Water pH limits extracellular but not intracellular pH compensation in the CO2-tolerant freshwater fish Pangasianodon hypophthalmus.

Preferentially regulating intracellular pH (pHi) confers exceptional CO2 tolerance on fish, but is often associated with reductions in extracellular pH (pHe) compensation. It is unknown whether these reductions are due to intrinsically lower capacities for pHe compensation, hypercarbia-induced reductions in water pH or other factors. To test how water pH affects capacities and strategies for pH compensation, we exposed the CO2-tolerant fish Pangasianodon hypophthalmus to 3 kPa PCO2  for 20 h at an ecologically relevant water pH of 4.5 or 5.8. Brain, heart and liver pHi was preferentially regulated in both treatments. However, blood pHe compensation was severely reduced at water pH 4.5 but not 5.8. This suggests that low water pH limits acute pHe but not pHi compensation in fishes preferentially regulating pHi Hypercarbia-induced reductions in water pH might therefore underlie the unexplained reductions to pHe compensation in fishes preferentially regulating pHi, and may increase selection for preferential pHi regulation.

Ontogeny and morphometrics of the gills and swim bladder of air-breathing striped catfish Pangasianodon hypophthalmus.

The air-breathing fish Pangasianodon hypophthalmus has been shown to have highly plastic branchial surfaces whose area (SA) increases with temperature and aquatic hypoxia. This modulation occurs through development of inter-lamellar cell mass (ILCM). Paradoxically, in conditions where this fish has been shown capable of covering its entire aerobic scope from the water phase, it has been shown to have a very small branchial SA. To address this paradox, we measured the SA, harmonic mean diffusion distance (τh) and calculated the anatomic diffusion factor (ADF) of the branchial and swim bladder surfaces in fish ranging from 3 to 1900 g at 27°C in normoxia. Since the lamellae were distinguishable from the ILCM, we measured the actual SA as well as the potential SA if ILCM were lost. As a result of low τh, P. hypophthalmus has a high capacity for branchial oxygen uptake with or without ILCM. Actual and potential gill ADF were 361 and 1002 cm2 µm-1 kg-1, respectively, for a 100 g fish and the ADF of the swim bladder was found to be 308 cm2 µm-1 kg-1 By swimming fish to exhaustion at different temperatures, we show that modulation of this SA is rapid, indicating that the apparent paradox between previous studies is eliminated. Regression analysis of log-log plots of respiratory SA in relation to body mass shows that the gill scales with mass similarly to the SA in active water-breathing fish, whereas the swim bladder scales with mass more like the mammalian lung does. This fish presents a combination of respiratory surfaces not previously seen in air-breathing fish.

Gill remodelling and growth rate of striped catfish Pangasianodon hypophthalmus under impacts of hypoxia and temperature.

Gill morphometric and gill plasticity of the air-breathing striped catfish (Pangasianodon hypophthalmus) exposed to different temperatures (present day 27°C and future 33°C) and different air saturation levels (92% and 35%) during 6weeks were investigated using vertical sections to estimate the respiratory lamellae surface areas, harmonic mean barrier thicknesses, and gill component volumes. Gill respiratory surface area (SA) and harmonic mean water - blood barrier thicknesses (HM) of the fish were strongly affected by both environmental temperature and oxygen level. Thus initial values for 27°C normoxic fish (12.4±0.8g) were 211.8±21.6mm2g-1 and 1.67±0.12µm for SA and HM respectively. After 5weeks in same conditions or in the combinations of 33°C and/or PO2 of 55mmHg, this initial surface area scaled allometrically with size for the 33°C hypoxic group, whereas branchial SA was almost eliminated in the 27°C normoxic group, with other groups intermediate. In addition, elevated temperature had an astounding effect on growth with the 33°C group growing nearly 8-fold faster than the 27°C fish.

High affinity and temperature sensitivity of blood oxygen binding in Pangasianodon hypophthalmus due to lack of chloride-hemoglobin allosteric interaction.

Air-breathing fishes represent interesting organisms in terms of understanding the physiological changes associated with the terrestrialization of vertebrates, and, further, are of great socio-economic importance for aquaculture in Southeast Asia. To understand how environmental factors, such as high temperature, affect O2 transport in air-breathing fishes, this study assessed the effects of temperature on O2 binding of blood and Hb in the economically important air-breathing fish Pangasianodon hypophthalmus. To determine blood O2 binding properties, blood was drawn from resting cannulated fishes and O2 binding curves made at 25°C and 35°C. To determine the allosteric regulation and thermodynamics of Hb O2 binding, Hb was purified, and O2 equilibria were recorded at five temperatures in the absence and presence of ATP and Cl(-). Whole blood had a high O2 affinity (O2 tension at half saturation P50 = 4.6 mmHg at extracellular pH 7.6 and 25°C), a high temperature sensitivity of O2 binding (apparent heat of oxygenation ΔH(app) = -28.3 kcal/mol), and lacked a Root effect. Further, the data on Hb revealed weak ATP binding and a complete lack of Cl(-) binding to Hb, which, in part, explains the high O2 affinity and high temperature sensitivity of blood O2 binding. This study demonstrates how a potent mechanism for increasing O2 affinity is linked to increased temperature sensitivity of O2 transport and provides a basic framework for a better understanding of how hypoxia-adapted species will react to increasing temperatures.