Type IV carbonic anhydrase is present in the gills of spiny dogfish (Squalus acanthias)

Physiological and biochemical studies have provided indirect evidence for a membrane-associated carbonic anhydrase (CA) isoform, similar to mammalian type IV CA, in the gills of dogfish ( Squalus acanthias). This CA isoform is linked to the plasma membrane of gill epithelial cells by a glycosylphosphatidylinositol anchor and oriented toward the plasma, such that it can catalyze the dehydration of plasma HCO[Formula: see text] ions. The present study directly tested the hypothesis that CA IV is present in dogfish gills in a location amenable to catalyzing plasma HCO[Formula: see text] dehydration. Homology cloning techniques were used to assemble a 1,127 base pair cDNA that coded for a deduced protein of 306 amino acids. Phylogenetic analysis suggested that this protein was a type IV CA. For purposes of comparison, a second cDNA (1,107 base pairs) was cloned from dogfish blood; it encoded a deduced protein of 260 amino acids that was identified as a cytosolic CA through phylogenetic analysis. Using real-time PCR and in situ hybridization, mRNA expression for the dogfish type IV CA was detected in gill tissue and specifically localized to pillar cells and branchial epithelial cells that flanked the pillar cells. Immunohistochemistry using a polyclonal antibody raised against rainbow trout type IV CA revealed a similar pattern of CA IV immunoreactivity and demonstrated a limited degree of colocalization with Na+-K+-ATPase immunoreactivity. The presence and localization of a type IV CA isoform in the gills of dogfish is consistent with the hypothesis that branchial membrane-bound CA with an extracellular orientation contributes to CO2 excretion in dogfish by catalyzing the dehydration of plasma HCO[Formula: see text] ions.

Branchial membrane-associated carbonic anhydrase activity maintains CO2 excretion in severely anemic dogfish

Plasma CO2 reactions in Pacific spiny dogfish ( Squalus acanthias) have access to plasma and gill membrane-associated carbonic anhydrase (CA). Acute severe experimental anemia and selective CA inhibitors were used to investigate the role of extracellular CA in CO2 excretion. Anemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering hematocrit from 14.2 ± 0.4% (mean ± SE; N = 31) to 5.2 ± 0.1% ( N = 31) had no significant impact on arterial or venous CO2 tensions (PaCO2 and PvCO2, respectively) over the subsequent 2 h. Pco2 was maintained despite the reduction in red cell number and a significant 32% increase in cardiac output (V̇b), both of which have been found to cause PaCO2 increases in teleost fish. By contrast, treatment of anemic dogfish with the CA inhibitors benzolamide (1.3 mg/kg) or F3500 (50 mg/kg), to selectively inhibit extracellular CA, elicited rapid and significant increases in PaCO2 of 0.68 ± 0.17 Torr ( N = 6) and 0.53 ± 0.11 Torr ( N = 7), respectively, by 30 min after treatment. These findings provide a functional context in which extracellular CA in dogfish contributes substantially to CO2 excretion. Additionally, the apparent lack of effect of V̇b changes on Pco2 suggests that, in contrast to teleost fish, CO2 excretion in dogfish does not behave as a diffusion-limited system.

Water Exchange Across Fish Gills: The Significance of Tritiated-Water Flux Measurements

1. Branchial turnover rates of tritiated water in the cyprinid teleost Carassius auratus are temperature dependent and show temperature acclimation with time. 2. Hypoxia produced abrupt increases in the rates of branchial water turnover. 3. The evidence suggests the importance of branchial blood flow parameters and ventilation on turnover rate. Membrane resistance is not a major barrier to water diffusion in these small fish. 4. Calculated apparent diffusional water permeabilities do not represent true branchial membrane permeabilities. Note: Address for correspondence after 1 September 1978: Department of Zoology, University of California, Berkeley, California 94720, U.S.A.

Comparative Effects of Temperature on the Sodium and Water Permeabilities of the Gills of a Stenohaline Freshwater Fish (Carassius Auratus) and a Stenohaline Marine Fish (Serranus Scriba, Serranus Cabrilla)

1. Comparative effects of temperature on the permeability of the gill to water and to sodium were studied in the marine sea perch Serranus and the freshwater goldfish Carassius. 2. The acclimation Q10 for the water fluxes is higher in the freshwater fish than in the marine fish. 3. In the goldfish the osmotic permeability (Pos) is greater than the diffusional permeability (Pdlf) at all acclimation temperatures, suggesting the presence of ‘waterfilled channels’ in the branchial membrane. In the sea perch, on the other hand, Pos/Pdlf is approximately 1, indicating that water movements probably occur by simple diffusion. 4. The permeabilities to water and to sodium are similar in the sea perch but very different in the goldfish. Considering these results together with those from a similar study on an elasmobranch, it would seem that the more perfect the semipermeability of the gill membrane and the weaker the transepithelial osmotic gradient, the greater is the branchial porosity. 5. Assuming that temperature changes do not cause modification of the branchial surface or relative permeabilities to water and to sodium the independence of the temperature-coefficient variations for water and for sodium indicates a certain dissociation between the movements of salt and of water, in the sea perch. In the goldfish, assuming a constant branchial surface and in view of the fact that Pos > Pdif the high temperature coefficients for the water fluxes suggest that the water in the ‘water-filled channels’ is in a highly organized state.