Chronic endothelin-1 infusion in rats increases glomerular permeability to albumin and proximal tubular albumin uptake as determined by intravital microscopy

David Pollock1, Mohamed Saleh1, Ruben Sandoval2, George Rhodes2, Silvia Campos2, Bruce Molitoris2. 1Georgia Health Sciences University, Augusta, GA 30912, United States, 2Indiana University Purdue University at Indianapolis, Indianapolis, IN 46202, United States.

Animal and human studies demonstrate the ability of ETA receptor blockade to rapidly reduce albuminuria and proteinuria suggesting an active effect of ET-1 to control glomerular permeability. Chronic infusion of endothelin-1 (ET-1) in rats increases glomerular permeability to albumin (Palb) in an ETA dependent manner; however, ET-1 had no effect on albuminuria or arterial pressure. Therefore, we utilized in vivo intravital 2-photon microscopy to determine whether ET-1 increases glomerular albumin filtration are accompanied by an increase in albumin uptake via the proximal tubule (PT). Simonsen Munich-Wistar rats were surgically prepared for in vivo imaging (n = 3). Rats were placed on the microscope stage with the exposed kidney placed in a coverslip-bottomed dish bathed in warm isotonic saline. Serum albumin conjugated to Alexa568 (AA) was acutely infused i.v. to visualize glomeruli and associated S1 segments before and after 2 wk of chronic ET-1 (2 pmol/kg/min; i.v. osmotic minipump). AA was observed to enter capillaries of superficial glomeruli, move into Bowman’s space, bind to the PT cell brush border and reabsorbed across the apical membrane. Glomerular-sieving coefficient (GSC) was calculated as the ratio of AA within the glomerular capillary vs. that in Bowman’s space. GSC was significantly increased in rats following chronic ET-1 infusion (0.039 ± 0.001 vs. 0.028 ± 0.001, p<0.05). Mean fluorescence intensity for AA in PTs was increased by ET-1: 118 ± 6 vs. 76 ± 3 pixel intensity (p<0.05). These findings provide direct in vivo evidence that chronic ET-1 in a non-diabetic rat increases Palb and that albuminuria is prevented by increased PT albumin uptake. (NIH P-30 O’Brien Center for Advanced Renal Microscopy)