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First published online August 23, 2004
Journal of Experimental Biology 207, 3381-3390 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01144

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Limited extracellular but complete intracellular acid-base regulation during short-term environmental hypercapnia in the armoured catfish, Liposarcus pardalis

C. J. Brauner^1,*, T. Wang², Y. Wang³, J. G. Richards¹, R. J. Gonzalez⁴, N. J. Bernier⁵, W. Xi⁴, M. Patrick³ and A. L. Val⁶

¹ Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, B.C., Canada V6T 1Z4
² Department of Zoophysiology, University of Aarhus, 8000 Aarhus C, Denmark
³ Department of Biology, Queens University, Kingston, Ontario, Canada K7L 3N6
⁴ Department of Biology, University of San Diego, San Diego, CA 92110, USA
⁵ Department of Zoology, University of Guelph, Guelph, ON, Canada N1G 2W1
⁶ National Institute for Research in the Amazon (INPA), Laboratory of Ecophysiology and Molecular Evolution, Manaus, AM, Brazil, CEP 69083-000

^* Author for correspondence (e-mail: brauner{at}zoology.ubc.ca)

Accepted 17 June 2004

Environmental hypercapnia induces a respiratory acidosis that is usually compensated within 24-96 h in freshwater fish. Water ionic composition has a large influence on both the rate and degree of pH recovery during hypercapnia. Waters of the Amazon are characteristically dilute in ions, which may have consequences for acid-base regulation during environmental hypercapnia in endemic fishes. The armoured catfish Liposarcus pardalis, from the Amazon, was exposed to a water P_CO2 of 7, 14 or 42 mmHg in soft water (in µmol l^-1: Na⁺, 15, Cl^-, 16, K⁺, 9, Ca²⁺, 9, Mg²⁺, 2). Blood pH fell within 2 h from a normocapnic value of 7.90±0.03 to 7.56±0.04, 7.34±0.05 and 6.99±0.02, respectively. Only minor extracellular pH (pH_e) recovery was observed in the subsequent 24-96 h. Despite the pronounced extracellular acidosis, intracellular pH (pH_i) of the heart, liver and white muscle was tightly regulated within 6 h (the earliest time at which these parameters were measured) via a rapid accumulation of intracellular HCO₃^-. While most fish regulate pH_i during exposure to environmental hypercapnia, the time course for this is usually similar to that for pH_e regulation. The degree of extracellular acidosis tolerated by L. pardalis, and the ability to regulate pH_i in the face of an extracellular acidosis, are the greatest reported to date in a teleost fish. The preferential regulation of pH_i in the face of a largely uncompensated extracellular acidosis in L. pardalis is rare among vertebrates, and it is not known whether this is associated with the ability to air-breathe and tolerate aerial exposure, or living in water dilute in counter ions, or with other environmental or evolutionary selective pressures. The ubiquity of this strategy among Amazonian fishes and the mechanisms employed by L. pardalis are clearly worthy of further study.

Key words: acid-base regulation, hypercapnia, pH_e, pH_i, intracellular pH regulation, catfish, Liposarcus pardalis, Amazon, water hardness, air-breathing