Abstract
A single injection of prolactin into seawater-acclimated flounder, Platichthys stellatus, caused a decrease in the permeability of the bladder to water (mucosa to serosa) with an increase in sodium absorption by the bladder and a decrease in urine osmolality and sodium concentration. A simultaneous increase occurred in plasma sodium and osmolality. Maximal effects were observed 2 days after the injection, and the effects disappeared after 4 days. Transfer of seawater fish to fresh water similarly resulted in a maximal decrease in bladder water permeability and in serum sodium concentration by 2 days after transfer. Serum sodium increased to the normal freshwater level by 14 days. Bladder water permeability increased significantly 2 days after transfer from fresh water to sea water, and attained the fully adapted level by 14 days, whereas plasma sodium increased maximally by 2 days after transfer and decreased to the normal level by 7 days. A change in pituitary prolactin content as determined by the xanthophore pigment-dispersion test was observed within 12 hr after salinity transfer. The parallelism between the time of response of serum and urine adjustments and urinary bladder permeability following prolactin treatment and that following changes in environmental salinity adds additional support for an osmoregulatory function for the urinary bladder in the starry flounder.
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