Stimulation of ANP by angiotensin-(1-9) via the angiotensin type 2 receptor

Angiotensin-(1-9) [Ang-(1-9)] and Ang-(1-7) are cleaved by Ang converting enzyme 2 forming Ang I and Ang II, respectively, and the truncated Angs play a role in regulating atrial natriuretic peptide (ANP) secretion. Previously, we found that Ang-(1-7) stimulates ANP secretion via the Mas receptor. However, the effect of Ang-(1-9) on ANP secretion is still unknown. The aim of the present study is to determine whether Ang-(1-9) stimulates ANP secretion and to characterize the signaling pathway involved in stimulating secretion. We examined the effects of Ang-(1-9) on ANP secretion and atrial contractility with and without inhibitors in isolated perfused atria. Ang-(1-9) stimulated ANP secretion and concentration without change in atrial contractility. Ang-(1-9)-induced-ANP secretion was increased from 5% to 60% by 3μM Ang-(1-9) during the low-stretch state of the atrium. This stimulatory effect of Ang-(1-9) on ANP secretion was attenuated by pretreatment with an Ang II type 2 receptor (AT2R) antagonist but not by AT1R or Mas receptor antagonist. In addition, pretreatment with inhibitors of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) blocked Ang-(1-9)-induced ANP secretion. In the high-stretch atrial state, Ang-(1-9)-induced ANP secretion was increased more than in the low-stretch state following addition of 1μM Ang-(1-9) (from 108% to 170%). In an in vivo experiment, acute infusion of Ang-(1-9) increased plasma ANP level without altering arterial blood pressure. This effect was attenuated by pretreatment with AT2R antagonist but not by Mas receptor antagonist. These results suggest that Ang-(1-9) stimulates ANP secretion via the AT2R-PI3K-Akt-NO-cGMP pathway.