Endothelial function in aorta segments of apolipoprotein E-deficient mice before development of atherosclerotic lesions

Acetylcholine (ACh)-induced relaxation declines in apolipoprotein E-deficient (apoE) mouse aortas, but only after atherosclerotic plaque formation. This study investigated intracellular calcium concentrations [Ca] and changes in phenylephrine-induced contractions as index of baseline nitric oxide (NO) bioavailability before plaque development. Isometric contractions of thoracic aorta rings of young (4 months) apoE and C57BL/6J (WT) mice were evoked by phenylephrine (3×10-3×10 M) in the presence and absence of endothelial cells (ECs) or NO synthase (NOS) inhibitors. [Ca ] (Fura-2 AM) and endothelium-dependent relaxation were measured at baseline and after ACh stimulation. Segments of apoE mice were significantly more sensitive and developed more tension than WT segments in response to phenylephrine. The differences disappeared after NOS inhibition or EC removal or upon increasing [Ca] in apoE strips with 10 M cyclopiazonic acid or 10 M Ca-ionophore A23187. Expression of endothelial NOS (eNOS) mRNA was similar in apoE and WT aorta segments. Basal [Ca] was significantly lower in apoE than in WT strips. Relaxation by ACh (3×10-10 M) was time- and dose-dependently related to [Ca], but neither ACh-induced relaxation nor Ca mobilization were diminished in apoE strips. In conclusion, basal, but not ACh-induced NO bioavailability, was compromised in lesion-free aorta of apoE mice. Decreased basal NO bioavailability was not related to lower eNOS expression, but most likely related to lower basal [Ca] . These findings further point to important differences between basal and stimulated eNOS activity. © 2007 Springer-Verlag.