Acid-base disequilibrium in the arterial blood of rainbow trout

Abstract

An extracorporeal blood circulation and a stopflow technique were used to examine the acid-base status of arterial blood in the rainbow trout, Oncorhynchus mykiss. Arterial blood was routed from the coeliac artery through an external circuit in which pH (pHa), partial pressure of oxygen (Pa O)in2) and partial pressure of carbond dioxide (Pa CO 2 ) were monitored continously. The stopflow condition was imposed by turning off the pump which drove the external loop. A radioisotopic CO 2 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 Pa CO 2 by 0.03–0.10. Torr, during the stopflow period. The disequilibrium appeared to arise primarly from the slow (uncatalyzed) rate of plasma H 2CO 3 dehydration. This was confirmed by the intra-arterial injection of bovine CA (22 mg kg −1) prior to the stopflow; the disequilibrium 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 Pa CO 2 of 0.57 Torr, was oberserved during the stopflow. These results can be explained by the acetazolamide-induced inhibition of rbc CA, which leads to continuing rbc CO 2 “excretion” in the post-branchial blood.