Over the last years, a large mass of information has accumulated indicating that changes in the serum insulin concentration can affect renal electrolyte excretion. We analyzed the response of the kidney to furosemide in 5 healthy men, in the presence of both normal physiological serum insulin levels and levels at the upper limit of the physiological range, obtained by the hyperinsulinemic-euglycemic clamp technique. After furosemide administration, glomerular filtration rate, urine flow, urine sodium excretion, free water clearance, urine pH, plasma renin activity and plasma aldosterone exhibited the same behavior in the presence of both serum insulin concentrations. The rise in urinary potassium excretion following furosemide administration was significantly lower in the presence of high insulin concentrations. Although we observed a slight decrease in plasma potassium levels during the equilibration phase of the clamp required before the administration of furosemide, a significantly lower increase in potassium fractional excretion indicated a direct tubular effect of insulin. Thus, in conditions in which natriuresis is mildly stimulated, as in the case of the administration of low doses of furosemide, insulin does not affect the rate of renal sodium reabsorption. Conversely, the hormone has an appreciable influence on the modulation of tubular potassium exchanges.
