Adenosinergic and cholinergic control mechanisms during hypoxia in the epaulette shark (Hemiscyllium ocellatum), with emphasis on branchial circulation

Fish gill chemosensing: knowledge gaps and inconsistencies

In this review, we explore the inconsistencies in the data and gaps in our knowledge that exist in what is currently known regarding gill chemosensors which drive the cardiorespiratory reflexes in fish. Although putative serotonergic neuroepithelial cells (NEC) dominate the literature, it is clear that other neurotransmitters are involved (adrenaline, noradrenaline, acetylcholine, purines, and dopamine). And although we assume that these agents act on neurons synapsing with the NECs or in the afferent or efferent limbs of the paths between chemosensors and central integration sites, this process remains elusive and may explain current discrepancies or species differences in the literature. To date it has been impossible to link the distribution of NECs to species sensitivity to different stimuli or fish lifestyles and while the gills have been shown to be the primary sensing site for respiratory gases, the location (gills, oro-branchial cavity or elsewhere) and orientation (external/water or internal/blood sensing) of the NECs are highly variable between species of water and air breathing fish. Much of what has been described so far comes from studies of hypoxic responses in fish, however, changes in CO2, ammonia and lactate have all been shown to elicit cardio-respiratory responses and all have been suggested to arise from stimulation of gill NECs. Our view of the role of NECs is broadening as we begin to understand the polymodal nature of these cells. We begin by presenting the fundamental picture of gill chemosensing that has developed, followed by some key unanswered questions about gill chemosensing in general.

Neurochemical Signalling Associated With Gill Oxygen Sensing and Ventilation: A Receptor Focused Mini-Review

Despite the large body of work describing vertebrate ventilatory responses to hypoxia, remarkably little is known about the receptors and afferent pathways mediating these responses in fishes. In this review, we aim to summarize all receptor types to date implicated in the neurotransmission or neuromodulation associated with O2 sensing in the gills of fish. This includes serotonergic, cholinergic, purinergic, and dopaminergic receptor subtypes. Recent transcriptomic analysis of the gills of zebrafish using single-cell RNA sequencing has begun to elucidate specific receptor targets in the gill; however, the absence of receptor characterization at the cellular level in the gill remains a major limitation in understanding the neurochemical control of hypoxia signalling.

Regulation of heart rate in vertebrates during hypoxia: A comparative overview

Acute exposure to low oxygen (hypoxia) places conflicting demands on the heart. Whilst an increase in heart rate (tachycardia) may compensate systemic oxygen delivery as arterial oxygenation falls, the heart itself is an energetically expensive organ that may benefit from slowing (bradycardia) to reduce work when oxygen is limited. Both strategies are apparent in vertebrates, with tetrapods (mammals, birds, reptiles, and amphibians) classically exhibiting hypoxic tachycardia and fishes displaying characteristic hypoxic bradycardia. With a richer understanding of the ontogeny and evolution of the responses, however, we see similarities in the underlying mechanisms between vertebrate groups. For example, in adult mammals, primary bradycardia results from the hypoxic stimulation of carotid body chemoreceptors that are overwhelmed by mechano-sensory feedback from the lung associated with hyperpnoea. Fish-like bradycardia prevails in the mammalian foetus (which, at this stage, is incapable of pulmonary ventilation), and in fish and foetus alike, the bradycardia ensues despite an elevation of circulating catecholamines. In both cases, the reduced heart rate may primarily serve to protect the heart. Thus, the comparative perspective offers fundamental insight into how and why different vertebrates regulate heart rate in different ways during periods of hypoxia.

Cardiophysiological responses of the air-breathing Alaska blackfish to cold acclimation and chronic hypoxic submergence at 5°C

The Alaska blackfish (Dallia pectoralis) remains active at cold temperatures when experiencing aquatic hypoxia without air access. To discern the cardiophysiological adjustments that permit this behaviour, we quantified the effect of acclimation from 15°C to 5°C in normoxia (15N and 5N fish), as well as chronic hypoxic submergence (6-8 weeks; ∼6.3-8.4 kPa; no air access) at 5°C (5H fish), on in vivo and spontaneous heart rate (fH), electrocardiogram, ventricular action potential (AP) shape and duration (APD), the background inward rectifier (IK1) and rapid delayed rectifier (IKr) K+ currents and ventricular gene expression of proteins involved in excitation-contraction coupling. In vivo fH was ∼50% slower in 5N than in 15N fish, but 5H fish did not display hypoxic bradycardia. Atypically, cold acclimation in normoxia did not induce shortening of APD or alter resting membrane potential. Rather, QT interval and APD were ∼2.6-fold longer in 5N than in 15N fish because outward IK1 and IKr were not upregulated in 5N fish. By contrast, chronic hypoxic submergence elicited a shortening of QT interval and APD, driven by an upregulation of IKr The altered electrophysiology of 5H fish was accompanied by increased gene expression of kcnh6 (3.5-fold; Kv11.2 of IKr), kcnj12 (7.4-fold; Kir2.2 of IK1) and kcnj14 (2.9-fold; Kir2.4 of IK1). 5H fish also exhibited a unique gene expression pattern that suggests modification of ventricular Ca2+ cycling. Overall, the findings reveal that Alaska blackfish exposed to chronic hypoxic submergence prioritize the continuation of cardiac performance to support an active lifestyle over reducing cardiac ATP demand.

Can variation among hypoxic environments explain why different fish species use different hypoxic survival strategies?

In aquatic environments, hypoxia is a multi-dimensional stressor that can vary in O₂ level (partial pressure of O₂ in water, Pw_O2 ), rate of induction and duration. Natural hypoxic environments can therefore be very different from one another. For the many fish species that have evolved to cope with these different hypoxic environments, survival requires adjusting energy supply and demand pathways to maintain energy balance. The literature describes innumerable ways that fishes combine aerobic metabolism, anaerobic metabolism and metabolic rate depression (MRD) to accomplish this, but it is unknown whether the evolutionary paths leading to these different strategies are determined primarily by species' phylogenetic histories, genetic constraint or their native hypoxic environments. We explored this idea by devising a four-quadrant matrix that bins different aquatic hypoxic environments according to their duration and Pw_O2  characteristics. We then systematically mined the literature for well-studied species native to environments within each quadrant, and, for each of 10 case studies, described the species' total hypoxic response (THR), defined as its hypoxia-induced combination of sustained aerobic metabolism, enhanced anaerobic metabolism and MRD, encompassing also the mechanisms underlying these metabolic modes. Our analysis revealed that fishes use a wide range of THRs, but that distantly related species from environments within the same matrix quadrant have converged on similar THRs. For example, environments of moderately hypoxic Pw_O2  favoured predominantly aerobic THRs, whereas environments of severely hypoxic Pw_O2  favoured MRD. Capacity for aerial emergence as well as predation pressure (aquatic and aerial) also contributed to these responses, in addition to other biotic and abiotic factors. Generally, it appears that the particular type of hypoxia experienced by a fish plays a major role in shaping its particular THR.

Blood pressure in the Greenland shark as estimated from ventral aortic elasticity

We conducted in vitro inflations of freshly excised ventral aortas of the Greenland shark, Somniosus microcephalus, and used pressure-diameter data to estimate the point of transition from high to low compliance, which has been shown to occur at the mean blood pressure in other vertebrates including fishes. We also determined the pressure at which the modulus of elasticity of the aorta reached 0.4 MPa, as occurs at the compliance transition in other species. From these analyses, we predict the average ventral aortic blood pressure in S. microcephalus to be about 2.3-2.8 kPa, much lower than reported for other sharks. Our results support the idea that this species is slow moving and has a relatively low aerobic metabolism. Histological investigation of the ventral aorta shows that elastic fibres are present in relatively low abundance and loosely connected, consistent with this aorta having high compliance at a relatively low blood pressure.

Mass Transport: Circulatory System with Emphasis on Nonendothermic Species

Mass transport can be generally defined as movement of material matter. The circulatory system then is a biological example given its role in the movement in transporting gases, nutrients, wastes, and chemical signals. Comparative physiology has a long history of providing new insights and advancing our understanding of circulatory mass transport across a wide array of circulatory systems. Here we focus on circulatory function of nonmodel species. Invertebrates possess diverse convection systems; that at the most complex generate pressures and perform at a level comparable to vertebrates. Many invertebrates actively modulate cardiovascular function using neuronal, neurohormonal, and skeletal muscle activity. In vertebrates, our understanding of cardiac morphology, cardiomyocyte function, and contractile protein regulation by Ca2+ highlights a high degree of conservation, but differences between species exist and are coupled to variable environments and body temperatures. Key regulators of vertebrate cardiac function and systemic blood pressure include the autonomic nervous system, hormones, and ventricular filling. Further chemical factors regulating cardiovascular function include adenosine, natriuretic peptides, arginine vasotocin, endothelin 1, bradykinin, histamine, nitric oxide, and hydrogen sulfide, to name but a few. Diverse vascular morphologies and the regulation of blood flow in the coronary and cerebral circulations are also apparent in nonmammalian species. Dynamic adjustments of cardiovascular function are associated with exercise on land, flying at high altitude, prolonged dives by marine mammals, and unique morphology, such as the giraffe. Future studies should address limits of gas exchange and convective transport, the evolution of high arterial pressure across diverse taxa, and the importance of the cardiovascular system adaptations to extreme environments. © 2017 American Physiological Society. Compr Physiol 7:17-66, 2017.

Determinants of coronary blood flow in sandbar sharks, Carcharhinus plumbeus

The coronary circulation first appeared in the chordate lineage in cartilaginous fishes where, as in birds and mammals but unlike most teleost fishes, it supplies arterial blood to the entire myocardium. Despite the pivotal position of elasmobranch fishes in the evolution of the coronary circulation, the determinants of coronary blood flow have never been investigated in this group. Elasmobranch fishes are of special interest because of the morphological arrangement of their cardiomyocytes. Unlike teleosts, the majority of the ventricular myocardium in elasmobranch fishes is distant to the venous blood returning to the heart (i.e., the luminal blood). Also, the majority of the myocardium is in close association with the coronary circulation. To determine the relative contribution of the coronary and luminal blood supplies to cardiovascular function in sandbar sharks, Carcharhinus plumbeus, we measured coronary blood flow while manipulating cardiovascular status using acetylcholine and adrenaline. By exploring inter- and intra-individual variation in cardiovascular variables, we show that coronary blood flow is directly related to heart rate (R ² = 0.6; P < 0.001), as it is in mammalian hearts. Since coronary blood flow is inversely related to coronary resistance both in vivo and in vitro, we suggest that in elasmobranch fishes, changes in heart rate mediate changes in coronary vascular resistance, which adjust coronary blood flow appropriately.

The adenylate energy charge as a new and useful indicator of capture stress in chondrichthyans

Quantifying the physiological stress response of chondrichthyans to capture has assisted the development of fishing practices conducive to their survival. However, currently used indicators of stress show significant interspecific and intraspecific variation in species' physiological responses and tolerances to capture. To improve our understanding of chondrichthyan stress physiology and potentially reduce variation when quantifying the stress response, we investigated the use of the adenylate energy charge (AEC); a measure of available metabolic energy. To determine tissues sensitive to metabolic stress, we extracted samples of the brain, heart, liver, white muscle and blood from gummy sharks (Mustelus antarcticus) immediately following gillnet capture and after 3 h recovery under laboratory conditions. Capture caused significant declines in liver, white muscle and blood AEC, whereas no decline was detected in the heart and brain AEC. Following 3 h of recovery from capture, the AEC of the liver and blood returned to "unstressed" levels (control values) whereas white muscle AEC was not significantly different to that immediately after capture. Our results show that the liver is most sensitive to metabolic stress and white muscle offers a practical method to sample animals non-lethally for determination of the AEC. The AEC is a highly informative indicator of stress and unlike current indicators, it can directly measure the change in available energy and thus the metabolic stress experienced by a given tissue. Cellular metabolism is highly conserved across organisms and, therefore, we think the AEC can also provide a standardised form of measuring capture stress in many chondrichthyan species.

Adenosinergic regulation of the cardiovascular system in the red-eared slider Trachemys scripta

Few studies have investigated adenosinergic regulation of the cardiovascular system in reptiles. The haemodynamic effect of a bolus intra-arterial adenosine injection (2.5 μM kg⁻¹) was investigated in nine anaesthetised red-eared sliders (Trachemys scripta). Adenosine caused a transient bradycardia, which was accompanied by systemic vasodilatation as evidenced by an increase in systemic flow and a decrease in systemic pressure. Meanwhile, pulmonary flow fell significantly. Both the bradycardia and increase in systemic conductance were significantly attenuated by theophylline (4 mg kg⁻¹), demonstrating an involvement of P₁ receptors. These results suggest that adenosine is likely to play a significant role in reptile cardiovascular physiology. In turtles specifically, adenosinergic regulation may be particularly relevant during periods of apnoea.

Hypoxia tolerance in elasmobranchs. II. Cardiovascular function and tissue metabolic responses during progressive and relative hypoxia exposures

Cardiovascular function and metabolic responses of the heart and other tissues during hypoxia exposure were compared between the hypoxia-tolerant epaulette shark (Hemiscyllium ocellatum) and the hypoxia-sensitive shovelnose ray (Aptychotrema rostrata). In both species, progressive hypoxia exposure caused increases in stroke volume and decreases in heart rate, cardiac output, cardiac power output (CPO, an assessment of cardiac energy demand) and dorsal aortic blood pressure, all of which occurred at or below each species' critical P(O2) for whole-animal O(2) consumption rate, M(O2) (P(crit)). In epaulette sharks, which have a lower P(crit) than shovelnose rays, routine levels of cardiovascular function were maintained to lower water P(O2) levels and the changes from routine levels during hypoxia exposure were smaller compared with those for the shovelnose ray. The maintenance rather than depression of cardiovascular function during hypoxia exposure may contribute to the superior hypoxia tolerance of the epaulette shark, presumably by improving O(2) delivery and waste removal. Compared with shovelnose rays, epaulette sharks were also better able to maintain a stable cardiac high-energy phosphate pool and to minimize metabolic acidosis and lactate accumulation in the heart (despite higher CPO) and other tissues during a 4 h exposure to 40% of their respective P(crit) (referred to as a relative hypoxia exposure), which results in similar hypoxaemia in the two species (∼16% Hb-O(2) saturation). These different metabolic responses to relative hypoxia exposure suggest that variation in hypoxia tolerance among species is not solely dictated by differences in O(2) uptake and transport but also by tissue-specific metabolic responses. In particular, lower tissue [lactate] accumulation in epaulette sharks than in shovelnose rays during relative hypoxia exposure suggests that enhanced extra-cardiac metabolic depression occurs in the former species. This could facilitate strategic utilization of available O(2) for vital organs such as the heart, potentially explaining the greater hypoxic cardiovascular function of epaulette sharks.

Cardiac responses to anoxia in the Pacific hagfish, Eptatretus stoutii

In the absence of any previous study of the cardiac status of hagfishes during prolonged anoxia and because of their propensity for oxygen-depleted environments, the present study tested the hypothesis that the Pacific hagfish Eptatretus stoutii maintains cardiac performance during prolonged anoxia. Heart rate was halved from the routine value of 10.4±1.3 beats min⁻¹ by the sixth hour of an anoxic period and then remained stable for a further 30 h. Cardiac stroke volume increased from routine (1.3±0.1 ml kg⁻¹) to partially compensate the anoxic bradycardia, such that cardiac output decreased by only 33% from the routine value of 12.3±0.9 ml min⁻¹ kg⁻¹. Cardiac power output decreased by only 25% from the routine value of 0.26±0.02 mW g⁻¹. During recovery from prolonged anoxia, cardiac output and heart rate increased to peak values within 1.5 h. Thus, the Pacific hagfish should be acknowledged as hypoxic tolerant in terms of its ability to maintain around 70% of their normoxic cardiac performance during prolonged anoxia. This is only the second fish species to be so classified.

Ecophysiology of neuronal metabolism in transiently oxygen-depleted environments: evidence that GABA is accumulated pre-synaptically in the cerebellum

Interactions between coral reef topography, tide cycles, and photoperiod provided selection pressure for adaptive physiological changes in sheltered hypoxic niches to be exploited by specialized tropical reef fish. The epaulette shark Hemiscyllium ocellatum withstands cyclic hypoxia in its natural environment, many hours of experimental hypoxia, and anoxia for up to 5h. It shows neuronal hypometabolism in response to 5% oxygen saturation. Northern-hemisphere hypoxia- and anoxia-tolerant vertebrates that over-winter under ice alter their inhibitory to excitatory neurotransmitter balance to forestall brain ATP depletion in the absence of oxidative phosphorylation. GABA immunochemistry, HPLC analysis and receptor binding studies in H. ocellatum cerebellum revealed a heterogeneous regional accumulation of neuronal GABA despite no change in its overall concentration, and a significant increase in GABA(A) receptor density without altered binding affinity. Increased GABA(A) receptor density would protect the cerebellum during reoxygenation when transmitter release resumes. While all hypoxia- and anoxia-tolerant teleosts examined to date respond to low oxygen levels by elevating brain GABA, the phylogenetically older epaulette shark did not, suggesting that it uses an alternative neuroprotective mechanism for energy conservation. This may reflect an inherent phylogenetic difference, or represent a novel ecophysiological adaptation to cyclic variations in the availability of oxygen.

Effect of anoxia on the electroretinogram of three anoxia-tolerant vertebrates

To survive anoxia, neural ATP levels have to be defended. Reducing electrical activity, which accounts for 50% or more of neural energy consumption, should be beneficial for anoxic survival. The retina is a hypoxia sensitive part of the central nervous system. Here, we quantify the in vivo retinal light response (electroretinogram; ERG) in three vertebrates that exhibit varying degrees of anoxia tolerance: freshwater turtle (Trachemys scripta), epaulette shark (Hemiscyllium ocellatum) and leopard frog (Rana pipiens). A virtually total suppression of ERG in anoxia, probably resulting in functional blindness, has previously been seen in the extremely anoxia-tolerant crucian carp (Carassius carassius). Surprisingly, the equally anoxia-tolerant turtle, which strongly depresses brain and whole-body metabolism during anoxia, exhibited a relatively modest anoxic reduction in ERG: the combined amplitude of turtle ERG waves was reduced by approximately 50% after 2 h. In contrast, the shark b-wave amplitude practically disappeared after 30 min of severe hypoxia, and the frog b-wave was decreased by approximately 75% after 40 min in anoxia. The specific A(1) adenosine receptor antagonist CPT significantly delayed the suppression of turtle ERG, while the hypoxic shark ERG was unaffected by the non-specific adenosine receptor antagonist aminophylline, suggesting adenosinergic involvement in turtle but not in shark.

Tribute to P. L. Lutz: a message from the heart--why hypoxic bradycardia in fishes?

The sensing and processing of hypoxic signals, the responses to these signals and the modulation of these responses by other physical and physiological factors are an immense topic filled with numerous novel and exciting discoveries. Nestled among these discoveries, and in contrast to mammals, is the unusual cardiac response of many fish to environmental hypoxia - a reflex slowing of heart rate. The afferent and efferent arms of this reflex have been characterised, but the benefits of the hypoxic bradycardia remain enigmatic since equivocal results have emerged from experiments examining the benefit to oxygen transfer across the gills. The main thesis developed here is that hypoxic bradycardia could afford a number of direct benefits to the fish heart, largely because the oxygen supply to the spongy myocardium is precarious (i.e. it is determined primarily by the partial pressure of oxygen in venous blood, Pv(O(2))) and, secondarily, because the fish heart has an unusual ability to produce large increases in cardiac stroke volume (V(SH)) that allow cardiac output to be maintained during hypoxic bradycardia. Among the putative benefits of hypoxic bradycardia is an increase in the diastolic residence time of blood in the lumen of the heart, which offers an advantage of increased time for diffusion, and improved cardiac contractility through the negative force-frequency effect. The increase in V(SH) will stretch the cardiac chambers, potentially reducing the diffusion distance for oxygen. Hypoxic bradycardia could also reduce cardiac oxygen demand by reducing cardiac dP/dt and cardiac power output, something that could be masked at cold temperature because of a reduced myocardial work load. While the presence of a coronary circulation in certain fishes decreases the reliance of the heart on Pv(O(2)), hypoxic bradycardia could still benefit oxygen delivery via an extended diastolic period during which peak coronary blood flow occurs. The notable absence of hypoxic bradycardia among fishes that breathe air during aquatic hypoxia and thereby raise their Pv(O(2)), opens the possibility that that the evolutionary loss of hypoxic bradycardia may have coincided with some forms of air breathing in fishes. Experiments are needed to test some of these possibilities. Ultimately, any potential benefit of hypoxic bradycardia must be placed in the proper context of myocardial oxygen supply and demand, and must consider the ability of the fish heart to support its routine cardiac power output through glycolysis.

Adenosine deaminase-related genes: molecular identification, tissue expression pattern and truncated alternative splice isoform in adult zebrafish (Danio rerio)

Adenosine deaminase (ADA) is responsible for cleaving the neuromodulator adenosine to inosine. Two members of ADA subfamilies, known as ADA1 and ADA2, were described and evidence demonstrated another similar protein group named ADAL (adenosine deaminase "like"). Although the identification of ADA members seems to be consistent, the expression profile of ADA1, ADA2 and ADAL genes in zebrafish has not yet been reported. The aim of the present study was to map the expression pattern of ADA-related genes in various tissues of adult zebrafish (Danio rerio). An extensive search on zebrafish genome followed by a phylogenetic analysis confirmed the presence of distinct ADA-related genes (ADA1, ADAL and two orthologous genes of ADA2). Specific primers for each ADA member were designed, optimized semi-quantitative RT-PCR experiments were conducted and the relative amount of transcripts was determined. The tissue samples (brain, gills, heart, liver, skeletal muscle and kidney) were collected and the expression of ADA1, ADAL and ADA2 genes was characterized. ADA1 had a similar expression pattern, whereas ADAL was less expressed in the heart. The highest relative amount of ADA2-1 transcripts was observed in the brain, liver and gills and it was less expressed in the heart. RT-PCR assays revealed that the other ADA2 form (ADA2-2) was expressed ubiquitously and at comparable levels in zebrafish tissues. The strategy adopted also allowed the identification of an ADA2-1 truncated alternative splice isoform (ADA2-1/T), which was expressed at different intensities. These findings demonstrated the existence of different ADA-related genes, their distinct expression pattern and a truncated ADA2-1 isoform, which suggest a high degree of complexity in zebrafish adenosinergic system.

Adenosine does not save the heart of anoxia-tolerant vertebrates during prolonged oxygen deprivation

Despite adenosine being regarded as an important signaling molecule capable of coordinating ATP supply and demand during periods of oxygen deprivation in anoxia-intolerant mammals, the importance of adenosinergic cardiovascular control in anoxia-tolerant vertebrates is poorly understood. Here, we report on adenosinergic cardiovascular control during normoxia and prolonged (hours to days) oxygen deprivation for three vertebrate species tolerant of severe hypoxia/anoxia, the closely related common (Cyprinus carpio) and crucian (Carassius carassius) carp, and the freshwater turtle (Trachemys scripta). Using an intra-arterial injection of the non-specific adenosine receptor antagonist aminophylline while measuring cardiac output (Q), heart rate (f(H)) and arterial blood pressure, we establish that adenosinergic cardiovascular control is unimportant during prolonged anoxia in the freshwater turtle (6 h at 21 degrees C and 14 d at 5 degrees C) and the crucian carp (5 d at 8 degrees C). In contrast, adenosinergic control contributes to the down-regulation of cardiac activity exhibited by 5 degrees C-acclimated common carp during a 12.5 h severe hypoxia (<0.3 mg O2 l(-1)) exposure. Specifically, aminophylline injection resulted in significant increases in f(H) and Q, and a decrease in total peripheral resistance. These species-specific differences in adenosinergic cardiovascular control during prolonged periods of oxygen deprivation may be related to differences displayed by these three species in their anoxia tolerance and survival strategies.

Regulation of the cardiorespiratory system of common carp (Cyprinus carpio) during severe hypoxia at three seasonal acclimation temperatures

Little is known of the cardiorespiratory control mechanisms utilized by hypoxia-tolerant teleost fish to tolerate prolonged periods (h) of near anoxic exposure. Here, we report on the cardiorespiratory control mechanisms of the common carp Cyprinus carpio L. during normoxia and prolonged, severe hypoxic (<0.3 mg O(2) L(-1)) exposure at acclimation temperatures of 5 degrees C, 10 degrees C, and 15 degrees C. Through serial intra-arterial injections of alpha - and beta -adrenergic, cholinergic, and purinergic antagonists while measuring cardiac output (Q), heart rate (f(H)), ventral aortic blood pressure, and respiration rate, we established that autonomic cardiovascular and respiratory control was preserved during severe hypoxia at all three acclimation temperatures and contributed to downregulation of cardiorespiratory activity. Specifically, inhibitory cholinergic tone mediated up to 76% reductions in f(H) and Q during hypoxia, whereas the accompanying arterial hypotension was attenuated by an upregulation of an alpha -adrenergically mediated peripheral vasoconstriction. Despite the overall cardiac downregulation, a large, stimulatory cardiac beta -adrenergic tone was present during prolonged, severe hypoxia, possibly to protect the heart from attendant acidotic conditions. Purinergic blockade, following alpha -adrenergic and cholinergic antagonists, showed that the hypoxic ventilatory depression, which reversed the 2.3- to 7.7-fold increases in respiration rate that occurred with the onset of hypoxia, was a result of purinergic inhibition at all three acclimation temperatures. In contrast, purinergic inhibition of cardiac activity during hypoxia might be important only at 5 degrees C. Finally, given that cardiac power output was reduced 72%-87% during prolonged, severe hypoxia and that glycolysis yields approximately 94% less ATP per mole glucose than oxidative phosphorylation, it seems unlikely that the common carp sufficiently reduces its cardiac energy demand to a level to preclude activation of a partial Pasteur effect. This means that glycogen stores will be used and waste products will accumulate at faster rates, a finding that may help explain why the common carp cannot tolerate such extended periods of severe hypoxia (weeks to months) at cold acclimation temperatures as the freshwater turtle, which is able to reduce its cardiac energy demand to a level that does not require a Pasteur effect and also blunts autonomic cardiovascular control.