Effect of ramipril on α-adrenoceptor-mediated oscillatory contractions in tail artery of hypertensive rats

Recent studies indicate that norepinephrine-induced contractile oscillations in the tail artery from stroke-prone spontaneously hypertensive rats (SHRSP) may be a vascular phenomenon independent of blood pressure level. The objectives of this study were: (1) to characterize pharmacologically the α-adrenoceptor mediating norepinephrine-induced oscillations in tail artery; and (2) to investigate the relationship between blood pressure level, altered by treatments with hydralazine/hydrochlorothiazide or the angiotensin converting enzyme inhibitor ramipril, and the observation of norepinephrine-induced oscillations in tail artery. The α₂-adrenoceptor agonists clonidine and guanabenz potently stimulated oscillatory contractions in the tail artery while the α₁-adrenoceptor agonists phenylephrine and methoxamine were considerably less potent. Yohimbine, an α₂-adrenoceptor antagonist, but not the α₁-adrenoceptor antagonist prazosin demonstrated high affinity for the receptor mediating norepinephrine-induced oscillatory contractions. These results support the hypothesis that norepinephrine-induced oscillatory contractions in the tail artery from SHRSP occur primarily through stimulation of α₂-adrenoceptors. Ramipril lowered blood pressure in SHRSP after 4 weeks of treatment during 6-10 weeks of life but did not alter the ability of the α₂-adrenoceptor agonist clonidine (10^-5 M) to induce contractile oscillations in tail arteries from SHRSP, indicating these oscillations are not a secondary effect of high blood pressure. These studies suggest that norepinephrine-induced oscillations in tail artery from SHRSP may be a vascular trait separate and distinct from blood pressure level and angiotensin II expression early in life.