Effect of hemodynamic factors on the distribution of anionic groups in the glomerular capillary wall

Using an ultrastructural colloidal iron technique, the distribution of anionic groups was examined in the rat glomerulus following fixation under different hemodynamic conditions. Whereas the glomerular capillary wall showed heavy staining for anionic groups following immersion fixation, superficial glomeruli fixed in situ during good blood flow showed little labeling within and beneath the endothelial fenestrae. This finding was unexpected in view of recent data indicating first that plasma albumin is normally held up at the level of the endothelial fenestrae and second that the presence of intrinsic negative groups in the glomerular capillary wall may be responsible for limiting the passage of polyanionic plasma proteins across the wall. The heavy labeling detected in the region of the endothelial fenestrae in immersion-fixed material is probably an artifact resulting from the clumping of albumin and other plasma polyanions following their diffusion into the glomerular capillary wall as blood flow stops. The epithelial layer and lamina rara externa label most heavily with colloidal iron whatever the method of fixation and may, by imposing a negatively charged field across the width of the basement membrane, play the most significant role in charge-related barrier function.