The Effect of Environmental Calcium and Ovine Prolactin on Sodium Balance in Fundulus Kansae

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Abstract
1. The sodium contents and rates of turnover of sodium have been determined in both intact and hypophysectomized Fundulus kansae in a variety of media. 2. A reduction of the calcium content of fresh water from 1.0 to 0.1 mM/l increased the rate of sodium efflux by half while transter to calcium-free water increased the rate several fold. 3. The rate of sodium efflux increased fourfold in sea water containing no magnesium and only 1 mm/l calcium. 4. Hypophysectomy increased the rate of efflux in fresh water but reduced it in sea water. 5. Intact fish adapted to sea water and treated with prolactin showed a net gain of sodium probably due to a reduction in the activity of the sodium efflux pump. 6. Both intact and hypophysectomized fish on transfer from sea water to fresh water reduced the rate of efflux of sodium in a series of stages. 7. Studies of the rate of efflux of sodium before and after transfer between various media showed no evidence of exchange diffusion or of a linked sodium-potassium efflux pump, but did show evidence or interaction between sodium and chloride fluxes probably as a consequence of potential changes. 8. The rate of efflux of sodium from F. kansae doubled following hypophysectomy. The ability of hypophysectomized F. kansae to survive in fresh water after hypophysectomy, unlike F. heteroclitus, is due to a more effective uptake system.