Original models proposed for Na+ and Cl− uptake, and their linkage with H+ and HCO3− secretion have changed substantially with the development of more sophisticated physiological techniques. At the freshwater fish gill,two dominant mechanisms for Na+ uptake from dilute environments have persisted in the literature. The use of
an apical Na+/H+ exchanger driven by a basolateral Na+/K+-ATPase versus an apical Na+ channel electrogenically coupled to an apical H+-ATPase have been the source of debate for a number of years. Advances in molecular biology have greatly enhanced our understanding of the basic ion transport mechanisms at the fish gill.
However, it is imperative to ensure that thermodynamic principles are followed in the development of new models for gill ion transport. This review will focus on the recent molecular advances for Na+ uptake in freshwaterfish. Emphasis will be placed on thermodynamic constraints that prevent electroneutral apical NHE
function in most freshwater environments. By combining recent advances in molecular and functional physiology offish gills with thermodynamic considerations of ion transport, our knowledge in thefield should continue to grow in a logical manner.