EVIDENCE FOR AN EFFECT OF OUABAIN ON THE BRANCHIAL SODIUM-EXCRETING PUMP OF MARINE TELEOSTS: INTERACTION BETWEEN THE INHIBITOR AND EXTERNAL Na AND K

The ability to maintain body fluids hypotonic to the environment is a characteristic of marine teleosts. Homer Smith (1930) showed that to replace body water lost along the osmotic gradient the teleost swallows the external medium continuously, absorbing ions and water from the gut. To abolish the salt load incurred by drinking sea water the fish excretes Na and Cl through the gills. By using radioactive tracers, however, the sodium outflux through the gill has in fact been found to be more than ten times greater than the sodium extrusion expected from the drinking rate (Motais & Maetz, 1965). On experimental grounds it has been suggested (Motais, 1967) that the branchial sodium efflux in flounder and eel may be divided into at least three components, as shown in Fig. 1. (1) A small net extrusion component balancing the intestinal absorption (amounting to about 100/i-equiv/h/ioo g body weight in the eel). (2) A very large fraction, dependent of the external sodium concentration in a manner suggesting Michaelis-Menten saturation kinetics, linked with the branchial sodium influx and considered as an exchange diffusion component (amounting to about 700/^equiv/h/ioo g body weight in the eel). (3) A residual outflux observed after rapid transfer to fresh water, i.e. independent of external cations (amounting to about 200 /i-equiv/h/100 g body weight in the eel). Indirect evidence suggests a close correlation between the first two components (Maetz, Motais & Mayer, 1969). Until recently the nature of the sodium-excreting pump was completely unknown. But Maetz (1969) showed in the flounder that the net Na extrusion rate is identical to the K influx and is blocked in K-free sea water. So he considered that the sodium-excreting pump is a sodium-potassium exchange pump. Furthermore, since external Na competes with K on the K site of the pump and since sea water contains 50 times more Na than K, he suggested that at least a part of the exchange-diffusion fluxes are the result of this competitive process. This model is significant for two reasons. First, it adequately explains the correlation between the Na-Na and Na-K exchanges mentioned above, and secondly, it agrees well with the hypothesis that Na-K-activated ATPase is involved in the excretion of sodium ions by the gills of marine teleosts indeed parallel variations of the enzyme activity and gill ionic transfers either during adaptation to salt water (see