Adenosine receptor blockade and hypoxia-tolerance in rainbow trout and Pacific hagfish. II. Effects on plasma catecholamines and erythrocytes

Long-term hypoxia exposure alters the cardiorespiratory physiology of steelhead trout (Oncorhynchus mykiss), but does not affect their upper thermal tolerance

It has been suggested that exposure to high temperature or hypoxia may confer tolerance to the other oxygen-limited stressor (i.e., 'cross-tolerance'). Thus, we investigated if chronic hypoxia-acclimation (>3 months at 40% air saturation) improved the steelhead trout's critical thermal maximum (CT_Max), or affected key physiological variables that could impact upper thermal tolerance. Neither CT_Max (24.7 vs. 25.3°C) itself, nor oxygen consumption ( [Formula: see text] ), haematocrit, blood haemoglobin concentration, or heart rate differed between hypoxia- and normoxia-acclimated trout when acutely warmed. However, the cardiac output (Q̇) of hypoxia-acclimated fish plateaued earlier compared to normoxia-acclimated fish due to an inability to maintain stroke volume (S_V), and this resulted in a ~50% lower maximum Q̇. Despite this reduced maximum cardiac function, hypoxia-acclimated trout were able to consume more O₂ per volume of blood pumped as evidenced by the equivalent [Formula: see text] . These results provide additional evidence that long-term hypoxia improves tissue oxygen utilization, and that this compensates for diminished cardiac pumping capacity. The limited S_V in hypoxia-acclimated trout in vivo was not associated with changes in cardiac morphology or in vitro maximum S_V, but the affinity and density of myocardial ß-adrenoreceptors were lower and higher, respectively, than in normoxia-acclimated fish. These data suggest that alterations in ventricular filling dynamics or myocardial contractility constrain cardiac function in hypoxia-acclimated fish at high temperatures. Our results do not support (1) 'cross-tolerance' between high temperature and hypoxia when hypoxia is chronic, or (2) that cardiac function is always the determinant of temperature-induced changes in fish [Formula: see text] , and thus thermal tolerance, as suggested by the oxygen- and capacity-limited thermal tolerance (OCLTT) theory.

Adenosine A1, but not A2, receptor blockade increases anxiety and arousal in Zebrafish

Adenosinergic systems have been implicated in anxiety-like states, as caffeine can induce a state of anxiety in human beings. Caffeine is an antagonist at A(1) and A(2) adenosine receptors but it remains unclear whether anxiety is mediated by one or both of these. As the adenosinergic system is rather conserved, we opted to pursue these questions using zebrafish, a widely used model organism in genetics and developmental biology. Zebrafish adenosine 1. 2A.1 and 2A.2 receptors conserve histidine residues in TM6 and TM7 that are responsible for affinity in bovine A1 receptor. We investigated the effects of caffeine, PACPX (an A(1) receptor antagonist) and 1,3-dimethyl-1-propargylxanthine (DMPX) (an A(2) receptor antagonist) on anxiety-like behaviour and locomotor activity of zebrafish in the scototaxis test as well as evaluated the effects of these drugs on pigment aggregation. Caffeine increased anxiety at the dose of 100 mg/kg, while locomotion at the dose of 10 mg/kg was increased. Both doses of 10 and 100 mg/kg induced pigment aggregation. PACPX, on the other hand, increased anxiety at a dose of 6 mg/kg and induced pigment aggregation at the doses of 0.6 and 6 mg/kg, but did not produce a locomotor effect. DMPX, in turn, increased locomotion at the dose of 6 mg/kg but did not produce any effect on pigment aggregation or anxiety-like behaviour. These results indicate that blockade of A(1)-R, but not A(2)-R, induces anxiety and autonomic arousal, while the blockade of A(2)-R induces hyperlocomotion. Thus, as in rodents, caffeine's anxiogenic and arousing effects are probably mediated by A(1) receptors in zebrafish and its locomotor activating effect is probably mediated by A(2) receptors.

Ontogeny of the corticotropin-releasing factor system in rainbow trout and differential effects of hypoxia on the endocrine and cellular stress responses during development

To further our understanding of the development of the stress axis and the responsiveness of embryonic and larval fish to environmental stressors, this study examined the ontogeny of whole-body cortisol levels and of the corticotropin-releasing factor (CRF) system in rainbow trout, as well as the endocrine and cellular stress responses to hypoxia. After depletion of a maternal deposit, de novo synthesis of cortisol increased exponentially between the 'eyed' stage and first feeding. Whole body CRF mRNA levels dominated over those of the related peptide urotensin I (UI) from hatch through complete yolk sac absorption. The mRNA levels of CRF-binding protein (CRF-BP) closely paralleled those of CRF and UI throughout ontogeny except at first feeding when an increase in CRF gene expression was not matched by change in CRF-BP transcript abundance. In the hypoxia challenge, fish were exposed to 15% O(2) saturation for either 90 min or 24h at three key developmental stages: hatch, swim up and first feeding. While the embryos were unaffected, chronic hypoxia elicited a transient 2-fold increase in whole-body cortisol levels in the larval stages. The hypoxia challenge also generally suppressed the mRNA levels of CRF and CRF-BP, had no effect on the expression of UI, but had a marked stimulatory effect on heat shock protein 70 (Hsp70) gene expression. Taken together, these results suggest a role for the CRF system in the ontogenic regulation of corticosteroidogenesis and show that hypoxia has developmental stage-specific effects on the endocrine and cellular stress responses in rainbow trout.

Anoxic survival of the Pacific hagfish (Eptatretus stoutii)

It is not known how the Pacific hagfish (Eptatretus stoutii) can survive extended periods of anoxia. The present study used two experimental approaches to examine energy use during and following anoxic exposure periods of different durations (6, 24 and 36 h). By measuring oxygen consumption prior to anoxic exposure, we detected a circadian rhythm, with hagfish being active during night and showing a minimum routine oxygen consumption (RMR) during the daytime. By measuring the excess post-anoxic oxygen consumption (EPAOC) after 6 and 24 h it was possible to mathematically account for RMR being maintained even though heme stores of oxygen would have been depleted by the animal's metabolism during the first hours of anoxia. However, EPAOC after 36 h of anoxia could not account for RMR being maintained. Measurements of tissue glycogen disappearance and lactate appearance during anoxia showed that the degree of glycolysis and the timing of its activation varied among tissues. Yet, neither measurement could account for the RMR being maintained during even the 6-h anoxic period. Therefore, two independent analyses of the metabolic responses of hagfish to anoxia exposure suggest that hagfish utilize metabolic rate suppression as part of the strategy for longer-term anoxia survival.

The autonomic nervous system and chromaffin tissue: neuroendocrine regulation of catecholamine secretion in non-mammalian vertebrates

If severe enough, periods of acute stress in animals may be associated with the release of catecholamine hormones (noradrenaline and adrenaline) into the circulation; a response termed the acute humoral adrenergic stress response. The release of catecholamines from the sites of storage, the chromaffin cells, is under neuroendocrine control, the complexity of which appears to increase through phylogeny. In the agnathans, the earliest branching vertebrates, the chromaffin cells which are localized predominantly within the heart, lack neuronal innervation and thus catecholamine secretion in these animals is initiated solely by humoral mechanisms. In the more advanced teleost fish, the chromaffin cells are largely confined to the walls of the posterior cardinal vein at the level of the head kidney where they are intermingled with the steroidogenic interrenal cells. Catecholamine secretion from teleost chromaffin cells is regulated by a host of cholinergic and non-cholinergic pathways that ensure sufficient redundancy and flexibility in the secretion process to permit synchronized responses to a myriad of stressors. The complexity of catecholamine secretion control mechanisms continues through the amphibians, reptiles and birds although neural (cholinergic) regulation may become increasingly important in birds. Discrete adrenal glands are present in the non-mammalian tetrapods but unlike in mammals, there is no clear division of a steroidogenic cortex and a chromaffin cell enriched medulla. However, in all groups, there is an obvious intermingling of chromaffin and steroiodogenic cells. The association of the two cell types may be particularly important in the amphibians and birds because like in mammals, the enzyme catalysing the methylation of noradrenaline to adrenaline, PNMT, is under the control of the steroid cortisol.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Increased systemic vascular resistance in Atlantic salmon, Salmo salar L., affected with amoebic gill disease

Previous investigations into the pathophysiology of amoebic gill disease (AGD) have suggested that there are probable cardiovascular effects associated with this disease. In the present study Atlantic salmon, Salmo salar L., were experimentally infected by cohabitation with diseased individuals. Two commonly used vasodilators, sodium nitroprusside (SNP) and captopril, the angiotensin-converting enzyme (ACE) inhibitor, were used as tools to investigate possible vasoconstriction and/or renin-angiotensin system (RAS) dysfunction in AGD-affected animals. Within the SNP trial, results showed that AGD-affected fish exhibited lowered cardiac output (Q), lowered cardiac stroke volume (V(S)) and a significantly elevated systemic vascular resistance (R(S)) compared with non-affected naïve counterparts. These effects were totally abolished following SNP administration (40 microg kg(-1)), however significant cardiovascular effects associated with SNP were not observed. Within the captopril trial, where AGD-affected fish were more diseased compared with the SNP trial, a significant hypertension was observed in AGD-affected fish. Captopril administration (10(-4) mol L(-1) at 1 mL kg(-1)) resulted in a significant drop in dorsal aortic pressure (P(DA)) for both AGD-affected and naïve control fish. In terms of peak individual responses, captopril administration effectively lowered P(DA) in both AGD-affected and naïve control groups equally. The drop in P(DA) following SNP administration however was significantly greater in AGD-affected fish potentially suggesting disease-related vasoconstriction. The lack of significant cardiovascular effects directly associated with both SNP and captopril administrations possibly relate to the 6 h recovery period following surgical procedures. However, while variable, these results do suggest that there are significant cardiovascular effects including vasoconstriction and hypertension associated with AGD.

Changes in plasma catecholamine concentration during salinity manipulation and anaesthesia in the hagfish Eptatretus cirrhatus

Plasma catecholamines were measured following surgery under anaesthesia and after exposing hagfish to 90 and 110% sea water (SW). Plasma noradrenaline (NA) concentration increased from a resting value of 7 to 818 nM l(-1) on anaesthesia. Plasma adrenaline (AD) did not change. NA concentrations also increased during volume depletion (110% SW), but to much lower values (26 nM l(-1 )at 100 min). AD concentrations were increased at 20 min, then fell. During volume loading (90% SW) NA fell, and AD increased to a maximum concentration of 511 nM l(-1) at 40 min (resting concentration 24 nM l(-1)). The data are consistent with a vasoconstrictory role for NA on central veins when venous pressures fall and a vasodilatory role for AD on volume expansion.

Cardiorespiratory and tissue adenosine responses to hypoxia and reoxygenation in the short-horned sculpinMyoxocephalus scorpius

Adenosine is a product of adenylate phosphate breakdown that can exert protective effects on tissues during energy limitation. Accumulation of cardiac adenosine under hypoxia is well documented in mammals but has not been shown in fish. Adenosine content was measured in heart and brain tissue from short-horned sculpin Myoxocephalus scorpius L. exposed to acute hypoxia and to graded hypoxia and reoxygenation at 8°C. Cardiorespiratory parameters were recorded along with plasma lactate, K+,Ca2+ and Na+ levels and their relationship to adenosine levels investigated. Sculpin exhibited a large bradycardia during hypoxia,with a concomitant drop in cardiac output that recovers fully with reoxygenation. Ventilation rate also declined with hypoxia, suggesting a depression of activity. Plasma lactate concentration was significantly elevated after 4 h at 2.0 mg l-1 dissolved oxygen while K+ levels increased during acute hypoxia. Adenosine levels were maintained in heart under acute and graded hypoxia. Brain levels fluctuated under hypoxia and showed no change with reoxygenation. It is concluded that a depression of cardiac activity in conjunction with an adequate anaerobic metabolism allow sculpin to avoid excessive adenosine accumulation under conditions of moderate hypoxia. Cardiac adenosine levels decreased and plasma K+ levels and heart rate increased significantly at reoxygenation.

A comparison of adrenergic stress responses in three tropical teleosts exposed to acute hypoxia

Experiments were performed to assess the afferent and efferent limbs of the hypoxia-mediated humoral adrenergic stress response in selected hypoxia-tolerant tropical fishes that routinely experience environmental O2depletion. Plasma catecholamine (Cat) levels and blood respiratory status were measured during acute aquatic hypoxia [water Po2(PwO2) = 10–60 mmHg] in three teleost species, the obligate water breathers Hoplias malabaricus (traira) and Piaractus mesopotamicus (pacu) and the facultative air breather Hoplerythrinus unitaeniatus (jeju). Traira displayed a significant increase in plasma Cat levels (from 1.3 ± 0.4 to 23.3 ± 15.1 nmol/l) at PwO2levels below 20 mmHg, whereas circulating Cat levels were unaltered in pacu at all levels of hypoxia. In jeju denied access to air, plasma Cat levels were increased markedly to a maximum mean value of 53.6 ± 19.1 nmol/l as PwO2was lowered below 40 mmHg. In traira and jeju, Cat release into the circulation occurred at abrupt thresholds corresponding to arterial Po2(PaO2) values of approximately 8.5–12.5 mmHg. A comparison of in vivo blood O2equilibration curves revealed low and similar P50values (i.e., PaO2at 50% Hb-O2saturation) among the three species (7.7–11.3 mmHg). Thus Cat release in traira and jeju occurred as blood O2concentration was reduced to approximately 50–60% of the normoxic value. Intravascular injections of nicotine (600 nmol/kg) elicited pronounced increases in plasma Cat levels in traira and jeju but not in pacu. Thus the lack of Cat release during hypoxia in pacu may reflect an inoperative or absent humoral adrenergic stress response in this species. When allowed access to air, jeju did not release Cats into the circulation at any level of aquatic hypoxia. The likeliest explanation for the absence of Cat release in these fish was that air breathing, initiated by aquatic hypoxia, prevented PaO2values from falling to the critical threshold required for Cat secretion. The ventilatory responses to hypoxia in each species were similar, consisting generally of increases in both frequency and amplitude. These responses were not synchronized with or influenced by plasma Cat levels. Thus the acute humoral adrenergic stress response does not appear to stimulate ventilation during acute hypoxia in these tropical species.

Gene expression profile of zebrafish exposed to hypoxia during development

Understanding how vertebrates respond to hypoxia can have important clinical implications. Fish have evolved the ability to survive long exposure to low oxygen levels. However, little is known about the specific changes in gene expression that result from hypoxia. In this study we used a zebrafish cDNA microarray to examine the expression of >4,500 genes in zebrafish embryos exposed to 24 h of hypoxia during development. We tested the hypotheses that hypoxia changes gene expression profile of the zebrafish embryos and that these changes can be reverted by reexposure to a normoxic (20.8% O(2)) environment. Our data were consistent with both of these hypotheses: indicating that zebrafish embryos undergo adaptive changes in gene expression in response to hypoxia. Our study provides a striking genetic portrait of the zebrafish embryos' adaptive responses to hypoxic stress and demonstrates the utility of the microarray technology as a tool for analyzing complex developmental processes in the zebrafish.

Haemoglobin function in vertebrates: evolutionary changes in cellular regulation in hypoxia

The evolution of erythrocytic hypoxia responses is reviewed by comparing the cellular control of haemoglobin-oxygen affinity in agnathans, teleost fish and terrestrial vertebrates. The most ancient response to hypoxic conditions appears to be an increase in cell volume, which increases the haemoglobin-oxygen affinity in lampreys. In teleost fish, an increase of cell volume in hypoxic conditions is also evident. The volume increase is coupled to an increase in erythrocyte pH. These changes are caused by an adrenergic activation of sodium/proton exchange across the erythrocyte membrane. The mechanism is important in acute hypoxia and is followed by a decrease in cellular adenosine triphosphate (ATP) and guanosine triphosphate (GTP) concentrations in continued hypoxia. In hypoxic bird embryos, the ATP levels are also reduced. The mechanisms by which hypoxia decreases cellular ATP and GTP concentrations remains unknown, although at least in bird embryos cAMP-dependent mechanisms have been implicated. In mammals, hypoxia responses appear to occur mainly via modulation of cellular organic phosphate concentrations. In moderate hypoxia, 2,3-diphosphoglycerate levels are increased as a result of alkalosis caused by increased ventilation.

The adrenergic stress response in fish: control of catecholamine storage and release

In fish, the catecholamine hormones adrenaline and noradrenaline are released into the circulation, from chromaffin cells, during numerous 'stressful' situations. The physiological and biochemical actions of these hormones (the efferent adrenergic response) have been the focus of numerous investigations over the past several decades. However, until recently, few studies have examined aspects involved in controlling/modulating catecholamine storage and release in fish. This review provides a detailed account of the afferent limb of the adrenergic response in fish, from the biosynthesis of catecholamines to the exocytotic release of these hormones from the chromaffin cells. The emphasis is on three particular topics: (1) catecholamine biosynthesis and storage within the chromaffin cells including the different types of chromaffin cells and their varying arrangement amongst species; (2) situations eliciting the secretion of catecholamines (e.g. hypoxia, hypercapnia, chasing); (3) cholinergic and non-cholinergic (i.e. serotonin, adrenocorticotropic hormone, angiotensin, adenosine) control of catecholamine secretion. As such, this review will demonstrate that the control of catecholamine storage and release in fish chromaffin cells is a complex processes involving regulation via numerous hormones, neurotransmitters and second messenger systems.