We measured glomerular ultrafiltration coefficient (Kf) of isolated superficial (S) and deep (D) glomeruli of normovolemic and volume-depleted rats. Filtration was induced in vitro, and Kf was calculated from the maximum rate of change in glomerular size. Basement membrane area (A) for each glomerulus was estimated from morphometric analyses, and glomerular capillary hydraulic conductivity (Lp) was calculated by the formula Lp = Kf/A. Kf of S and D glomeruli of normovolemic rats were 2.98 +/- 0.98 and 4.25 +/- 0.07 nl . min-1 . mmHg-1, respectively. In hypovolemic rats, Kf of S glomeruli fell by approximately 50% to 1.52 +/- 0.14 nl . min-1 . mmHg-1 (P less than 0.001), whereas Kf of D glomeruli remained unchanged at 4.28 +/- 0.10 nl . min-1 . mmHg-1. Lp, calculated using the peripheral capillary area, averaged 1.98 +/- 0.09 and 1.98 +/- 0.06 microliter . min-1 . mmHg-1 . cm-2 in S and D glomeruli of normovolemic rats and 1.89 +/- 0.11 microliter . min-1 . mmHg-1 . cm-2 in D glomeruli of hypovolemic rats. Lp of S glomeruli of volume-depleted rats (0.90 +/- 0.03 microliter . min-1 . mmHg-1 . cm-2) was lower than in any of the other three samples. Mild hypovolemia causes the Kf of S glomeruli to decline, whereas Kf of D glomeruli remains constant. The decrease in Kf occurs without an alteration in capillary area and is most likely due to a decrease in Lp.
Ultrafiltration coefficient (Kf) was measured in vitro using glomeruli isolated from 28 human renal biopsies in order to asses both the relationship between glomerular structure and filtration characteristics and the relationship between Kf and patients' clinical state. The patients, ages 1 to 72 years, had a wide variety of renal diseases and serum creatinines of 1 to 16 mg/dl. Glomeruli were examined by light, immunofluorescence and electron microscopy, and glomerular alterations were measured. Filtration was induced in isolated glomeruli by an oncotic gradient and Kf calculated. Glomerular diameter (D), averaged for each patient, varied from 131 to 315 microns, and Kf varied from 5.7 to 51 nl/min mm Hg. Hydraulic conductivity (Lp) in 15 biopsies averaged 1.45 mu.min-1.mm Hg-1.cm-2. In order to identify the significant predictors of Kf and delineate their relationships, stepwise multiple regression analysis was performed. Kf increased with increasing glomerular size and with increasing degree of glomerular hypercellularity. D, in turn, increased with body surface area, urinary protein, and degree of capillary damage, and decreased with percent senescent glomeruli and degree of epithelial foot process broadening. Kf did not significantly correlate with clinical measures of renal function.
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