Renal functional reserve in diabetic patients without clinical nephropathy: comparisons with renal morphology

The acute effect of a protein loading test (1.5 g kg-1) on glomerular filtration rate (GFR) was examined in 10 Type 1 diabetic patients without clinical nephropathy to evaluate the renal functional reserve. GFR was measured before (baseline) and after (test) ingestion of the protein load. Two groups of diabetic patients were distinguished: those who exhibited normal renal reserve equal to that in healthy individuals (change in GFR 35 +/- 17 (+/- SD)%) and those who had no renal reserve (change in GFR -20 +/- 20%). These groups did not differ in the duration and control of diabetes nor in the level of urinary albumin excretion. However baseline GFR was 120 +/- 34 and 209 +/- 46 ml min-1 in diabetic patients with and without renal reserve, respectively. Renal morphology revealed hilar glomerular lesions composed of severely expanded mesangium only in diabetic patients without renal reserve. Minimal structural nephropathy was observed in those who exhibited normal renal reserve. We suggest that the impaired response of GFR to protein loading precedes other subclinical manifestations of renal lesions in diabetic patients.