Apparent diffusion limitations for CO2 excretion in rainbow trout are relieved by injections of carbonic anhydrase

Experiments were performed in vivo to elucidate the underlying mechanism(s) of apparent diffusion limitations for CO2 excretion in rainbow trout (Onco rhynchus mykiss). Ligation of two gill arches and the associated expected r eduction in gill surface area of 30% caused pronounced respiratory acidosis as indicated by elevated arterial blood P-CO2 (Pa-CO2) and reduced arteria l blood pH. Under conditions of normoxia, arterial blood P-O2 (Pa-O2) was n ot significantly (statistically) reduced. However, during hypoxia (water P- O2 = 70-80 mmHg), the apparent trend for reduced Pa-O2 values became statis tically significant in fish with 15% surface area reduction. To determine w hether the elevated Pa-CO2 in fish with reduced surface area (30%) reflecte d true diffusion limitations or chemical equilibrium limitations imposed by the relatively slow rate of red blood cell Cl-/HCO3- exchange, fish were i njected with carbonic anhydrase (CA) to permit catalysis of HCO3- dehydrati on within the plasma. Injection of CA caused a lowering of Pa-CO2 by 0.87 /- 0.32 mmHg within 120 min and thus essentially eliminated the increase in Pa-CO2 (1.04 +/- 0.33 mmHg) that was caused by the reduction in surface ar ea. These results clearly demonstrate that the elevation in Pa-CO2 evoked b y gill surface area reduction is a consequence of chemical equilibrium limi tations rather than true diffusion limitations, per se. (C) 2000 Elsevier S cience B.V. All rights reserved.