Evolution de la balance minérale du sodium chez Fundulus heteroclitus au cours du transfert d'eau de mer en eau douce: Effects de l'hypophysectomie et de la prolactine

Abstract

The normally euryhaline killifish Fundulus heteroclitus cannot survive in fresh water after hypophysectomy. Injections of mammalian pituitary preparations with prolactin activity protect such fish against death in fresh water. Radioisotopic methods have been used to measure sodium balance in intact and hypophysectomized F. heteroclitus in sea water (SW) and after transfer to fresh water (FW). Intact fish adapted to SW exchange Na with the environment at a rate equivalent to 27 to 35% of their total exchangeable Na per hour. This fractional turnover rate is lower in hypophysectomized fish (18.5%). Sheep prolactin (125 mU per gram of body weight per day for 3 days) does not change Na turnover rates in hypophysectomized fish kept in SW. When intact fish are transferred from SW to FW, Na influx falls abruptly to levels found in fish long-adapted to FW. Outflux does not fall as rapidly, and positive Na balance is achieved only on the second day after transfer. Na balance in hypophysectomized fish resembles that of intact fish during the first few hours after transfer to FW. On the second day, however, Na outflux is about twice, and Na influx about half of, the rates found in intact fish. Hypophysectomized fish remain in negative Na balance. Prolactin was injected into hypophysectomized fish for 4 days before transfer to FW. This reduced outflux rates to normal levels on the second day after transfer. Na influx rates were not increased by such treatment, but these fish did attain an equilibrated Na balance on the second day in FW. These results may help to explain the progressive fall in plasma osmolality in hypophysectomized F. heteroclitus exposed to FW. Prolactin appears to prevent this fall by limiting Na outflux and thus permits survival. The site (or sites) at which this hormone acts to limit Na loss is discussed.