E Prostanoid (EP) receptors play an important role in urinary Na+ excretion. In the kidney, the epithelial sodium channel (ENaC) is the rate-limiting-step for Na+ reabsorption. We hypothesized that activation of EP1/EP3 regulates the expression of ENaC in the face of renin-angiotensin-aldosterone-system (RAAS) activation. In primary cultures of inner medullary collecting duct (IMCD) cells, sulprostone (EP1 > EP3 agonist, 1 mu M) and 17 Phenyl trinor (17 Pt, EP1 agonist, 10 mu M) prevented the up-regulation of alpha ENaC mRNA induced by aldosterone (10 nM). In Sprague-Dawley rats infused with angiotensin II (0.4 mu g/kg/min), alpha ENaC expression was up-regulated in renal cortex and medulla coincidently with high plasma aldosterone levels. Sulprostone and/or 17 Pt prevented this effect in renal medulla but not in cortex. Immunocytochemistry demonstrated that IMCD cells express EP1. Our results suggest that specific activation of EP1 receptor during RAAS activation antagonizes the action of aldosterone on alpha ENaC expression in the renal medulla. (C) 2009 Elsevier Inc. All rights reserved.