1. The uptake and transflux of sodium by goldfish intestines has been
compared under different experimental conditions. Uptake was measured
from a 1 min contact of the intestinal mucosa with radioactive sodium
chloride solution. Transflux was measured over a period of 2 hr using the
everted sac technique.
2. Keeping goldfish in saline reduced the transflux of sodium to one
quarter the value found for fresh-water fish. The uptake of sodium was
halved by this treatment. Cortisol injected previously into saline-adapted
fish changed neither the transflux nor the uptake of sodium measured
subsequently.
3. In fresh-water fish hypophysectomy reduced sodium transflux while
leaving the uptake of sodium unchanged. Injection of cortisol restored
sodium transflux to control levels without producing any additional
effects on the uptake of sodium.
4. It is suggested that adaptation to saline involves regulation of sodium
movement across the microvillar membrane of the mucosal cell. Cortisol
would appear to play no part in this type of regulation.
5. The presence of cortisol, or possibly other steroids with similar
actions, has however been shown to be essential for the normal operation
of sodium transport in this tissue. It is not clear exactly how cortisol
exerts this effect. What evidence there is suggests that cortisol exerts a
metabolic control rather than changing directly the membrane permeability
of the cell.