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

Krogh (1939) first suggested that in freshwater animals branchial or cutaneous excretion of ammonia may be related to sodium absorption. In both freshwater Crustacea (Astacus pallipes) and teleosts (Carassius auratus) independent active transport of Na and Cl~ from the outside medium have been definitely established (Shaw, 1960 a—c; Garcia Romeu & Maetz, 1964). Sodium in particular may be absorbed at very high rates without an accompanying anion. The maintenance of the electroneutrality of the external and internal media necessitates an exchange with an endogenous ion during sodium movement. The possibility that ammonium ions may fulfil this role is suggested by the fact that the gill is the main site of ammonia excretion (Smith, 1929,1953) and indirect experimental evidence has supported this hypothesis. Addition of ammonia to the outside medium, a procedure which inhibits ammonia excretion, interferes with the sodium uptake (Shaw, 1960a; Maetz & Garcia Romeu, 1964). Injection of ammonium ions, a procedure which increases ammonia excretion by the gill, produces an increase of sodium uptake in the goldfish (Maetz & Garcia Romeu, 1964) and in the eel (Garcia Romeu & Motais, 1966). No stoichiometric relationship between ammonia excretion rate and sodium net uptake in relation to experimental external salinity changes is, however, observed in the crayfish (Shaw, 1960A). Thus the Na/NHJ exchange cannot be obligatory. In particular when ammonia excretion falls short of sodium uptake, a second exchange process, Na against H, is thought to be operative, or when sodium uptake is blocked at low external sodium concentration, ammonium is thought to be accompanied by endogenous bicarbonate ions excreted by the gill (Shaw, 19606). The occurrence of a Na/H exchange has recently been demonstrated by Garcia Romeu, Salibian & Pezzani-Hernandez (1969) working on the amphibian Calyptocephallela gayi. This type of exchange is thought to be related to the switch from ammonotelism to ureotelism accompanying amphibiosis. In the teleosts, Maetz & Garcia Romeu (1964) considered that the Na/NHi" exchange was obligatory on grounds of the evidence presented by Goldstein & Forster (1961) indicating that enzymic deamination and deamidation within the gill tissue accounts for most of the ammonia excreted by the gills. This point, however, was challenged by Pequin & Serfaty (1963) who demonstrated in the carp that most if not all of the excreted ammonia originates in the liver and that the gill is simply the