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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 (P_NHNH3) and NH₄⁺ (NH₄⁺) gradients and sodium transport (J^Na) across the gills were employed to analyse the normal mechanism(s) of branchial ammonia excretion (J^Amm_net) in trout acclimated to fresh water of pH 8.0. Control J^Amm_net occurred in the absence of, or against, an apparent PNH₃ gradient, while NH₄⁺ was positive. Severe acid exposure (pH = 4.06) raised PNH₃ and NH₄⁺, abolished J^Na_in, and reduced J^Amm_net by 28%, while moderate acidity (pH = 6.64), which also elevated PNH₃ had no significant influence on J^Na_in and J^Amm_net. Severe alkaline exposure (pH = 9.54) raised NH₄⁺, reduced PNH₃ to a very negative value, and decreased J^Na_in and J^Amm_net by equimolar amounts, representing 55% and 80% of control levels respectively. Moderate alkalinity (pH = 8.69) had similar effects on PNH₃ and NH₄⁺, but reduced J^Na_in and J^Amm_net 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 J^Na_in and J^Amm_net but did not alter PNH₃ or 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 (J^Amm_net). 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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