QUALITATIVE AND QUANTITATIVE DIFFERENCES BETWEEN THE POSTSYNAPTIC ALPHA-ADRENOCEPTORS OF RABBIT EAR ARTERY AND THORACIC AORTA

Abstract

The .alpha.-adrenoceptor properties of the rabbit ear artery and thoracic aorta were assessed using isolated blood vessel rings mounted in tissue baths. Labetalol, an .alpha. and .beta.-receptor antagonist, caused dose-dependent contractions in control, reserpinized and surgically denervated ear arteries. This contraction was inhibited by phentolamine and abolished by the irreversible .alpha.-receptor antagonist, N,N''-bis-(O-methoxybenzylaminohexyl)cystamine tetrahydrochloride. Thoracic aorta failed to respond to labetalol. Using labetalol as an antagonist against methoxamine, labetalol pA2 [competitive antagonist activity] values were 7.4 .+-. 0.3 (95% confidence interval) and 7.13 .+-. 0.25 in ear artery and aorta, respectively. Labetalol had the same affinity for the .alpha.-receptors of these 2 vessels but was an .alpha.-agonist only in the ear artery. Norepinephrine ED50 values and dissociation constants (KA) were determined by analysis of dose-response data with and without partial inactivation of .alpha.-receptors by phenoxybenzamine. Ear artery and aorta norepinephrine ED50 values, 4.24 (2.24-8.03) .times. 10-8 M and 2.48 (1.64-3.76) .times. 10-8 M, respectively, were not significantly different. Norepinephrine KA values differed by a factor of 32, 4.11 (3.02-5.60) .times. 10-6 M and 1.29 (0.85-1.94) .times. 10-7 M, respectively. Receptor reserves were also markedly different in these vessels. ED50 was achieved with 1% receptor occupancy in ear artery as compared to 16% receptor occupancy in aorta. The .alpha.-receptors of ear artery and aorta are evidently qualitatively and quantitatively different.