NATURE OF THE GLOMERULAR CAPILLARY INJURY IN HUMAN MEMBRANOUS GLOMERULOPATHY

A differential solute clearance technique was used to evaluate glomerular capillary wall function in 20 patients with membranous glomerulopathy and massive proteinuria. The clearance of inulin, the filtration fraction, and the fractional clearance of uncharged dextrans of a radius of 28-48 .ANG. were depressed significantly below control values in 20 healthy volunteers (P < 0.01). In contrast, the fractional clearance of dextrans of radius > 50 .ANG. was elevated markedly. A theoretical model of solute transport that depicts the major portion of the glomerular capillary wall as an isoporous membrane and the minor portion as a nondiscriminatory shunt pathway revealed the calculated glomerular ultrafiltration coefficient to be five times lower and mean pore radius of the major membrane component to be 4 .ANG. smaller than control values. However, the fraction of filtrate volume permeating the shunt pathway was three- to fourfold above control values and correlated strongly in individual patients with the fractional clearance of albumin (r = 0.76) and of IgG (r = 0.80). Lowering renal plasma flow by 24% during indomethacin therapy in seven patients resulted in a 74% reduction in proteinuria accompanied by a corresponding diminution of filtrate formed through the shunt pathway. Morphometric analysis of glomerular ultrastructure revealed the magnitude of depression of the glomerular filtration rate and of urinary protein leakage to be related strongly to changes in the epithelial layer of the glomerular capillary wall, but not to the density of subepithelial immune deposits. We conclude (a) that glomerular capillaries in membranous glomerulopathy are characterized by a loss of ultrafiltration capacity and of barrier size-selectivity, and (b) that subepithelial immune deposits do not provide a structural basis for these functional alterations.