- / Background: Endothelin-1 (ET-1), a strong vasoconstrictive substance acting via stimulation of specific receptors (ET-A and ET-B), has been implicated in the development of renal scarring. Activation of endothelin system was observed in experimental models of glomerular diseases and this was attributed to the toxic action of proteinuria to the tubular epithelial cells. However, we have not enough information about the role of endothelin system in human glomerular diseases and in renal diseases without proteinuria like obstructive nephropathy. The aim of this study was to examine the endothelin system in patients with primary glomerular diseases and in experimental animals with unilateral ureteric obstruction. Patients and Methods: Thirty-seven patients with different types of primary glomerulonephritides and 14 controls were included in the study. Patients presented by either nephrotic syndrome (n=25) or mild proteinuria (<1g/24h, n=12). The expression of ET-A and ET-B receptors in the renal tissue was examined immunohistochemically. At the time of biopsy, urinary ET-1 was determined by RIA. Experimental animals and Methods: Twenty –day old opossum pups (n=6) underwent surgical ligation of the left ureter. Sham operated animals, non-operated controls and normal human kidneys were also used. Animals were sacrificed at 2 (n=2), 3 (n=1), 4 (n=1), 5(n=1) and 8 (n=1) weeks post surgery and their kidneys were examined. Sham operation was performed at equivalent times in pups that served as control. The expression of ET-A and ET-B receptors in the renal tissue was examined immunohistochemically. Results: The expression of both receptors was mainly localized within tubular epithelial cells and was significantly higher in patients with glomerulonephritides compared to controls. The expression of ET-B receptors was higher in nephrotic compared to non-nephrotic patients while no difference was observed in the expression of ET-A receptors. Urinary excretion of ET-1 was increased in patients compared to healthy subjects (579±146 ng/24h vs. 410±78 ng/24h, p<0.01) and it was higher in nephrotic compared to non-nephrotic patients (617±167 ng/24h vs. 485±71 ng/24h, p<0.05). A significant positive correlation of the excreted ET-1 with the degree of proteinuria (r= 0.338, p<0.05) and the extent of immunostaining for ET-B receptors (r=0.427, p<0.05) was observed. The expression of ET-B receptors and the excretion of ET-1 were significantly decreased in patients who present remission of the nephrotic syndrome under immunosuppressive therapy. In tubular epithelial cells of the experimental animals there was a temporal increase in the expression of ET-A receptors with duration of obstruction while there was no significant difference between the expression of ET-B receptors in obstructed kidneys and controls. Conclusions: this study provides evidence that the endothelin system is activated in renal diseases and proteinuria seems to be related only in part to this activation. Further investigation is needed to ascertain if the activation of endothelin system has a causative role in the progression of renal diseases.