Role of inducible nitric oxide synthase in endothelium-independent relaxation to raloxifene in rat aorta

Abstract

Background and Purpose: Raloxifene can induce both endothelium-dependent and -independent relaxation in different arteries. However, the underlying mechanisms by which raloxifene triggers endothelium-independent relaxation are still incompletely understood. The purpose of present study was to examine the roles of NOSs and Ca2+ channels in the relaxant response to raloxifene in the rat isolated, endothelium-denuded aorta. Experimental Approach: Changes in isometric tension, cGMP, nitrite, inducible NOS protein expression and distribution in response to raloxifene in endothelium-denuded aortic rings were studied by organ baths, radioimmunoassay, Griess reaction, western blot and immunohistochemistry respectively. Key Results: Raloxifene reduced the contraction to CaCl2 in a Ca2+-free, high K+-containing solution in intact aortic rings. Raloxifene also acutely relaxed the aorta primarily through an endothelium-independent mechanism involving NO, mostly from inducible NOS (iNOS) in vascular smooth muscle layers. This effect of raloxifene involved the generation of cGMP and nitrite. Also, it was genomic in nature, as it was inhibited by a classical oestrogen receptor antagonist and inhibitors of RNA and protein synthesis. Raloxifene-induced stimulation of iNOS gene expression was partly mediated through activation of the NF-κB pathway. Raloxifene was more potent than 17β-estradiol or tamoxifen at relaxing endothelium-denuded aortic rings by stimulation of iNOS. Conclusions and Implications: Raloxifene-mediated vasorelaxation in rat aorta is independent of a functional endothelium and is mediated by oestrogen receptors and NF-κB. This effect is mainly mediated through an enhanced production of NO, cGMP and nitrite, via the induction of iNOS and inhibition of calcium influx through Ca2+ channels in rat aortic smooth muscle.