1. To elucidate the mechanisms by which cyclosporin A diminishes proteinuria, we studied 20 patients with severe nephrotic syndrome. Biopsy-established pathologies included minimal change disease (n = 5), membranous glomerulopathy (n = 6), membranoproliferative glomerulonephritis (n = 5) and focal segmental glomerulosclerosis (n = 4). Before, at the end of a 90 day course of cyclosporin A, and finally 1 month after stopping cyclosporin A we determined 24 h protein excretion. Measurements of glomerular filtration rate, effective renal plasma flow, fractional clearance rates of albumin and immunoglobulins with different charges and the transglomerular sieving of uncharged dextrans of broad size distribution were used to study the effects of cyclosporin A on renal perfusion and the glomerular filtration barrier. The findings were analysed with a theoretical model of solute transport.

2. Among the different forms of glomerulopathy the response to low-dose cyclosporin A (trough levels 32.0–36.9 ng/ml) varied markedly. In minimal change disease, proteinuria decreased from 9.5 ± 3.1 to 1.3 ± 0.2 g/24 h (mean ± sem, P < 0.01). This response was due to restoration of the charge selectivity of the glomerular barrier. The depressed value of the glomerular permeability coefficient also returned to normal. Glomerular filtration rate, effective renal plasma flow and renal vascular resistance did not change. Proteinuria returned after stopping cyclosporin A, although it did not reach pretreatment levels. In membranous glomerulopathy, proteinuria fell from 9.9 ± 1.5 to 1.8 ± 0.3 g/24 h (P < 0.01). Changes in protein excretion and dextran sieving were compatible with an increase in glomerular permselectivity and a decrease in filtrate flow through the ‘shunt’ pathway. Glomerular filtration rate was maintained, although effective renal plasma flow fell significantly. Proteinuria relapsed after stopping cyclosporin A. In membranoproliferative glomerulonephritis and focal segmental glomerulosclerosis proteinuria did not respond to cyclosporin A, although cyclosporin A exerted important haemodynamic effects.

3. In minimal change disease and membranous glomerulopathy cyclosporin A exerts its beneficial effects on proteinuria through changes in the properties of the glomerular barrier, resulting in increased charge and size selectivity, respectively.

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