Changes in urinary bladder and kidney function in the starry flounder ( Platichthys stellatus) in response to prolactin and to freshwater transfer

Abstract

The effect of ovine prolactin treatment and of acclimation to freshwater on renal hydromineral metabolism were investigated in the seawater starry flounder, Platichthys stellatus. Isolated urinary bladders of seawater, freshwater, and prolactin-injected seawater fish readily transport sodium and chloride but not potassium, magnesium, and calcium. As was established earlier, urinary bladders of seawater flounder show considerable mucosa-to-serosa water movement whereas prolactin treatment or freshwater acclimation markedly decreases bladder water permeability. The flounder kidney was studied by cannulating the urinary bladder in vivo. Prolactin treatment and freshwater acclimation caused increased urine production, increased sodium excretion, and decreased urine osmolarity. These changes suggest diminished fluid reabsorption resulting from decreased water permeability of the kidney tubule. Furthermore, prolactin treatment lowered urine magnesium content without affecting net renal magnesium excretion. Kidneys of freshwater flounder excreted considerably less magnesium and produced urine of lower magnesium concentration than kidneys of seawater flounders. Maximal effects were manifested 48 hrs after a single prolactin injection; thereafter, values slowly returned toward initial levels. A primary and inital action of prolactin on renal excretion in the starry flounder appears to be a decrease in water permeability of the kidney tubules and urinary bladder.