AJP - Regulatory, Integrative and Comparative Physiology, Vol 240, Issue 5 364-R369, Copyright © 1981 by American Physiological Society
Exchange diffusion, active transport, and diffusional components of transbranchial Na and cl fluxes
L. B. Kirschner and D. Howe
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.
