Renal functional reserve in patients with IgA glomerulopathy

Seven patients with histologically proven IgA nephropathy and modest impairment of renal function, and 2 patients with IgA nephropathy and nephrotic syndrome were investigated, compared to a control group of 9 healthy individuals, to study the effects of amino acids on glomerular and tubular function, and to evaluate renal functional reserve in IgA nephropathy with different clinical course. Inulin and PAH clearances were used to evaluate glomerular filtration rate (GFR) and effective renal plasma flow (ERPF); proximal and distal tubular fluid delivery and reabsorption were measured by lithium clearance, before and after submission of a standardized amino acid solution. GFR and ERPF increased significantly during amino acid load in healthy individuals and patients without nephrotic syndrome, while filtration fraction (GFR/ERPF) remained constant. Lithium clearance (CLi) and fractional lithium excretion (CLi/GFR) rose significantly in both groups, whereas the reabsorbed volume of fluid in the proximal tubule did not change. In the distal tubule, fractional volume excretion decreased significantly during amino acid load whereas the reabsorbed volume significantly increased. Baseline values of the two groups did not differ significantly. Two patients with nephrotic course of IgA nephropathy showed a distinct decrease in glomerular and tubular function, and a loss of renal functional reserve after amino acid load.

Despite distinct alterations in renal biopsy, IgA nephropathy without nephrotic course presents with a still adequately preserved kidney function and renal functional reserve. A single determination of renal function with noninvasive functional tests does not give valid prognostic information concerning glomerular and tubular function. Therefore, a repeated measurement of renal function at defined intervals might reveal clinical progression of renal disease. The results of the lithium clearance might indicate an increase in tubular function after amino acid load, indicating a tubular adaptation in state of hyperfiltration.