Acetylcholine (ACh)-induced relaxation declines in apolipoprotein E-deficient (apoE-/-) mouse aortas, but only after atherosclerotic plaque formation. This study investigated intracellular calcium concentrations [Ca2+]i 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 (3x10(-9)-3x10(-5) M) in the presence and absence of endothelial cells (ECs) or NO synthase (NOS) inhibitors. [Ca2+]i (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 [Ca2+]i in apoE-/- strips with 10(-6) M cyclopiazonic acid or 10(-7) M Ca2+-ionophore A23187. Expression of endothelial NOS (eNOS) mRNA was similar in apoE-/- and WT aorta segments. Basal [Ca2+]i was significantly lower in apoE-/- than in WT strips. Relaxation by ACh (3x10(-9)-10(-5) M) was time- and dose-dependently related to [Ca2+]i, but neither ACh-induced relaxation nor Ca2+ 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 [Ca2+]i. These findings further point to important differences between basal and stimulated eNOS activity.