Near maximally swimming schoolmaster snapper (Lutjanus apodus) have a greater metabolic capacity, and only a slightly lower thermal tolerance, than when tested at rest.

To assess the relationship among various measures of thermal tolerance and performance suggested for use in fish, we determined the critical thermal maximum (CTmax), Ucrit, maximum thermal tolerance while swimming [CTSmax] and realistic aerobic scope (ASR) of juvenile schoolmaster snapper (Lutjanus apodus). Their CTSmax (37.5±0.1°C) was only slightly lower than their CTmax (38.9±0.1°C), and this is likely because their maximum metabolic rate (MMR) and ASR during the former test were∼42 and 65% higher, respectively. Further, we did not observe a transition to unsteady (i.e., anaerobically fueled) swimming in the CTSmax test as we did in the Ucrit protocol. These data strongly suggest that thermal tolerance tests on fishes whose lifestyle involves schooling or sustained activity should be performed at ecologically-relevant swimming speeds. Further, they do not support the hypothesis that failure during a CTSmax test is due to a fish's inability to meet its tissue oxygen demands.

Hyperoxia Does Not Improve the Acute Upper Thermal Tolerance of a Tropical Marine Fish (Lutjanus apodus).

Fish can experience hyperoxia in shallow environments due to photosynthetic activity, and this has been suggested to provide them with a metabolic refuge during acute warming. However, this hypothesis has never been tested on a tropical marine species. Thus, we fitted 29°C-acclimated wild schoolmaster snapper (Lutjanus apodus; a species known to experience diel hyperoxia in mangrove creeks and coastal waters) with Transonic® flow probes, and exposed them to an acute increase in temperature (at 1°C h-1) in respirometers under normoxia and hyperoxia (150% air saturation), until their critical thermal maximum (CTmax). The CTmax of both groups was ∼39°C, and no differences in maximum cardiac function were recorded as the fish were warmed. However, temperature-induced factorial aerobic scope was significantly greater in fish tested under hyperoxia. These data suggest that hyperoxia will not protect coastal tropical fish species during marine heat waves, despite its effects on metabolic scope / capacity.

Hypoxic acclimation negatively impacts the contractility of steelhead trout (Oncorhynchus mykiss) spongy myocardium.

Cardiac stroke volume (SV) is compromised in Atlantic cod and rainbow trout following acclimation to hypoxia (i.e., 40% air saturation; ~8 kPa O2) at 10-12°C, and this is not due to changes in heart morphometrics or maximum achievable in vitro end-diastolic volume. To examine if this diminished SV may be related to compromised myocardial contractility, we used the work-loop method to measure work and power in spongy myocardial strips from normoxic- and hypoxic-acclimated steelhead trout when exposed to decreasing Po2 levels (21 to 1.5 kPa) at several frequencies (30-90 contractions/min) at 14°C (their acclimation temperature). Work required to lengthen the muscle, as during filling of the heart, was strongly frequency dependent (i.e., increased with contraction rate) but was not affected by hypoxic acclimation or test Po2. In contrast, although shortening work was less frequency dependent, this parameter and network (and power) 1) were consistently lower (by ~30-50 and ~15%, respectively) in strips from hypoxic-acclimated fish and 2) fell by ~40-50% in both groups from 20 to 1.5 kPa Po2, despite the already-reduced myocardial performance in the hypoxic-acclimated group. In addition, strips from hypoxic-acclimated trout showed a poorer recovery of net power (by ~15%) when returned to normoxia. These results strongly suggest that hypoxic acclimation reduces myocardial contractility, and in turn, may limit SV (possibly by increasing end-systolic volume), but that this diminished performance does not improve the capacity to maintain myocardial performance under oxygen limiting conditions.

Functional support for a novel mechanism that enhances tissue oxygen extraction in a teleost fish.

A successful spawning migration in salmon depends on their athletic ability, and thus on efficient cardiovascular oxygen (O2) transport. Most teleost fishes have highly pH-sensitive haemoglobins (Hb) that can release large amounts of O2 when the blood is acidified at the tissues. We hypothesized that plasma-accessible carbonic anhydrase (paCA; the enzyme that catalyses proton production from CO2) is required to acidify the blood at the tissues and promote tissue O2 extraction. Previous studies have reported an elevated tissue O2 extraction in hypoxia-acclimated teleosts that may also be facilitated by paCA. Thus, to create experimental contrasts in tissue O2 extraction, Atlantic salmon were acclimated to normoxia or hypoxia (40% air saturation for more than six weeks), and the role of paCA in enhancing tissue O2 extraction was tested by inhibiting paCA at rest and during submaximal exercise. Our results show that: (i) in both acclimation groups, the inhibition of paCA increased cardiac output by one-third, indicating a role of paCA in promoting tissue O2 extraction during exercise, recovery and at rest; (ii) the recruitment of paCA was plastic and increased following hypoxic acclimation; and (iii) maximal exercise performance in salmon, and thus a successful spawning migration, may not be possible without paCA.

Effects of Loma morhua (Microsporidia) infection on the cardiorespiratory performance of Atlantic cod Gadus morhua (L).

The microsporidian Loma morhua infects Atlantic cod (Gadus morhua) in the wild and in culture and results in the formation of xenomas within the gill filaments, heart and spleen. Given the importance of the two former organs to metabolic capacity and thermal tolerance, the cardiorespiratory performance of cod with a naturally acquired infection of Loma was measured during an acute temperature increase (2 °C h(-1)) from 10 °C to the fish's critical thermal maximum (CT(Max)). In addition, oxygen consumption and swimming performance were measured during two successive critical swimming speed (U(crit)) tests at 10 °C. While Loma infection had a negative impact on cod cardiac function at warm temperatures, and on metabolic capacity in both the CT(Max) and U(crit) tests (i.e. a reduction of 30-40%), it appears that the Atlantic cod can largely compensate for these Loma-induced cardiorespiratory limitations. For example, (i) CT(Max) (21.0 ± 0.3 °C) and U(crit) (~1.75 BL s(-1)) were very comparable to those reported in previous studies using uninfected fish from the same founder population; and (ii) our data suggest that tissue oxygen extraction, and potentially the capacity for anaerobic metabolism, is enhanced in fish infected with this microsporidian.

Cold-acclimation leads to differential regulation of the steelhead trout (Oncorhynchus mykiss) coronary microcirculation.

The regulation of vascular resistance in fishes has largely been studied using isolated large conductance vessels, yet changes in tissue perfusion/vascular resistance are primarily mediated by the dilation/constriction of small arterioles. Thus we adapted mammalian isolated microvessel techniques for use in fish and examined how several agents affected the tone/resistance of isolated coronary arterioles (<150 µm ID) from steelhead trout (Oncorhynchus mykiss) acclimated to 1, 5, and 10°C. At 10°C, the vessels showed a concentration-dependent dilation to adenosine (ADE; 61 ± 8%), sodium nitroprusside (SNP; 35 ± 10%), and serotonin (SER; 27 ± 2%) (all values maximum responses). A biphasic response (mild contraction then dilation) was observed in vessels exposed to increasing concentrations of epinephrine (EPI; 34 ± 9% dilation) and norepinephrine (NE; 32 ± 7% dilation), whereas the effect was less pronounced with bradykinin (BK; 12.5 ± 3.5% constriction vs. 6 ± 6% dilation). Finally, a mild constriction was observed after exposure to acetylcholine (ACh; 6.5 ± 1.4%), while endothelin (ET)-1 caused a strong dose-dependent increase in tone (79 ± 5% constriction). Acclimation temperature had varying effects on the responsiveness of vessels. The dilations induced by EPI, ADE, SER, and SNP were reduced/eliminated at 5°C and/or 1°C as compared with 10°C. In contrast, acclimation to 5 and 1°C increased the maximum constriction induced by ACh and the sensitivity of vessels to ET-1 (but not the maximum response) at 1°C was greater. Acclimation temperature had no effect on the response to NE, and responsiveness to BK was variable.

Steelhead trout Oncorhynchus mykiss metabolic rate is affected by dietary Aloe vera inclusion but not by mounting an immune response against formalin-killed Aeromonas salmonicida.

The oxygen consumption (MO2) of two groups of 10° C acclimated steelhead trout Oncorhynchus mykiss was measured for 72 h after they were given a 100 µl kg(-1) intraperitoneal injection of formalin-killed Aeromonas salmonicida (ASAL) or phosphate-buffered saline (PBS). In addition, plasma cortisol levels were measured in fish from both groups prior to, and 1 and 3 h after, they were given a 30 s net stress. The first group was fed an unaltered commercial diet for 4 weeks, whereas the second group was fed the same diet but with 0·5% (5 g kg(-1) ) Aloe vera powder added; A. vera has potential as an immunostimulant for use in aquaculture, but its effects on basal and acute phase response (APR)-related metabolic expenditures and stress physiology, are unknown. Injection of ASAL v. PBS had no measurable effect on the MO2 of O. mykiss indicating that the APR in this species is not associated with any net increase in energy expenditure. In contrast, incorporating 0·5% A. vera powder into the feed decreased routine metabolic rate by c. 8% in both injection groups and standard metabolic rate in the ASAL-injected group (by c. 4 mg O2 kg(-1) h(-1) ; 5%). Aloe vera fed fish had resting cortisol levels that were approximately half of those in fish on the commercial diet (c. 2·5 v. 5·0 ng ml(-1) ), but neither this difference nor those post-stress reached statistical significance (P > 0·05).

Seasonal temperatures in South Eleuthera, The Bahamas, have considerable impacts on the cardiorespiratory function and swimming performance of Nassau grouper (Epinephelus striatus).

Surprisingly, the impacts of environmental changes on the physiology of tropical/subtropical marine fishes have received limited attention. Given that (i) temperature is considered to be a key factor controlling the biology of fishes; (ii) no published data are available on the swimming performance, metabolic capacity or cardiac function of any of the ~165 grouper species worldwide; and (iii) the Nassau grouper is an endangered species of great ecological and socioeconomic significance in The Bahamas, we investigated how current summer/early fall (30°C) and winter (22°C) temperatures in South Eleuthera affected the aerobic metabolism and heart function of wild Nassau grouper when swum to exhaustion (i.e. to their critical swimming speed, Ucrit). The Nassau grouper had a very low Ucrit at 30°C (i.e. <1 body lengths s-1), and a 30% lower swimming performance during the winter (at 22°C), and this was that was indicative of a reduced absolute aerobic scope (~185 vs. 290 mg O2 kg-1 h-1) and values of maximum heart rate ([Formula: see text]HMax) and scope for [Formula: see text]H that were only one-half of that achieved at 30°C (~60 vs. 120 and 29 vs. 61 beats min-1, respectively). Overall, these data reveal that the Nassau grouper's aerobic and swimming capacity are well below values reported for other tropical/subtropical fishes and suggest that, despite a compensatory (~30-40%) increase in stroke volume, constraints on [Formula: see text]H near this species' lower thermal limit negatively affect its cardiac output and swimming performance. These findings have considerable ecological implications as Bahamian grouper populations migrate over long distances to spawn during the winter months, and given the predicted increase in temperature variability with climate change.

Acute and chronic cold exposure differentially affect cardiac control, but not cardiorespiratory function, in resting Atlantic salmon (Salmo salar).

No studies have examined the effects of cold temperatures (∼0-1 °C) on in vivo cardiac function and control, and metabolism, in salmonids. Thus, we examined: 1) how acclimation to 8 °C vs. acclimation (>3 weeks) or acute exposure (8-1 °C at 1 °C h-1) to 1 °C influenced cardiorespiratory parameters in resting Atlantic salmon; and 2) if/how the control of cardiac function was affected. Oxygen consumption ( M ˙ O 2 ) and cardiac function [i.e., heart rate (f H) and cardiac output ( Q ˙ ) ] were 50% lower in the acutely cooled and 1oC-acclimated salmon as compared to 8 °C fish, whereas stroke volume (VS) was unchanged. Intrinsic f H was not affected by whether the fish were acutely exposed or acclimated to 1 °C (values ∼51, 24 and 21 beats min-1 in 8 and 1 °C-acclimated fish, and 8-1 °C fish, respectively), and in all groups f H was primarily under adrenergic control/tone (cholinergic tone 13-18%; adrenergic tone 37-70%). However, ß-adrenergic blockade resulted in a 50% increase in VS in the 1oC-acclimated group, and this was surprising as circulating catecholamine levels were ∼1-3 nM in all groups. Overall, the data suggest that this species has a limited capacity to acclimate to temperatures approaching 0 °C. However, we cannot exclude the possibility that cardiac and metabolic responses are evoked when salmon are cooled to ∼ 0-1 °C, and that this prevented further declines in these parameters (i.e., they 'reset' quickly). Our data also provide further evidence that VS is temperature insensitive, and strongly suggest that changes in adrenoreceptor mediated control of venous pressure/capacitance occur when salmon are acclimated to 1 °C.

In situ cardiac function in Atlantic cod (Gadus morhua): effects of acute and chronic hypoxia.

Recent in vivo experiments on Atlantic cod (Gadus morhua) acclimated to chronic hypoxia (6-12 weeks at 10 degrees C; Pw(O(2)) approximately 8-9 kPa) revealed a considerable decrease in the pumping capacity of the heart. To examine whether this diminished cardiac performance was due to the direct effects of chronic moderate hypoxia on the myocardium (as opposed to alterations in neural and/or hormonal control), we measured the resting and maximum in situ function of hearts from normoxia- and hypoxia-acclimated cod: (1) when initially perfused with oxygenated saline; (2) at the end of a 15 min exposure to severe hypoxia (P(O(2)) approximately 0.6 kPa); and (3) 30 min after the hearts had been reperfused with oxygenated saline. Acclimation to hypoxia did not influence resting (basal) in situ cardiac performance during oxygenated or hypoxic conditions. However, it caused a decrease in maximum cardiac output (Q(max)) under oxygenated conditions (from 49.5 to 40.3 ml min(-1) kg(-1); by 19%), that was due to diminished values for maximum stroke volume (V(S)) and scope for V(S). Severe hypoxia reduced in both groups to approximately 20 ml min(-1) kg(-1), yet, the hearts of hypoxia-acclimated fish were better able to sustain this level of Q under hypoxia, and the recovery of Q(max) (as compared with initial values under oxygenated conditions) was significantly improved (94% vs 83%). These data show that acclimation to hypoxia has a direct effect on cod myocardial function and/or physiology, and suggest that the cod heart shows some adaptations to prolonged hypoxia.

Effect of acute and chronic hypoxia on the swimming performance, metabolic capacity and cardiac function of Atlantic cod (Gadus morhua).

Low water oxygen content (hypoxia) is a common feature of many freshwater and marine environments. However, we have a poor understanding of the degree to which diminished cardiac function contributes to the reduction in fish swimming performance concomitant with acute exposure to hypoxia, or how fish cardiorespiratory physiology is altered by, or adapts to, chronic hypoxia. Thus, we acclimated adult Atlantic cod (Gadus morhua) to either approximately 8-9 kPa O(2) (40-45% air saturation) or approximately 21 kPa O(2) (100% air saturation; normoxia) for 6-12 weeks at 10 degrees C, and subsequently measured metabolic variables [routine oxygen consumption (M(O(2)), maximum (M(O(2)), metabolic scope] and cardiac function (cardiac output, Q; heart rate, f(H); and stroke volume, V(S)) in these fish during critical swimming speed (U(crit)) tests performed at both levels of water oxygenation. Although surgery (flow probe implantation) reduced the U(crit) of normoxia-acclimated cod by 14% (from 1.74 to 1.50 BL s(-1)) under normoxic conditions, exposure to acute hypoxia lowered the U(crit) of both groups (surgery and non-surgery) by approximately 30% (to 1.23 and 1.02 BL s(-1), respectively). This reduction in swimming performance was associated with large decreases in maximum M(O(2)) and metabolic scope (> or = 50%), and maximum f(H) and Q (by 16 and 22%), but not V(S). Long-term acclimation to hypoxia resulted in a significant elevation in normoxic metabolic rate as compared with normoxia-acclimated fish (by 27%), but did not influence normoxic or hypoxic values for U(crit), maximum M(O(2)) or metabolic scope. This was surprising given that resting and maximum values for Q were significantly lower in hypoxia-acclimated cod at both levels of oxygenation, because of lower values for V(S). However, hypoxia-acclimated cod were able to consume more oxygen for a given cardiac output. These results provide important insights into how fish cardiorespiratory physiology is impacted by short-term and prolonged exposure to hypoxia, and further highlight the tremendous capacity of the fish cardiorespiratory system to deal with environmental challenges.

Exhaustive exercise does not affect the preferred temperature for recovery in juvenile rainbow trout (Oncorhynchus mykiss).

We tested the hypothesis that juvenile rainbow trout (Oncorhynchus mykiss) would select a temperature colder than their acclimation temperature (16 deg +/-1 deg C) to minimize postexhaustive exercise metabolic demands and enhance oxygen availability. After an initial 3-h exploratory period in a thermal gradient (6 degrees -25 degrees C), fish selected a temperature of approximately 14 degrees C and had a baseline exploratory swimming activity of approximately 60 cm min(-1). Subsequently, experimental (chased) fish were individually removed, exhaustively exercised for 1.5 min, and replaced. Both control (unchased) and experimental fish were allowed to explore the thermal gradient for another 2 h. Immediately after being chased, trout had a metabolic profile that was consistent with being exhausted; levels of plasma and muscle lactate were 4.38+/-0.25 mmol L(-1) and 28.0+/-2.0 mmol kg(-1), respectively, and levels of muscle glycogen, adenosine triphosphate, and phosphocreatine were 3.89+/-0.95, 4.23+/-0.62, and 3.07+/-0.73 mmol kg(-1), respectively. Although exploratory swimming activity of the chased fish was significantly lower (by 81%) as compared with control fish during the first 5 min postchase, differences in the mean, median, and mode values for selected temperatures during the next 2 h were neither large (<1 degrees C) nor significant (P>0.05). Contrary to our initial hypothesis, these findings suggest that juvenile rainbow trout do not select a colder temperature to decrease metabolic rate following exhaustive exercise. Instead, rainbow trout selected a temperature marginally cooler than their acclimation temperature (16 degrees C) regardless of whether they had been previously exhausted.

Hemoglobin genotype has minimal influence on the physiological response of juvenile Atlantic cod (Gadus morhua) to environmental challenges.

Hemoglobin (Hb) polymorphism in cod is associated with temperature-related differences in biogeographical distribution, and several authors have suggested that functional characteristics of the various hemoglobin isoforms (HbIs) directly influence phenotypic traits such as growth rate. However, no study has directly examined whether Hb genotype translates into physiological differences at the whole animal level. Thus, we generated a family of juvenile Atlantic cod consisting of all three main Hb genotypes (HbI-1/1, HbI-2/2, and HbI-1/2) by crossing a single pair of heterozygous parents, and we compared their metabolic and cortisol responses to an acute thermal challenge (10 degrees C to their critical thermal maximum [CTM] or 22 degrees C, respectively) and tolerance of graded hypoxia. There were no differences in routine metabolism (at 10 degrees C), maximum metabolic rate, metabolic scope, CTM (overall mean 22.9 degrees +/- 0.2 degrees), or resting and poststress plasma cortisol levels among Hb genotypes. Further, although the HbI-1/1 fish grew more (by 15%-30% during the first 9 mo) when reared at 10 degrees +/- 1 degrees C and had a slightly enhanced hypoxia tolerance at 10 degrees C (e.g., the critical O(2) levels for HbI-1/1, HbI-2/2, and HbI-1/2 cod were 35.56% +/- 1.24% , 40.56% +/- 1.99%, and 40.20% +/- 1.19% air saturation, respectively), these results are contradictory to expectations based on HbI functional properties. Thus, our findings (1) do not support previous assumptions that growth rate differences among cod Hb genotypes result from a more efficient use of the oxygen supply-that is, reduced standard metabolic rates and/or increased metabolic capacity-and (2) suggest that in juvenile cod, there is no selective advantage to having a particular Hb genotype with regards to the capacity to withstand ecologically relevant environmental challenges.

Expression levels of genes associated with oxygen utilization, glucose transport and glucose phosphorylation in hypoxia exposed Atlantic cod (Gadus morhua).

Expression level of genes associated with oxygen [cytochrome oxidase 1 (COX1) and myoglobin (Mb)] and glucose utilization [glucose transporters (GLUTs) and hexokinases (HKs)] along with metabolic indices were determined in Atlantic cod (Gadus morhua) subjected to an hypoxic challenge of <45% oxygen saturation for 24 days. There were two closely related HKs considered to be homologues of mammalian HKIs. HKIa and HKIb share 86% sequence identity and are both ubiquitously expressed. Mb was also expressed in many tissues with highest levels occurring in heart. Over the first 15 days of hypoxia there were transient increases in plasma lactate in hypoxic relative to normoxic fish associated with a significant decrease in liver glycogen. Over days 1-6, there were in ten of eleven cases, increased average (with a number of conditions being statistically significant) expression levels of GLUTs (1, 2, 4) and HKs (1a and b) in gill, heart, liver, and white muscle in hypoxic relative to normoxic fish. There were significant increases in COX1 and Mb expression levels in gill by day 24 but no changes in these aerobic indicators in heart or liver. Overall the data suggest a transient increase in genes associated with glucose utilization during the early part of the hypoxic challenge followed by alterations in gene expression in gill.

The immune and stress responses of Atlantic cod to long-term increases in water temperature.

Sea-caged cod are limited in their movements in the water column, and thus can be exposed to large seasonal ( approximately 0-20 degrees C) temperature fluctuations. To investigate the physiological response of Atlantic cod to summer-like increases in temperature, we exposed 10 degrees C acclimated juvenile cod to a graded thermal challenge (1 degrees C increase every 5 days) and measured: (1) plasma cortisol and glucose levels; (2) the respiratory burst activity of blood leukocytes; and (3) the expression of specific immune-related genes [MHC Class I, Interleukin-1beta (IL-1beta), beta2-microglobulin (beta2-M), Immunoglobulin M (IgM)-light (L) and -heavy (H) chains] in the blood using quantitative reverse transcription-polymerase chain reaction (QRT-PCR). The experiment was stopped at 19.1 degrees C, with 26.7% of the fish surviving to this point. Plasma glucose levels increased slightly at 16 and 18 degrees C (by 1.39- and 1.74-fold, respectively), in contrast, cortisol levels were elevated significantly (by 2.9-fold) at 16 degrees C but returned to control levels thereafter. The effect of increasing temperature on the expression of immune related genes in blood cells (leukocytes) was variable and depended on the gene of interest. The expression of IgM-H remained stable for the duration of the experiment. In contrast, IL-1beta expression was increased significantly (by approximately 25-fold) at 19 degrees C as compared to time-matched control fish, and changes in the expression of beta2-M, MHC Class I and IgM-L followed a pattern similar to that seen for cortisol: increasing at 16 degrees C (by 4.2-, 5.3- and 17-fold, respectively), but returning to pre-stress levels by 19 degrees C. Interestingly, increasing temperatures had no effect on respiratory burst activity. This study is the first to examine the effects of a chronic regimen of increasing temperature on the stress physiology and immunology of a marine teleost, and suggests that immune function is influenced by complex interactions between thermal effects and temperature-induced stress (elevated circulating cortisol levels).

Changes in free and total plasma cortisol levels in juvenile haddock (Melanogrammus aeglefinus) exposed to long-term handling stress.

We measured changes in free and total plasma cortisol levels, plasma glucose, gill hsp70 levels, and growth in haddock (Melanogrammus aeglefinus) subjected to a long-term handling stress (15 s out of water, each day, for 4 weeks), and the effect of this long-term stress on the ability of haddock to respond to an acute stressor. The acute stressor was a single handling stress, and fish were sampled at 1, 6, and 12 h post-stress. During the long-term stress study, free and total plasma cortisol levels increased significantly (10-fold) in the stressed group after the second week. However, the percentage of free cortisol was already significantly elevated by the first week (control 17%, stressed 55%), and remained high during the second week (control 35% and stressed 65%). After 3 and 4 weeks of handling, both free and total cortisol declined in stressed fish to levels that were not significantly different from pre-stress values. Control fish grew significantly more than stressed fish (by 32% and 18%, respectively) over the 4 week study, and condition factor only increased in control fish. Although fish from the control group showed elevated total plasma cortisol levels (to 47 ng mL(-1)) 1 h after the acute stress, and the levels in stressed fish were comparable to those for the control fish, no significant increase in plasma cortisol was measured in the group subjected to the long-term stress. Free plasma cortisol levels did not increase significantly in either group following the acute stress. However, free plasma cortisol levels were significantly higher in long-term stress group, as compared with the control group, at 6 h post-stress. Plasma glucose and gill hsp70 levels were not altered by either the long-term stress or acute stressor. Our data indicate that cortisol (free and total), but not glucose or hsp70, appears to be adequate to assess short- and long-term stress in haddock.

Responses to hypoxia and recovery: repayment of oxygen debt is not associated with compensatory protein synthesis in the Amazonian cichlid, Astronotus ocellatus.

Oxygen consumption, as an indicator of routine metabolic rate (RoMR), and tissue-specific changes in protein synthesis, as measured by (3)H-labelled phenylalanine incorporation rates, were determined in Astronotus ocellatus to investigate the cellular mechanisms behind hypoxia-induced metabolic depression and recovery. RoMR was significantly depressed, by approximately 50%, when dissolved oxygen levels reached 10% saturation (0.67+/-0.01 mg l(-1) at 28+/-1 degrees C). This depression in RoMR was accompanied by a 50-60% decrease in liver, heart and gill protein synthesis, but only a 30% decrease in brain protein synthesis. During recovery from hypoxia, an overshoot in RoMR to 270% of the normoxic rate was observed, indicating the accumulation of an oxygen debt during hypoxia. This conclusion was consistent with significant increase in plasma lactate levels during the hypoxic exposure, and the fact that lactate levels rapidly returned to pre-hypoxic levels. In contrast, a hyperactivation of protein synthesis did not occur, suggesting the overshoot in oxygen consumption during recovery is attributed to an increase in cellular processes other than protein synthesis.

Cardiovascular and haematological responses of Atlantic cod (Gadus morhua) to acute temperature increase.

For fish to survive large acute temperature increases (i.e. >10.0 degrees C) that may bring them close to their critical thermal maximum (CTM), oxygen uptake at the gills and distribution by the cardiovascular system must increase to match tissue oxygen demand. To examine the effects of an acute temperature increase ( approximately 1.7 degrees C h(-1) to CTM) on the cardiorespiratory physiology of Atlantic cod, we (1) carried out respirometry on 10.0 degrees C acclimated fish, while simultaneously measuring in vivo cardiac parameters using Transonic probes, and (2) constructed in vitro oxygen binding curves on whole blood from 7.0 degrees C acclimated cod at a range of temperatures. Both cardiac output (Q) and heart rate (fh) increased until near the fish's CTM (22.2+/-0.2 degrees C), and then declined rapidly. Q(10) values for Q and fh were 2.48 and 2.12, respectively, and increases in both parameters were tightly correlated with O(2) consumption. The haemoglobin (Hb)-oxygen binding curve at 24.0 degrees C showed pronounced downward and rightward shifts compared to 20.0 degrees C and 7.0 degrees C, indicating that both binding capacity and affinity decreased. Further, Hb levels were lower at 24.0 degrees C than at 20.0 degrees C and 7.0 degrees C. This was likely to be due to cell swelling, as electrophoresis of Hb samples did not suggest protein denaturation, and at 24.0 degrees C Hb samples showed peak absorbance at the expected wavelength (540 nm). Our results show that cardiac function is unlikely to limit metabolic rate in Atlantic cod from Newfoundland until close to their CTM, and we suggest that decreased blood oxygen binding capacity may contribute to the plateau in oxygen consumption.

Cardiorespiratory modifications, and limitations, in post-smolt growth hormone transgenic Atlantic salmon Salmo salar.

In recent years, there has been a great deal of interest in how growth hormone (GH) transgenesis affects fish physiology. However, the results of these studies are often difficult to interpret because the transgenic and non-transgenic fish had very different environmental/rearing histories. This study used a stable line of size-matched GH Atlantic salmon (Salmo salar) that were reared in a shared tank with controls (at 10 degrees C, for approximately 9 months) to perform a comprehensive examination of the cardiorespiratory physiology of GH transgenic salmon, and serves as a novel test of the theory of symmorphosis. The GH transgenic salmon had a 3.6x faster growth rate, and 21 and 25% higher values for mass-specific routine and standard oxygen consumption (M(O(2))), respectively. However, there was no concurrent increase in their maximum M(O(2)), which resulted in them having an 18% lower metabolic scope and a 9% reduction in critical swimming speed. This decreased metabolic capacity/performance was surprising given that the transgenics had a 29% larger heart with an 18% greater mass-specific maximum in situ cardiac output, a 14% greater post-stress blood haemoglobin concentration, 5-10% higher red muscle and heart aerobic enzyme (citrate synthase or cytochrome oxidase) activities, and twofold higher resting and 1.7x higher post-stress, catecholamine levels. However, gill surface area was the only cardiorespiratory parameter that was not enhanced, and our data suggest that gill oxygen transfer may have been limiting. Overall, this research: (1) shows that there are significant metabolic costs associated with GH transgenesis in this line of Atlantic salmon; (2) provides the first direct evidence that cardiac function is enhanced by GH transgenesis; (3) shows that a universal upregulation of post-smolt (adult) GH transgenic salmon cardiorespiratory physiology, as suggested by symmorphosis, does not occur; and (4) supports the idea that whereas differences in arterial oxygen transport (i.e. cardiac output and blood oxygen carrying capacity) are important determinants of inter-specific differences in aerobicity, diffusion-limited processes must be enhanced to achieve substantial intra-specific improvements in metabolic and swimming performance.

Beta-adrenoreceptors in the trout (Oncorhynchus mykiss) heart: characterization, quantification, and effects of repeated catecholamine exposure.

Specific binding of the hydrophilic radioligand [3H]CGP-12177 to cell surface (functional) beta-adrenoreceptors was quantified in ventricular micropunches (2 mm diameter, 350 microns thickness) from seawater-acclimated rainbow trout held at 7-9 degrees. Binding was stereospecific, saturable, of high affinity, and displaceable by appropriate agonists and antagonists. Phentolamine failed to displace [3H]CGP at concentrations up to 10(-4) M, indicating an absence of [3H]CGP binding to alpha-adrenergic receptors. Trout ventricular beta-adrenoreceptors are exclusively of the beta 2 type. This conclusion is based on (1) the IC50 value for the beta 2-antagonist ICI 118551 (2.9 x 10(-6) M); (2) the inability of the beta 1-antagonist atenolol to displace [3H]CGP from beta-adrenoreceptors; and (3) the order of agonist-binding affinity (isoproterenol > epinephrine >> norepinephrine). The Bmax and Kd values for [3H]CGP binding to myocardial tissue were approximately 0.04 fmol micrograms protein-1 and 0.25 nM, respectively. The Bmax value indicates that the density of cell surface (functional) beta-adreno-receptors in the ventricle was 12,000 sites per cell or 3.38 sites per microns 2 of sarcolemma. The Kd and Bmax values for [3H]CGP binding to ventricular beta-adrenoreceptors were unaffected by the in vivo administration of five bolus catecholamine injections (4.0 micrograms kg-1 epinephrine, 2.0 micrograms kg-1 norepinephrine). This suggests that stress-induced increases in plasma catecholamines are unlikely to cause the down-regulation of heart beta-adrenoreceptors in fish. The method described here represents a simple but powerful technique for the quantification and characterization of adrenergic receptors in the fish heart.