Different red blood cell characteristics in a primitive agnathan (M. glutinosa) and a more recent teleost (O. mykiss) influence their strategies for blood CO2 transport

The physiological significance of plasma-accessible carbonic anhydrase in the respiratory systems of fishes

Carbonic anhydrase (CA) activity is ubiquitously found in all vertebrate species, tissues and cellular compartments. Most species have plasma-accessible CA (paCA) isoforms at the respiratory surfaces, where the enzyme catalyzes the conversion of plasma bicarbonate to carbon dioxide (CO2) that can be excreted by diffusion. A notable exception are the teleost fishes that appear to lack paCA at their gills. The present review: (i) recapitulates the significance of CA activity and distribution in vertebrates; (ii) summarizes the current evidence for the presence or absence of paCA at the gills of fishes, from the basal cyclostomes to the derived teleosts and extremophiles such as the Antarctic icefishes; (iii) explores the contribution of paCA to organismal CO2 excretion in fishes; and (iv) the functional significance of its absence at the gills, for the specialized system of O2 transport in most teleosts; (v) outlines the multiplicity and isoform distribution of membrane-associated CAs in fishes and methodologies to determine their plasma-accessible orientation; and (vi) sketches a tentative time line for the evolutionary dynamics of branchial paCA distribution in the major groups of fishes. Finally, this review highlights current gaps in the knowledge on branchial paCA function and provides recommendations for future work.

The roles of plasma accessible and cytosolic carbonic anhydrases in bicarbonate (HCO3-) excretion in Pacific hagfish (Eptatretus stoutii)

Pacific hagfish (Eptatretus stoutii) are marine scavengers and feed on decaying animal carrion by burrowing their bodies inside rotten carcasses where they are exposed to several threatening environmental stressors, including hypercapnia (high partial pressures of CO₂). Hagfish possess a remarkable capacity to tolerate hypercapnia, and their ability to recover from acid-base disturbances is well known. To deal with the metabolic acidosis resulting from exposure to high CO₂, hagfish can mount a rapid elevation of plasma HCO₃^- concentration (hypercarbia). Once PCO₂ is restored, hagfish quickly excrete their HCO₃^- load, a process that likely involves the enzyme carbonic anhydrase (CA), which catalyzes HCO₃^- dehydration into CO₂ at the hagfish gills. We aimed to characterize the role of branchial CA in CO₂/HCO₃^- clearance from the plasma at the gills of E. stoutii, under control and high PCO₂ (hypercapnic) exposure conditions. We assessed the relative contributions of plasma accessible versus intracellular (cytosolic) CA to gill HCO₃^- excretion by measuring in situ [¹⁴C]-HCO₃^- fluxes. To accomplish this, we employed a novel surgical technique of individual gill pouch arterial perfusion combined with perifusion of the gill afferent to efferent water ducts. [¹⁴C]-HCO₃^- efflux was measured at the gills of fish exposed to control, hypercapnic (48 h) and recovery from hypercapnia conditions (6 h), in the presence of two well-known pharmacological inhibitors of CA, the membrane impermeant C18 (targets membrane bound, plasma accessible CA) and membrane-permeant acetazolamide, which targets all forms of CA, including extracellular and intracellular cytosolic CAs. C18 did not affect HCO₃^- flux in control fish, whereas acetazolamide resulted in a significant reduction of 72%. In hypercapnic fish, HCO₃^- fluxes were much higher and perfusion with acetazolamide caused a reduction of HCO₃^- flux by 38%. The same pattern was observed for fish in recovery, where in all three experimental conditions, there was no significant inhibition of plasma-accessible CA. We also observed no change in CA enzyme activity (measured in vitro) in any of the experimental PCO₂ conditions. In summary, our data suggests that there are additional pathways for HCO₃^- excretion at the gills of hagfish that are independent of plasma-accessible CA.

Comparative analysis of nitrite uptake and hemoglobin-nitrite reactions in erythrocytes: sorting out uptake mechanisms and oxygenation dependencies

Nitrite uptake into red blood cells (RBCs) precedes its intracellular reactions with hemoglobin (Hb) that forms nitric oxide (NO) during hypoxia. We investigated the uptake of nitrite and its reactions with Hb at different oxygen saturations (So(2)), using RBCs with (carp and rabbit) and without (hagfish and lamprey) anion exchanger-1 (AE1) in the membrane, with the aim to unravel the mechanisms and oxygenation dependencies of nitrite transport. Added nitrite rapidly diffused into the RBCs until equilibrium. The distribution ratio of nitrite across the membrane agreed with that expected from HNO(2) diffusion and AE1-mediated facilitated NO(2)(-) diffusion. Participation of HNO(2) diffusion was emphasized by rapid transmembrane nitrite equilibration also in the natural AE1 knockouts. Following the equilibration, nitrite was consumed by reacting with Hb, which created a continued inward diffusion controlled by intracellular reaction rates. Changes in nitrite uptake with So(2), pH, or species were accordingly explained by corresponding changes in reaction rates. In carp, nitrite uptake rates increased linearly with decreasing So(2) over the entire So(2) range. In rabbit, nitrite uptake rates were highest at intermediate So(2), producing a bell-shaped relationship with So(2). Nitrite consumption increased approximately 10-fold with a 1 unit decrease in pH, as expected from the involvement of protons in the reactions with Hb. The reaction of nitrite with deoxyhemoglobin was favored over that with oxyhemoglobin at intermediate So(2). We propose a model for RBC nitrite uptake that involves both HNO(2) diffusion and AE1-mediated transport and that explains both the present and previous (sometimes puzzling) results.

Membrane-associated carbonic anhydrase in the respiratory system of the Pacific hagfish (Eptatretus stouti)

Like other agnathans, the Pacific hagfish (Eptatretus stouti) lacks red blood cell (RBC) Cl(-)/HCO(3)(-) exchange. Despite this absence of anion exchange, the majority (86.7+/-1.4%) of the total CO(2) carried in the blood is found within the plasma as HCO(3)(-), and thus presumably is inaccessible to RBC carbonic anhydrase (CA). As such, a branchial plasma-accessible CA isozyme in hagfish would be beneficial for mobilizing the considerable plasma HCO(3)(-) stores for CO(2) excretion and blood acid-base balance. The current study used a combination of molecular and biochemical methods to identify two membrane-associated CA isozymes in the respiratory system of E. stouti. Using homology cloning methods, CA IV and XV-like isozymes were identified in the gill and RBC, respectively. Real-time PCR analysis of relative mRNA expression revealed that CA IV was specific to the gill, while CA XV was found in several tissues including the RBC, gill, liver, heart and muscle. Isolation of subcellular fractions of gill and RBC verified the presence of membrane-associated CA activity in each tissue that persisted after standard washing protocols. Unlike CA activity associated with the cytoplasmic fractions, the activity in gill membranes was not inhibited by sodium dodecyl sulphate, while RBC membrane activity was inhibited to a lesser degree than the cytoplasmic fraction. Additionally, incubation of gill membrane fractions with phosphatidylinositol-specific phospholipase C released significant CA activity into the supernatant indicating the presence of a glycophosphatidyl inositol-linkage to the membrane, as found with other CA IV and XV isozymes. These results demonstrate that Pacific hagfish possess gill and RBC plasma-accessible membrane-associated CA that may play important roles in respiratory gas exchange and acid-base balance.

High plasma buffering and the absence of a red blood cell beta-NHE response in brown bullhead (Ameiurus nebulosus)

The high non-bicarbonate buffer capacity of brown bullhead (Ameiurus nebulosus) plasma was postulated to function as an alternative mechanism for the protection of red blood cell (RBC) intracellular pH (pHi) in the absence or attenuation of a RBC adrenergic response. The requirement for protecting RBC pHi arises from the presence of a Root effect haemoglobin in bullhead. In support of this hypothesis, bullhead RBCs incubated in vitro with isoproterenol (10(-8)-10(-5) mol l(-1)) or forskolin (10(-4) mol l(-1)) exhibited significant cyclic AMP accumulation, but failed to exhibit cell swelling or significant Na(+) or Cl(-) accumulation; plasma pH (pHe) was also unaffected. Similarly, no significant effect on RBC water content, Na(+) or Cl(-) concentration, or pHe was detected in bullhead blood incubated with 8-bromo cyclic AMP (10(-4)-10(-2) mol l(-1)) in vitro. These results suggest that while bullhead RBCs possess a beta-adrenoreceptor linked to cyclic AMP formation, stimulation of this adrenergic receptor does not result in measurable activation of a Na(+)/H(+) exchanger.

Hagfish (Myxine glutinosa) red cell membrane exhibits no bicarbonate permeability as detected by (18)O exchange

The bicarbonate permeability of the plasma membrane of intact hagfish (Myxine glutinosa) red blood cells and the intracellular carbonic anhydrase activity of these cells were determined by applying the (18)O exchange reaction using a special mass spectrometric technique. When the macromolecular carbonic anhydrase inhibitor Prontosil-Dextran was used to suppress any extracellular carbonic anhydrase activity, the mean intracellular acceleration of the CO(2) hydration/HCO(3)(−) dehydration reaction over the uncatalyzed reaction (referred to as intracellular carbonic anhydrase activity A(i)) was 21 320+/−3000 at 10 degrees C (mean +/− s.d., N=9). The mean bicarbonate permeability of the red blood cell membrane (P(HCO3)-) was indistinguishable from zero. It can be concluded that CO(2) transport within hagfish blood does not follow the classical scheme of CO(2) transport in vertebrate blood. It is suggested that the combination of considerable intraerythrocytic carbonic anhydrase activity and low P(HCO3)- may serve to enhance O(2) delivery to the tissue in the exceptionally hypoxia-tolerant hagfish.

Haemoglobin H+ equilibria in lamprey (Lampetra fluviatilis) and hagfish (Myxine glutinosa)

Agnathans, comprising lamprey and hagfish species, have been reported to be practically devoid of HCO3-/Cl- exchange across the red blood cell membrane. This suggests that the capacity of their haemoglobin (Hb) to remove H+ is essential for obtaining a high CO2-carrying capacity in the blood. Hydrogen ion titrations were performed on oxygenated and deoxygenated composite Hbs from river lamprey and from Atlantic hagfish at 15 degrees C and an ionic strength of 0.1 (0.1 mol l-1 KCl). Lamprey Hb was characterised by very low buffer values when the degree of oxygenation was constant, whereas the fixed-acid Haldane effect was large (uptake of approximately 0.9 H+ per monomer upon deoxygenation). Hagfish Hb, in contrast, had large buffer values and a moderate fixed-acid Haldane effect. In deoxygenated Hb, the low buffer values in lamprey correlated with the presence of only 1–1.5 titratable ‘neutral’ groups (normally histidines and α -amino groups) per monomer, whereas there were 4–5 titratable ‘neutral’ groups per monomer in hagfish. The large differences in Hb/H+ equilibria between the two species reflect the early evolutionary divergence between lampreys and hagfish. With respect to CO2 transport, the special Hb/H+ equilibria and the high red blood cell pH in lamprey ensure a high concentration of free HCO3- inside the red cells in venous blood, which compensates for the absence of a shift of HCO3- to the plasma. The Hb/H+ equilibria in hagfish are less effective in ensuring a high CO2-carrying capacity given the virtual absence of a red blood cell HCO3-/Cl- exchange, and other adaptations may be involved.