Changes in the intestinal transport of sodium induced by exposure of goldfish to a saline environment.

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.