Osmorespiratory Compromise in Zebrafish (Danio rerio): Effects of Hypoxia and Acute Thermal Stress on Oxygen Consumption, Diffusive Water Flux, and Sodium Net Loss Rates

Hypoxia alters the upper thermal limits and blood physiology in zebrafish, Danio rerio

Hypoxic aquatic environments occur more frequently as a result of climate change, thereby exerting challenges on the physiological and metabolic functions of aquatic animals. In this study, a model fish, zebrafish (Danio rerio) was used to observe the climate-induced hypoxic effect on the upper thermal limit (critical thermal maximum; CTmax), hemoglobin, and blood glucose levels, and abnormalities of erythrocytes at cellular and nuclear level. The value of CTmax decreased significantly under hypoxia (39.10 ± 0.96 °C) compared to normoxia (43.70 ± 0.91 °C). At CTmax, hemoglobin levels were much lower (9.33 ± 0.60 g/dL) and blood glucose levels were significantly higher (194.20 ± 11.33 mg/L) under hypoxia than they were under normoxia and at the beginning of the experiment. Increased frequencies of abnormalities in the erythrocytes at both cellular (fusion, twin, elongated, spindle and tear drop shaped) and nuclear (micronucleus, karyopyknosis, binuclei, nuclear degeneration and notched nuclei) levels were also found under hypoxia compared to normoxia. These results suggest that hypoxic conditions significantly alter the temperature tolerance and subsequent physiology in zebrafish. Our findings will aid in the development of effective management techniques for aquatic environments with minimum oxygen availability.

Osmo-respiratory compromise in the mosshead sculpin (Clinocottus globiceps): effects of temperature, hypoxia, and re-oxygenation on rates of diffusive water flux and oxygen uptake

In nature, mosshead sculpins (Clinocottus globiceps) are challenged by fluctuations in temperature and oxygen levels in their environment. However, it is unclear how mosshead sculpins modulate the permeability of their branchial epithelia to water and O2 in response to temperature or hypoxia stress. Acute decrease in temperature from 13 to 6 oC reduced diffusive water flux rate by 22% and ṀO2 by 51%, whereas acute increase in temperature from 13 to 25 oC increased diffusive water flux rate by 217% and ṀO2 by 140%, yielding overall Q10 values of 2.08 and 2.47 respectively. Acute reductions in oxygen tension from >95% to 20% or 10% air saturation did not impact diffusive water flux rates, however, ṀO2 was reduced significantly by 36% and 65% respectively. During 1-h or 3-h recovery periods diffusive water flux rates were depressed while ṀO2 exhibited overshoots beyond the normoxic control level. Many responses differed from those seen in our parallel earlier study on the tidepool sculpin, a cottid with similar hypoxia tolerance but much smaller gill area that occupies a similar environment. Overall, our data suggest that during temperature stress, diffusive water flux rates and ṀO2 follow the traditional osmo-respiratory compromise pattern, but during hypoxia and re-oxygenation stress, diffusive water flux rates are decoupled from ṀO2.

Taurine depletion impairs cardiac function and affects tolerance to hypoxia and high temperatures in brook char (Salvelinus fontinalis)

Physiological and environmental stressors can cause osmotic stress in fish hearts, leading to a reduction in intracellular taurine concentration. Taurine is a β-amino acid known to regulate cardiac function in other animal models but its role in fish has not been well characterized. We generated a model of cardiac taurine deficiency (TD) by feeding brook char (Salvelinus fontinalis) a diet enriched in β-alanine, which inhibits cardiomyocyte taurine uptake. Cardiac taurine levels were reduced by 21% and stress-induced changes in normal taurine handling were observed in TD brook char. Responses to exhaustive exercise and acute thermal and hypoxia tolerance were then assessed using a combination of in vivo, in vitro and biochemical approaches. Critical thermal maximum was higher in TD brook char despite significant reductions in maximum heart rate. In vivo, TD brook char exhibited a lower resting heart rate, blunted hypoxic bradycardia and a severe reduction in time to loss of equilibrium under hypoxia. In vitro function was similar between control and TD hearts under oxygenated conditions, but stroke volume and cardiac output were severely compromised in TD hearts under severe hypoxia. Aspects of mitochondrial structure and function were also impacted in TD permeabilized cardiomyocytes, but overall effects were modest. High levels of intracellular taurine are required to achieve maximum cardiac function in brook char and cardiac taurine efflux may be necessary to support heart function under stress. Taurine appears to play a vital, previously unrecognized role in supporting cardiovascular function and stress tolerance in fish.

Effects of Acute Hypoxic Stress on Physiological and Hepatic Metabolic Responses of Triploid Rainbow Trout (Oncorhynchus mykiss)

This experiment simulated the hypoxic environment caused by actual production operations in fish farming (i.e., catching, gathering, transferring, and weighting) to study the effects of acute hypoxic conditions on the physiological and metabolic responses of triploid rainbow trout (O. mykiss). Two groups of fish weighting 590 g were sampled in the normoxia group (dissolved oxygen above 7 mg/L) and hypoxia group (dissolved oxygen ranged from 2 to 5 mg/L for 10 min). The results showed that 1) regarding stress response, hypoxia increased plasma levels of cortisol, heat shock protein 70 (HSP-70), lysozyme, alanine aminotransferase (ALT), aspartate aminotransferase (AST) and creatine phosphokinase (CPK); induced the expression of hepatic genes encoding nuclear factor erythroid 2 related factor 2 (Nrf2), interferon γ (IFN-γ) and interleukin-1β (IL-1β). 2) Regarding metabolism response, hypoxia increased plasma levels of globulin (GLOB), glucose (GLU), triglyceride (TG) and lactate dehydrogenase (LDH); upregulated the hepatic gene expression of phosphoenolpyruvate carboxykinase, (PEPCK), pyruvate dehydrogenase kinase (PDK1), acetyl-CoA carboxylase (ACC) and acetyl-CoA oxidase (ACO); downregulated the hepatic gene expression of carnitine palmitoyl transferase 1 (CPT1); and unchanged the expression of hepatic genes in glycolysis and autophagy. 3) In response to hypoxia-inducible factors (HIFs), the hepatic HIF-2α gene was activated in the hypoxia group, but HIF-1α gene expression remained unchanged. Thus, during acute hypoxic stress, triploid rainbow trout were in a defensive state, with an enhanced immune response and altered antioxidant status. Additionally, the hepatic mitochondrial oxidation of glucose- and lipid-derived carbon in trout was suppressed, and hepatic gluconeogenesis and lipid synthesis were activated, which might be regulated by the HIF-2α pathway.

The osmorespiratory compromise in the fish gill

August Krogh made fundamental discoveries about both respiratory gas exchange and osmo/iono-regulation in fish gills. Dave Randall and co-workers identified a tradeoff between these two functions such that high functional surface area and low diffusion distance would favour O₂ uptake (e.g. exercise, hypoxia), whereas low functional surface area and high diffusion distance would favour osmo/iono-regulation (rest, normoxia). Today we call this concept the "osmorespiratory compromise" and realize that it is much more complex than originally envisaged. There are at least 6 mechanisms by which fish can change functional branchial area and diffusion distance. Three involve reorganizing blood flow pathways: (i) flow redistribution within the secondary (respiratory) lamellae; (ii) flow shunting between "respiratory" and "ionoregulatory" pathways in the filament; (iii) opening up more distal lamellae on the filament and closing non-respiratory pathways. Three more involve "reversible gill remodeling": (iv) proliferation of the interlamellar gill cell mass (ILCM); (v) proliferation of ionocytes up the sides of the lamellae; (vi) covering over the apical exposure of ionocytes by extension of pavement cells. In ways that remain incompletely understood, these mechanisms allow dynamic regulation of the osmorespiratory compromise, such that ion and water fluxes can be decoupled from O₂ uptake during continuous exercise. Furthermore, hypoxia-tolerant species can reduce branchial ion and water fluxes below normoxic levels despite hyperventilating during hypoxia. In marine fish, the osmorespiratory conflict is intensified by the greater ionic and osmotic gradients from seawater to blood, but underlying mechanisms remain poorly understood.