Comparative study of postsynaptic alpha-adrenoceptors in aorta obtained from human and other mammalian species

The alpha-adrenoceptor populations in aortic strips from humans, rats, guinea-pigs and rabbits were investigated in vitro, using specific agonists and antagonists. In rabbit and human preparations the activities of the two agonists tested, noradrenaline and methoxamine, were competitively antagonized by prazosin, whereas in the other animal species prazosin showed a competitive antagonism for methoxamine induced contraction, but an uncompetitive behaviour against noradrenaline. The alpha 2-selective agonists B-HT 920 and detomidine did not elicit any effect on aortic strips up to 10(-3) M. On the basis of these results, alpha 1-receptors seem to represent an homogeneous population in human and rabbit aortae, but not in rat and guinea-pig tissues. On the other hand, the contractile response of noradrenaline and methoxamine on aortic strips from the four animal species examined cannot be ascribed to the activation of postsynaptic alpha 2-receptors. Moreover we present preliminary evidence that rat and guinea-pig aortae do not contain a clear subdivision in alpha 1a- and alpha 1b-receptors.