Does gill boundary layer carbonic anhydrase contribute to carbon dioxide excretion: A comparison between dogfish (Squalus acanthias) and rainbow trout (Oncorhynchus mykiss)

In vivo experiments were conducted on spiny dogfish (Squalus acanthias) and rainbow trout (Oncorhynchus mykiss) in sea water to determine the potentia l role of externally oriented or gill boundary layer carbonic anhydrase in carbon dioxide excretion. This was accomplished by assessing pH changes in expired water using a stopped-flow apparatus. In dogfish, expired water was in acid-base disequilibrium as indicated by a pronounced acidification (De lta pH=-0.11+/-0.01; N=22; mean +/- S.E.M.) during the period of stopped fl ow; inspired water, however, was in acid-base equilibrium (Delta pH=-0.002/-0.01; N=22), The acid-base disequilibrium in expired water was abolished (Delta pH=-0.005+/-0.01; N=6) by the addition of bovine carbonic anhydrase (5 mgl(-1)) to the external medium. Addition of the carbonic anhydrase inhi bitor acetazolamide (1 mmol l(-1)) to the water significantly reduced the m agnitude of the pH disequilibrium (from -0.133+/-0.03 to -0.063+/-0.02; N=4 ), However, after correcting for the increased buffering capacity of the wa ter caused by acetazolamide, the acid-base disequilibrium during stopped fl ow was unaffected by this treatment (control Delta[H+]=99.8+/-22.8 mu mol l (-1); acetazolamide Delta[H+]=81.3+/-21.5 mu mol l(-1)). In rainbow trout, expired water displayed an acid-base disequilibrium (Delta pH=0.09+/-0.01; N=6) that also was abolished by the application of external carbonic anhydr ase (Delta pH=0.02+/-0.01). The origin of the expired water disequilibrium was investigated further in acid-base disequilibrium was investigated further in dogfish, Intravascular injection of acetazolamide (40 mg kg(-1)) to inhibit internal carbonic anh ydrase activity nonspecifically and thus CO2 excretion significantly dimini shed the extent of the expired water disequilibrium pH after 30 min (from - 0.123+/-0.01 to -0.065+/-0.01; N=6). Selective inhibition of extracellular carbonic anhydrase activity using a low intravascular dose (1.3 mg kg(-1)) of the inhibitor benzolamide caused a significant reduction in the acid-bas e disequilibrium after 5 min (from -0.11+/-0.01 to -0.07+/-0.01; N=14). The se results demonstrate that the expired water acid-base disequilibrium orig inates, at least in part, from excretory CO2 and that extracellular carboni c anhydrase in dogfish may have a significant role in carbon dioxide excret ion. However, externally oriented carbonic anhydrase (if present in dogfish ) plays no role in catalysing the hydration of the excretory CO2 in water f lowing over the gills and thus is unlikely to facilitate CO2 excretion.