Characterization of the adrenoceptors in coronary arteries of pigs

Abstract

The effects of phenylephrine and norepinephrine and the influence of adrenoceptor blockers on them were investigated on isolated strips of pig coronary arteries of different diameter. Phenylephrine contracted large coronary arteries, an effect inhibited by phentolamine, but was ineffective on small coronary arteries. Norepinephrine either relaxed large coronary arteries or occasionally contracted them. Phentolamine potentiated the relaxing effect of norepinephrine and propranolol inhibited or even reversed it into a contraction. Small coronary arteries were without exception relaxed by norepinephrine. This effect was not influenced by phentolamine but was inhibited by propranolol. These findings prove the presence of α-adrenoceptors in pig coronary arteries with a large diameter although β-adrenoceptors predominate; α-adrenoceptors are not found in coronary arteries with a small diameter. The classification of β-adrenoceptors in the coronary musculature into one of the two β-types was determined by comparing the affinities of propranolol, practolol and H 35/25 on the β-adrenoceptors of isolated atrial preparation of guinea pigs and of strips of pig coronary arteries. While the affinities of propranolol were the same to both organs the affinity of practolol to the myocardium was 100 times higher than to the coronary arteries. On the other hand the affinity of H 35/25 to the coronary arteries was three times higher than to the myocardium. The selective effects of the β 1-adrenoceptor blocker practolol on the myocardium and the β 2-adrenoceptor blocker H 35/25 on the coronary arteries indicate that the β-adrenoceptors of the coronary arteries should be classified as β 2-adrenoceptors.