Adaptation of renal function to hypotonic medium in the winter flounder (Pseudopleuronectes americanus)

The kidneys of winter flounders transferred to hypotonic medium were investigated for glomerular and tubular handling of fluid and electrolytes and for the urinary excretion of proteins. Media were sea water (925 mosm X kg-1) and brackish water (70 mosm X kg-1). In sea water, the urine was hypertonic to the plasma in 7 fish of this study. Urine flow rate was correlated with the GFR. After adaptation to brackish water a delay of 1 to 3 days was observed until the kidneys switched from fluid retention to the excretion of large amounts of dilute urine. GFR and urine flow rate were increased from 0.61 +/- 0.08 to 1.58 +/- 0.29 ml X h-1 X kg-1 and from 0.14 +/- 0.02 to 0.68 +/- 0.08 ml X h-1 X kg-1, respectively (mean +/- SEM). With increased filtered load the tubular reabsorption of fluid decreased from 74 +/- 2.4% to 45 +/- 11.2%. The excretion rates of sodium and potassium were increased due to decreased fractional sodium and potassium reabsorption. The urinary excretion of divalent cations, however, was reduced because the net tubular reabsorption of calcium was increased and the net secretion of magnesium was diminished. Both the urinary total protein concentration and the protein pattern showed no significant change, but the rate of protein excretion was increased from 0.21 +/- 0.04 to 0.60 +/- 0.05 mg X h-1 X kg-1. The comparison of protein patterns obtained from urine and serum samples revealed that high molecular weight (HMW) proteins prevail in the serum whereas low molecular weight (LMW) proteins dominate in the urine. The diminished quantity of the HMW-protein fraction in the urine thus may reflect size selectivity of the glomerular filtration barrier for serum proteins also in the winter flounder.