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

In Pacific spiny dogfish (Squalus acanthias), plasma CO2 reactions have acc ess to plasma carbonic anhydrase (CA) and gill membrane-associated CA. The objectives of this study were to characterise the gill membrane-bound CA an d investigate whether extracellular CA contributes significantly to CO2 exc retion in dogfish. A subcellular fraction containing membrane-associated CA activity was isolated from dogfish gills and incubated with phosphatidylin ositol-specific phospholipase C. This treatment caused significant release of CA activity from its membrane association, a result consistent with iden tification of the dogfish gill membrane-bound CA as a type IV isozyme. Inhi bition 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 concentratio n difference, a significant increase in Pco(2) and an acidosis, without aff ecting blood flow or ventilation. No effect of benzolamide on any measure o f CO2 excretion was detected in rainbow trout (Oncorhynchus mykiss). These results indicate that extracellular CA contributes substantially to CO2 exc retion in the dogfish, an elasmobranch, and confirm that CA is not availabl e to plasma CO2 reactions in rainbow trout, a teleost.