Exchange diffusion, active transport, and diffusional components of transbranchial Na and cl fluxes

Sodium efflux across the gills of the sculpin Leptocottus armatus average about 900 mumol.100 g-1.h-1 in seawater (SW). When external Na+ was replaced by Tris [tris(hydroxymethyl)aminomethane] the efflux dropped about 35% and the voltage across the gill (TEP) decreased from +20.3 to -2.3 mV. The electrical change accounted, almost exactly, for the diminution of efflux, suggesting that most, if not all, of the Na+ efflux in this fish is diffusive. Chloride efflux in SW was about 300 mumol.100 g-1.h-1. When external Cl- was replaced by gluconate, efflux fell to about one-half the SW value. This could not be due to a change in TEP and is therefore attributed to exchange diffusion. Injection of thiocyanate further reduced the efflux to about 15% of the SW rate. This fraction of the total efflux is active extrusion. The remaining efflux (exchange and active transport eliminated) is diffusive. It is also shown that substitution of gluconate for chloride reduces the activity coefficient for Na+. A small decrease in TEP, noted in this substitution, can be explained by the activity change. A few experiments with isethionate suggest that it does not have this effect and hence is a better chloride substitute in single-ion replacement experiments.