Role of insulin and atrial natriuretic peptide in sodium retention in insulin-treated IDDM patients during isotonic volume expansion

Because insulin shows an antinatriuretic effect in healthy humans, insulin therapy resulting in circulating hyperinsulinemia may lead to sodium retention and in turn to hypertension in individuals with insulin-dependent diabetes mellitus (IDDM). Moreover, it has been proved that atrial natriuretic peptide (ANP) plays a major role in modulating natriuresis in humans. This study investigated the relationship between insulin and ANP in modulating sodium metabolism in normotensive and hypertensive IDDM subjects compared with control groups of normotensive and hypertensive nondiabetic subjects. IDDM normotensive and hypertensive subjects had mean ± SE duration of IDDM of 7 ± 2 and 8 ± 2 yr, respectively, and had no clinical features of diabetic nephropathy. All subjects received a saline infusion (2 mmol · kg−1 · 90 min−1) during euglycemia. IDDM normotensive and hypertensive subjects received a subcutaneous insulin infusion (15 mU · kg−1 · h−1), resulting in twofold higher plasma free-insulin levels (16 ± 2 and 19 ± 3 μU/ml, respectively) than in nondiabetic normotensive and hypertensive subjects (7 ± 2 and 8 ± 2 μU/ml, respectively). During saline challenge, sodium excretion increased by 22 ± 4% in normotensive and 49 ± 9% in hypertensive nondiabetic subjects but by only 11 ± 0.4% in normotensive (P < 0.01) and 8 ± 2% in hypertensive (P < 0.01) IDDM subjects. The impaired natriuretic response to saline challenge was mainly due to greater rates of sodium reabsorption by kidney proximal tubules in IDDM than nondiabetic subjects. At baseline, plasma ANP concentrations were significantly higher in both IDDM groups than in control groups (normotensive IDDM and control subjects: 38 ± 4 and 19 ± 2 pg/ml, respectively, P < 0.01 ; hypertensive IDDM and control subjects: 45 ± 6 and 27 ± 4 pg/ml, respectively, P < 0.05). After saline challenge, ANP concentrations rose to 39 ± 4 pg/ml in normotensive and 49 ± 5 pg/ml in hypertensive control subjects, whereas no significant change above baseline value was seen in IDDM subjects. Both IDDM groups showed a 10–12% greater exchangeable Na+pool than control subjects regardless of the presence of hypertension. Subcutaneous insulin infusion, resulting in circulating plasma free-insulin levels in normotensive control subjects comparable to those in IDDM patients, inhibited natriuresis, increased proximal tubule sodium reabsorption at the level of the kidney, and inhibited an adequate ANP stimulation by saline challenge. We conclude that hyperinsulinemia leads to increased proximal tubule sodium reabsorption and impaired ANP response during saline administration. Both mechanisms account for sodium retention in normotensive and hypertensive IDDM patients. Sodium retention seems to characterize insulin-treated diabetes regardless of the presence of hypertension, suggesting that insulin-induced expansion of the Na+ pool alone cannot produce hypertension and requires the concomitant interaction of other pathogenetic factors.