Effect And The Mechanisms Of Endothelin-1on ANP Secretion In Beating Rabbit Atria

Atrial natriuretic peptide (ANP) is discovered in the1981and has been shown to regulate blood pressure as well as blood volume. Sequently, the members in the family of natriuretic peptides (NPs) have been discovered by one after another, including brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). ANP is synthesized and secreted in the atrial myocytes. Natriuretic peptide receptors (NPR) have been classified in three subtypes including NPR-A, NPR-B and NPR-C. ANP or BNP is mainly binding to the NPR-A and CNP is mainly binding to the NPR-B. NPR-A and NPR-B exhibit intrinsic GC activity and mediate many of the well-known biological functions of NPs. NPR-C (clearance receptor subtype) has no GC activity but binds and internalizes all kinds of endogenous NPs for clearance. ANP has been shown to participate in natriuresis, vascular relaxation, blood pressure regulation, lipolysis, antiproliferation of cells, anti-fibrosis, antioxygen, and cardiac protection. Many factors are closely related to the regulation of atrial ANP secretion, such as atrial dynamics, humoral and neural factors. However, atrial stretch is the critical factor in regulation of ANP secretion. There are contradictory reports in the regulation of Ca2+on atrial ANP secretion.Endothelin (ET), which is discovered by Yanagisawa in the1988, is a potent vasoactive peptide of21amino acids that was originally isolated from a culture medium of porcine aortic endothelial cells. There are three isomers of endothelin (ET) such as ET-1, ET-2and ET-3. ET receptors have been identified of two seven-transmembrane G protein-coupled endothelin receptors, endothelin A (ETRA) and endothelin B (ETRB) receptor. In endothelial cells, ETRA has been shown to induce vascular contraction and ETRB causes vasorelaxation via nitric oxide production. It is demonstrated that both of ETRA and ETRB mRNA are distributed in the heart.It is generally believed that Ca2+is an important factor in ET-1-dependent ANP secretion. There is report indicated that L-type Ca2+channel blocker inhibits ET-1-increased ANP secretion. In addition, ET receptors are functionally coupled to phospholipase C (PLC). ET binds to the receptors leading to increased phosphatidylinositol4,5-bisphosphate hydrolysis, which in turn promotes both inositol1,4,5-trisphosphate-mediated Ca2+mobilization and the diacylglycerol-dependent activation of protein kinase C (PKC). In the near future, it is reported that a protein tyrosine kinase antagonist genistein may inhibits ET-1-induced positive inotropic effects. Furthermore, it is demonstrated that the detailed molecular mechanisms of endothelin are correlated with protein tyrosine kinases (PTKs) activity and phosphoinositide-3-kinase (PI3K) signaling pathway. However, the effects of PTKs and PI3K signaling pathway on the regulation of ET-1-increased atrial ANP secretion are not well known.Therefore, the purpose of the present study is to investigate the effect of Ca2+, PLC, PKC, PTKs, and PI3K signaling pathway on the regulation of ET-1-induced ANP secretion in isolated perfuased rabbit atria.The data of the present study is showed that,1. ET-1(3.0nmol/L,30.0nmol/L,300.0nmol/L) significantly increased atrial ANP secretion and atrial stroke volume by dose-dependent manner (P<0.001vs control period respectively).2. An ETRA blocker BQ123(0.3μmol/L) partially attenuated ET-1-increased ANP secretion but higher than control levels (P<0.01vs control period). An ETRB antagonist BQ788(0.3μmol/L) completely blocked the effect of ET-1-increased atrial ANP secretion (P>0.05vs control period).3. Nifedipine (1.0μmol/L), an inhibitor of L-type Ca2+channel, slightly attenuated the effect of ET-1-increased ANP secretion only (P<0.001vs control period), while atrial stroke volume is inhibited by nifedipine (P<0.001vs control period).4. A PKC antagonist chelerythrine (10.0μmol/L) is also slightly attenuated the effect of ET-1-increased ANP secretion only (P<0.01vs control period), while atrial stroke volume is increased by chelerythrine plus ET-1(P<0.001vs control period).5. U73122(10.0μmol/L), an inhibitor of PLC, accentuated the effect of ET-1-increased ANP secretion and the atrial stroke volume (P<0.001vs control period respectively).6. Genistein (30.0μmol/L), an inhibitor of PTKs, completely blocked the effect of ET-1-increased ANP secretion (P>0.05vs control) without changes in atrial stroke volume (P>0.05vs control).7. Pretreatment with LY294002(30.0μmol/L), an antagonist of PI3K, genistein (30.0μmol/L) plus ET-lleading to an increase in ANP secretion (increased about57%) and in atrial stroke volume (increased about50%)(P<0.01vs control respectively).8. Pretreatment with LY294002plus genistein, an inhibitor of cyclooxygenase indomethacin (30.0umol/L) completely blocked the effect of ET-1-induced ANP secretion (P<0.05vs control) and atrial stroke volume (P<0.05vs control).The results of the present study are indicated that,1.ET-1increases ANP secretion mainly via the ETRB-mediated pathway and the ETRA is in partially involved in it.2.Protein tyrosine kinases or PI3K signaling pathway may play an important role in the regulation of ET-1-induced atrial ANP secretion.3.Cyclooxygenase may participate in the regulation of ET-1-induced ANP secretion when protein tyrosine kinases as well as PI3K have been inhibited in beating rabbit atria.