The cultured branchial epithelium of the rainbow trout as a model for diffusive fluxes of ammonia across the fish gill

A novel branchial epithelial preparation grown in L-15 medium in culture was used as a model system for understanding the diffusion of ammonia across the gills of the rainbow trout Oncorhynchus mykiss. The epithelium is known to contain both respiratory and mitochondria-rich cells in the approximate proportion in which they occur in vivo and to exhibit diffusive fluxes of Na⁺ and Cl^– similar to in vivo values, but does not exhibit active apical-to-basolateral transport of Na⁺. Transepithelial resistance and paracellular permeability are also known to increase when the apical medium is changed from L-15 medium (symmetrical conditions) to fresh water (asymmetrical conditions). In the present study, net basolateral-to-apical ammonia fluxes increased as basolateral total ammonia concentration, basolateral-to-apical pH gradients and basolateral-to-apical P_NH3 gradients were experimentally increased and were greater under asymmetrical than under symmetrical conditions. The slope of the relationship between ammonia flux and P_NH3 gradient (i.e. NH₃ permeability) was the same under both conditions and similar to values for other epithelia. The higher fluxes under asymmetrical conditions were explained by an apparent diffusive flux of NH₄⁺ that was linearly correlated with transepithelial conductance and was probably explained by the higher electrochemical gradient and higher paracellular permeability when fresh water was present on the apical surface. In this situation, NH₄⁺ diffusion was greater than NH₃ diffusion under conditions representative of in vivo values, but overall fluxes amounted to only approximately 20 % of those in vivo. These results suggest that branchial ammonia excretion in the intact animal is unlikely to be explained by diffusion alone and, therefore, that carrier-mediated transport may play an important role.