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Journal of Experimental Biology 114,329-353 (1985)
Published by Company of Biologists 1985

An Analysis of Branchial Ammonia Excretion in the Freshwater Rainbow Trout: Effects of Environmental pH change and Sodium Uptake Blockade

P. A. Wright 1 and C. M. Wood 2

1 Department of Biology, McMaster University, Hamilton, Ontario, Canada, L8S 4K1: Department of Zoology, University of British Columbia, Vancouver, B.C., Canada V6T 2A9.
2 Department of Biology, McMaster University, Hamilton, Ontario, Canada, L8S 4K1

Short-term treatments (3h) designed to change the relative NH3 ({Delta}PNHNH3) and NH4+ ({Delta}NH4+) gradients and sodium transport (JNa) across the gills were employed to analyse the normal mechanism(s) of branchial ammonia excretion (JAmmnet) in trout acclimated to fresh water of pH ~= 8.0. Control JAmmnet occurred in the absence of, or against, an apparent {Delta}PNH3 gradient, while {Delta}NH4+ was positive. Severe acid exposure (pH = 4.06) raised {Delta}PNH3 and {Delta}NH4+, abolished JNain, and reduced JAmmnet by 28%, while moderate acidity (pH = 6.64), which also elevated {Delta}PNH3 had no significant influence on JNain and JAmmnet. Severe alkaline exposure (pH = 9.54) raised {Delta}NH4+, reduced {Delta}PNH3 to a very negative value, and decreased JNain and JAmmnet by equimolar amounts, representing 55% and 80% of control levels respectively. Moderate alkalinity (pH = 8.69) had similar effects on {Delta}PNH3 and {Delta}NH4+, but reduced JNain and JAmmnet by only 25%. The sodium transport inhibitor amiloride (10-4moll-1 in the external water, pH ~= 8.0) had very similar effects to pH = 4.06 on both JNain and JAmmnet but did not alter {Delta}PNH3 or {Delta}NH4+. The results discount the quantitative importance of NH4+ diffusion and favour a flexible combination of NH3 diffusion and Na+/NH4+ exchange as the major mechanisms of (JAmmnet). with the latter dominating under the particular control conditions of the present study.

Key words: Trout, ammonia, gills, amiloride, pH.

Accepted on July 9, 1984




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