Printed in Great Britain FISH BRANCHIAL Na+/NH+ EXCHANGE IS VIA

Krogh (1939) first proposed that freshwater organisms extract needed Na+ in exchange for NHJ to maintain near-electroneutrality across the skin or gills. Maetz & Garcia-Romeu (1964) provided an indirect demonstration of Na+/NH^ " exchange in the goldfish {Carassius auratus) by showing that injected NH4C1 stimulated Na+ uptake, but addition of NH4C1 to the freshwater inhibited Na+ uptake. They pro-posed that Na+/NHi " exchange was apical (on the mucosal surface of the transporting epithelium, facing the fresh water) and that blood NH3 entered the basolateral surface of the cell, combined with a proton generated by the carbonic anhydrase hydration of CO2 (and the subsequent dissociation of carbonic acid), and left the cell in exchange for Na+. The role for carbonic anhydrase was indicated by their finding that injection of acetazolamide inhibited Na+ uptake. Subsequently, Kerstetter, Kirschner & Rafuse (1970) demonstrated that acetazolamide injection inhibited the influx of Na+ into the irrigated gills of the trout (Salmo gairdneri). However, ammonia efflux was not significantly inhibited, while acid efflux was. They concluded that Na+/H+, rather than Na+/NH^ " exchange, occured at the apical surface. Mor