Mechanisms of reduced nitric oxide/cGMP-mediated vasorelaxation in transgenic mice overexpressing endothelial nitric oxide synthase

NO, constitutively produced by endothelial NO synthase (eNOS), plays a key regulatory role in vascular wall homeostasis. We generated transgenic (Tg) mice overexpressing eNOS in the endothelium and reported the presence of re duced NO-elicited relaxation. The purpose of this study was to clarify mech anisms of the reduced response to NO-mediated vasodilators in eNOS-Tg mice. Thoracic aortas of Tg and control mice were surgically isolated for vasomo tor studies. Relaxations to acetylcholine and sodium nitroprusside were sig nificantly reduced in Tg vessels compared with control vessels. Relaxations to atrial natriuretic peptide and 8-bromo-cGMP were also significantly red uced in Tg vessels. Reduced relaxations to these agents were restored by ch ronic N-G-nitro-L-arginine methyl ester treatment, Basal cGMP levels of aor tas were higher in Tg mice than in control mice, whereas soluble guanylate cyclase (sGC) activity in Tg vessels was approximate to 50% of the activity in control vessels. Moreover, cGMP-dependent protein kinase (PKG) protein levels and PKG enzyme activity were decreased in Tg vessels. These observat ions indicate that chronic overexpression of cNOS in the endothelium result ed in resistance to the NO/cCMP-mediated vasodilators and that at least 2 d istinct mechanisms might be involved: one is reduced sGC activity, and the other is a decrease in PKG protein levels. We reported for the first time t hat increased NO release from the endothelium reduces sGC and PKG activity in mice. These data may provide a new insight into the mechanisms of nitrat e tolerance and cross tolerance to nitrovasodilators.