Extracellular carbonic anhydrase in the dogfish, Squalus acanthias:: A role in CO2 excretion

In Pacific spiny dogfish (Squalus acanthias), plasma CO2 reactions have access to plasma carbonic anhydrase (CA) and gill membrane-associated CA. The objectives of this study were to characterise the gill membrane-bound CA and investigate whether extracellular CA contributes significantly to CO2 excretion in dogfish. A subcellular fraction containing membrane-associated CA activity was isolated from dogfish gills and incubated with phosphatidylinositol-specific phospholipase C. This treatment caused significant release of CA activity from its membrane association, a result consistent with identification of the dogfish gill membrane-bound CA as a type IV isozyme. Inhibition constants (K-i) against acetazolamide and benzolamide were 4.2 and 3.5 nmol L-1, respectively. Use of a low dose (1.3 mg kg(-1) or 13 mu mol L-1) of benzolamide to selectively inhibit extracellular CA in vivo caused a significant 30%-60% reduction in the arterial-venous total CO2 concentration difference, a significant increase in Pco(2) and an acidosis, without affecting blood flow or ventilation. No effect of benzolamide on any measure of CO2 excretion was detected in rainbow trout (Oncorhynchus mykiss). These results indicate that extracellular CA contributes substantially to CO2 excretion in the dogfish, an elasmobranch, and confirm that CA is not available to plasma CO2 reactions in rainbow trout, a teleost.