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

Abstract

ABSTRACT Short-term treatments (3 h) designed to change the relative NH3 and NH4+ (ΔNH4+) gradients and sodium transport (JNa) 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.