EFFECT OF INSULIN ON SYSTEMIC AND RENAL HANDLING OF ALBUMIN IN NONDIABETIC AND NIDDM SUBJECTS

Insulin resistance and hyperinsulinemia cluster with microalbuminuria in both diabetic and nondiabetic subjects, but the mechanism underlyin g this association is unknown. To test the hypothesis that insulin inf luences protein permeability, we measured the albumin transcapillary e scape rate (TER) by the I-131-labeled albumin technique in 12 healthy volunteers and 12 normoalbuminuric NIDDM patients (fasting plasma gluc ose, 10.9 +/- 1.3 mmol/l) during 4 h of isoglycemia with high (1.1 mU . min(-1) . kg(-1)) or, on a different day, low (0.1 mU . min(-1) . kg (-1)) insulin infusion. In both patients and control subjects, high in sulin was associated with a 7% decrease in blood volume (P = 0.006) an d a 6% decrease in diastolic blood pressure (P < 0.02), these two chan ges being related to one another (r = 0.56, P < 0.01). Basal albumin T ER was similar in patients (8.4 +/- 0.5% . h(-1)) and control subjects (7.7 +/- 0.7% . h(-1)) and was not significantly changed by high insu lin in either group (patients vs. control subjects, 7.3 +/- 0.9 vs. 6. 2 +/- 0.4% . h(-1); NS vs. low insulin). In contrast, high insulin inc reased renal albumin excretion (from 3.6 +/- 0.8 to 5.4 +/- 1.1 pg/min , P < 0.01) and clearance rate (0.09 +/- 0.02 to 0.13 +/- 0.03 mu l/mi n, P < 0.001) in patients but not in control subjects. To localize the effect of insulin along the nephron, we measured the urinary excretio n of N-acetyl-beta-D-glucosaminidase (beta-NAG), released by the proxi mal tubule; retinol-binding protein (RBP), reabsorbed by the proximal tubule; and Tamm-Horsfall protein (THP) and epidermal growth factor (E GF), both secreted by the distal tubule. For both beta-NAG and RBP, bu t not EGF or THP, insulin enhanced urinary excretion (diabetics vs. co ntrols: beta-NAG, 0.48 vs. -0.15 mu U/min [P = 0.03]; BBP, 78 vs. -32 ng/min [P = 0.05]). In conclusion, physiological hyperinsulinemia does not affect systemic albumin permeability in healthy subjects or normo albuminuric NIDDM patients. In contrast, in NIDDM patients, but not in healthy subjects, insulin increases the urinary excretion of albumin and protein markers of proximal tubular function. The significance of this finding for the pathogenesis of diabetic nephropathy remains to b e established.