[Regulation of natriuresis in diabetic nephropathy]

The aim of this study was to determine the influence of metabolic control of diabetes on natriuresis, the effect of natriuretic peptides and renal kallikrein on the kidney and the participation of proximal and distal tubules in natriuresis. The study was done in 41 individuals: 27 IDDM patients and 14 healthy controls. The patients were on insulin only, had normal blood pressure, and were prescribed a standard diabetic diet without sodium or protein restriction. Diabetic patients were assigned to subgroups, depending on the stage of nephropathy and level of metabolic control. Urine collection was done three times daily in all participants. The first collection was done after 500 mg lithium carbonate (p.o.) and was followed by 10 mg amilorid (Midamor, Thomas Morson Pharmaceuticals). The third collection of urine was used to evaluate excretion of cGMP. In addition to sodium, lithium, potassium and creatinine clearances, excretion of renal kallikrein, and levels of microalbuminuria, fructosamine and glycated hemoglobin were also determined. Lithium clearance was used to evaluate tubular sodium transport. The influence of diuretic peptides--ANP and urodilatin, on natriuresis was reflected by urinary cGMP excretion. Function of the kallikrein-kinin system was studied on the basis of excretion of kallikrein. Amilorid was used to test the effect of blocking amiloride-sensitive sodium channels in distal tubules on natriuresis (Tab. 1). A statistically significant decrease in mean lithium clearance was observed in IDDM patients as compared to healthy controls. Creatinine clearance was the same in both groups (Tab. 2). Lower lithium clearance was observed in the subgroup of diabetic patients with "silent" nephropathy. Diabetic patients with "silent" and early nephropathy had significantly higher levels of fractional sodium reabsorption in the proximal tubule when compared with controls (Tab. 3). Moreover, lower daily excretion of kallikrein was observed in patients with stage II nephropathy in comparison to the control group (Tab. 4). Amilorid uptake had no influence on urinary kallikrein. However, natriuresis after amilorid was significantly higher in diabetic patients than in controls. In conclusion, reabsorption in the proximal tubule is increased in patients with "silent" diabetic nephropathy, as revealed by decreased lithium clearance and unchanged creatinine clearance. Hyperactivity of the proximal tubule in stage II and III of diabetic nephropathy results in increased sodium reabsorption in the proximal tubule, as reflected by the increase in fractional sodium reabsorption in this tubule. Amilorid, a distal tubule blocker, reduces distal tubule activity independently of urinary kallikrein excretion. Elevated natriuresis was observed after amilorid without any change in urinary kallikrein excretion.