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VASCULAR BIOLOGY

Estrogen and the Ca²⁺-mobilizing agonist ATP evoke acute NO synthesis via distinct pathways in an individual human vascular endothelium-derived cell

¹Smooth Muscle Research Group, Libin Cardiovascular Institute and Department of Pharmacology and Therapeutics; and ²Department of Physiology and Biophysics, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada

Submitted 25 November 2007 ; accepted in final form 25 March 2008

In this study, we have systematically evaluated the signaling mechanisms underlying stimulated nitric oxide (NO) synthesis by estrogen (E₂) and other vasoactive agents at the level of a single endothelium-derived cell. To do so, we have characterized and contrasted rapid E₂-evoked NO synthesis with that of ATP using single-cell microfluorimetry and patch-clamp recordings to monitor stimulated changes in cellular NO synthesis (via 4-amino-5-methylamino-2',7'-difluorofluorescein), Ca²⁺ transients (via Fluo-3), and membrane hyperpolarization in cultured human EA.hy926 cells. E₂-evoked NO synthesis in single cells (EC₅₀ 0.3 nM) was blocked by the E₂ receptor antagonist ICI 182,780 and the NO synthase inhibitor N-nitro-L-arginine methyl ester. Although both E₂ and ATP stimulated comparable Ca²⁺ transients, E₂-induced NO synthesis was insensitive to intracellular BAPTA-AM or removal of external Ca²⁺. In contrast, ATP-evoked NO production was abolished by either one of these treatments. ATP-evoked hyperpolarizations (20 mV) and NO production were both inhibited by the respective small-conductance and intermediate-conductance calcium- activated K⁺ channel blockers apamin and charybdotoxin. E₂ minimally affected membrane potential, and stimulated NO synthesis was insensitive to calcium-activated K⁺ channel blockers. Exposure to either the phosphatidylinositol 3-kinase inhibitor LY-294001 or the MAP kinase inhibitor PD-98059 abolished the NO response to E₂, but not that to ATP. Finally, the NO response evoked by a combined stimulus of E₂ plus ATP was similar to that of ATP alone. In conclusion, our data directly demonstrate that an individual human EA.hy926 cell contains at least two distinct mechanisms for stimulated NO synthesis that depend on either calcium or protein kinase signaling events.

nitric oxide; endothelial function; calcium; signal transduction