Hormonal changes, substrate mobilization and energy metabolism were studied in turbot Scophthalmus maximus exposed to 3 hypoxic conditions (oxygen partial pressure in water, PwO 2 = 90, 60 and 30 mm Hg) followed by recovery under normoxia. Measurements of the blood pH, total CO 2 concentration, arterial oxygen partial pressure, hematocrit, glucose, lactate, and 'stress' hormones (cortisol, adrenaline and noradrenaline) plasmatic concentrations were performed. Highenergy phosphorylated compounds, glycogen, glucose and lactate concentrations were also determined in liver and white muscle tissues. Exposure to 90 or 60 mm Hg did not induce any major physiological change, as hyperventilation by itself could compensate for the decrease in water oxygen tension. At 30 mm Hg, marked increases in cortisol, adrenaline and noradrenaline concentrations, associated with a decrease in blood arterial oxygen partial pressure, were observed. During exposure to 30 mm Hg, turbot resorted to anaerobic metabolism, resulting in liver glycogen depletion and lactate production. This mechanism appeared to be efficient enough to produce energy, as no significant change in phosphorylated compounds and adenylate energy charges in muscle and liver could be observed. These results indicate an absence of metabolic depression in turbot down to 30 mm Hg and confirm the high capacity of this species to cope with low water oxygen tension.

Hypoxia is a pervasive problem in coastal environments and is predicted to have enduring impacts on aquatic ecosystems. Intraspecific variation in hypoxia tolerance is well documented in fish; however, the factors underlying this variation remain unknown. Here, we investigate the role of the heart in individual hypoxia tolerance of the European sea bass (Dicentrarchus labrax). We found individual whole-animal hypoxia tolerance is a stable trait in sea bass for more than 18 months (duration of study). We next examined in vitro cardiac performance and found myocardial muscle from hypoxia-tolerant individuals generated greater force, with higher rates of contraction and relaxation, than hypoxic-sensitive individuals during hypoxic exposure. Thus, whole-animal hypoxia tolerance is associated with cardiac hypoxia tolerance. As the occurrence of aquatic hypoxia is expected to increase in marine ecosystems, our experimental data suggest that cardiac performance may influence fish survival and distribution.

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The effect of hyposmotic shock on exocytosis was examined in isolated hepatocytes of turbot, a marine flatfish, using the molecular probe FM1-43. Sudden exposure to a reduced osmolality caused an increase in cell exocytic activity related to the osmotic gradient between intra- and extracellular fluids. Cytoskeletal microtubules could contribute to this hyposmotic-induced exocytosis since colchicine inhibited the process. Protein kinase C, phosphatidylinositol-3 kinase, phospholipases A2, C and D could constitute key enzymes in the mechanism since their inhibition by specific agents altered the hyposmotic-induced exocytic activity. Moreover, arachidonic acid and derivates from the 5-lipoxygenase pathway as well as calcium could participate in the process. As regulatory volume decrease (RVD) exhibited by turbot hepatocytes following hyposmotic stimulation involves similar features, a potential role of exocytosis in volume regulation is suggested. In particular, exocytosis could serve RVD by contributing to ATP release since this latter process similarly appeared to be phospholipase D-dependent and related to the osmotic gradient. This study provides the first evidence of a volume-sensitive exocytosis that could aim at volume constancy in a marine teleost fish cell type.

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Here we explored the hypothesis that individual variation in thermal tolerance in the European sea bass is associated with other key performance traits involved in determining individual Darwinian fitness. Our finding that whole animal temperature tolerance positively correlates with cardiac size could suggest selection on the cardiorespiratory system may benefit thermal tolerance.

The relationship between hypothermia induction time and survival duration following sepsis was studied on 31 male Sprague-Dawley rats (median weight 311 g, range 260-356 g). After anesthesia and when the target temperature was reached (normothermia: 38°C or mild induced hypothermia: 34°C), sepsis was induced by cecal ligation and perforation. Five experimental groups were used. In groups 1 and 2, temperature of septic rats was maintained throughout the experiment at 38°C (seven rats) or 34°C (six rats), respectively. In groups 3, 4, and 5, septic rats (six per group) were maintained at 38°C for 1, 2, and 3 hours, respectively, and then placed in mild hypothermia (34°C). For each group, the survival duration was determined and blood samples were performed at the tail to measure tumor necrosis factor-α (TNF-α) plasma concentration. Whatever the experimental group, a decrease in temperature from 38°C to 34°C significantly increased the survival duration of septic rats compared with those maintained at 38°C throughout the experiment. The delay between the onset of sepsis and induction of hypothermia was also crucial. Thus, hypothermia induced after 1 hour of sepsis at 38°C significantly increased the survival duration of septic rats (12 hours 37 minutes±1 hour 4 minutes; group 3) compared with hypothermia induced after 3 hours of sepsis (8 hours 56 minutes±1 h 20 minutes; group 5). Moreover, except for group 5, survival duration improvement of septic rats observed in hypothermia was related to a lower increase of TNF-α plasma concentration compared with septic rats in normothermia. During sepsis, mild induced hypothermia significantly increased the survival duration of septic rats. The earlier hypothermia was applied, the longer the septic rats survived. According to these results, hypothermia may therefore provide the necessary time to apply a proper treatment.

Background information. ATP is released from many cell types exposed to hypo-osmotic shock and is involved in RVD (regulatory volume decrease). Purinergic signalling events have been extensively investigated in mammals, but not in marine teleosteans.Results. The effect of hypo-osmotic shock on ATP release was examined in isolated hepatocytes from turbot (Scophthalmus maximus), a marine flatfish. Hypo-osmotic stress (240 mOsm · kg −1 ) induced a significant increase in ATP efflux, and was inhibited by a potential CFTR (cystic fibrosis transmembrane conductance regulator) inhibitor, glibenclamide, but not by the MDR1 (multidrug resistance 1) P-glycoprotein inhibitor, verapamil. ATP efflux could be a cAMP-dependent process, as IBMX (isobutylmethylxanthine) and forskolin triggered the process under isoosmotic conditions. Protein kinases, including protein kinase C, could also be involved, as staurosporine and chelerythrine inhibited the mechanism. Calcium could contribute to ATP efflux as ionomycin, a calcium ionophore, elicited a rapid release under iso-osmotic conditions, and chelation using EGTA abolished ATP release under hypo-osmotic conditions. RVD was partially abolished by apyrase, an ATP scavenger, and suramin, a purinoceptor antagonist. Moreover, hypo-osmotic shock induced a rise in intracellular calcium which could be involved in RVD. Since extracellular ATP triggered an increase in cellular free-calcium content under iso-osmotic conditions, our results could indicate that hypo-osmotic-induced ATP efflux contributes to RVD in turbot hepatocytes by stimulating purinergic receptors, which may lead to activation of a calcium signalling pathway.Conclusions. These data provide the first evidence of volume-sensitive ATP signalling for volume maintenance in a marine teleost fish cell type.