Branchial Sodium Exchange and Ammonia Excretion in the Goldfish Carassius Auratus. Effects of Ammonia-Loading and Temperature Changes

Abstract

1. Sodium influx and efflux and ammonia excretion by the gill have been studied as a function of external sodium chloride concentration in Carassius auratus before and after loading the fish with ammonia. 2. No correlation between net sodium uptake and ammonia excretion is observed, either when the net uptake changes with an external sodium change or when net uptake increases with ammonia-loading. Branchial handling of chlroide ions cannot explain this absence of correlation. 3. Comparison of the concentrations of free base ammonia (NH3) and of ammonium ions (NH4+) in both (dorsal aorta) and external medium at the end of the closed-circuit experiments on control or ammonia-loaded fish demonstrates that the gill is permeable to the ionized form of ammonia. 4. An abrupt temperature decrease (16 → 6 °C) affects the sodium influx (Q10 = 3) much more than the sodium efflux (Q10 = 1.7). Sodium balance becomes negative unless the fish is ammonia-loaded. The observed effects of temperature are reversible when the fish is returned to 16 °C. Branchial ammonia excretion is highly temperature-sensitive (Q10 = 4) in control fish when metabolic production limits ammonia excretion. After ammonia-loading, when most of the ammonia cleared by the gill is exogenous, the effect of temperature on branchial permeability to ammonia (Q10 = 1.9) suggests a passive transfer of ammonium ions. 5. The contributors of the kidney and the gill in sodium loss and ammonia excretion are compared in intact and ammonia-loaded fish.