Insulin increases loop segment chloride reabsorption in the euglycemic rat.

To elucidate the nephron segment(s) responsible for insulin's antinatriuretic effect in the mammalian kidney, late proximal, early distal, and late distal tubule micropuncture was performed during base-line and intravenous insulin infusion in glucose-clamped, volume-expanded rats. During insulin infusion, urinary sodium chloride excretion fell. Blood glucose, arterial pressure, whole kidney and single-nephron inulin clearance, fluid, and chloride delivery out of the proximal convoluted tubule remained unchanged. Early distal chloride delivery decreased (P less than 0.001) during insulin administration. Insulin infusion increased calculated loop chloride reabsorption compared with both base-line values and values for time-control rats. Distal convoluted tubule chloride reabsorption was unchanged during insulin administration. We conclude that 1) euglycemic insulin administration reduces urinary chloride excretion in volume-expanded rats by tubular mechanisms; 2) in superficial nephrons, insulin has no effect on proximal tubule fluid or chloride reabsorption but markedly stimulates chloride reabsorption in the loop; and 3) chloride reabsorption in the distal convoluted tubule was unaltered by insulin administration.