ELSEVIER Respiration Physiology 96 (1994) 259-272 RESPIRATION PHYSIOLOGY Acid-base disequilibrium in the arterial blood of rainbow trout Kathleen M. Gilmour *, D.J. Randall I and Steve F. Perry Department of Biology, Universityof Ottawa, 30 Marie Curie, Ottawa Ontario, KIN 6N5 Canada Accepted 13 December 1993 Abstract An extracorporeal blood circulation and a stopflow technique were used to examine the acid- base status of arterial blood in the rainbow trout, Oneorhynchus mykiss. Arterial blood was routed from the coeliac artery through an external circuit in which pH (pHa), partial pressure of oxy- gen (Pao2) and partial pressure of carbon dioxide (Paco2) were monitored continuously. The stopflow condition was imposed by turning off the pump which drove the external loop. A radioisotopic CO2 excretion assay was performed on blood samples collected periodically to evaluate plasma carbonic anhydrase (CA) activity and hence red blood cell (rbc) lysis. An acid-base disequilibrium was found in the post-branchial blood; pHa increased by 0.04-0.06 units, and Pac% by 0.03-0.10 Torr, during the stopflow period. The disequilibrium appeared to arise primarily from the slow (uncatalyzed) rate of plasma HzCO 3 dehydration. This was con- firmed by the intra-arterial injection of bovine CA (22 mg kg -1) prior to the stopflow; the dis- equilibrium was abolished. When the CA inhibitor acetazolamide (30 mg kg -1) was injected, a negative pH disequilibrium of 0.04 units, accompanied by a rise in Paco 2 of 0.57 Torr, was observed during the stopflow. These results can be explained by the acetazolamide-induced in- hibition of rbc CA, which leads to continuing rbc CO 2 "excretion" in the post-branchial blood. Key words: Acid-base equilibrium, arterial blood; Carbon dioxide; excretion, acid-base disequi- librium; Carbonic anhydrase, fish blood; Fish; trout (Oncorhynchus mykiss) 1. Introduction The process of carbon dioxide excretion in fish has been described (Fig. 1) (see re- views by Perry, 1986; Perry and Wood, 1989; Perry and Laurent, 1990). Dehydratio n of H2CO3 in the red blood cell (rbc) is rapid, owing to the presence of the enzyme * Corresponding author. Tel.: (613) 564-6890; Fax: (613) 564-5014. Present address: Dept. of Zoology, University of British Columbia Vancouver, B.C. Canada. 0034-5687/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0034-5687(94)00006-L