Extending The Dogfish Model Of CO2 Excretion Broadly Across Chondrichthyan Fishes

Abstract

The Pacific spiny dogfish (Squalus suckleyi) has been widely used as a representative species for chondrichthyan CO2 excretion. S. suckleyi has a slow red blood cell (RBC) carbonic anhydrase (CA) isoform, extracellular CA activity, no endogenous plasma CA inhibitor and plasma accessible CAIV at the gills. In dogfish, both the RBC and plasma compartments contribute to HCO3- dehydration at the gills for CO2 excretion, in contrast to teleost fishes where HCO33 dehydration is restricted to the RBC. This study determines CA activity levels, subcellular localization and presence of plasma CA inhibitors in the blood and gills of 13 chondrichthyan species to examine the hypothesis that this “dogfish model of CO2 excretion” applies broadly to chondrichthyan fishes. In general, blood samples from the 12 other chondrichthyan species examined had lower RBC CA activity than teleosts, some extracellular CA activity and no endogenous plasma CA inhibitor. While type IV-like membrane-associated CA was found in the gills in all four of the chondrichthyan species examined, S. suckleyi had 3x more CA activity (183 13. 2 µmol CO2 min-1 mg protein-1) in the microsomal (membrane) fraction of gills than the other three. In addition, unexpected variation in CA characteristics was observed between chondrichthyan species.

Thus, in general, it appears that CO2 excretion in fishes can be categorized as either chondrichthyan or teleost models, however, further studies should examine the functional significance of the within-group differences observed.

Introduction

Carbon dioxide (CO2) is a byproduct of aerobic metabolism that must be excreted. Since it is a weak acid, 90-95% of CO2 is transported in the blood as bicarbonate ions (HCO3-) at normal blood pH (Perry et al. , 1986) then dehydrated back to molecular CO2 at the gills of fish. Blood transit time through the gills is short (0. 5-6. 5 seconds; Edsall, 1969; Cameron and Polhemus, 1974; Randall and Daxboeck, 1984; Bhargava et al. , 1992) yet the uncatalyzed dehydration rate constant of HCO3- is 25-90 seconds at physiological pH and temperature (Edsall, 1969; Heming, 1984). CO2 excretion is thus dependent on the catalyzed conversion of HCO3- to CO2 by the enzyme carbonic anhydrase (CA). CA is typically at high concentrations in the red blood cell (RBC) where there is an interaction between O2 and CO2, but some groups have CA at other locations in the gills and blood that have the potential to contribute to CO2 excretion (Brauner and Randall, 1998).

Carbonic anhydrase is present in the pulmonary capillaries of mammals (Whitney and Briggle, 1982; Waheed et al. , 1992), birds, reptiles and amphibians (Stabenau et al. , 1996; Stabenau and Heming, 1999; Stabenau and Vietti, 2002; Stabenau and Heming, 2003) and is available to catalyze plasma CO2 reactions (Zhu and Sly, 1990). This high activity isoform, found attached to membranes by a glycosylphosphatidylinositol (GPI) anchor, has been identified as CA IV.