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Strategies for acid-base regulation in fishes Iwama, George Katsushi


Three sets of in vivo experiments were conducted to investigate several aspects of acid-base regulation in fishes. There are two possible ways that involve the gills of fishes in which the acid-base regulation of the extracellular fluid can be adjusted. First, CO₂ excretion can be adjusted by altering gill water flow to increase or decrease the PCO₂ tensions in the blood. The second mechanism would involve the exchange of ions across the gill epithelium to change the concentrations of H⁺, HCO₃⁻ or NH₄⁺ in the blood. The first two sets of experiments were, respectively, designed to investigate these two possibilites. The third set of experiments investigated the role that plasma catecholamines might play in regulating the pH of the extracellular fluid as well as the intracellular compartment of the red blood cell. Experimental manipulation of ventilation in rainbow trout in steady state showed that gill water flow affected CO₂ excretion only at levels lower than about 100ml/min. Carbon dioxide excretion was retarded and blood PCO₂ pressures increased at these levels of gill ventilation. Increasing gill water flow above control levels effected neither O₂ or CO₂ exchange across the gill. Dogfish, subjected to environmental hyperoxia and various levels of hypercapnia, showed the best correlation between gill ventilation and plasma pH. There was a very weak correlation with plasma PCO₂ tension and plasma HCO₃⁻ concentrations did not affect ventilation at all. Gill ventilation increased exponentially as plasma pH declined. Experiments that involved the fresh water trout and the sea water conger eel showed that water salinity had a direct effect on the acid-base regulation of the plasma. Recovery of plasma pH in both species, after an initial decline in response to exposure to environmental hypercapnia, was dependent on water salinity. The recovery was effected by an increase in plasma HCO₃⁻ concentration. There was also an associated decrease in plasma Cl⁻ concentration in both species, indicating the possible involvement of a Cl⁻/ HCO₃⁻ exchange process. When carp were exposed to environmental hypercapnia, a reduction in the active uptake of water Cl⁻, while maintaining normal efflux rates, caused the reduction of the plasma concentration of this ion. Therefore, it seems that the modulation of this active Cl⁻/ HCO₃⁻ exchange process effected the HCO₃⁻ accumulation in the carp, and probably also in the trout and conger in fresh water. Consistent with the data from the above carp experiment, further analyses of the electrochemical gradients for Cl⁻ in trout exposed to environmental hypercapnia at the three salinities showed that active exchange processes must have accumulated the plasma HCO₃⁻ by the proposed Cl⁻/ HCO₃⁻ mechanism. These analyses also showed that the trout gill was about 2.5 times more permeable to Na⁺ than to Cl⁻ in steady state control conditions. Furthermore, Na⁺ is maintained out of electrochemical equilibrium more than Cl⁻ by a factor of about 1.5 - 2.0. This latter calculation was based on the comparison between the measured plasma concentrations of these ions and the expected concentrations based on a distribution according to the existing electrochemical gradents Catecholamines are released in trout immediately after acid infusion. This release is proportional to the change in plasma pH relative to control values and functions to maintain the oxygen carrying capacity of the blood which would otherwise be compromised due to the Root shift. This data supports existing data showing that some of the effects which catecholamines have on the physiology of fishes include those which enhance the regulation of the acid-base status of the extracellular and red cell compartments. This data also suggests that the release of catecholamines during burst exercise is due, at least partially, to the excess proton load from the lactacidosis.

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