Qualitative and quantitative differences between the postsynaptic alpha adrenoceptors of rabbit ear artery and thoracic aorta.

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-methoxy-benzylaminohexyl) cystamine tetrahydrochloride. Thoracic aorta failed to respond to labetalol. Using labetalol as an antagonist against methoxamine, labetalol pA2 values were 7.4 +/- 0.3 (95% confidence interval) and 7.13 +/- 0.25 in ear artery and aorta, respectively. Thus, labetalol had the same affinity for the alpha receptors of these two 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) X 10(-8) M and 2.48(1.64-3.76) X 10(-8) M, respectively, were not significantly different. In contrast, norepinephrine KA values differed by a factor of 32, 4.11 (3.02-5.60) X 10(-6) M and 1.29 (0.85-1.94) X 10(-7) M, respectively. Receptor reserves were also markedly different in these vessels. Thus, ED50 was achieved with 1% receptor occupancy in ear artery as compared to 16% receptor occupancy in aorta. It is concluded that the alpha receptors of ear artery and aorta are both qualitatively and quantitatively different.