Tamoxifen dilates porcine coronary arteries: roles for nitric oxide and ouabain-sensitive mechanisms

Background and purpose: Experiments were designed to determine the mechanism of the relaxation induced by tamoxifen in porcine coronary arteries at the tissue, cellular and molecular levels. Experimental approach: Porcine left circumflex coronary arteries were isolated and isometric tension was measured. [Ca(2+)](i) in native endothelial cells of intact arteries was determined by a calcium fluorescence imaging technique and eNOS ser 1177 phosphorylation was assayed by Western blotting. Key results: Tamoxifen induced an endothelium-dependent relaxation that was antagonized by ICI 182,780 and abolished by N(G)-nitro-L-arginine methyl ester (L-NAME) or 1H-[1,2,4] oxadizolo[4,3-] quinoxalin-1-one ( ODQ). L-Arginine reversed the effect of L-NAME while indomethacin was without effect. Tamoxifen-induced relaxation was attenuated by charybdotoxin (CTX) plus apamin, ouabain or by incubation in a K(+)-free solution. Moreover, tamoxifen triggered extracellular Ca(2+) dependent increases in endothelial [Ca(2+)](i) and this effect was abolished by ICI 182,780. Endothelium-independent relaxation to sodium nitroprusside was also inhibited by ouabain or in a K(+)-free solution. Furthermore, tamoxifen increased endothelial nitric oxide synthase (eNOS) phosphorylation at Ser-1177 and ICI 182,780 prevented this effect. Conclusions and Implications: The present results suggest that tamoxifen mainly induces endothelium-dependent relaxation and that endothelial nitric oxide (NO) is the primary mediator of this effect. NO-dependent responses may result from elevated [Ca(2+)](i) in endothelial cells; an effect abolished by ICI 182,780. NO activates Na(+)/K(+)-ATPase in vascular smooth muscle, leading to relaxation. These results suggest that tamoxifen is able to modulate eNOS phosphorylation directly.