An Analysis of Branchial Ammonia Excretion in the Freshwater Rainbow Trout: Effects of Environmental Ph Change and Sodium Uptake Blockade Available to Purchase

Short-term treatments (3 h) designed to change the relative NH₃ and NH₄⁺ (ΔNH4⁺) gradients and sodium transport (J^Na) across the gills were employed to analyse the normal mechanism(s) of branchial ammonia excretion in trout acclimated to fresh water of pH ≃8·0. Control occurred in the absence of, or against, an apparent gradient, while ΔNH4⁺ was positive. Severe acid exposure (pH = 4·06) raised and ΔNH₄⁺, abolished ⁠, and reduced by 28%, while moderate acidity (pH = 6·64), which also elevated had no significant influence on and ⁠. Severe alkaline exposure (pH = 9·4) raised ΔNH4⁺, reduced to a very negative value, and decreased and by equ1molar amounts, representing 55% and 80% of control levels respectively. Moderate alkalinity (pH = 8·69) had similar effects on and ⁠, but reduced and 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 and ⁠, but did not alter or ⁠. 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 ⁠, with the latter dominating under the particular control conditions of the present study.