Albuminuria represents excessive macromolecular permeability of the glomerular filtration barrier (GFB) and is a common feature of kidney disease. Angiopoietin-1 (Ang1) is the only podocyte-produced molecule shown to reduce the reflection coefficient (σ) for albumin (σ_alb) of systemic microvessels¹. We hypothesised that Ang1 would decrease glomerular albumin permeability. We previously described a refinement of a technique to measure water permeability (L_pA) in single isolated glomeruli, in which an oncotic pressure gradient drives fluid flux across the GFB, resulting in changes in glomerular volume². We have adapted this technique to measure glomerular σ_alb³. σ describes the fraction of molecules retained by a membrane: for very large molecules that are perfectly retained by the GFB (e.g. >100kDa dextran), σ = 1; for albumin, σ of the GFB is just less than 1. A solution of albumin molecules will therefore exert a lower oncotic pressure across the GFB than an equivalent solution of dextran molecules. The ratio of these effective oncotic pressures is used to calculate σ_alb. 250kDa dextran was dialysed across a 100kDa membrane to eliminate smaller fragments that can cross the GFB. We created BSA and dextran solutions iso-oncotic to mouse plasma [π=32.2cm H₂O]⁴. Kidneys were extracted from humanely sacrificed Tie-2GFP mice. Glomeruli were isolated by flushing renal cortical tissue through graded metal sieves. Glomeruli were aspirated onto a micropipette within a flow-controlled observation chamber, then perifused with BSA and examined under static conditions for 2min. Perifusate was then switched to dextran. Glomerular volume during each perifusate incubation period (dextran: V_dex; albumin: V_alb) was recorded on video, then calculated using ImageJ and Adobe Photoshop software. The ratio V_dex/V_alb describes σ_alb. Paired measurements of σ_alb were obtained in glomeruli before and after exposure to either 200ng/ml Ang1 or control solution for 1 hour. Whereas treatment of glomeruli with control solution did not alter σ_alb [from 0.9608 to 0.9639, p>0.35, paired t test, n=15], treatment with Ang1 significantly increased σ_alb [from 0.9725 to 0.9804, p<0.05, paired t test, n=15].

These results indicate a reduction in the macromolecular permeability of the GFB after treatment with Ang1 and support further research with Ang1 as a potential novel therapy in kidney disease.