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The Journal of Experimental Biology 205, 1153-1160 (2002)
© 2002 The Company of Biologists Limited

Changes in metabolic rate and N excretion in the marine invertebrate Sipunculus nudus under conditions of environmental hypercapnia : identifying effective acid—base variables

M. Langenbuch and H. O. Pörtner^*

Alfred-Wegener-Institut für Polar- und Meeresforschung, Ökophysiologie und Ökotoxikologie, Postfach 120161, D-27515 Bremerhaven, Germany

^* Author for correspondence (e-mail: hpoertner{at}awi-bremerhaven.de )

Accepted 31 January 2002

Increased CO₂ partial pressures (hypercapnia) as well as hypoxia are natural features of marine environments like the intertidal zone. Nevertheless little is known about the specific effects of CO₂ on metabolism, except for the well-described effects on acid—base variables and regulation. Accordingly, the sediment-dwelling worm Sipunculus nudus was used as an experimental model to investigate the correlation of acid—base-induced metabolic depression and protein/amino acid catabolism, by determining the rates of oxygen consumption, ammonia excretion and O/N ratios in non-perfused preparations of body wall musculature at various levels of extra- and intracellular pH, P_CO2 and [HCO₃^-]. A decrease in extracellular pH from control level (7.9) to 6.7 caused a reduction in aerobic metabolic rate of both normocapnic and hypercapnic tissues by 40-45 %. O/N ratios of 4.0-4.5 under control conditions indicate that amino acid catabolism meets the largest fraction of aerobic energy demand. A significant 10-15 % drop in ammonia excretion, a simultaneous reduction of O/N ratios and a transient accumulation of intracellular bicarbonate during transition to extreme acidosis suggest a reduction in net amino acid catabolism and a shift in the selection of amino acids used, favouring monoamino dicarboxylic acids and their amines (asparagine, glutamine, aspartic and glutamic acids). A drop in intracellular pH was identified as mediating this effect. In conclusion, the present data provide evidence for a regulatory role of intracellular pH in the selection of amino acids used by catabolism.

Key words: Sipunculus nudus, hypercapnia, acid—base variable, intracellular pH, amino acid catabolism

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