Measurement of albumin reflection coefficient with isolated rat glomeruli

Macromolecular permeability of the glomerular capillary has been inferred from the clearance of endogenous protein or infused macromolecules. Permeability is increased after treatment with polycations as well as after renal injury. It has previously been shown that the capillaries of glomeruli isolated from normal mammals expand or collapse in response to transcapillary albumin gradients and that the magnitude of the changes in capillary volume and in total glomerular volume are directly proportional to the applied oncotic gradients. In the experiments presented here, the volume responses of control glomeruli and of glomeruli treated with protamine (100 to 600 μg|mL for up to 60 min) were used to calculate the albumin reflection coefficient, σ_albumin, and the convectional permeability, P_{COnvectional albumin} = (1 - σ_albumin), of the capillary wall. σ_albumin for normal glomeruli was about 1 (P_{convectional albumin} = 0); σ_albumin fell to a minimum of 0.2 ± 0.1 (P_{convectional albumin} = 0.8 ± 0.1) after incubation with protamine sulfate (600 μg/ mL) for 30 min. Retraction and fusion of podocyte foot processes and denudation of the underlying matrix was seen on scanning electron micrographs of protamine-treated glomeruli. These results confirm that it is possible to study macromolecular permeability of the glomerular capillary in vitro and to calculate albumin independent of hemodynamic and systemic humoral influences. This method will permit the assessment of the effects of individual mediators of glomerular injury and the study of glomeruli from kidneys affected by experimentlaly induced or naturally occurring renal diseases.