat (i.e. regional endothermy), an increased gill surface area with a decreased blood-water barrier thickness, a higher maximum blood oxygen carrying capacity, and greater muscle aerobic and anaerobic enzyme activities at in vivo temperatures. The suite of morphological, physiological, and biochemical specializations that define "high-performance fishes" have been extensively characterized in the tunas. This review examines the convergent features of lamnid sharks and tunas in order to gain insight into the extent that comparable environmental selection pressures have led to the independent origin of similar suites of functional characteristics in these two distinctly different taxa. We propose that, despite differences between teleost and elasmobranch fishes, lamnid sharks and tunas have evolved morphological and physiological specializations that enhance their swimming performance relative to other sharks and most other high performance pelagic fishes.