Fish red blood cell carbon dioxide transport in vitro: A comparative study

Blood and Gill Carbonic Anhydrase in the Context of a Chondrichthyan Model of CO2 Excretion

Pacific spiny dogfish (Squalus suckleyi) have been widely used as a representative species for chondrichthyan CO₂ excretion. Pacific spiny dogfish have a slower red blood cell (RBC) carbonic anhydrase (CA) isoform than teleost fishes, extracellular CA activity, no endogenous plasma CA inhibitor, and plasma-accessible CA IV at the gills. Thus, both the RBC and plasma compartments contribute to bicarbonate ion (HCO3-) dehydration at the gills for CO₂ excretion in contrast to teleost fishes, in which HCO3- dehydration is restricted to RBCs. We compared CA activity levels, subcellular localization, and presence of plasma CA inhibitors in the blood and gills of 13 chondrichthyans to examine the hypothesis that the dogfish model of CO₂ excretion applies broadly to chondrichthyans. In general, blood samples from the 12 other chondrichthyans 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 chondrichthyans examined, S. suckleyi had three times 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 the pattern of CA distribution in fishes can be generally categorized as either chondrichthyan or teleost models. However, further studies should examine the functional significance of the within-chondrichthyan differences we observed and investigate whether CO₂ excretion patterns exist along a continuum or in discrete groups.

New insights into the many functions of carbonic anhydrase in fish gills

Carbonic anhydrase (CA) is a zinc metalloenzyme that catalyzes the reversible reactions of carbon dioxide and water: CO(2) + H(2)O ↔ H(+) + HCO(3)(-). It has long been recognized that CA is abundant in the fish gill, with attention focused on the role of CA in catalyzing the hydration of CO(2) to provide H(+) and HCO(3)(-) for the branchial ion transport processes that underlie systemic ionic and acid-base regulation. Recent work has explored the diversity of CA isoforms in the fish gill. By linking these isoforms to different cell types in the gill, and by exploiting the diversity of fish species available for study, this work is increasing our understanding of the many roles that CA plays in the fish gill. In particular, recent work has revealed that fish utilize more than one model of CO(2) excretion, that to understand the role of CA and the gill in ionic regulation and acid-base balance means characterizing the transporter and CA complement of individual cell types, and that CA plays roles in branchial sensory mechanisms. The goal of this brief review is to summarize these new developments, while at the same time highlighting key areas in which further research is needed.

Extracellular carbonic anhydrase in the dogfish, Squalus acanthias: a role in CO2 excretion

In Pacific spiny dogfish (Squalus acanthias), plasma CO(2) reactions have access to plasma carbonic anhydrase (CA) and gill membrane-associated CA. The objectives of this study were to characterise the gill membrane-bound CA and investigate whether extracellular CA contributes significantly to CO(2) excretion in dogfish. A subcellular fraction containing membrane-associated CA activity was isolated from dogfish gills and incubated with phosphatidylinositol-specific phospholipase C. This treatment caused significant release of CA activity from its membrane association, a result consistent with identification of the dogfish gill membrane-bound CA as a type IV isozyme. Inhibition 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 micromol L(-1)) of benzolamide to selectively inhibit extracellular CA in vivo caused a significant 30%-60% reduction in the arterial-venous total CO(2) concentration difference, a significant increase in Pco(2) and an acidosis, without affecting blood flow or ventilation. No effect of benzolamide on any measure of CO(2) excretion was detected in rainbow trout (Oncorhynchus mykiss). These results indicate that extracellular CA contributes substantially to CO(2) excretion in the dogfish, an elasmobranch, and confirm that CA is not available to plasma CO(2) reactions in rainbow trout, a teleost.

The interaction between O2 and CO2 exchange in rainbow trout during graded sustained exercise

A quantitative analysis of O2 and CO2 transport was conducted in resting and exercising rainbow trout, and these data were used to quantify the magnitude of coupling between O2 and CO2 exchange, in vivo. The release of Bohr protons during haemoglobin-oxygenation was non-linear over the Hb-O2 equilibrium curve used in trout subjected to different levels of sustained exercise. At low swimming speeds, when venous blood O2 content (CvO2) was high, there was a small acidosis as blood passed through the gills, indicating more protons were released during oxygenation of Hb than were consumed during HCO3- dehydration. At higher swimming speeds, when CvO2 was low, there was a significant alkalosis in arterial relative to venous blood, indicating that fewer protons were released upon oxygenation than HCO3- ions were dehydrated to CO2. Haldane coefficients (moles of protons released per mole of O2 which binds to Hb), calculated from steady state arterial and mixed-venous parameters, revealed that under resting conditions all blood CO2 removed from the blood during gill transit was stoichiometrically related to O2 uptake through the release of Bohr protons during Hb oxygenation. The magnitude of coupling between CO2 excretion and O2 uptake decreased from 100% to less than 40% at the maximal swimming velocity when the largest region of the Hb-O2 equilibrium curve was used for gas exchange. The non-linear release of Bohr protons over the range of Hb-O2 saturation in the blood reduces HCO3- dehydration at the gills during greater work loads elevating arterial P(CO2) levels, leading to an increase in HCO3- buffer capacity of the blood and tissues.

Effect of haemoglobin oxygenation on Bohr proton release and CO2 excretion in the rainbow trout

CO2 excretion in trout is dependent upon the Haldane effect and the release of protons (Bohr protons) from haemoglobin during oxygenation of the blood. Oxygenation of whole blood from trout in vitro, resulted in a non-linear release of Bohr protons over the haemoglobin-oxygen (Hb-O2) equilibrium curve, where the majority of Bohr protons were released between 60 and 100% of Hb oxygen saturation (SO2). Oxygenation of the blood over this region of the Hb-O2 equilibrium curve elevated the HCO3- flux rate across the HCO3-/CI- exchanger on the red cell membrane by about 30% during CO2 excretion in vitro. Oxygenation of the Hb between 0 and 60% SO2 did not elevate CO2 excretion rate in vitro. These data indicate that utilization of different regions of the Hb-O2 dissociation curve in trout in vivo will influence CO2 excretion by altering the rate of HCO3- entry into the red cell during gill blood transit.