Abstract
ABSTRACT Krogh (1939) first proposed that freshwater organisms extract needed Na+ in exchange for to maintain near-electroneutrality across the skin or gills. Maetz & Garcia-Romeu (1964) provided an indirect demonstration of 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 proposed that 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 exchange, occured at the apical surface. More recent evidence supporting the model for basolateral uptake of NH3 and apical exchange was presented by Payan (1978). He used the isolated, perfused, head of the trout (Salmo gairdneri) and showed that: (1) addition of amiloride to the solution irrigating the gills inhibited both Na+ influx and ammonia efflux, (2) acetazolamide added to the perfusate inhibited ammonia efflux, and (3) reduction in the NH3 concentration of the perfusate (by lowering pH) inhibited ammonia efflux as well as Na+ influx. Furthermore, addition of ouabain to the perfusate inhibited ammonia efflux, presumably because Na+-K+-activated ATPase at the basolateral surface mediates the final transfer of Na+ into the serosal medium, and is ultimately limiting in the regulation of apical exchange.
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