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Journal of Experimental Biology 92,243-254 (1981)
Published by Company of Biologists 1981

Crab Gill Intra-Epithelial Carbonic Anhydrase Plays a Major Role in Haemolymph CO₂ and Chloride Ion Regulation

LOUIS E. BURNETT ¹, PAUL B. J. WOODSON ¹, MARTHA G. RIETOW ², and VICTORIA C. VILICICH ¹

¹ Department of Biology, University of San Diego, Alcala Park, San Diego, California 92110, U.S.A.
² Department of Biology, University of San Diego, Alcala Park, San Diego, California 92110, U.S.A.; Department of Physiology and Pharmacology, University of California, San Diego, Jolla, California 92037.

The distribution and function of the enzyme carbonic anhydrase in the crab Pachygrapsus crassipes was investigated. Carbonic anhydrase was found in gill epithelial tissue but not in the haemolymph, muscle, heart, hepatopancreas or gonads of male crabs.

Enzyme activity was completely inhibited in vitro by 2 x 10^-4M sodium acetazolamide (Diamox). Radiolabelled Diamox (2 x 10^-4M) in sea water was found to equilibrate with the intact crab's haemolymph within one hour. Haemolymph CO₂ content, pH and chloride ion concentration were measured in crabs acclimated to different salinities and exposed to 2 x 10^-4M Diamox.

Haemolymph CO₂ content increased at all salinities, especially low salinities, while pH remained unchanged, except at low salinities where it increased. Diamox impaired the regulation of haemolymph chloride at low salinities, but had no effect on chloride regulation at high salinities. Measurement of O₂ uptake (estimating CO₂ production) in crabs before and after Diamox exposure confirmed that elevated haemolymph CO₂ was not due to increases in tissue CO₂ production.