AN ANALYSIS OF BRANCHIAL AMMONIA EXCRETION IN THE FRESHWATER RAINBOW TROUT: EFFECTS OF ENVIRONMENTAL pH CHANGE AND SODIUM UPTAKE BLOCKADE

Short-term treatments (3 h) designed to change the relative NH3 (AP N H 3 ) and NH4 + (ANH4 ) gradients and sodium transport (J) across the gills were employed to analyse the normal mechanism(s) of branchial ammonia excretion (J^eT) i trout acclimated to fresh water of pH = 8-0. Control Jne? occurred in the absence of, or against, an apparent APNH gradient, while ANH4 + was positive. Severe acid exposure (pH = 4-06) raised APNHj and ANH4 , abolished J^, and reduced } ^ m by 28%, while moderate acidity (pH = 664), which also elevated APNH3I had no significant influence on J^ and JneTSevere alkaline exposure (pH = 9-54) raised ANH4 , reduced APNH] to a very negative value, and decreased J,>J and ]^TM by equimolar amounts, representing 55 % and 80% of control levels respectively. Moderate alkalinity (pH = 8-69) had similar effects on APNH and ANH4 , but reduced Jin and JneTTM by only ~ 2 5 % . The sodium transport inhibitor amiloride (10~moll 1 in the external water, pH = 8-0) had very similar effects to pH = 4-06 on both jf̂ 8 and Jne?> but did not alter A P N H 3 or ANH4 . The results discount the quantitative importance of NH4 + diffusion and favour a flexible combination of NH3 diffusion and Na + /NH 4 + exchange as the major mechanisms of J^T". with the latter dominating under the particular control conditions of the present study.