Cannulation techniques

A method for measuring meaningful physiological variables in fish blood without surgical cannulation

Gaining meaningful blood samples from water-breathing fish is a significant challenge. Two main methods typically used are grab 'n' stab and surgical cannulation. Both methods have benefits, but also significant limitations under various scenarios. Here we present a method of blood sampling laboratory fish involving gradual induction of anaesthesia within their home tank, avoiding physical struggling associated with capture, followed by rapid transfer to a gill irrigation system to maintain artificial ventilation via adequate gill water flow and then followed by sampling the caudal vasculature. This method negates many blood chemistry disturbances associated with grab 'n' stab (i.e., low pH and oxygen, elevated lactate, CO₂ and stress hormones) and generates results that are directly comparable to cannulated fish under a wide range of experimentally-induced acid-base scenarios (acidosis and alkalosis). Crucially this method was successful in achieving accurate acid-base blood measurements from fish ten times smaller than are typically suitable for cannulation. This opens opportunities not previously possible for studies that relate to basic physiology, sustainable aquaculture, ecotoxicology, conservation, and climate change.

Hormetic-like dose-response induced by alternagin-C, a protein isolated from urutu snake (Rhinocerophis alternatus) venom, in fish (Hoplias malabaricus) cardiac contractility

The aim of this study was to evaluate the effects of different doses of alternagin-C, a disintegrin-like protein from Rhinocerophis alternatus venom, on myocardial contractility of the freshwater fish Hoplias malabaricus, an alternative model to contractile function studies. Alternagin-C treatment exhibited a hormetic-like dose-response curve with a strong positive inotropism and enhanced cardiac pumping capacity at low dose, whereas a modest inotropism and a left shift in the force-frequency relationship was registered at high dose.

The baroreflex in aquatic and amphibious teleosts: Does terrestriality represent a significant driving force for the evolution of a more effective baroreflex in vertebrates?

All vertebrates have baroreflexes that provide fast regulation of arterial blood pressure (P_A) to maintain adequate tissue perfusion and avoid vascular lesions from excessive pressures. The baroreflex is a negative feedback loop, where altered P_A results in reciprocal changes in heart rate (f_H) and systemic vascular conductance to restore pressure. In terrestrial environments, gravity usually leads to blood pooling in the lower body reducing venous return, cardiac filling, cardiac output and P_A. Conversely, in aquatic environments, the hydrostatic pressure of surrounding water mitigates blood pooling and prevents vascular distensions. In this context, we aimed to test the hypothesis that vertebrate species that were exposed to gravity-induced hemodynamic disturbances throughout their evolutionary histories have a more effective barostatic reflex than those that were not. We examined the cardiac baroreflex of fish that perform (Clarias gariepinus and Hoplerythrinus unitaeniatus) and do not perform (Hoplias malabaricus and Oreochromis niloticus) voluntary terrestrial sojourns, using pharmacological manipulations of P_A to characterize reflex changes in f_H using a four-variable sigmoidal logistic function (i.e. the "Oxford technique"). Our results revealed that amphibious fish exhibit higher baroreflex gain and responsiveness to hypotension than strictly aquatic fish, suggesting that terrestriality and the gravitational circulatory stresses constitute a relevant driving force for the evolution of a more effective baroreflex in vertebrates. We also demonstrate that strictly aquatic teleosts have considerable baroreflex gain, supporting the view that the baroreflex is an ancient cardiovascular trait that appeared before vertebrates colonized the gravity-dominated realm of land.

Best practices for non-lethal blood sampling of fish via the caudal vasculature

Blood sampling through the caudal vasculature is a widely used technique in fish biology for investigating organismal health and physiology. In live fishes, it can provide a quick, easy and relatively non-invasive method for obtaining a blood sample (cf. cannulation and cardiac puncture). Here, a general set of recommendations are provided for optimizing the blood sampling protocol that reflects best practices in animal welfare and sample integrity. This includes selecting appropriate use of anaesthetics for blood sampling as well as restraint techniques for situations where sedation is not used. In addition, ideal sampling environments where the fish can freely ventilate and strategies for minimizing handling time are discussed. This study summarizes the techniques used for extracting blood from the caudal vasculature in live fishes, highlighting the phlebotomy itself, the timing of sampling events and acceptable blood sample volumes. This study further discuss considerations for selecting appropriate physiological metrics when sampling in the caudal region and the potential benefits that this technique provides with respect to long-term biological assessments. Although general guidelines for blood sampling are provided here, it should be recognized that contextual considerations (e.g., taxonomic diversity, legal matters, environmental constraints) may influence the approach to blood sampling. Overall, it can be concluded that when done properly, blood sampling live fishes through the caudal vasculature is quick, efficient and minimally invasive, thus promoting conditions where live release of focal animals is possible.

Extreme blood boosting capacity of an Antarctic fish represents an adaptation to life in a sub-zero environment

Blood doping, the practice of boosting the oxygen carrying capacity of blood, is an illegal strategy used by human athletes to enhance aerobic capacity and athletic performance. Interestingly, the practice of boosting blood oxygen carrying capacity is also naturally prevalent in the animal kingdom via the splenic release of stored erythrocytes. Here we demonstrate that an Antarctic notothenioid fish, the bald notothen (Pagothenia borchgrevinki), is a master of this practice. Due to the sub-zero environment these fish inhabit, they sequester a large proportion of erythrocytes in the spleen during times of inactivity to reduce the energetic and physiological costs associated with continuously pumping highly viscous blood around the body. However, in response to metabolically demanding situations (i.e. exercise and feeding), these fish contract the spleen to eject stored erythrocytes into circulation, which boosts blood oxygen carrying capacity by up to 207% (c.f. exercise-induced increases of ∼40-60% in a range of other vertebrates and ∼5-25% in blood-doping athletes). By evaluating cardiorespiratory differences between splenectomized (unable to release erythrocytes from the spleen) and sham-operated individuals, we demonstrate the metabolic benefits (i.e. aerobic scope increased 103%) and the cardiovascular trade-offs (i.e. ventral aortic blood pressure and cardiac workload increased 12% and 30%, respectively) associated with the splenic blood boosting strategy. In conclusion, this strategy provides bald notothens with an extraordinary facultative aerobic scope that enables an active lifestyle in the extreme Antarctic marine environment, while minimizing the energetic and physiological costs of transporting highly viscous blood during times of reduced energetic demand.

Cardioprotective effects of alternagin-C (ALT-C), a disintegrin-like protein from Rhinocerophis alternatus snake venom, on hypoxia-reoxygenation-induced injury in fish

Alternagin-C (ALT-C) is a disintegrin-like peptide purified from Rhinocerophis alternatus snake venom with the property of inducing vascular endothelial growth factor (VEGF) expression, endothelial cell proliferation and migration, and angiogenesis. Therefore, this protein could be interesting as a new approach for ischemic heart diseases, an imbalance between myocardial oxygen supply and demand, leading to cardiac dysfunction. We investigated the effects of a single dose of alternagin-C (0.5 mg kg^-1, via intra-arterial), after 7 days, on hypoxia/reoxygenation challenge in isolated ventricle strips and on morphological changes and density of blood vessels of the heart, using fish as an alternative experimental model. ALT-C treatment provided protection of cardiomyocytes against hypoxia/reoxygenation-induced negative inotropism. ALT-C also stimulated angiogenesis and improved excitation-contraction coupling during hypoxic conditions. Our results provide a new insight into a functional role of ALT-C against hypoxia/reoxygenation-induced cardiomyocyte injury pointing out to a potential therapeutic strategy for ischemia-related diseases.

Seawater acclimation affects cardiac output and adrenergic control of blood pressure in rainbow trout (Oncorhynchus mykiss)-implications for salinity variations now and in the future

Greater salinity variations resulting from ongoing climate change requires consideration in conservation management as this may impact on the performance of aquatic organisms. Euryhaline fish exhibit osmoregulatory flexibility and can exploit a wide range of salinities. In seawater (SW), they drink and absorb water in the intestine, which is associated with increased gastrointestinal blood flow. Yet, detailed information on other cardiovascular changes and their control across salinities is scant. Such knowledge is fundamental to understand how fish are affected during migrations between environments with different salinities, as well as by increased future salinity variability. We used rainbow trout (Oncorhynchus mykiss) as a euryhaline model species and determined dorsal aortic blood pressure, cardiac output and systemic vascular resistance in vivo after chronic freshwater-or SW-acclimation. We also assessed α-adrenergic control of blood pressure using pharmacological tools. Dorsal aortic blood pressure and systemic vascular resistance were reduced, whereas cardiac output increased in SW. α-Adrenergic stimulation with phenylephrine caused similar dose-dependent increases in resistance and pressure across salinities, indicating unaltered α-adrenoceptor sensitivity. α-Adrenergic blockade with prazosin decreased resistance and pressure across salinities, but the absolute reduction in resistance was smaller in SW. Yet, both pressure and resistance after prazosin remained consistently lower in SW. This shows that SW-acclimation lowers systemic resistance through reduced vascular α-adrenergic tone, along with other unknown vasodilating factors. The marked changes in adrenergic regulation of the vasculature across salinities discovered here may have implications for cardiovascular and aerobic performance of fishes, with possible impacts on fitness-related traits like digestion and exercise capacity. Moreover, the evolution of more complex circulatory control systems in teleost fishes compared with elasmobranchs and cyclostomes may have been an important factor in the evolution of euryhalinity, and may provide euryhaline teleosts with competitive advantages in more variable salinity environments of the future.

In vivo aerobic metabolism of the rainbow trout gut and the effects of an acute temperature increase and stress event

The fish gut is responsible for numerous potentially energetically costly processes, yet little is known about its metabolism. Here, we provide the first in vivo measurements of aerobic metabolism of the gut in a teleost fish by measuring gut blood flow, as well as arterial and portal venous oxygen content. At 10°C, gut oxygen uptake rate was 4.3±0.5 ml O₂ h^-1 kg^-1 (∼11% of whole-animal oxygen uptake). Following acute warming to 15°C, gut blood flow increased ∼3.4-fold and gut oxygen uptake rate increased ∼3.7-fold (16.0±3.3 ml O₂ h^-1 kg^-1), now representing ∼25% of whole-animal oxygen uptake. Although gut blood flow decreased following an acute stress event at 15°C, gut oxygen uptake remained unchanged as a result of a ∼2-fold increase in oxygen extraction. The high metabolic thermal sensitivity of the gut could have important implications for the overall aerobic capacity and performance of fish in a warming world and warrants further investigation.

Alternagin-C (ALT-C), a Disintegrin-Like Cys-Rich Protein Isolated from the Venom of the Snake Rhinocerophis alternatus, Stimulates Angiogenesis and Antioxidant Defenses in the Liver of Freshwater Fish, Hoplias malabaricus

Alternagin-C (ALT-C) is a disintegrin-like protein isolated from Rhinocerophis alternatus snake venom, which induces endothelial cell proliferation and angiogenesis. The aim of this study was to evaluate the systemic effects of a single dose of alternagin-C (0.5 mg·kg⁻¹, via intra-arterial) on oxidative stress biomarkers, histological alterations, vascular endothelial growth factor (VEGF) production, and the degree of vascularization in the liver of the freshwater fish traíra, Hoplias malabaricus, seven days after the initiation of therapy. ALT-C treatment increased VEGF levels and hepatic angiogenesis. ALT-C also enhanced hepatic antioxidant enzymes activities such as superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase, decreasing the basal oxidative damage to lipids and proteins in the fish liver. These results indicate that ALT-C improved hepatic tissue and may play a crucial role in tissue regeneration mechanisms.

Bigger is not better: cortisol-induced cardiac growth and dysfunction in salmonids

Stress and elevated cortisol levels are associated with pathological heart growth and cardiovascular disease in humans and other mammals. We recently established a link between heritable variation in post-stress cortisol production and cardiac growth in salmonid fish too. A conserved stimulatory effect of the otherwise catabolic steroid hormone cortisol is probably implied, but has to date not been established experimentally. Furthermore, whereas cardiac growth is associated with failure of the mammalian heart, pathological cardiac hypertrophy has not previously been described in fish. Here, we show that rainbow trout (Oncorhynchus mykiss) treated with cortisol in the diet for 45 days have enlarged hearts with lower maximum stroke volume and cardiac output. In accordance with impaired cardiac performance, overall circulatory oxygen-transporting capacity was diminished as indicated by reduced aerobic swimming performance. In contrast to the well-known adaptive/physiological heart growth observed in fish, cortisol-induced growth is maladaptive. Furthermore, the observed heart growth was associated with up-regulated signature genes of mammalian cardiac pathology, suggesting that signalling pathways mediating cortisol-induced cardiac remodelling in fish are conserved from fish to mammals. Altogether, we show that excessive cortisol can induce pathological cardiac remodelling. This is the first study to report and integrate the etiology, physiology and molecular biology of cortisol-induced pathological remodelling in fish.

Physiological constraints to climate warming in fish follow principles of plastic floors and concrete ceilings

Understanding the resilience of aquatic ectothermic animals to climate warming has been hindered by the absence of experimental systems experiencing warming across relevant timescales (for example, decades). Here, we examine European perch (Perca fluviatilis, L.) from the Biotest enclosure, a unique coastal ecosystem that maintains natural thermal fluctuations but has been warmed by 5–10 °C by a nuclear power plant for over three decades. We show that Biotest perch grow faster and display thermally compensated resting cardiorespiratory functions compared with reference perch living at natural temperatures in adjacent waters. However, maximum cardiorespiratory capacities and heat tolerance limits exhibit limited or no thermal compensation when compared with acutely heated reference perch. We propose that while basal energy requirements and resting cardiorespiratory functions (floors) are thermally plastic, maximum capacities and upper critical heat limits (ceilings) are much less flexible and thus will limit the adaptive capacity of fishes in a warming climate.

Understanding climatic adaptation in fish is limited by a lack of large-scale, long term acclimation studies. Here, Sandblom et al. show that fish exposed to a 5-10 °C increase in water temperature next to a nuclear power plant display contrasting upper and lower cardiorespiratory thermal compensation limits.

Cardiac reflexes in a warming world: Thermal plasticity of barostatic control and autonomic tones in a temperate fish

Thermal plasticity of cardiorespiratory function allows ectotherms like fish to cope with seasonal temperature changes and is critical for resilience to climate change. Yet, the chronic thermal effects on cardiovascular homeostatic reflexes in fish are little understood although this may have important implications for physiological performance and overall resilience to climate warming. We compared cardiac autonomic control and baroreflex regulation of heart rate in perch (Perca fluviatilis L.) from a reference area in the Baltic Sea at 18–19°C with conspecifics from the ‘Biotest enclosure’, a chronically heated ecosystem receiving warmed effluent water (24–25°C) from a nuclear power plant. Resting heart rates of Biotest fish displayed clear thermal compensation and were 58.3±2.3 beats min−1 compared with 52.4±2.6 beats min−1 in reference fish at their respective environmental temperatures (Q10: 1.2). The thermally-compensated heart rate of Biotest fish was a combined effect of elevated inhibitory cholinergic tone (105% in Biotest fish versus 70% in reference fish) and reduced intrinsic cardiac pacemaker rate. A barostatic response was evident in both groups, as pharmacologically-induced increases and decreases in blood pressure resulted in atropine-sensitive bradycardia and tachycardia, respectively. Yet, the tachycardia in Biotest fish was significantly greater, presumably due to the larger scope for vagal release. Acclimation of Biotest fish to 18°C for 3 weeks abolished differences in intrinsic heart rate and autonomic tones, suggesting considerable short-term thermal plasticity of cardiovascular control in this species. The heightened hypotensive tachycardia in Biotest perch may represent an important mechanism of ectothermic vertebrates that safeguards tissue perfusion pressure when tissue oxygen demand is elevated by environmental warming.

Alternagin-C (ALT-C), a disintegrin-like protein from Rhinocerophis alternatus snake venom promotes positive inotropism and chronotropism in fish heart

Alternagin-C (ALT-C) is a disintegrin-like protein purified from the venom of the snake, Rhinocerophis alternatus. Recent studies showed that ALT-C is able to induce vascular endothelial growth factor (VEGF) expression, endothelial cell proliferation and migration, angiogenesis and to increase myoblast viability. This peptide, therefore, can play a crucial role in tissue regeneration mechanisms. The aim of this study was to evaluate the effects of a single dose of alternagin-C (0.5 mg kg(-1), via intra-arterial) on in vitro cardiac function of the freshwater fish traíra, Hoplias malabaricus, after 7 days. ALT-C treatment increased the cardiac performance promoting: 1) significant increases in the contraction force and in the rates of contraction and relaxation with concomitant decreases in the values of time to the peak tension and time to half- and 90% relaxation; 2) improvement in the cardiac pumping capacity and maximal electrical stimulation frequency, shifting the optimum frequency curve upward and to the right; 3) increases in myocardial VEGF levels and expression of key Ca(2+)-cycling proteins such as SERCA (sarcoplasmic reticulum Ca(2+)-ATPase), PLB (phospholamban), and NCX (Na(+)/Ca(2+) exchanger); 4) abolishment of the typical negative force-frequency relationship of fish myocardium. In conclusion, this study indicates that ALT-C improves cardiac function, by increasing Ca(2+) handling efficiency leading to a positive inotropism and chronotropism. The results suggest that ALT-C may lead to better cardiac output regulation indicating its potential application in therapies for cardiac contractile dysfunction.

Disruption of the rainbow trout reproductive endocrine axis by the polycyclic aromatic hydrocarbon benzo[a]pyrene

Successful reproduction in salmonids depends on a complex and highly regulated interplay between the pharmacokinetics and pharmacodynamics of naturally circulating sex steroids. The effects of a single intraperitoneal administration of the model PAH benzo[a]pyrene (B[a]P) on the kinetics of circulating levels of estradiol and testosterone through 7d post-injection in mature male and female rainbow trout (Oncorhynchus mykiss) in pre-spawning and spawning condition were investigated. Detailed measurements of the time course of injected E2 and excretion into the bile followed by pharmacokinetic modeling techniques were used to aid in identifying the potential mechanism of ED caused by B[a]P exposure. Plasma E2 and T concentrations were reduced significantly in both male and female trout. Administration of the GnRH analogue des-Gly(10)[D-Ala(6)]LH-RH-ethylamide, to induce spawning steroid profiles increased plasma E2 concentrations in control females, but not in B[a]P-treated fish. The mechanism underlying reductions in sex steroids in pre-spawning and spawning salmonids appears to be unrelated to the induction of P450 and related biotransformation enzymes by B[a]P. Induced biotransformation enzyme activities did not result in altered [(3)H]estradiol pharmacokinetics (e.g. terminal half-life) or elimination of steroid in bile, suggesting that B[a]P alters plasma E2 and T concentrations by other ED mechanisms in an anti-estrogenic manner.

Resolving shifting patterns of muscle energy use in swimming fish

Muscle metabolism dominates the energy costs of locomotion. Although in vivo measures of muscle strain, activity and force can indicate mechanical function, similar muscle-level measures of energy use are challenging to obtain. Without this information locomotor systems are essentially a black box in terms of the distribution of metabolic energy. Although in situ measurements of muscle metabolism are not practical in multiple muscles, the rate of blood flow to skeletal muscle tissue can be used as a proxy for aerobic metabolism, allowing the cost of particular muscle functions to be estimated. Axial, undulatory swimming is one of the most common modes of vertebrate locomotion. In fish, segmented myotomal muscles are the primary power source, driving undulations of the body axis that transfer momentum to the water. Multiple fins and the associated fin muscles also contribute to thrust production, and stabilization and control of the swimming trajectory. We have used blood flow tracers in swimming rainbow trout (Oncorhynchus mykiss) to estimate the regional distribution of energy use across the myotomal and fin muscle groups to reveal the functional distribution of metabolic energy use within a swimming animal for the first time. Energy use by the myotomal muscle increased with speed to meet thrust requirements, particularly in posterior myotomes where muscle power outputs are greatest. At low speeds, there was high fin muscle energy use, consistent with active stability control. As speed increased, and fins were adducted, overall fin muscle energy use declined, except in the caudal fin muscles where active fin stiffening is required to maintain power transfer to the wake. The present data were obtained under steady-state conditions which rarely apply in natural, physical environments. This approach also has potential to reveal the mechanical factors that underlie changes in locomotor cost associated with movement through unsteady flow regimes.

Sympathetic, parasympathetic and enteric regulation of the gastrointestinal vasculature in rainbow trout (Oncorhynchus mykiss) under normal and postprandial conditions

The control of the gastrointestinal hyperemia that occurs after feeding in most animals is of fundamental importance for the subsequent absorption, metabolism and redistribution of nutrients. Yet, in fish, it has received little attention and the nature of it is far from clear. We sought to investigate the importance of extrinsic and intrinsic innervation of the gastrointestinal tract in the regulation of gastrointestinal blood flow in rainbow trout (Oncorhynchus mykiss). The contribution of the extrinsic innervation, i.e. by the sympathetic and the parasympathetic nervous system, was examined by comparing the response to the injection of a predigested nutrient diet into the proximal intestine of untreated fish with the response in fish in which the splanchnic and vagal innervation of the gut had been removed. We also injected the predigested nutrient diet into anaesthetized fish treated with tetrodotoxin that would block the intrinsic innervation of the gut (i.e. enteric nervous system). Our results confirm the notion that the sympathetic portion of the extrinsic innervation maintains the basal vascular tone, but neither the splanchnic nor the vagal innervation is fundamental to the postprandial hyperemia. However, the tetrodotoxin treatment completely abolished the postprandial hyperemia, indicating the importance of the enteric nervous system. In conclusion, it seems as though the enteric nervous system is essential to the regulation of the postprandial hyperemia, and that the extrinsic innervation is involved mainly in the regulation of gastrointestinal blood flow under normal conditions and in response to central coordination with other organs.

Transcriptome responses to heat stress in the nucleated red blood cells of the rainbow trout (Oncorhynchus mykiss)

The retention of a nucleus in the mature state of fish red blood cells (RBCs) and the ability to easily collect and manipulate blood in nonterminal experiments make blood an ideal tissue on which to study the cellular stress response in fish. Through the use of the cGRASP 16K salmonid microarray, we investigated differences in RBC global gene transcription in fish held under control conditions (11°C) and exposed to heat stress (1 h at 25°C followed by recovery at 11°C). Repeated blood sampling (via a dorsal aorta cannula) enables us to examine the individual stress response over time. Samples were taken preheat stress (representing individual control) and at 4 and 24 h postheat stress (representing early and late transcriptional regulation). Approximately 3,000 microarray features had signal above threshold when hybridized with RBC RNA-derived targets, and cannulation did not have a detectable effect on RBC mRNA expression at the investigated time points. Genes involved in the stress response, immune response, and apoptosis were among those showing the highest dysregulation during both early and late transcriptional regulation. Additionally, genes related to the differentiation and development of blood cells were transcriptionally upregulated at the 24 h time point. This study provides a broader understanding of the mechanisms underpinning the stress response in fish and the discovery of novel genes that are regulated in a stress specific manner. Moreover, salmonid transcripts that are consistently dysregulated in blood in response to heat stress are potential candidates of nonlethal biomarkers of exposure to this particular stressor.

Cholecystokinin as a regulator of cardiac function and postprandial gastrointestinal blood flow in rainbow trout (Oncorhynchus mykiss)

We have studied the potential role of CCK as a regulator/modulator of the postprandial increase in gastrointestinal blood flow. Rainbow trout ( Oncorhynchus mykiss ) were instrumented with pulsed Doppler flow probes to measure the effects of CCK on cardiac output and gastrointestinal blood flow. Furthermore, vascular preparations were used to study the direct effects of CCK on the vessels. In addition, we used in situ perfused hearts to further study the effects of CCK on the cardiovascular system. When the sulfated form of CCK-8 was injected at a physiological concentration (0.19 pmol/kg) in vivo, there was a significant increase in the gastrointestinal blood flow (18 ± 4%). This increase in gastrointestinal blood flow was followed by a subsequent increase in cardiac output (30 ± 6%). When the dose was increased to 0.76 pmol/kg, there was only a 14 ± 6% increase in gastrointestinal blood flow; possibly due to a dose-dependent increase in the gill vascular resistance as previously reported or a direct effect on the heart. Nevertheless, CCK did not affect the isolated vessel preparations, and thus, it seems unlikely that CCK has a direct effect on the blood vessels of the second or third order. CCK did, however, have profound effects on the dynamics of the heart, and without a change in cardiac output, there was a significant increase in the amplitude (59 ± 4%) and rate (dQ/d t: 55 ± 4%; -dQ/d t: 208 ± 49%) of the phasic flow profile. If and how this might be coupled to a postprandial gastrointestinal hyperemia remains to be determined. We conclude that CCK has the potential as a regulator of the postprandial gastrointestinal blood flow in fish and most likely has its effect by inducing a gastrointestinal hyperemia. The mechanism by which CCK acts is at present unknown.

Effects of environmental hypoxia on cardiac energy metabolism and performance in tilapia

The ability of an animal to depress ATP turnover while maintaining metabolic energy balance is important for survival during hypoxia. In the present study, we investigated the responses of cardiac energy metabolism and performance in the hypoxia-tolerant tilapia (Oreochromis hybrid sp.) during exposure to environmental hypoxia. Exposure to graded hypoxia (> or =92% to 2.5% air saturation over 3.6 +/- 0.2 h) followed by exposure to 5% air saturation for 8 h caused a depression of whole animal oxygen consumption rate that was accompanied by parallel decreases in heart rate, cardiac output, and cardiac power output (CPO, analogous to ATP demand of the heart). These cardiac parameters remained depressed by 50-60% compared with normoxic values throughout the 8-h exposure. During a 24-h exposure to 5% air saturation, cardiac ATP concentration was unchanged compared with normoxia and anaerobic glycolysis contributed to ATP supply as evidenced by considerable accumulation of lactate in the heart and plasma. Reductions in the provision of aerobic substrates were apparent from a large and rapid (in <1 h) decrease in plasma nonesterified fatty acids concentration and a modest decrease in activity of pyruvate dehydrogenase. Depression of cardiac ATP demand via bradycardia and an associated decrease in CPO appears to be an integral component of hypoxia-induced metabolic rate depression in tilapia and likely contributes to hypoxic survival.

An investigation of the role of carbonic anhydrase in aquatic and aerial gas transfer in the African lungfishProtopterus dolloi

Experiments were performed on bimodally breathing African lungfish Protopterus dolloi to examine the effects of inhibition of extracellular vs total (extracellular and intracellular) carbonic anhydrase (CA) activity on pulmonary and branchial/cutaneous gas transfer. In contrast to previous studies on Protopterus, which showed that the vast majority of CO2 is excreted into the water through the gill and/or skin whereas O2 uptake largely occurs via the lung, P. dolloi appeared to use the lung for the bulk of both O2uptake (91.0±2.9%) and CO2 excretion (76.0±6.6%). In support of the lung as the more important site of CO2 transfer,aerial hypercapnia (PCO2=40 mmHg) caused a significant rise in partial pressure of arterial blood CO2(PaCO2) whereas a similar degree of aquatic hypercapnia was without effect on PaCO2. Intravascular injection of low levels (1.2 mg kg-1) of the slowly permanent CA inhibitor, benzolamide, was without effect on red blood cell CA activity after 30 min, thus confirming its suitability as a short-term selective inhibitor of extracellular CA. Benzolamide treatment did not affect CO2 excretion, blood acid–base status or any other measured variable within the 30 min measurement period. Injection of the permeant CA inhibitor acetazolamide (30 mg kg-1) resulted in the complete inhibition of red cell CA activity within 10 min. However, CO2excretion (measured for 2 h after injection) and arterial blood acid–base status (assessed for 24 h after injection) were unaffected by acetazolamide treatment. Intra-arterial injection of bovine CA (2 mg kg-1) caused a significant increase in overall CO2excretion (from 0.41±0.03 to 0.58±0.03 mmol kg-1h-1) and an increase in air breathing frequency (from 19.0±1.3 to 24.7±1.8 breaths min-1) that was accompanied by a slight, but significant, reduction in PaCO2 (from 21.6±1.6 to 19.6±1.8 mmHg).

The findings of this study are significant because they (i) demonstrate that, unlike in other species of African lungfish that have been examined, the gill/skin is not the major route of CO2 excretion in P. dolloi, and (ii) suggest that CO2 excretion in Protopterus may be less reliant on carbonic anhydrase than in most other fish species.

Cardiorespiratory responses to hypercarbia in tambaquiColossoma macropomum: chemoreceptor orientation and specificity

Experiments were carried out to test the hypothesis that ventilatory and cardiovascular responses to hypercarbia (elevated water PCO2) in the tambaqui Colossoma macropomum are stimulated by externally oriented receptors that are sensitive to water CO2 tension as opposed to water pH. Cardiorespiratory responses to acute hypercarbia were evaluated in both the absence and presence of internal hypercarbia (elevated blood PCO2), achieved by treating fish with the carbonic anhydrase inhibitor acetazolamide. Exposure to acute hypercarbia (15 min at each level, final water CO2 tensions of 7.2,15.5 and 26.3 mmHg) elicited significant increases in ventilation frequency(at 26.3 mmHg, a 42% increase over the normocarbic value) and amplitude(128%), together with a fall in heart rate (35%) and an increase in cardiac stroke volume (62%). Rapid washout of CO2 from the water reversed these effects, and the timing of the changes in cardiorespiratory variables corresponded more closely to the fall in water PCO2(PwCO2) than to that in blood PCO2(PaCO2). Similar responses to acute hypercarbia (15 min,final PwCO2 of 13.6 mmHg) were observed in acetazolamide-treated (30 mg kg-1) tambaqui. Acetazolamide treatment itself, however, increased PaCO2 (from 4.81±0.58 to 13.83±0.91 mmHg, mean ± s.e.m.; N=8) in the absence of significant change in ventilation, heart rate or cardiac stroke volume. The lack of response to changes in blood PCO2 and/or pH were confirmed by comparing responses to the bolus injection of hypercarbic saline(5% or 10% CO2; 2 ml kg-1) into the caudal vein with those to the injection of CO2-enriched water (1%, 3%, 5% or 10%CO2; 50 ml kg-1) into the buccal cavity. Whereas injections of hypercarbic saline were ineffective in eliciting cardiorespiratory responses, changes in ventilation and cardiovascular parameters accompanied injection of CO2-laden water into the mouth. Similar injections of CO2-free water acidified to the corresponding pH of the hypercarbic water (pH 6.3, 5.6, 5.3 or 4.9, respectively) generally did not stimulate cardiorespiratory responses. These results are in agreement with the hypothesis that in tambaqui, externally oriented chemoreceptors that are predominantly activated by increases in water PCO2,rather than by accompanying decreases in water pH, are linked to the initiation of cardiorespiratory responses to hypercarbia.

Baroreflex mediated control of heart rate and vascular capacitance in trout

The baroreflex was triggered by altering branchial blood pressure with pre-and post-branchial occlusions for 30 s in rainbow trout Oncorhynchus mykiss. The cardiac limb of the baroreflex was monitored by continuous heart rate (fH) measurements. Responses of venous capacitance vessels were assessed, immediately following either occlusion, by measuring mean circulatory filling pressure (MCFP). Arterial responses were evaluated as the change in dorsal aortic blood pressure(Pda) before and after pre-branchial occlusion. In untreated fish pre-branchial occlusion resulted in tachycardia(62.4±2.4 to 69.1±1.7 beats min–1), decreased venous capacitance reflected as an increase in MCFP (0.17±0.03 to 0.27±0.03 kPa) and increased Pda (4.0±0.2 kPa compared to 3.2±0.1 kPa before occlusion). Post-branchial occlusion somewhat reversed the responses since fH decreased(62.4±2.4 to 53.0±3.1 beats min–1), whereas MCFP remained unaltered. Treatment with the α-adrenergic blocker prazosin (1 mg kg–1) increased resting MCFP to 0.33±0.03 kPa and appeared to abolish both venous and arterial responses to branchial occlusion. Subsequent atropine treatment (1.2 mg kg–1) abolished all chronotropic responses. We present for the first time ample evidence for baroreflex-mediated control of cardiovascular homeostasis, including both the chronotropic and the vascular limb of the baroreflex in an unanaesthetized fish. Furthermore, a novel technique to cannulate and occlude the dorsal aorta, using a Fogarty thru-lumen embolectomy catheter, is explained.

The use of power spectral analysis to determine cardiorespiratory control in the short-horned sculpinMyoxocephalus scorpius

Anaesthesia and minor surgery to place electrocardiogram recording electrodes in the short-horned sculpin caused a decrease in mean normal beat(R–R) interval and heart rate variability (HRV), measured as the standard deviation in the R–R interval (SDRR). Mean R–R interval increased to a steady state value (1.9±2.9 s) 72 h post-surgery, but SDRR took 120 h to stabilise (0.56±0.09 s). Power spectral analysis applied to recordings of instantaneous heart rate showed no spectral peaks immediately after surgery, with the development of twin peaks (at 0.02 and 0.05 Hz) that also became stable 120 h post surgery. Bilateral cardiac vagotomy abolished the variability in beat-to-beat interval, and both the high and low frequency peaks, suggesting that much of the regulation of heart rate and HRV in sculpin was under parasympathetic, cholinergic control that was withdrawn as a result of surgical and handling stress. Rate of oxygen consumption \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(({\dot{M}}_{\mathrm{O}_{2}})\) \end{document} and heart rate (fH) were monitored simultaneously and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{M}}_{\mathrm{O}_{2}}\) \end{document} showed a good correlation with both mean R–R interval(r2=–0.89) and SDRR (r2=0.93),although a more significant (ANCOVA, P=0.02) covariance existed between the post-surgical decrease in \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{M}}_{\mathrm{O}_{2}}\) \end{document} and increase in SDRR. These data suggest that sculpin use fHas a way of moderating oxygen consumption, fine-tuned on a beat-to-beat basis by cholinergic control. We conclude that power spectral analysis is a useful method of determining HRV in fish, and that HRV is a more sensitive measure of recovery from disturbance than fH alone.

Apparent diffusion limitations on branchial CO2 transfer are revealed by severe experimental anaemia in brown bullhead (Ameiurus nebulosus)

In this study, the rapid (within 2 h) effects of acute anaemia on blood gas and acid-base status, as well as cardiorespiratory variables, were examined in brown bullhead (Ameiurus nebulosus). Anaemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering haematocrit from the control value of 23.5+/-1.0% (mean+/-S.E.M.; N=37) to 5.9+/-0.3% (N=37) resulted in a significant increase (by 2.63+/-0.51 torr; N=7) in arterial CO(2) tension (PaCO(2)) over the subsequent 2-h period in the absence of a change in arterial O(2) tension (PaO(2)). Treatment with bovine carbonic anhydrase (CA) reduced the extent of the PaCO(2) increase to the point where it was not statistically significant. In both control and CA-treated fish, arterial pH decreased during acute anaemia; the acidosis was of mixed respiratory and metabolic origin in control fish and primarily metabolic in CA-treated fish. Inducing anaemia caused increases in both cardiac output (V*b) and heart rate that were similar in control and CA-treated fish. Experimental elevation of V*b equivalent to that observed during anaemia, but in the absence of lowered haematocrit, increased PaCO(2) significantly by 1.49+/-0.74 to 1.64+/-0.78 torr (N=5) without affecting PaO(2). These findings suggest that CO(2) excretion in bullhead, as in rainbow trout, is effectively diffusion-limited, and that approximately half of the increase in PaCO(2) measured during the initial 2 h of anaemia results from the impact of increased blood flow (hence decreased gill transit time) in a diffusion-limited system.

Evidence for glutamatergic mechanisms in the vagal sensory pathway initiating cardiorespiratory reflexes in the shorthorn sculpin Myoxocephalus scorpius

Glutamate is a major neurotransmitter of chemoreceptor and baroreceptor afferent pathways in mammals and therefore plays a central role in the development of cardiorespiratory reflexes. In fish, the gills are the major sites of these receptors, and, consequently, the terminal field (sensory area) of their afferents (glossopharyngus and vagus) in the medulla must be an important site for the integration of chemoreceptor and baroreceptor signals. This investigation explored whether fish have glutamatergic mechanisms in the vagal sensory area (Xs) that could be involved in the generation of cardiorespiratory reflexes. The locations of the vagal sensory and motor (Xm) areas in the medulla were established by the orthograde and retrograde axonal transport of the neural tract tracer Fast Blue following its injection into the ganglion nodosum. Glutamate was then microinjected into identified sites within the Xs in an attempt to mimic chemoreceptor- and baroreceptor-induced reflexes commonly observed in fish. By necessity, the brain injections were performed on anaesthetised animals that were fixed by 'eye bars' in a recirculating water system. Blood pressure and heart rate were measured using an arterial cannula positioned in the afferent branchial artery of the 3rd gill arch, and ventilation was measured by impedance probes sutured onto the operculum. Unilateral injection of glutamate (40-100 nl, 10 mmol l(-1)) into the Xs caused marked cardiorespiratory changes. Injection (0.1-0.3 mm deep) in different rostrocaudal, medial-lateral positions induced a bradycardia, either increased or decreased blood pressure, ventilation frequency and amplitude and, sometimes, an initial apnea. Often these responses occurred simultaneously in various different combinations but, occasionally, they appeared singly, suggesting specific projections into the Xs for each cardiorespiratory variable and local determination of the modality of the response. Response patterns related to chemoreceptor reflex activation were predominantly located rostral of obex, whereas patterns related to baroreceptor reflex activation were more caudal, around obex. The glutamate-induced bradycardia was N-methyl-D-aspartate (NMDA) receptor dependent and atropine sensitive. Taken together, our data provide evidence that glutamate is a putative player in the central integration of chemoreceptor and baroreceptor information in fish.

Cardiorespiratory adjustments during hypercarbia in rainbow troutOncorhynchus mykissare initiated by external CO2receptors on the first gill arch

Experiments were performed to test the hypothesis that the marked ventilatory and cardiovascular responses to hypercarbia in rainbow trout Oncorhynchus mykiss arise from specific stimulation of chemoreceptors localised to the first gill arch. This was accomplished by measuring cardiorespiratory variables during acute hypercarbia (20 min at PCO2=8 mmHg; 1 mmHg=0.133 kPa) in fish subjected to selective bilateral extirpation of the first gill arch. The cardiovascular responses to hypercarbia in the intact fish included a significant bradycardia (from 75.0±1.6 to 69.0±2.0 beats min-1; means ± S.E.M.; N=16), an increase in dorsal aortic blood pressure (from 30.8±1.5 to 41.9±2.5 mmHg; N=16) and a rise in systemic vascular resistance (from 1.1±0.1 to 1.4±0.1 mmHg ml-1 kg-1 min-1; N=16). Removal of the first gill arch or pre-treatment with the muscarinic receptor antagonist atropine prevented the hypercarbic bradycardia without affecting the pressure or resistance responses. Correlation analysis,however, revealed shallow but significant inverse relationships between water PCO2 and cardiac frequency in both atropinised(r2=0.75) and gill-extirpated(r2=0.90) fish, suggesting a direct mild effect of CO2 on cardiac function. The ventilatory response to hypercarbia in the intact fish consisted of an increase in ventilation amplitude (from 0.62±0.06 to 1.0±0.13 cm; N=16) with no change in breathing frequency. Removal of the first gill arch lowered resting breathing frequency and prevented the statistically significant elevation of breathing amplitude. Gill extirpation, however, did not totally abolish the positive correlation between water PCO2 and ventilation amplitude (r2=0.84), suggesting the presence of additional(although less important) chemoreceptive sites that are not confined to the first gill arch. Plasma catecholamine levels were elevated during hypercarbia,and this response was unaffected by prior gill extirpation.

To assess whether the CO2 chemoreceptors of the first gill arch were sensing water and/or blood PCO2, bolus injections of CO2-enriched water or saline were made into the buccal cavity or caudal vein, respectively. Injections of CO2-enriched water to preferentially stimulate external receptors evoked catecholamine release and cardiorespiratory responses that closely resembled the responses to hypercarbia. As in hypercarbia, extirpation of the first gill arch prevented the bradycardia and the increase in ventilation amplitude associated with externally injected CO2-enriched water. Except for a slight decrease in cardiac frequency (from 73.0±2.8 to 70.3±3.5 beats min-1; N=11), injection of CO2-enriched saline to preferentially stimulate internal chemoreceptors did not affect any measured variable. Taken together, these data indicate that, in rainbow trout, the bradycardia and hyperventilation associated with hypercarbia are triggered largely by external CO2chemoreceptors confined to the first gill arch.

The role of angiotensin II in regulating catecholamine secretion during hypoxia in rainbow troutOncorhynchus mykiss

Experiments were performed in vivo on chronically cannulated adult rainbow trout (Oncorhynchus mykiss) to assess the involvement of serotonergic or muscarinic receptor stimulation or activation of the renin–angiotensin system in eliciting catecholamine release during acute hypoxia during periods of nicotinic receptor desensitisation.Despite nicotinic receptor desensitisation induced by intravenous infusion of nicotine (1.3×10–5 mol kg–1 h–1), plasma catecholamine levels were increased to levels (adrenaline plus noradrenaline 125–200 nmol l–1) similar to those in control fish during severe hypoxia (40–45 mmHg; 5.3–6.0 kPa). Blockade of serotonergic receptors using methysergide or of muscarinic receptors using atropine did not affect the ability of fish to elevate circulating catecholamine levels during hypoxia. However, selective blockade of the renin–angiotensin system, using lisinopril to inhibit angiotensin-converting enzyme, prevented the elevation of both angiotensin II and circulating catecholamine levels in acutely hypoxic fish experiencing nicotinic receptor desensitisation. In fish possessing functional nicotinic receptors, angiotensin-converting enzyme blockade attenuated but did not prevent the elevation of plasma catecholamine levels during hypoxia. The results of this study indicate that the renin–angiotensin system is activated during hypoxia and plays a role in eliciting catecholamine release that is secondary to activation of nicotinic receptors. However, under conditions of nicotinic receptor desensitisation, activation of the renin–angiotensin system during hypoxia is a prerequisite for catecholamine release.

The relative roles of external and internal CO2versusH+ in eliciting the cardiorespiratory responses ofSalmo salarandSqualus acanthiasto hypercarbia

Fish breathing hypercarbic water encounter externally elevated PCO2 and proton levels ([H+]) and experience an associated internal respiratory acidosis, an elevation of blood PCO2 and [H+]. The objective of the present study was to assess the potential relative contributions of CO2versus H+ in promoting the cardiorespiratory responses of dogfish (Squalus acanthias) and Atlantic salmon (Salmo salar) to hypercarbia and to evaluate the relative contributions of externally versus internally oriented receptors in dogfish.

In dogfish, the preferential stimulation of externally oriented branchial chemoreceptors using bolus injections (50 ml kg–1) of CO2-enriched (4 % CO2) sea water into the buccal cavity caused marked cardiorespiratory responses including bradycardia (–4.1±0.9 min–1), a reduction in cardiac output (–3.2±0.6 ml min–1 kg–1), an increase in systemic vascular resistance (+0.3±0.2 mmHg ml min–1 kg–1), arterial hypotension (–1.6±0.2 mmHg) and an increase in breathing amplitude (+0.3±0.09 mmHg) (means ± s.e.m., N=9–11). Similar injections of CO2-free sea water acidified to the corresponding pH of the hypercarbic water (pH 6.3) did not significantly affect any of the measured cardiorespiratory variables (when compared with control injections). To preferentially stimulate putative internal CO2/H+ chemoreceptors, hypercarbic saline (4 % CO2) was injected (2 ml kg–1) into the caudal vein. Apart from an increase in arterial blood pressure caused by volume loading, internally injected CO2 was without effect on any measured variable.

In salmon, injection of hypercarbic water into the buccal cavity caused a bradycardia (–13.9±3.8 min–1), a decrease in cardiac output (–5.3±1.2 ml min–1 kg–1), an increase in systemic resistance (0.33±0.08 mmHg ml min–1 kg–1) and increases in breathing frequency (9.7±2.2 min–1) and amplitude (1.2±0.2 mmHg) (means ± s.e.m., N=8–12). Apart from a small increase in breathing amplitude (0.4±0.1 mmHg), these cardiorespiratory responses were not observed after injection of acidified water.

These results demonstrate that, in dogfish and salmon, the external chemoreceptors linked to the initiation of cardiorespiratory responses during hypercarbia are predominantly stimulated by the increase in water PCO2 rather than by the accompanying decrease in water pH. Furthermore, in dogfish, the cardiorespiratory responses to hypercarbia are probably exclusively derived from the stimulation of external CO2 chemoreceptors, with no apparent contribution from internally oriented receptors.

The effects of C-type natriuretic peptide on catecholamine release in the pacific spiny dogfish (Squalus acanthias)

The interaction between homologous C-type natriuretic peptide (dfCNP) and catecholamine release in cardiovascular control was assessed in the marine dogfish (Squalus acanthias). This was accomplished by evaluation of the dynamics of the dfCNP-elicited secretion of catecholamines in situ and in vivo. With an in situ saline-perfused postcardinal sinus preparation, it was demonstrated that perfusion with saline containing dfCNP (10(-9) mol x L(-1)) did not affect the secretion of either noradrenaline or adrenaline. However, the presence of dfCNP in the perfusate significantly enhanced carbachol-evoked secretion of noradrenaline. In vivo, intravascular injection of dfCNP (10(-9) mol x kg(-1)) caused a biphasic pressor-depressor response consisting of a brief increase in caudal artery blood pressure (P(CA)) followed by a prolonged reduction in P(CA). Furthermore, although systemic resistance initially increased, it was subsequently maintained at baseline values in the face of persistent decreases in both P(CA) and cardiac output. Bolus injection of dfCNP elicited significant increases in plasma noradrenaline levels that peaked within 10 min; plasma adrenaline levels were unaffected. The release of noradrenaline elicited by dfCNP was unaffected by prior blockade of the renin-angiotensin system (RAS) (with the angiotensin converting enzyme inhibitor lisinopril) or by pretreatment with the nicotinic receptor blocker hexamethonium. The delayed decrease in P(CA) was not observed in the hexamethonium-treated fish. Prior blockade of beta-adrenoreceptors (with sotalol) or alpha-adrenoreceptors (with prazosin) either significantly reduced (sotalol) or abolished (prazosin) the increase in plasma noradrenaline levels after dfCNP injection. The results of this investigation demonstrate that the elevation of plasma noradrenaline levels observed in vivo following dfCNP injection is not caused by a direct effect of dfCNP on catecholamine secretion from axillary body chromaffin cells. Furthermore, the dfCNP-mediated increase of plasma noradrenaline appears to be unrelated to changes in P(CA) and is insensitive to blockade of the RAS or nicotinic receptors. However, stimulation of adrenergic receptors, in particular the alpha-adrenoreceptors, appears to be a key mechanism underlying the dfCNP-elicited secretion of noradrenaline.

Glucose tolerance and peripheral glucose utilization in rainbow trout (Oncorhynchus mykiss), American eel (Anguilla rostrata), and black bullhead catfish (Ameiurus melas)

This study tests the hypothesis that glucose tolerance in fish is related to nutrient preference and is correlated with white muscle glucose transporter and phosphorylation (hexokinase) activities. Glucose clearance was investigated in the carnivorous rainbow trout (Oncorhynchus mykiss) and American eel (Anguilla rostrata) (feeding and fasting) and the omnivorous black bullhead catfish (Ameiurus melas). Glucose tolerance was assessed by an intravenous glucose tolerance test, injecting 250 mg glucose/kg body weight and tracking blood glucose concentrations over 24 h. Both feeding eel and feeding catfish returned plasma glucose levels to baseline within 60 min of glucose injection. Glucose values remained elevated for more than 360 min in both the food-deprived eel and the feeding rainbow trout. Glucose transport studies in white muscle membrane vesicles provided evidence for the presence of a stereospecific, saturable glucose transporter in all three species. Affinity constants (K(m)) ranged from 8 to 14 mM while V(max) values ranged from 75 to 150 pmol/s/mg protein. Neither kinetic parameter differed significantly between species. Cytochalasin B and phloretin did not significantly inhibit glucose transport, implying that these transporters are unlike the mammalian muscle glucose transporters (GLUT). In fact, Northern and Western blot analyses of mRNA and protein from white and red muscles and heart did not detect a mammalian-type GLUT-1 or -4 in any of the species examined. Glucose phosphorylation indicated the presence of a hexokinase activity (low K(m) enzyme) but again there were no differences in kinetic parameters between species. These studies demonstrate that glucose tolerance in fish is species-dependent but none of the parameters examined clearly differentiate between the species examined. Certainly a stereospecific glucose transporter exists in white skeletal muscle of the fish studied but no molecular or kinetic similarities to the mammalian GLUTs were found. Whether these transporters are insulin-sensitive or contribute to glucose tolerance requires further molecular characterization.