Sivritas S-H, Ploth DW, Fitzgibbon WR. Blockade of renal medullary bradykinin B2 receptors increases tubular sodium reabsorption in rats fed a normal-salt diet. Am J Physiol Renal Physiol 295: F811-F817, 2008. First published July 16, 2008 doi:10.1152/ajprenal.90225.2008.-The present study was performed to test the hypothesis that under normal physiological conditions and/or during augmentation of kinin levels, intrarenal kinins act on medullary bradykinin B2 (BKB2) receptors to acutely increase papillary blood flow (PBF) and therefore Na ϩ excretion. We determined the effect of acute inner medullary interstitial (IMI) BKB2 receptor blockade on renal hemodynamics and excretory function in rats fed either a normal (0.23%)-or a low (0.08%)-NaCl diet. For each NaCl diet, two groups of rats were studied. Baseline renal hemodynamic and excretory function were determined during IMI infusion of 0.9% NaCl into the left kidney. The infusion was then either changed to HOE-140 (100 g ⅐ kg Ϫ1 ⅐ h Ϫ1 , treated group) or maintained with 0.9% NaCl (time control group), and the parameters were again determined. In rats fed a normal-salt diet, HOE-140 infusion decreased left kidney Na ϩ excretion (urinary Na ϩ extraction rate) and fractional Na ϩ excretion by 40 Ϯ 5% and 40 Ϯ 4%, respectively (P Ͻ 0.01), but did not alter glomerular filtration rate, inner medullary blood flow (PBF), or cortical blood flow. In rats fed a low-salt diet, HOE-140 infusion did not alter renal regional hemodynamics or excretory function. We conclude that in rats fed a normal-salt diet, kinins act tonically via medullary BKB2 receptors to increase Na ϩ excretion independent of changes in inner medullary blood flow.inner medullary collecting duct; inner medullary blood flow; renal hemodynamics; renal excretion THE TISSUE KALLIKREIN-KININ-KININASE cascade is a complex enzymatic pathway for the generation of kinin peptides (4). Two of these peptides, bradykinin and lys-bradykinin (kallidin), induce local vasodilation and inhibit renal electrolyte reabsorption by acting as paracrine/autocrine ligands for G proteincoupled bradykinin B 2 (BKB 2 ) receptors (see reviews in Refs. 4 and 18).The kallikrein-kinin systems in brain, kidney, and cardiovascular tissue contribute to the regulation of cardiovascular function and blood pressure (3,5,18,22). However, the renal kallikrein-kinin system (through its action to increase electrolyte and water excretion) plays a primary role in the long-term regulation of blood pressure under conditions of hypertensive insult, especially high salt intake (1,17,19,20,26,41).Paradoxically, although renal kinins are natriuretic, high salt intake suppresses the renal kallikrein-kinin system (28, 44). This suppression is rapid. Cortical and medullary interstitial levels of kinins are markedly reduced within 24 h of exposure to a high-salt diet (36). Conversely, cortical interstitial levels of bradykinin and the activity of the renal kallikrein-kinin system are markedly augmented by low salt intake (28,36,44). This augmentation of t...