Participation of α₁- and β₁-Adrenoceptors in Norepinephrine-Induced Contraction and Relaxation of Isolated Equine Coronary Artery in Vitro

Abstract

In coronary arterial rings isolated from horse, norepinephrine (NE)(10^-7-10^-5 M) induced concentration-dependent contractions which were not influenced by endothelial denudation. Prazosin (α₁-antagonist) inhibited the contraction, but yohimbine (α₂-antagonist) did not, and propranolol (β-antagonist) enhanced the contraction. Pretreatment with phentolamine (10^-5 M)(α-antagonist) converted the contraction induced by NE to relaxation in coronary rings precontracted with ONO11113 (thromboxane A₂ derivative). The relaxation was not influenced by removal of the endothelium, and was inhibited by propranolol and atenolol (β₁-antagonist) but not by butoxamine (β₂-antagonist). These results suggest that in equine coronary arteries, the contractile response to NE is mediated by stimulation of (α₁-adrenoceptors on the smooth muscle, and that stimulation of β₁-adrenoceptors on the smooth muscle modifies the contraction by inducing relaxation.