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Journal of Experimental Biology 158,1-18 (1991)
Published by Company of Biologists 1991

Effect Of Environmental Water Salinity on Acid-Base Regulation During Environmental Hypercapnia in the Rainbow Trout (Oncorhynchus Mykiss)

GEORGE K. IWAMA ¹ and NORBERT HEISLER ²

¹ Abteilung Physiologie, Max-Planck-Institut für experimentelle Medizin D-3400 Göttingen, FRG; Department of Animal Science, University of British Columbia, 248-2357 Main Mall, Vancouver, BC, Canada V6T 2A2
² Abteilung Physiologie, Max-Planck-Institut für experimentelle Medizin D-3400 Göttingen, FRG

Acid-base regulation in rainbow trout acclimated to about 3, 100 and 500 mmol l^-1 Na⁺ and Cl^-, at constant water [HCO₃^-], was assessed during 24h of exposure to 1% CO₂ and during recovery. The respiratory acidosis induced by a rise in plasma P_COCO2 to about 1.15kPa (8.5mmHg, 3mmol l^-1), 1.33kPa (10mmHg, 100 mmol l^-1) or 1.5 kPa (11.2 mmHg, 500 mmol l^-1) was partially compensated for by accumulation of plasma HCO₃^-. The degree of pH compensation depended on the salinity of the environmental water, being about 61, 82 and 88% at 3, 100 and 300 mmol l^-1 Na⁺ and Cl^-, respectively. [HCO₃^-] in animals acclimated to 100 and 500 mmol l^-1 rose to higher values than that in fish at 3 mmol l^-1.

Plasma [Cl^-] decreased during hypercapnia as compared to control concentrations in all groups of fish. Plasma [Na⁺] rose during the first 8 h of hypercapnia in fish acclimated to all three salinities, but recovered towards control values during the remainder of hypercapnia. The rise in plasma [HCO₃^-] was significantly related to the fall in plasma [Cl^-], whereas the changes in plasma [Na⁺] were unaffected by simultaneous changes in plasma [HCO₃^-]. Time courses of changes in plasma [Na⁺] and total ammonia concentration, [T_amm], were similar but in opposite directions.

The transepithelial potential (TEP) of blood relative to water was negative, close to zero and positive, averaging -21, -5.8 and +6.2 mV for fish acclimated to 3, 100 and 300 mmol l^-1 Na⁺, respectively. After initiation of hypercapnia, which caused a quite heterogeneous response among groups, a clear trend towards depolarization was observed during the remainder of hypercapnia.

These results confirm the role of active HCO₃^-/Cl^- exchange processes for the compensation of extracellular pH during respiratory acidoses in fish.

Key words: acid-base regulation, water salinity, hypercapnia, rainbow trout, Oncorhynchus mykiss, transepithelial potential

Accepted on March 27, 1991

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