Abstract
Endothelial beta(2)-adrenoceptor (beta(2)AR) stimulation increases nitric oxide (NO) generation, but the underlying cellular mechanisms are unclear. We examined the role of l-arginine transport and of phosphorylation of NO synthase 3 (NOS-3) in beta(2)AR-mediated NO biosynthesis by human umbilical vein endothelial cells (HUVEC). To this end, we assessed l-arginine uptake, NOS activity (from l-arginine to l-citrulline conversion), membrane potential (using [(3)H]tetraphenylphosphonium), as well as serine phosphorylation of NOS-3 (by Western blotting and mass spectrometry), in HUVEC treated with betaAR agonists or cyclic AMP-elevating agents. beta(2)AR stimulation increased l-arginine transport, as did cyclic AMP elevation with either forskolin or dibutyryl cyclic AMP, and this increase was inhibitable by N-ethylmaleimide. Blockade of l-arginine uptake by l-lysine inhibited NOS activity and, conversely, blockade of NOS using N(omega)-nitro-l-arginine methyl ester (l-NAME) inhibited l-arginine transport. beta(2)AR stimulation also caused a membrane hyperpolarization inhibitable by l-NAME, suggesting that the increase in l-arginine uptake occurred in response to NO-mediated hyperpolarization. beta(2)AR activation also increased NOS activity and phosphorylation of NOS-3 on serine-1177, and these increases were attenuated by inhibition of protein kinase A (PKA), phosphatidylinositol 3-kinase (PI3K) or Akt, and abolished by coinhibition of PKA and Akt. These findings suggest that beta(2)AR-mediated NOS-3 activation in HUVEC is mediated through phosphorylation of NOS-3 on serine-1177 through both the PKA and the PI3K/Akt systems, and is sustained by an increase in l-arginine uptake resulting from NO-mediated membrane hyperpolarization.