Impairment of smooth muscle function of rat thoracic aorta in an endothelium-independent manner by long-term administration of N ^G-nitro-L-arginine methyl ester

In this study, we aimed to elucidate whether the daily hypertensive dose of long-term N^G-nitro-L-arginine methyl ester (L-NAME) treatment, could make a difference between endothelial and smooth muscle functions in rat thoracic aorta. We test the hypothesis that high-dose, long-term L-NAME treatment has a depressive effect on vascular smooth muscle contractile activity which is not related with nitric oxide (NO) synthesis inhibition. After 14 days of treatment, isometric tension and ⁴⁵Ca²⁺ influx were measured in aortic tissues isolated from L-NAME₁₀ and L-NAME ₁₀₀ hypertensive (10 and 100 mg/kg/day, systolic blood pressures 167 ± 7 and 172 ± 10 mmHg, respectively) and control normotensive rats (132 ± 7 mmHg). In L-NAME₁₀- and L-NAME₁₀₀-treated rats, acetylcholine-induced relaxation in aortic rings was suppressed with no significant difference between the treatments. L-NAME₁₀₀ (but not L-NAME₁₀) treatment, significantly inhibited contractile responses to phenylephrine, angiotensin II, and K⁺ (80 mM) in endothelium-intact tissues. The effect of L-NAME₁₀₀ on phenylephrine-induced contractile responses was not observed after 3 days of treatment. In endothelium-denuded aortic tissues of L-NAME₁₀₀ (but not L-NAME₁₀)-treated rats, phenylephrine (1 × 10^-6 M)- and K⁺ (80 mM)-induced contractions and ⁴⁵Ca²⁺ influxes were significantly reduced. In Ca²⁺-free medium (0.1 mM EDTA), on the contrary, the transient contractions obtained by either phenylephrine (1 × 10^-6 M) or caffeine (1 × 10^-6 M), or the sustained contractions induced by 12-o-tetradecanoylphorbol-13-acetate (1 × 10 ^-6 M; a protein kinase C activator) in endothelium-denuded aortic rings, were not modified by both L-NAME treatments. These results indicate that in aortic rings from L-NAME hypertensive rats, low and high doses, long-term L-NAME administration may be associated with equivalent inhibition in NO-dependent vasodilator tone (corresponding to equivalent hypertension values); whereas only high-dose, long-term L-NAME administration produces an endothelium-independent decrease in vasocontrictor activity, at least partly explained by a reduction in extracellular Ca²⁺ influx.