Recovery of the energy metabolism after a hypoxic challenge at different temperature conditions: a 31P-nuclear magnetic resonance spectroscopy study with common carp

Simulating the effects of fluctuating dissolved oxygen on growth, reproduction, and survival of fish and shrimp

Individuals are commonly exposed to fluctuating levels of stressors, while most laboratory experiments focus on constant exposures. We develop and test a mathematical model for predicting the effects of low dissolved oxygen (hypoxia) on growth, reproduction, and survival using laboratory experiments on fish and shrimp. The exposure-effects model simulates the hourly reductions in growth and survival, and the reduction in reproduction (fecundity) at times of spawning, of an individual as it is exposed to constant or hourly fluctuating dissolved oxygen (DO) concentrations. The model was applied to seven experiments involving fish and shrimp that included constant and fluctuating DO exposures, with constant exposures used for parameter estimation and the model then used to simulate the growth, reproduction, and survival in the fluctuating treatments. Cumulative effects on growth, reproduction, and survival were predicted well by the model, but the model did not replay the observed episodic low survival days. Further investigation should involve the role of acclimation, possible inclusion of repair effects in reproduction and survival, and the sensitivity of model predictions to the shape of the immediate effects function. Additional testing of the model with other taxa, different patterns of fluctuating exposures, and different stressors is needed to determine the model's generality and robustness.

Differential recovery from exercise and hypoxia exposure measured using 31P- and 1H-NMR in white muscle of the common carp Cyprinus carpio

Phosphocreatine (PCr) was reduced to equivalent levels in carp white muscle by high-intensity exhaustive exercise and exposure to hypoxia at 15 degrees C and 25 degrees C in order to assess the influence of intracellular pH (pH(i)), temperature and lactate levels on PCr recovery in vivo. High-intensity exercise resulted in a significantly lower pH(i) compared with hypoxia exposure and the rate of PCr depletion and tissue acidification during hypoxia exposure was significantly higher in carp held at 25 degrees C compared with those at 15 degrees C. During recovery, PCr and pH(i) returned towards normoxia/resting levels at a faster rate following hypoxia exposure than after exercise. The lower pH(i) in exercised carp caused a greater perturbation to cellular energy status (assessed as the free energy of ATP hydrolysis; DeltafG') and resulted in a higher [ATP]/[ADP(free)] ratio, which may limit mitochondrial ATP production and contribute to the slower recovery from exercise compared with recovery from hypoxia exposure. Rates of recovery from exercise and hypoxia exposure were not affected by acclimation temperature (15 and 25 degrees C), suggesting that the processes involved in acclimation compensate for the Q(10) effects of temperature on metabolic processes. Finally, using a dual 31P-NMR and 1H-NMR analysis technique, we demonstrated that the greater tissue acidification observed after high-intensity exercise compared with hypoxia exposure occurred at similar white muscle lactate concentrations.

Critical temperatures in the cephalopodSepia officinalisinvestigated usingin vivo31P NMR spectroscopy

The present study was designed to test the hypothesis of an oxygen limitation defining thermal tolerance in the European cuttlefish (Sepia officinalis). Mantle muscle organ metabolic status and pHiwere monitored using in vivo31P NMR spectroscopy, while mantle muscle performance was determined by recording mantle cavity pressure oscillations during ventilation and spontaneous exercise.

Under control conditions (15°C), changes in muscle phospho-l-arginine (PLA) and inorganic phosphate (Pi)levels could be linearly related to frequently occurring, high-pressure mantle contractions with pressure amplitudes (MMPA) of >0.2 kPa. Accordingly,mainly MMPA of >2 kPa affected muscle PLA reserves, indicating that contractions with MMPA of <2 kPa only involve the thin layers of aerobic circular mantle musculature. On average, no more than 20% of muscle PLA was depleted during spontaneous exercise under control conditions.

Subjecting animals to acute thermal change at an average rate of 1 deg. h–1 led to significant Pi accumulation (equivalent to PLA breakdown) and decrements in the free energy of ATP hydrolysis(dG/dζ) at both ends of the temperature window, starting at mean critical temperatures (Tc) of 7.0 and 26.8°C,respectively. Frequent groups of high-pressure mantle contractions could not(in the warm) or only partially (in the cold) be related to net PLA breakdown in mantle muscle, indicating an oxygen limitation of routine metabolism rather than exercise-related phosphagen use. We hypothesize that it is mainly the constantly working radial mantle muscles that become progressively devoid of oxygen. Estimates of very low dG/dζ values (–44 kJ mol–1) in this compartment, along with correlated stagnating ventilation pressures in the warm, support this hypothesis. In conclusion, we found evidence for an oxygen limitation of thermal tolerance in the cuttlefish Sepia officinalis, as indicated by a progressive transition of routine mantle metabolism to an anaerobic mode of energy production.

Metabolic changes in Japanese medaka (Oryzias latipes) during embryogenesis and hypoxia as determined by in vivo 31P NMR

In vivo (31)P nuclear magnetic resonance spectroscopy (NMR) was used to determine phosphometabolite changes in medaka (Oryzias latipes) during embryogenesis and hypoxia. NMR data were acquired using a flow-through NMR tube perfusion system designed to both deliver oxygenated water to embryos and accommodate a hypoxic challenge. Measurements of embryogenesis at 12- and 24-h intervals throughout 8 days of development (n = 3 per time point, 900 embryos per replicate) and during acute hypoxia (n = 6, 900 embryos at Iwamatsu stage 37 per replicate) were performed via NMR, and replicate samples (n = 4, 250 embryos each) were flash frozen for HPLC analysis. The hypoxic challenge experiment consisted of data acquisition with recirculating water (pre-hypoxic control period; 1 h), without recirculating water (hypoxic challenge; 1 h), then again with recirculating water (recovery period; 1.3 h). Concentrations of ATP, phosphocreatine (PCr), orthophosphate (P(i)), phosphomonoesters (PME), phosphodiesters (PDE), and intracellular pH (pH(i)) were determined by NMR, and ATP, ADP, AMP, GTP, GDP, and PCr were also determined via HPLC. During embryogenesis, [ATP] and [PCr] as determined by HPLC increased from 1-day post fertilization (DPF) levels of 0.93+/-0.08 and 2.48+/-0.21 micromol/mg (dry tissue), respectively, to 7.24+/-0.77 and 15.66+/-1.08 micromol/mg, respectively, by day 8. [ATP] and [PCr] measured by both NMR and HPLC fluctuated over 1-3 DPF, then increased significantly (p<0.05) over 3-8 DPF, while [PME] and [PDE] decreased (p<0.05) throughout embryogenesis. NMR and HPLC measurements revealed 1-3, 4-5, and 6-8 DPF as periods of embryogenesis significantly different from each other (p<0.05), and representing important transitions in metabolism and growth. During hypoxic challenge, [ATP] and [PCr] declined (p<0.05), [PME] and [PDE] decreased slightly, and [P(i)] increased (p<0.05). All phosphometabolites returned to pre-hypoxia concentrations during recovery. The pH(i) decreased (p<0.05) from 7.10+/-0.03 to 6.94+/-0.03 as a result of hypoxia, and failed to return to pre-hypoxic levels within the 1.3-h recovery phase. Results demonstrate the utility of in vivo (31)P NMR to detect significant alterations in phosphorylated nucleotides and phosphometabolites at specific developmental stages during medaka development and that late-stage medaka utilize PCr to generate ATP under hypoxic conditions.

Oxygen limited thermal tolerance in fish?--Answers obtained by nuclear magnetic resonance techniques

In various phyla of marine invertebrates limited capacities of both ventilatory and circulatory performance were found to set the borders of the thermal tolerance window with limitations in aerobic scope and onset of hypoxia as a first line of sensitivity to both cold and warm temperature extremes. The hypothesis of oxygen limited thermal tolerance has recently been investigated in fish using a combination of non-invasive nuclear magnetic resonance (NMR) methodology with invasive techniques. In contrast to observations in marine invertebrates arterial oxygen tensions in fish were independent of temperature, while venous oxygen tensions displayed a thermal optimum. As the fish heart relies on venous oxygen supply, limited cardio-circulatory capacity is concluded to set the first level of thermal intolerance in fish. Nonetheless, maximized ventilatory capacity is seen to support circulation in maintaining the width of thermal tolerance windows. The interdependent setting of low and high tolerance limits is interpreted to result from trade-offs between optimized tissue functional capacity and baseline oxygen demand and energy turnover co-determined by the adjustment of mitochondrial densities and functional properties to a species-specific temperature range. At temperature extremes, systemic hypoxia will elicit metabolic depression, thereby widening the thermal window transiently sustained especially in those species preadapted to hypoxic environments.