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Children with MCD, although massively proteinuric, do not have a generalized gbomerular leak to macromolecules ( 1 ). The clearance of neutral macromolecules in MCD is actually less than normal over a range of molecular radii. In contrast, the clearance of anionic macromolecules is significantly increased. This and several other lines of evidence suggest that protein- uria results from a loss of the fixed negative charges of anionic glycosaminoglycans in the glomerular capillary wall (1). The mechanism by which these charges are lost has not been defined. A highly cationic protein in the plasma and urine of children with MCD has been recently described, but the patho- genic significance of this protein has not been determined。 Edema The traditional view has been that massive albuminuria in NS causes a decrease in intravascular oncotic pressure, which allows extravasation of fluid, resulting in hypovolemia, in- creased aldosterone and antidiuretic hormone secretion, and renal salt and water retention. Consistent with this mechanism are the observations that, in MCD of childhood, edema seldom occurs when serum albumin levels are above 2.0 g/dL and the elevated hematocrit, prerenal azotemia, and fluid retention during relapse may be improved by intravenous infusions of salt-poor albumin. Evidence against this model (2) includes the failure of some patients to respond to intravenously administered albumin, the fact that steroid-induced diuresis often occurs before a dra- matic rise in serum albumin concentration, and the observa- tions that plasma volumes are normal to increased and plasma renin and aldosterone levels are decreased in many patients with heavy proteinuria and edema. Finally. patients with con- genital analbuminemia typically have little or no edema. An alternative explanation for retention of salt and water in NS is a decreased GFR, with a decreased filtration fraction. |
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