Effect of insulin on sodium reabsorption in hypertensive patients

The effect of insulin on sodium reabsorption was studied with the hyperinsulinemic euglycemic clamp technique, using lithium clearance as a marker of sodium reabsorption in the proximal tubule. The subjects were 12 nondiabetic hypertensive patients who were divided into two groups. In the first group, the effect of insulin on sodium and lithium reabsorption was examined. The second group served as a control. Blood glucose was continuously monitored from 9 AM to 10 AM (baseline period). Hyperinsulinemic euglycemic clamp was performed with an artificial endocrine pancreas by infusing insulin at a rate of 2 mU/kg.min from 10 AM to noon. In the control group the protocol was exactly the same except that glucose clamp was not performed. Urine was collected hourly from 9 AM to noon. In the glucose clamp group, urinary sodium excretion (UNaV) decreased by 42% during the second hour of the glucose clamp (P < .02). Clearance of lithium (CLi) and fractional distal sodium reabsorption [(CLi-CNa)/CLi] increased, and CNa/CLi decreased significantly during the glucose clamp. In the control group, UNaV and (CLi-CNa)/CLi remained unchanged, and CLi decreased significantly. As shown by the increase of CLi and (CLi-CNa/CLi, and the decrease of UNaV and CNa/CLi in the glucose clamp group, insulin decreases urinary sodium excretion by increasing distal sodium reabsorption, whereas it inhibits sodium reabsorption in the proximal tubule.