The Effects of Prolactin and Divalent Ions on the Permeability to Water of Fundulus Kansae

Fundulus kansae (Cyprinidae) is widely distributed in the western parts of the Mississipi basin. Although a continental form the fish is a powerful osmo-regulator. It can survive in the laboratory in solutions containing as little as 0-2 mM/1 of sodium and oi mM/1 of calcium up to concentrations as high as 250% normal sea water, and some individuals can survive in 300% sea water. The fish can stand immediate transfer from a more concentrated to a more dilute medium, but not all individuals can survive an immediate transfer from fresh water up to full strength sea water or higher. The animals described below were obtained from an area containing saline springs and lived in water of variable salinity. The fish has often been the subject of physiological researches, particularly by Fleming and his co-workers (Fleming & Stanley, 1965; Fleming, Stanley & Meier, 1964; Stanley & Fleming, 1964a, b, 1966a, b, 1967). The fish is easily hypophysectomized and differs from F. heterocktus in that it survives in fresh water after such treatment (Pickford, Pang, Stanley & Fleming, 1966). Hypophysectomized F. kansae living in fresh water had a slightly lower blood concentration and a markedly reduced rate of urine production (Stanley & Fleming, 1966 a, b). Injections of mammalian prolactin restored urine production approximately to the level in control animals. Stanley & Fleming (1967) suggested that prolactin increased the permeability of the body wall to water. Lahlou & Sawyer have recently reported (1969) that hypophysectomized goldfish, Carassius auratus, have lower rates of turnover of tritiated water than have intact fish. It has been noted that a number of fishes, including Tilapia mossambica, the yellow eel and the flounder, are more permeable to water when in fresh water than when in sea water (Potts, Foster, Rudy & Parry Ho wells, 1967) and in a systematic study of a variety of fishes from both sea water and fresh water it was noted that in general the fresh-water fishes are more permeable to water than are marine species (Evans, 1969). This evidence that teleosts in fresh water are more permeable to water than teleosts in sea water, together with the observation that prolactin is found in fresh-water teleosts, but is rare or absent in marine teleosts (Ensor & Ball, 1968; Dharmamba & Nishioka, 1968) and in euryhaline teleosts adapted to fresh water, strengthens the hypothesis that permeability to water in teleosts is increased by the prolactin hormone. The ability of Fundulus kansae to survive indefinitely in fresh water after hypophysectomy, in the absence of prolactin treatment, makes it a very convenient animal for the examination of this hypothesis.