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Tolerance of chronic hypercapnia by the European eel Anguilla anguilla
1 School of Biosciences, University of Birmingham, Birmingham, B15 2TT,
UK
2 Department of Pharmacological Sciences, Via Balzaretti, 9, University of
Milan, 20133 Milan, Italy
3 Marine Biological Laboratory, University of Copenhagen, Strandpromenaden
5, DK-3000 Helsingør, Denmark
* Author for correspondence at present address: CREMA-l'Houmeau, B.P. 5, 17137 l'Houmeau, France (e-mail: David.McKenzie{at}ifremer.fr)
Accepted 5 March 2003
European eels were exposed for 6 weeks to water CO2 partial pressures (PCO2) from ambient (approx. 0.8 mmHg), through 15±1 mmHg and 30±1 mmHg to 45±1 mmHg in water with a total hardness of 240 mg l–1 as CaCO3, pH 8.2, at 23±1°C. Arterial plasma PCO2 equilibrated at approximately 2 mmHg above water PCO2 in all groups, and plasma bicarbonate accumulated up to 72 mmol l–1 in the group at a water PCO2 of 45 mmHg. This was associated with an equimolar loss of plasma Cl–, which declined to 71 mmol l–1 at the highest water PCO2. Despite this, extracellular acid–base compensation was incomplete; all hypercapnic groups tolerated chronic extracellular acidoses and reductions in arterial blood O2 content (CaO2), of progressive severity with increasing PCO2. All hypercapnic eels, however, regulated the intracellular pH of heart and white muscle to the same levels as normocapnic animals. Hypercapnia had no effect on such indicators of stress as plasma catecholamine or cortisol levels, plasma osmolality or standard metabolic rate. Furthermore, although CaO2 was reduced by approximately 50% at the highest PCO2, there was no effect of hypercapnia on the eels' tolerance of hypoxia, aerobic metabolic scope or sustained swimming performance. The results indicate that, at the levels tested, chronic hypercapnia was not a physiological stress for the eel, which can tolerate extracellular acidosis and extremely low Cl– levels while compensating tissue intracellular pH, and which can meet the O2 requirements of routine and active metabolism despite profound hypoxaemia.
Key words: hypercapnia, European eel, Anguilla anguilla, acid–base balance, aerobic scope, hypoxia, metabolic rate, stress, swimming performance
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