Attenuation of Intrinsic Active Tone by Endothelium-Derived Nitric Oxide in Aortae of Spontaneously Hypertensive Rats with Different Levels of Blood Pressure

The influences of endothelium on the basal tone of aortae from various strains of spontaneously hypertensive rats with different blood pressure (SHR, SHRSP, MSHRSP) were studied. Endothelium-intact preparations of aortac from spontaneously hypertensive rats exhibited spontaneous active tone, which was greater in the order of SHR < SHRSP < M-SHRSP. The active tone of the MSHRSP preparations was about 40% of high-K⁺-induced contraction, while that of normotensive WKY was less than 5%. The active tone was enhanced by the removal of endothelium. The active tone was sensitive to extracellular Ca²⁺ and abolished by verapamil. The application of N^G-monomethyl-L-arginine caused the increase in the active tone which was counteracted by L-arginine. These results indicate that the active tone of smooth muscle increases as the blood pressure of the rat increases, and that endothelium attenuates the active tone by releasing nitric oxide (NO) spontaneously. It was also demonstrated that the attenuating action of endothelium was impaired depending on the blood pressure level.