Endothelin-Mediated Vasoconstriction in Early Atherosclerosis Is Markedly Increased in ApoE −/− Mouse but Prevented by Atorvastatin

Abstract

We have discovered that endothelin-1 (ET-1) vasoconstriction is significantly enhanced in aortas of young (8–16-week-old) apolipoprotein E–deficient (ApoE^−/−) mice devoid of atherosclerotic lesions (maximum response expressed as a percentage of the mean response to 100 mM KCl (E_MAX) = 55.7% ± 19.5% KCl, n = 5) compared to age-matched C57BL/6/J control animals (E_MAX = 12.6% ± 2.5% KCl, n = 8), indicating that alterations in the endothelin system may contribute to disease progression, at least in this animal model. There was no difference in the potency of ET-1 to contract aorta from the two groups (C57BL/6/J pD₂ = 8.74 ± 0.30; ApoE^−/− pD₂ = 8.50 ± 0.15, P > 0.05). This increased response was specific to ET-1, as it was not observed with phenylephrine or U46619, nor was it due to a non–receptor mediated increase in contractile sensitivity, as there was no change in response to KCl between the two groups. [¹²⁵I]ET-1 bound with subnanomolar affinity (K_D) to aorta (K_D == 0.018 ± 0.002 nM, n == 4) and, with an order of magnitude lower affinity, to heart (K_D == 0.47 ± 0.05, n == 5) of C57BL/6/J mice with binding densities (B_MAX) of 9.3 ± 2.4 fmol mg⁻¹protein and 100 ± 14 fmol mg⁻¹ protein, respectively. Alterations in vascular reactivity to ET-1 could not be explained by increased endothelin receptor density or affinity, as these were not altered in aorta (K_D == 0.011 ± 0.003 nM; B_MAX == 10.1 ± 3.9 fmol mg⁻¹, n == 4) and heart (K_D == 0.43 ± 0.04 nM; B_MAX == 115 ± 26 fmol mg⁻¹, n == 6) of ApoE^−/− animals. The ratio of ET_A to ET_B receptors in heart of control and ApoE^−/− mice was similar, comprising 89% and 85% ET_A receptors, respectively. In isolated aorta from ApoE^−/− mice on the Western diet, which more closely resembled more advanced stages of the disease in man, the augmented ET-1 vasoconstrictor response was maintained (E_MAX == 25.2% ± 6.8% KCl, n == 9); however, it was completely prevented in animals that had received 10 weeks of oral atorvastatin (30 mg kg⁻¹ day⁻¹) (E_MAX == 4.0% ± 1.5% KCl, n == 5), a concentration that was chosen because it did not affect plasma cholesterol and triglyceride levels. Therefore, this protective prevention of enhanced ET-1 vasoconstriction in ApoE^−/− mice by atorvastatin was independent of its lipid-lowering properties.