Effects of temperature on acid-base regulation, gill ventilation and air breathing in the clown knifefish, Chitala ornata.

Chitala ornata is a facultative air-breathing fish, which at low temperatures shows an arterial PCO2  (PaCO2 ) level only slightly elevated above that of water breathers. By holding fish with in-dwelling catheters at temperatures from 25 to 36°C and measuring blood gasses, we show that this animal follows the ubiquitous poikilotherm pattern of reducing arterial pH with increasing temperature. Surprisingly, the temperature increase caused an elevation of PaCO2  from 5 to 12 mmHg while the plasma bicarbonate concentration remained constant at around 8 mmol l-1 The temperature increase also gave rise to a larger fractional increase in air breathing than in gill ventilation frequency. These findings suggest that air breathing, and hence the partitioning of gas exchange, is to some extent regulated by acid-base status in air-breathing fish and that these bimodal breathers will be increasingly likely to adopt respiratory pH control as temperature rises, providing an interesting avenue for future research.

Clown knifefish (Chitala ornata) oxygen uptake and its partitioning in present and future temperature environments.

It has been argued that tropical ectotherms are more vulnerable to the projected temperature increases than their temperate relatives, because they already live closer to their upper temperature limit. Here we examine the effects of a temperature increase in environmental temperature to 6°C above the present day median temperature (27°C) in the freshwater air-breathing fish Chitala ornata, on aspects of its respiratory physiology in both normoxia and in hypoxia. We found no evidence of respiratory impairment with elevated temperature. The standard metabolic rate (SMR) and routine metabolic rate (RMR) in the two temperatures in normoxia and hypoxia increased with Q10 values between 2.3 and 2.9, while the specific dynamic action (SDA) and its coefficient increased from 7.8 to 14.7% in 27°C and 33°C, respectively. In addition, Chitala ornata exhibited significantly improved growth at the elevated temperature in both hypoxic and normoxic water. While projected temperature increases may negatively impact other essential aspects in this animal's environment, we see no evidence of a negative impact on this species itself.

High capacity for extracellular acid-base regulation in the air-breathing fish Pangasianodon hypophthalmus.

The evolution of accessory air-breathing structures is typically associated with reduction of the gills, although branchial ion transport remains pivotal for acid-base and ion regulation. Therefore, air-breathing fishes are believed to have a low capacity for extracellular pH regulation during a respiratory acidosis. In the present study, we investigated acid-base regulation during hypercapnia in the air-breathing fish Pangasianodon hypophthalmus in normoxic and hypoxic water at 28-30°C. Contrary to previous studies, we show that this air-breathing fish has a pronounced ability to regulate extracellular pH (pHe) during hypercapnia, with complete metabolic compensation of pHe within 72 h of exposure to hypoxic hypercapnia with CO2 levels above 34 mmHg. The high capacity for pHe regulation relies on a pronounced ability to increase levels of HCO3(-) in the plasma. Our study illustrates the diversity in the physiology of air-breathing fishes, such that generalizations across phylogenies may be difficult.

Ventilatory responses of the clown knifefish, Chitala ornata, to arterial hypercapnia remain after gill denervation.

The aim of this study was to corroborate the presence of CO2/H+-sensitive arterial chemoreceptors involved in producing air-breathing responses to aquatic hypercarbia in the facultative air-breathing clown knifefish (Chitala ornata) and to explore their possible location. Progressively increasing levels of CO2 mixed with air were injected into the air-breathing organ (ABO) of one group of intact fish to elevate internal PCO2 and decrease blood pH. Another group of fish in which the gills were totally denervated was exposed to aquatic hypercarbia (pH ~ 6) or arterial hypercapnia in aquatic normocarbia (by injection of acetazolamide to increase arterial PCO2 and decrease blood pH). Air-breathing frequency, gill ventilation frequency, heart rate and arterial PCO2 and pH were recorded during all treatments. The CO2 injections into the ABO induced progressive increases in air-breathing frequency, but did not alter gill ventilation or heart rate. Exposure to both hypercarbia and acetazolamide post-denervation of the gills also produced significant air-breathing responses, but no changes in gill ventilation. While all treatments produced increases in arterial PCO2 and decreases in blood pH, the modest changes in arterial PCO2/pH in the acetazolamide treatment produced the greatest increases in air-breathing frequency. These results strengthen the evidence that internal CO2/H+ sensing is involved in the stimulation of air breathing in clown knifefish and suggest that it involves extra-branchial chemoreceptors possibly situated either centrally or in the air-breathing organ.

Ventilatory responses of the clown knifefish, Chitala ornata, to hypercarbia and hypercapnia.

The aim of the present study was to determine the roles of externally versus internally oriented CO2/H+-sensitive chemoreceptors in promoting cardiorespiratory responses to environmental hypercarbia in the facultative air-breathing fish, Chitala ornata (the clown knifefish). Fish were exposed to environmental acidosis (pH ~ 6.0) or hypercarbia (≈ 30 torr PCO2) that produced changes in water pH equal to the pH levels of the acidotic water to distinguish the relative roles of CO2 versus H+. We also injected acetazolamide to elevate arterial levels of PCO2 and [H+] in fish in normocarbic water to distinguish between internal and external stimuli. We measured changes in gill ventilation frequency, air breathing frequency, heart rate and arterial blood pressure in response to each treatment as well as the changes produced in arterial PCO2 and pH. Exposure to normocarbic water of pH 6.0 for 1 h did not produce significant changes in any measured variable. Exposure to hypercarbic water dramatically increased air breathing frequency, but had no effect on gill ventilation. Hypercarbia also produced a modest bradycardia and fall in arterial blood pressure. Injection of acetazolamide produced similar effects. Both hypercarbia and acetazolamide led to increases in arterial PCO2 and falls in arterial pH although the changes in arterial PCO2/pH were more modest following acetazolamide injection as were the increases in air breathing frequency. The acetazolamide results suggest that the stimulation of air breathing was due, at least in part, to stimulation of internally oriented CO2/H+ chemoreceptors monitoring blood gas changes.