THE USE OF AMILORIDE TO UNCOUPLE BRANCHIAL SODIUM AND PROTON FLUXES IN THE BROWN TROUT, SALMO-TRUTTA

Resting proton, ammonium and sodium fluxes in Salmo trutta were 492.6 +/- 19.5 (n = 29); 122.9 +/- 34.2 (n = 28) and 277.1 +/- 18.5 (n = 50) mu mol . kg(-1) . h(-1), respectively. The resting transepithelial po tential was found to be composed of three successive potentials, the o utermost averaging -7.36 +/- 0.19mV, the second, -14.3 +/- 1.4 mV and the third -37 +/- 1.7 mV. Amiloride inhibits the proton, ammonium and sodium fluxes in a dose-dependent manner at concentrations of 0.5 mmol . l(-1) and 0.1 mmol . l(-1), but at 0.01 mmol . l(-1), proton and am monium fluxes remained at control levels whilst the sodium was reduced to 70.59 +/- 7.29 mu mol . kg(-1) . h(-1). The trans-epithelial poten tial was effected in a bi-phasic manner by 0.5 mmol . l(-1) amiloride. An initial hyperpolarisation of ca. 6 mV was followed by a sustained depolarisation of ca. 14 mV (towards zero) which persisted until the a miloride was washed off the gill. The initial hyperpolarisation was th ought to reflect a rapid inhibition of a positive inward sodium curren t and the subsequent depolarisation was due to the inhibition of a pos itive outward current (proton) which would abolish the transepithelial potential. However, at 0.01 mmol . l(-1) only the hyperpolarisation w as seen, due to the inhibition of only the inward sodium current. Acet azolamide (0.1 mmol . l(-1)) was found to have no significant effect o n the proton, ammonium and sodium fluxes. These results indicate that the proton and sodium fluxes across the gill of the freshwater trout a re not tightly linked. While this suggests that the trout gill resembl es the model of Ehrenburg et al. (1985) of sodium uptake in frog skin, the apical potentials measured in the pavement epithelial cell(s) are too low to account for sodium uptake unless the activity of the sodiu m in the cells is very low.