The modes of ammonia transport across the gill epithelium of the marine teleost Opsanus beta were investigated using an isolated, perfused head preparation. During perfusion for approximately 2h there was no macro- or microscopic damage to the exterior surface of the gill filaments or lamellae, no swelling of lamellae, the structural leak was only 0.3%, the afferent perfusion pressure was constant, and the transepithelial electrical potential (TEP) approximated that measured in vivo. The rate of ammonia efflux was similar to that measured in vivo, was constant for at least 80 min, and approached zero when the perfusate was nominally ammonia-free. Addition of ouabain or bumetanide to the perfusate indicated that basolateral Na+/NH4+ exchange, but not Na+/NH4+/2Cl cotransport, was important in transbranchial ammonia transport. No evidence was found for apical Na+/NH4+ exchange. Specific increases in perfusate NH3 were nearly 100 times as effective as increases in NH4+ concentrations in stimulating ammonia efflux. Alteration of TEPs could not account for any of these effects. Calculated relative permeabilities to NH3 and NH4+ indicate that the toadfish gill s relatively permeable to NH4+ compared to the shark gill and other ‘tight’ epithelia, consistent with the general conclusion that marine teleost gills are relatively cation-‘leaky’. Computation of the relative roles of the three delimited pathways for ammonia efflux indicate that 22% is via basolateral Na+/NH4+ exchange, 57% via non-ionic diffusion of NH3 and 21% via ionic diffusion of NH4+. Comparison with other published data suggests that the relative roles of non-ionic and ionic diffusion of ammonia across epithelia may be correlated with relative ionic permeabilities.

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