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Journal of Experimental Biology 143,33-51 (1989)
Published by Company of Biologists 1989

PHYSIOLOGICAL RESPONSES OF THE CRAYFISH PACIFASTACUS LENIUSCULUS TO ENVIRONMENTAL HYPEROXIA: I. EXTRACELLULAR ACID-BASE AND ELECTROLYTE STATUS AND TRANSBRANCHIAL EXCHANGE

MICHÉLE G. WHEATLY ¹

¹ Department of Zoology, University of Florida Gainesville, FL 32611, USA

Extracellular acid--base and ionic status, and transbranchial exchange of acidic equivalents and electrolytes, were monitored in freshwater crayfish (Pacifastacus leniusculus) during control normoxia (P_O2 = 148 mmHg; 1 mmHg = 133.3 Pa), 72 h of hyperoxia (P_O2 = 500 mmHg) and 24 h of recovery. An initial (3 h) respiratory acidosis of 0.2 _pH units was completely compensated within 48 h by a 50% increase in metabolic [HCO₃^-+CO₃^2-] accompanied by a significant reduction in circulating [Cl^-]. In addition, the original increase in P_co2 was partially accommodated. The time course of transbranchial acidic equivalent exchange paralleled the change in extracellular metabolic base load with a significant branchial output of H+ during the first 48 h of hyperoxia. This was associated with net branchial effluxes of Cl^- and Mg²⁺. Unidirectional flux analysis revealed parallel reductions in Na⁺ influx and efflux during initial hyperoxic exposure, reflecting an alteration in exchange diffusion. The net Cl^- efflux was due to an initial increase in efflux followed by a reduction in influx. The reverse sequence of events occurred more rapidly when normoxia was reinstated: metabolic base was removed from the haemolymph and control haemolymph acid--base and ion levels were re-established within 24 h. Transbranchial fluxes of acidic equivalents similarly recovered within 24 h although net Na⁺ output and Cl^- uptake persisted.

The study attempted to identify relationships between branchial net H⁺ exchange and components of Na⁺ and Cl^- exchange and quantitatively to correlate changes in the acidic equivalent and electrolyte concentrations in the extracellular fluid compartment with those in the external water.

Key words: acid--base balance, ionoregulation, transbranchial exchange

Accepted on October 19, 1988

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