Unidirectional water and sodium fluxes and respiratory metabolism in the African lung-fish,Protopterus annectens

Summary Simultaneous measurements have been made of the efflux rates of tritiated water and radiosodium from the lung-fishProtopterus annectens under conditions in which the fish can either use its gills, its lungs or both for respiration. Tritiated water efflux rate was similar (43–44% h^–1) when the fish was in oxygenated water with or without access to air but decreased slightly in deoxygenated water. An efflux rate constant of 44% h^–1 obtained in oxygenated water is similar to those of other inactive fish in fresh water. The efflux rate constant for sodium was 0.84% h^–1 and total sodium flux was comparable to that of fresh-water fish. The decrease in both tritiated water and sodium efflux rates in deoxygenated water are explained in terms of diversion of blood flow away from the functional gills to the lungs. Oxygen consumption was highest (38.7 ml kg^–1 h^–1) in deoxygenated water and lowest when the fish was out of water. Despite the reduced functional significance of the gill for respiration, it is still as permeable to water and electrolytes as that of fresh-water teleosts.This work was supported by a travel grant from the Commonwealth Universities Interchange Scheme of the British Council and laboratory facilities of the Department of Biological Sciences, University of Lancaster