Hypoxic cardiorespiratory reflexes in the facultative air-breathing fish jeju (Hoplerythrinus unitaeniatus): role of branchial O2 chemoreceptors

In one series of experiments, heart frequency (f _H), blood pressure (P _a), gill ventilation frequency (f _R ), ventilation amplitude (V _AMP) and total gill ventilation (V _TOT) were measured in intact jeju (Hoplerythrinus unitaeniatus) and jeju with progressive denervation of the branchial branches of cranial nerves IX (glossopharyngeal) and X (vagus) without access to air. When these fish were submitted to graded hypoxia (water PO₂ ~140, normoxia to 17 mmHg, severe hypoxia), they increased f _R , V _AMP, V _TOT and P _a and decreased f _H. In a second series of experiments, air-breathing frequency (f _RA), measured in fish with access to the surface, increased with graded hypoxia. In both series, bilateral denervation of all gill arches eliminated the responses to graded hypoxia. Based on the effects of internal (caudal vein, 150 μg NaCN in 0.2 mL saline) and external (buccal) injections of NaCN (500 μg NaCN in 1.0 mL water) on f _R , V _AMP, V _TOT, P _a and f _H we conclude that the O₂ receptors involved in eliciting changes in gill ventilation and associated cardiovascular responses are present on all gill arches and monitor the O₂ levels of both inspired water and blood perfusing the gills. We also conclude that air breathing arises solely from stimulation of branchial chemoreceptors and support the hypothesis that internal hypoxaemia is the primary drive to air breathing.     

Cardiorespiratory responses to hypoxia in the African catfish, <Emphasis Type="Italic">Clarias gariepinus</Emphasis> (Burchell 1822), an air-breathing fish

The African catfish, Clarias gariepinus, possesses a pair of suprabranchial chambers located in the dorsal-posterior part of the branchial cavity having extensions from the upper parts of the second and fourth gill arches, forming the arborescent organs. This structure is an air-breathing organ (ABO) and allows aerial breathing (AB). We evaluated its cardiorespiratory responses to aquatic hypoxia. To determine the mode of air-breathing (obligate or accessory), fish had the respiratory frequency (f _R) monitored and were subjected to normoxic water (PwO₂ = 140 mmHg) without becoming hyperactive for 30 h. During this period, all fish survived without displaying evidences of hyperactivity and maintained unchanged f _R, confirming that this species is a facultative air-breather. Its aquatic O₂ uptake ( [(V)\dot]\textO₂ \dot{V}{\text{O}}_{2} ) was maintained constant down to a critical PO₂ (PcO₂) of 60 mmHg, below which [(V)\dot]\textO₂ \dot{V}{\text{O}}_{2} declined linearly with further reductions of inspired O₂ tension (PiO₂). Just above the PcO₂ the ventilatory tidal volume (V _T) increased significantly along with gill ventilation ( [(V)\dot]_\textG \dot{V}_{\text{G}} ), while f _R changed little. Consequently, the water convection requirement ( [(V)\dot]_\textG /[(V)\dot]\textO₂ ) \left( {\dot{V}_{\text{G}} /\dot{V}{\text{O}}_{2} } \right) increased steeply. This threshold applied to a cardiac response that included reflex bradycardia. AB was initiated at PiO₂ = 140 mmHg (normoxia) and air-breathing episodes increased linearly with more severe hypoxia, being significantly higher at PiO₂ tensions below the PcO₂. Air-breathing episodes were accompanied by bradycardia pre air-breath, to tachycardia post air-breath.     

Respiration of juvenile Arctic cod (Boreogadus saida): effects of acclimation,temperature, and food intake

Oxygen consumption (VO₂) of juvenile Arctic cod (Boreogadus saida) was investigated at low tempera tures (six temperatures; range -0.5 to 2.7°C). Small (mean wt. 6–8 g) and large (mean wt. 14 g) fish were acclimated, or adjusted to a constant temperature (0.4°C), for 5 months and then tested for metabolic cold adaptation (elevated metabolic rates in polar fishes). Short-term (2 weeks) acclimated fish showed elevated VO₂ similar to previously established values for polar fishes, but there was no such evidence after longterm acclimation. Long-term acclimation caused VO₂ values to drop significantly (from 86.0 to 46.5 mg O₂·kg^–1·h^–1, at 0.4°C), which showed that metabolic cold adaptation was a phenomenon caused by insufficien: acclimation time for fish in respiration experiments. We also measured the effects of temperature and feeding on VO₂. A temperature increase of 2.3°C resulted in relatively large increases in VO₂ for both longand short-term acclimated fish (Q₁₀ = 6.7 and 7.1, respectively), which suggests that metabolic processes are strongly influenced by temperature when it is close to zero. Feeding individuals to satiation caused significant increases in VO₂ above pre-fed values (34–60% within 1–2 days after feeding). Respiration budgets of starved and fed Arctic cod at ambient temperatures in Resolute Bay N.W.T., Canada, were used to model annual respiration costs and potential weight loss. Low respiration costs for Arctic cod at ambient temperatures result in high growth efficiency during periods of feeding and low weight loss during periods of starvation.     

Combined ventilatory responses to aerial hypoxia and temperature in the South American lungfish Lepidosiren paradoxa

The control of pulmonary ventilation in South American lungfish Lepidosiren paradoxa is poorly understood. Interactions between temperature and hypoxia are particularly relevant due to large seasonal variations of its habitat. Therefore, we tested the hypothesis that the ventilatory responses to aerial hypoxia of Lepidosiren are highly dependent on ambient temperature. We used a pneumotachograph to measure pulmonary ventilation (V_E), tidal volume (V_T) and respiratory frequency (f_R) during normoxic (21% O₂) and hypoxic (12%, 10% and 7% O₂) conditions at two temperatures (25 and 35 °C). Blood gases, arterial PO₂ (PaO₂), arterial PCO₂ (PaCO₂) and arterial pH (pH_a) were also evaluated. At 25 °C, V_E increased significantly at 10% and 7% hypoxic levels when compared to the control value (21% O₂). At 35 °C, all hypoxic levels elicited a significant increase of V_E relative to control values. V_E is augmented mostly by increases of respiratory frequency (f_R), and there were significant interactions (p<0.001) between aerial hypoxia and temperature. PaCO₂ increased from ∼22 mmHg (normoxic value at 25 °C) to ∼32 mmHg (normoxic value at 35 °C). Concomitantly, the pH_a decreased from 7.51 (25 °C) to 7.38 (35 °C). Hypoxia-induced hyperventilation caused a reduction in PaCO₂ and an increase in pH_a, which were more pronounced at 35 °C than at 25 °C, reflecting an increased hyperventilation under the high temperature. In conclusion, the magnitude of ventilatory response is highly temperature-dependent in L. paradoxa, which is important for an animal experiencing large seasonal variations.     

Response of heart rate and cloacal ventilation in the bimodally respiring freshwater turtle, Rheodytes leukops, to experimental changes in aquatic PO2

Changes in heart rate (f _H) and cloacal ventilation frequency (f _C) were investigated in the Fitzroy turtle, Rheodytes leukops, under normoxic (17.85 kPa) and hypoxic (3.79 kPa) conditions at 25°C. Given R. leukops’ high reliance on aquatic respiration via the cloacal bursae, the objective of this study was to examine the effect of varying aquatic PO₂ levels upon the expression of a bradycardia in a freely diving, bimodally respiring turtle. In normoxia, mean diving f _H and f _C for R. leukops remained constant with increasing submergence length, indicating that a bradycardia failed to develop during extended dives of up to 3 days. Alternatively, exposure to aquatic hypoxia resulted in the expression of a bradycardia as recorded by a decreasing mean diving f _H with increasing dive duration. The observed bradycardia is attributed to a hypoxic-induced metabolic depression, possibly facilitated by a concurrent decrease in f _C. Results suggest that R. leukops alters its strategy from aquatic O₂ extraction via cloacal respiration in normoxia to O₂ conservation when exposed to aquatic hypoxia for the purpose of extending dive duration. Upon surfacing, a significant tachycardia was observed for R. leukops regardless of aquatic PO₂, presumably functioning to rapidly equilibrate blood and tissue gas tensions with alveolar gas to reduce surfacing duration.     

Hypoxia and interdemic variation in Poecilia latipinna

Variation in respiratory traits was quantified between two populations of the sailfin molly Poecilia latipinna (one from a periodically hypoxic salt marsh, Cedar Key, and one from a chronically normoxic river site, Santa Fe River). Two suites of characters were selected: traits that may show both short‐term acclimation response and interdemic variation in acclimation response (metabolic rate, critical oxygen tension and respiratory behaviour), and those that are not likely to respond to short‐term acclimation but may vary among populations (gill morphometric characters). Sailfin mollies from the salt marsh, acclimated to hypoxia (1 mg l⁻¹, c . 20 mmHg) for 6 weeks, spent less time conducting aquatic surface respiration and had lower gill ventilation rates than hypoxia‐acclimated conspecifics from the well‐oxygenated river site. Poecilia latipinna acclimated to hypoxia exhibited a lower critical oxygen tension ( P _c) than fish acclimated to normoxia; however, there was also a significant population effect. Poecilia latipinna from Cedar Key exhibited a lower P _c than fish from the Santa Fe River, regardless of acclimation. Cedar Key fish had a 14% higher mean gill surface area relative to fish from the Santa Fe River, a character that could account, at least in part, for their greater tolerance to hypoxia.     

Cardiorespiratory reflexes and aquatic surface respiration in the neotropical fish tambaqui (<Emphasis Type="Italic">Colossoma macropomum</Emphasis>): acute responses to hypercarbia

We examined the cardiorespiratory responses to 6 h of acute hypercarbia (1, 2.5, and 5% CO₂) in intact and gill-denervated (bilateral denervation of branchial branches of cranial nerves IX and X) tambaqui, Colossoma macropomum. Intact fish exposed to 1 and 2.5% CO₂ increased respiratory frequency (f_R) and ventilation amplitude (V_AMP) slowly over a 1- to 3-h period. Denervated fish did not show this response, suggesting that tambaqui possess receptors in the gills that will produce excitatory responses to low levels of hypercarbia (1 and 2.5% CO₂) if the exposure is prolonged. The cardiac response to stimulation of these receptors with this level of CO₂ was a tachycardia and not a bradycardia. During exposure to 5% CO₂, intact fish increased f_R and V_AMP, and showed a pronounced bradycardia after 1 h. After 2 h, the heart rate (f_H) started to increase, but returned to control values after 6 h. In denervated fish, the increase in f_R was abolished. The slow increase in V_AMP and the bradycardia were not abolished, suggesting that these changes arose from extra-branchial receptors. Neither intact nor denervated fish developed the swelling of the lower lip or performed aquatic surface respiration, even after 6 h, suggesting that these are unique responses to hypoxia and not hypercarbia.Abbreviations ASR aquatic surface respiration - f_H heart frequency - f_R respiratory frequency - V_AMP ventilation amplitude - _TOT total ventilation     

The relationship between O2 chemoreceptors, cardio-respiratory reflex and hypoxia tolerance in the neotropical fish Hoplias lacerdae

The localization, distribution and orientation of O₂ chemoreceptors associated with the control of cardio-respiratory responses were investigated in the neotropical, Hoplias lacerdae. Selective denervation of the cranial nerves (IX and X) was combined with chemical stimulation (NaCN) to characterize the gill O₂ chemoreceptors, and the fish were then exposed to gradual hypoxia to examine the extent of each cardio-respiratory response. Changes in heart rate (f_H) and ventilation amplitude (V_amp) were allied with chemoreceptors distributed on both internal and external surfaces of all gill arches, while ventilation rate (f_R) was allied to the O₂ chemoreceptors located only in the internal surface of the first gill arch. H. lacerdae exposed to gradual hypoxia produced a marked bradycardia (45%) and 50% increase in V_amp, but only a relatively small change in f_R (32%). Thus, the low f_R response yet high V_amp were in accord with the characterization of the O₂ chemoreceptors. Comparing these results from H. lacerdae with hypoxia-tolerant species revealed a relationship existent between general oxygenation of the individual species environment, its cardio-respiratory response to hypoxia and the characterization of O₂ chemoreceptors.     

Functional acclimation of Japanese quail to simulated high-altitude

Summary Japanese quail,Coturnix coturnix japonica, which were acclimated to a simulated altitude of 6100 m for six weeks were compared with control quail maintained at sea level. Body weight initially decreased in both groups, and by the end of six weeks the altitude quail weighed an average of 8% less than the control quail (Fig. 1). Compared with the controls, the altitude birds exhibited mean increases in hematocrit ratio and blood hemoglobin concentration of 31 and 37% respectively (Fig. 2). Blood volume was 36% higher in the altitude acclimated quail, while plasma volume was unchanged (Fig. 3). Exposure to chronic hypoxia resulted in hypertrophy of the right ventricle, while left ventricular mass was unchanged (Fig. 4). Rates of O₂ consumption (VO₂) at 5°C were continuously recorded as ambient PO₂ was slowly reduced from 155 to 30 torr. VO₂ was significantly higher at any given PO₂ in the altitude quail (Fig. 5). Exposure to reduced PO₂ resulted in a significantly greater decrease in body temperature in the control quail than in the altitude acclimated birds (Fig. 6).     

Effect of N-Methyl-D-Aspartate Receptor Blockade on the Control of Cerebral O2 Supply/Consumption Balance During Hypoxia in Newborn Pigs

Using dizocilpine (MK-801), we tested the hypothesis that N-methyl-D-aspartate (NMDA) receptors are important controllers of cerebral O₂ supply/consumption balance in newborn piglets both during normoxia and hypoxia. Twenty-five 2 to 7-day-old piglets were anesthetized and divided into four groups: (1) Normoxia (n = 6), (2) Normoxia + MK-801 (n = 6), (3) Hypoxia (n = 6), and (4) Hypoxia + MK-801 (n = 7). Regional cerebral blood flow (rCBF) in ml/min/100 g was measured using ¹⁴C-iodoantipyrine, and we determined arterial and venous O₂ saturations by microspectrophotometry, calculating cerebral O₂ consumption (VO₂) in ml O₂/min/100 g in the cortex, hypothalamus and pons. MK-801 did not significantly affect regional VO₂ or rCBF in normoxic piglets. Hypoxia resulted in an increase in local rCBF compared to controls: from 41 ± 6 to 103 ± 18 in the cortex; 34 ± 7 to 101 ± 20 in the hypothalamus; and 45 ± 10 to 95 ± 11 in the pons. Pretreatment with MK-801 abolished this hypoxic flow effect in the cortex (51 ± 2) and hypothalamus (49 ± 5), but not in the pons (91 ± 17). Similar results were observed for VO₂ with control values of 1.9 ± 0.3, 1.6 ± 0.2 and 2.1 ± 0.3 for the cortex, hypothalamus and pons respectively. Hypoxia resulted in an increase in the VO₂ to 3.9 ± 0.4 (cortex), 3.8 ± 0.6 (hypothalamus) and 3.9 ± 0.8 (pons). Pretreatment with MK-801 prior to hypoxia abolished these effects in the cortex (2.1 ± 0.2) and hypothalamus (2.1 ± 0.2), but not in the pons (2.9 ± 0.2). These findings suggest that NMDA receptors may play a role in the control of cerebral metabolism during hypoxia in this immature porcine model.     

Sub-lethal temperature thresholds indicate acclimation and physiological limits in brook trout Salvelinus fontinalis

The upper thermal tolerance of brook trout Salvelinus fontinalis was estimated using critical thermal maxima (CT_max) experiments on fish acclimated to temperatures that span the species' thermal range (5–25°C). The CT_max increased with acclimation temperature but plateaued in fish acclimated to 20, 23 and 25°C. Plasma lactate was highest, and the hepato-somatic index (I_H) was lowest at 23 and 25°C, which suggests additional metabolic costs at those acclimation temperatures. The results suggest that there is a sub-lethal threshold between 20 and 23°C, beyond which the fish experience reduced physiological performance.     

Ontogeny of hypoxic modulation of cardiac performance and its allometry in the African clawed frog Xenopus laevis

The ontogeny of cardiac hypoxic responses, and how such responses may be modified by rearing environment, are poorly understood in amphibians. In this study, cardiac performance was investigated in Xenopus laevis from 2 to 25 days post-fertilization (dpf). Larvae were reared under either normoxia or moderate hypoxia (PO₂ = 110 mmHg), and each population was assessed in both normoxia and acute hypoxia. Heart rate (f _h ) of normoxic-reared larvae exhibited an early increase from 77 ± 1 beats min^?1 at 2 dpf to 153 ± 1 beats min^?1 at 4 dpf, followed by gradual decreases to 123 ± 3 beats min^?1 at 25 dpf. Stroke volume (SV), 6 ± 1 nl, and cardiac output (CO), 0.8 ± 0.1 μl min^?1, at 5 dpf both increased by more than 40-fold to 25 dpf with rapid larval growth (~30-fold increase in body mass). When exposed to acute hypoxia, normoxic-reared larvae increased f _h and CO between 5 and 25 dpf. Increased SV in acute hypoxia, produced by increased end-diastolic volume (EDV), only occurred before 10 dpf. Hypoxic-reared larvae showed decreased acute hypoxic responses of EDV, SV and CO at 7 and 10 dpf. Over the period of 2–25 dpf, cardiac scaling with mass showed scaling coefficients of ?0.04 (f _h ), 1.23 (SV) and 1.19 (CO), contrary to the cardiac scaling relationships described in birds and mammals. In addition, f _h scaling in hypoxic-reared larvae was altered to a shallower slope of ?0.01. Collectively, these results indicate that acute cardiac hypoxic responses develop before 5 dpf. Chronic hypoxia at a moderate level can not only modulate this cardiac reflex, but also changes cardiac scaling relationship with mass.     

Acclimation of leaf respiration consistent with optimal photosynthetic capacity

Plant respiration is an important contributor to the proposed positive global carbon‐cycle feedback to climate change. However, as a major component, leaf mitochondrial (‘dark’) respiration (R_d) differs among species adapted to contrasting environments and is known to acclimate to sustained changes in temperature. No accepted theory explains these phenomena or predicts its magnitude. Here we propose that the acclimation of R_d follows an optimal behaviour related to the need to maintain long‐term average photosynthetic capacity (V_cmax) so that available environmental resources can be most efficiently used for photosynthesis. To test this hypothesis, we extend photosynthetic co‐ordination theory to predict the acclimation of R_d to growth temperature via a link to V_cmax, and compare predictions to a global set of measurements from 112 sites spanning all terrestrial biomes. This extended co‐ordination theory predicts that field‐measured R_d and V_cmax accessed at growth temperature (R_d,tg and V_cmax,tg) should increase by 3.7% and 5.5% per degree increase in growth temperature. These acclimated responses to growth temperature are less steep than the corresponding instantaneous responses, which increase 8.1% and 9.9% per degree of measurement temperature for R_d and V_cmax respectively. Data‐fitted responses proof indistinguishable from the values predicted by our theory, and smaller than the instantaneous responses. Theory and data are also shown to agree that the basal rates of both R_d and V_cmax assessed at 25°C (R_d,25 and V_cmax,25) decline by ~4.4% per degree increase in growth temperature. These results provide a parsimonious general theory for R_d acclimation to temperature that is simpler—and potentially more reliable—than the plant functional type‐based leaf respiration schemes currently employed in most ecosystem and land‐surface models.     

The effect of hypoxia on ventilation frequency in startled common sole Solea solea

Ventilation frequency (F_V) in motionless common sole Solea solea was measured before and after a startling stimulus in normoxia and in hypoxia (15% air saturation). Startling reduced F_V in normoxia (from mean ±s.e. 41 ± 3·3 beats min^?1 to near zero, i.e. 2·0 ± 1·8 beats min^?1) and in hypoxia (from mean ±s.e. 80 ± 4·4 to 58·8 ± 12·9 beats min^?1). It is suggested that the maintenance of high F_V in hypoxia may increase the probability of detection by predators compared to normoxia.     

Respiratory gas exchange in the airbreathing fish,Synbranchus marmoratus

Synopsis The partitioning of O₂ uptake between aquatic and aerial gas exchange and its dependence on ambient water PO₂ was studied in the facultative air breathing teleost Synbranchus marmoratus, after acclimation to well aerated water and after acute and chronic exposure to hypoxic water. O₂ uptake was also studied following acute air exposure and after prolonged entrapment in soil. Breathing rates during water and air breathing in response to reduced water PO₂ and tidal volume during air breathing were also studied. S. marmoratus satisfies its O₂ requirement by water breathing alone until water PO₂ falls below 30–50 mm Hg (switching PO₂) depending on the acclimation history. Below the switching PO₂, air breathing is adopted while active water breathing stops. The O₂ uptake varied little for all groups when the principal mode of gas exchange changed at the switching PO₂. The highest O₂ uptake prevailed when the fish employed the mode of gas exchange in operation during the acclimation period (i.e. water breathing for normoxia-acclimated, air breathing for hypoxic-acclimated).Acclimation to chronic hypoxia gave a much higher switching PO₂ 55 mm Hg) than for the other groups (about 30 mm Hg). S. marmoratus maintained its O₂ uptake when acutely exposed to air. When entrapped in soil in an aestivating state, the O₂ uptake was reduced to 25% of that in water or during acute air exposure. The overall gas exchange ratio for air breathing was very low (R_E 0.1).Branchial water pumping increased with lowering of water PO₂. The rate of air breathing was independent of water PO₂.The findings are discussed in the light of the ecophysiological conditions confronting S. marmoratus.     

Evidence of phenotypic plasticity and local adaption in metabolic rates between components of the Icelandic cod (Gadus morhua L.) stock

Phenotypic plasticity and local adaptations are important considerations in delineating population structure of marine fishes and critical to their conservation and management. We compared the weight-specific oxygen consumption rates (VO₂/M) of juvenile cod from the northern and southern components of the Icelandic stock acclimated to 4.0°C, 8.5°C, and 12.6°C and their metabolic response to abrupt temperatures changes within this range. Southern individuals exhibited VO₂/M up to 50% higher than their northern counterparts when tested at their acclimation temperature. However, northern fish generally experienced greater changes in VO₂/M, three to six-fold increases, relative to that expected at acclimation when moved to higher temperatures. Southern cod showed a greater decrease in VO₂/M when exposed to lower temperatures. Our results indicate physiological differences exist between the northern and southern components of the Icelandic cod stock and warrant considering them as two distinct populations.     

Membrane and calcium clock mechanisms contribute variably as a function of temperature to setting cardiac pacemaker rate in zebrafish Danio rerio

Here, we show that heart rate in zebrafish Danio rerio is dependent upon two pacemaking mechanisms and it possesses a limited ability to reset the cardiac pacemaker with temperature acclimation. Electrocardiogram recordings, taken from individual, anaesthetised zebrafish that had been acclimated to 18, 23 or 28°C were used to follow the response of maximum heart rate (f_Hmax) to acute warming from 18°C until signs of cardiac failure appeared (up to c. 40°C). Because f_Hmax was similar across the acclimation groups at almost all equivalent test temperatures, warm acclimation was limited to one significant effect, the 23°C acclimated zebrafish had a significantly higher (21%) peak f_Hmax and reached a higher (3°C) test temperature than the 18°C acclimated zebrafish. Using zatebradine to block the membrane hyperpolarisation-activated cyclic nucleotide–gated channels (HCN) and examine the contribution of the membrane clock mechanisms to cardiac pacemaking, f _Hmax was significantly reduced (by at least 40%) at all acute test temperatures and significantly more so at most test temperatures for zebrafish acclimated to 28°C vs. 23°C. Thus, HCN channels and the membrane clock were not only important, but could be modified by thermal acclimation. Using a combination of ryanodine (to block sarcoplasmic calcium release) and thapsigargin (to block sarcoplasmic calcium reuptake) to examine the contribution of sarcoplasmic reticular handling of calcium and the calcium clock, f _Hmax was again consistently reduced independent of the test temperature and acclimation temperature, but to a significantly lesser degree than zatebradine for zebrafish acclimated to both 28 and 18°C. Thus, the calcium clock mechanism plays an additional role in setting pacemaker activity that was independent of temperature. In conclusion, the zebrafish cardiac pacemaker has a limited temperature acclimation ability compared with known effects for other fishes and involves two pacemaking mechanisms, one of which was independent of temperature.     

Effects of environmental hypoxia on oxygen consumption rate and swimming activity in juvenile white sturgeon, Acipenser transmontanus, in relation to temperature and life intervals

Routine oxygen consumption rates (MO₂) and swimming activity rates of juvenile white sturgeon were determined using closed respirometers at life-interval-appropriate temperatures: 10° C (0.2 g mean wet weight), 16° C (1.9 g mean wet weight), and 20° C (63.1 g mean wet weight) under normoxic (PO₂ > 140 mmHg) and moderately hypoxic (PO₂=80 ± 5.0 mmHg) water conditions. At all temperatures and body sizes, hypoxia significantly depressed (p < 0.05) MO₂ (57% mean reduction) and swimming activity (70% mean reduction). Overall mean MO₂ was 228 µg O₂ g^-1 wet weight h^-1 (normoxia) and 99 µg O₂ g^-1 wet weight h^-1 (hypoxia). Thus, juvenile white sturgeon appear to decrease overall energy expenditures (hypometabolism) during hypoxia via reductions in spontaneous swimming activity. This is a life style that may increase survival during widespread or prolonged environmental hypoxia.     

White muscle 20S proteasome activity is negatively correlated to growth rate at low temperature in the spotted wolffish Anarhichas minor

The effect of temperature and mass on specific growth rate (G) was examined in spotted wolffish Anarhichas minor of different size classes (ranging from 60 to 1500 g) acclimated at different temperatures (4, 8 and 12° C). The relationship between G and 20S proteasome activity in heart ventricle, liver and white muscle tissue was then assessed in fish acclimated at 4 and 12° C to determine if protein degradation via the proteasome pathway could be imposing a limitation on somatic growth. Cardiac 20S proteasome activity was not affected by acclimation temperature nor fish mass and had no correlation with G. Hepatic 20S proteasome activity was higher at 12° C but did not show any relationship with G. Partial correlation analysis showed that white muscle 20S proteasome activity was negatively correlated to G (partial Pearson's r = ?0·609) but only at cold acclimation temperature (4° C). It is suggested that acclimation to cold temperature involves compensation of the mitochondrial oxidative capacity which would in turn lead to increased production of oxidatively damaged proteins that are degraded by the proteasome pathway and ultimately negatively affects G at cold temperature.     

The effects of feeding on the swimming performance and metabolic response of juvenile southern catfish,Silurus meridionalis,acclimated at different temperatures

To test whether the effects of feeding on swimming performance vary with acclimation temperature in juvenile southern catfish (Silurus meridionalis), we investigated the specific dynamic action (SDA) and swimming performance of fasting and feeding fish at acclimation temperatures of 15, 21, 27, and 33 °C. Feeding had no effect on the critical swimming speeding (U_crit) of fish acclimated at 15 °C (p = 0.66), whereas it elicited a 12.04, 18.70, and 20.98% decrease in U_crit for fish acclimated at 21, 27 and 33 °C, respectively (p < 0.05). Both the maximal postprandial oxygen consumption rate (VO_2peak) and the active metabolic rate (VO_2active, maximal aerobic sustainable metabolic rate of fasting fish) increased significantly with temperature (p < 0.05). The postprandial maximum oxygen consumption rates during swimming (VO_2max) were higher than the VO_2active of fasting fish at all temperature groups (p < 0.05). The VO_2max increased with increasing temperature, but the relative residual metabolic scope (VO_2max? VO_2peak) during swimming decreased with increasing in temperature. The present study showed that the impairment of postprandial swimming performance increased with increasing temperature due to the unparalleled changes in the catfish's central cardio-respiratory, peripheral digestive and locomotory capacities. The different metabolic strategies of juvenile southern catfish at different temperatures may relate to changes in oxygen demand, imbalances in ion fluxes and dissolved oxygen levels with changes in temperature.