Assay Limitations Of Osmotic Reflection Coefficient Of Isolated Mouse Glomeruli Ex Vivo

Glomerular reflection coefficient (σ) to albumin (σ alb ) represents glomerular filtration barrier (GFB) integrity. For molecules perfectly retained (e.g. >100kDa dextran), σ=1. If σ alb <1, an albumin solution exerts lower trans‐GFB oncotic pressure (π) than iso‐oncotic dextran solution. We refined and critiqued an assay calculating σ alb from the ratio of these effective π, indicated by relative glomerular volumes in each solution. We created a BSA solution iso‐oncotic to purified >100kDa dextran solution. Glomeruli were sieved from C57bl6 mouse kidney and perifused with BSA in a flow‐controlled chamber, then perifusate was switched to dextran. Volume was calculated from video stills of glomeruli during zero‐flow for each perifusate (dextran: V dex ; BSA: V BSA ). V dex /V BSA was taken to describe σ alb . Surprisingly, pronase treatment (0.1mg/ml, 2min) did not change σ alb [p=0.97, n=7]. V dex /V BSA =σ alb assumes V represents glomerular capillary volume, but we are limited to calculation of total glomerular volume. Unknown change of non‐capillary glomerular volume fractions confounds calculation of σ alb . V dex /V BSA =σ alb also assumes capillary protein retention. Leakage during lowered σ alters effective to an unknown extent, obscuring the relation of π to V. V dex /V BSA =σ alb is invalid in this case. Data must be interpreted cautiously in the case of increased σ. Kidney Research UK, British Heart Foundation