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Carbon dioxide excretion and acid-base regulation in the freshwater rainbow trout (Salmo gairdneri) : involvement of the branchial epithelium and red blood cell Perry, Steve F.


Carbon dioxide excretion and acid-base regulation were studied in the freshwater rainbow trout, Salmo gairdneri. Two different models for CO₂ excretion (Ṁ[sub CO₂]) were investigated. One model proposes that erythrocytes are not involved, and that the rate-limiting step in CO₂ excretion is the entry of plasma bicarbonate (HCO₃⁻) into the gill epithelium where it is dehydrated to molecular CO₂ by branchial carbonic anhydrase. According to this model, the counter ions, HCO₃⁻ and H⁺, for branchial apical CI⁻/HCO₃⁻ and Na⁺/H⁺(NH₄⁺) exchanges, arise from plasma. The other model adopts a typical mammalian role for fish erythrocytes, whereby HCO₃⁻ is converted to CO₂ by erythrocytic carbonic anhydrase. In this scheme carbon dioxide is thought to enter the gill epithelium as molecular CO₂ where it is catalytically hydrated to HCO₃⁻ and H⁺ thereby supplying the counter ions for CI⁻/HCO₃⁻ and Na⁺/H⁺(NH₄⁺) exchanges. Experiments using an isolated, saline-perfused head preparation demonstrated that branchial CI⁻/HCO₃⁻ exchange is related only to the partial pressure of CO₂ (P[sub CO₂]) in the perfusate and not to HCO₃⁻ concentration. These results indicate that CO₂ entry and not HCO₃⁻ entry into the gill epithelium is the dominant pathway. Furthermore, investigations using various other saline-perfused gill preparations have shown that Ṁ[sub CO₂] is not related to perfusate HCO₃⁻ concentration, but only to P[sub CO₂]. These results show that entry of plasma bicarbonate into the gill epithelium is not a major pathway for CO₂ excretion. Measurements of a constant trans-membrane potential (basal membrane of gill epithelial cells) at all concentrations of perfusate HCO₃⁻ support the conclusion that the basal membrane is impermeable to this ion. A spontaneously ventilating, blood-perfused trout preparation was developed to study the role of the red blood cell in CO₂ excretion. Unlike saline-perfused preparations, blood-perfused fish excreted CO₂ at rates comparable to in vivo values. The importance of the red blood cell for CO₂ excretion was demonstrated by perfusing with blood of various haematocrits (Hcts) as well as plasma. A linear relationship was observed between Hct and Ṁ[sub CO₂] while plasma perfusion completely abolished Ṁ[sub CO₂]. CO₂ excretion in blood-perfused fish was stimulated by increased blood HCO₃⁻ concentration. This was due to increased entry of HCO₃⁻ into the erythrocyte and not into the gill epithelium. Similarly, SITS, an anion transport inhibitor, reduced Ṁ[sub CO₂] as a result of reduced entry of HCO₃⁻ into erythrocytes. Clearly, HCO₃⁻ entry into the red blood cell is the rate-limiting step in CO₂ excretion. The branchial ion exchange processes (CI⁻/HCO₃⁻ and Na⁺/H⁺(NH₄⁺)) have been implicated in both maintainance of internal pH and regulation of acid-base disturbances. Studies using pharmacological inhibitors of these exchange processes indeed have demonstrated that branchial ion exchange is extremely important in maintaining steady-state internal acid-base status. I have postulated that proton movement from plasma into the gill epithelium is controlled by intracellular pH, which in turn is governed by the rates of apical ion exchange. Thus, perturbations of these 'pumps' affect blood acid-base status by altering proton movement into the gill epithelium. Branchial CI⁻/HCO₃⁻ exchange in saline-perfused heads is controlled by levels of circulating catecholamines. Stimulation of adrenergic β receptors inhibit CI⁻ uptake while stimulation of ck receptors stimulate CI⁻ uptake. These are direct effects and are not due to accompanying haemodynamic changes. Despite the likelihood that catecholamine levels increase during hypercapnia and the relationship that exists between ionic exchange and acid-base balance, I was unable to demonstrate that modulations of these exchanges are involved in the regulation of hypercapnic acidosis. Another possibility for acid-base regulation during hypercapnic acidosis is a reduction of CO₂ excretion. Because the gill epithelium is permeable to protons, a reduction of CO₂ excretion will not affect H⁺ ion excretion and will result in accumulation of plasma HCO₃⁻. Inhibition of HCO₃⁻ entry into erythrocytes was observed using levels of adrenaline associated with stress in fish. Control of this pathway by catecholamines, leading to a reduction of CO₂ excretion, may be an important process in the regulation of hypercapnic acidosis.

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