Effects of insulin on renal handling of sodium, potassium, calcium and phosphate were studied in man while maintaining the blood glucose concentration at the fasting level by negative feedback servocontrol of a variable glucose infusion. In studies on 6 water loaded normal subjects in a steady state of water diuresis, insulin was administered i.v. to raise the plasma insulin concentration to between 98 and 193 μU/ml and infused at a constant rate of 2 mU/kg per min over a total period of 120 min. The blood glucose concentration was not significantly altered, and there was no change in the filtered load of glucose; glomerular filtration rate (C(in)) and renal plasma flow (C(PAH)) were unchanged. Urinary sodium excretion (U(Na)V) decreased from 401±46 (SEM) to 213±18 μeq/min during insulin administration, the change becoming significant (P<0.02) within the 30 to 60 min collection period. Free water clearance (C(H2O)) increased from 10.6±0.6 to 13±0.5 ml/min (P<0.025); osmolar clearance decreased and urine flow was unchanged. There was no change in plasma aldosterone concentration, which was low throughout the studies, and a slight reduction was observed in plasma glucagon concentration. Urinary potassium (U(k)V) and phosphate (U(p)V) excretion were also both decreased during insulin administration; U(k)V decreased from 66±9 to 21±1 μeq/min (P<0.005), and U(p)V decreased from 504±93 to 230±43 μg/min (P<0.01). The change in U(k)V was associated with a significant reduction in plasma potassium concentration. There was also a statistically significant but small reduction in plasma phosphate concentration which was not considered sufficient alone to account for the large reduction in U(p)V. Urinary calcium excretion (U(Ca)V) increased from 126±24 to 200±17 μg/min (P<0.01).
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