UBC Theses and Dissertations
Carbonic anhydrase in the gills and blood of chondrichthyan fishes McMillan, Olivia Jenna Lindsay
Carbon dioxide (CO₂) is continuously produced as a result of aerobic respiration and must be excreted to maintain internal acid-base balance. Most CO₂ is carried in the blood as HCO₃- and must be converted back to molecular CO₂ at the respiratory surface to diffuse out into the environment. The uncatalyzed rate of HCO₃- dehydration is too slow to excrete CO₂ at a physiologically relevant rate and therefore it must be catalyzed by the enzyme carbonic anhydrase. The distribution of carbonic anhydrase in the blood and gills of fish therefore provides important information about general patterns of gas exchange and acid-base balance. Teleost fishes have a fast CA in the red blood cell (RBC), no extracellular CA activity, an endogenous plasma CA inhibitor and a relatively low plasma buffer value so HCO₃- dehydration is largely restricted to the RBC. Pacific spiny dogfish (Squalus suckleyi), however, have a slow RBC CA, extracellular CA activity, no endogenous plasma CA inhibitor, plasma accessible CAIV at the gills and a relatively high plasma buffer value, implying that both the RBC and plasma compartments may contribute to HCO₃- dehydration. This thesis uses biochemical assays, subcellular localization and immunohistochemistry on blood and gill samples from 13 chondrichthyan species to examine whether the characteristics of the dogfish model of CO₂ excretion apply to chondrichthyan fishes in general. Overall, the results of this study were consistent with the proposed chondrichthyan model of CO₂ excretion because most chondrichthyans had lower RBC CA activity than teleosts, some extracellular CA activity, no endogenous plasma CA inhibitor, higher plasma buffer values and type IV-like CA at the gills. Pacific spiny dogfish had 3x more microsomal CA activity (183 ± 13.2 µmol CO₂ min-1 mg protein-1) in the gills than the other three species examined for this trait, indicating that dogfish may not be a representative species to compare with other vertebrate groups. Overall, the results of this thesis suggest that all chondrichthyans have the capacity to use both the plasma and RBC compartments for CO₂ excretion and these data provide important information about general patterns of gas exchange and acid-base balance in fishes.
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