eNOS translocation but not eNOS phosphorylation is dependent on intracellular Ca2+ in human atrial myocardium

In endothelial cells, two ways of endothelial nitric oxide (NO) synthase (eNOS) activation are known: 1) translocation and 2) Akt- dependent phosphorylation of the enzyme at Ser1177 (Ser1177 eNOS). We have recently shown that agonist-induced Ser1177 eNOS phosphorylation also occurs in human myocardium (10). In this study, we investigated the Ca2+ dependency of these two mechanisms in human atrium. Therefore, atrial tissue was obtained from patients who underwent coronary artery bypass operations. In immunohistochemical experiments, the translocated form of eNOS and phosphorylated Ser1177 eNOS were labeled using specific antibodies. eNOS translocation was measured in the absence and presence of the Ca2+ chelator BAPTA before and after application of BRL 37344 ( BRL), a beta(3)-adrenoceptor agonist that increases eNOS activity ( 34). In the absence of BAPTA, BRL time dependently increased the staining intensity of translocated eNOS, whereas in the presence of BAPTA, this effect was blunted. In contrast, BRL clearly increased the staining of phosphorylated Ser1177 eNOS even in the presence of BAPTA. This observation was confirmed using Western blot analysis. Using the NO-sensitive dye diaminofluorescein, we have demonstrated that BRL induced a strong NO release. This effect was completely abolished in the presence of BAPTA but was unaffected by LY-292004, an inhibitor of phosphatidylinositol 3-kinase activity and eNOS phosphorylation. Although Ca2+ dependent, neither the translocation of eNOS nor NO release was changed by the adenylate cyclase activator forskolin. In conclusion, 1) in human atrial myocardium, BRL-induced eNOS translocation but not Ser1177 eNOS phosphorylation is dependent on intracellular Ca2+. 2) In atrial myocardium, eNOS-translocation and not Ser1177 eNOS phosphorylation is responsible for generating the main amount of NO. 3) Although Ca2+ dependent, eNOS translocation and NO release could not be mimicked by adenylate cyclase activation as a mediator of beta-adrenergic stimulation.