Heterogeneity of vascular α-adrenoceptors in man
The knowledge on α-adrenoceptors has expanded enormously in the last decade, mostly as a result of in vitro and in vivo animal experiments. In the face of considerable species differences we have tested several of the newly developed concepts on α-adrenoceptors in the circulation of the human forearm. In this model we were able to show the presence of postsynaptic α1-and α2-adrenoceptors, both contributing to resting vascular tone. Although adrenaline and noradrenaline were shown to have affinity for each α-adrenoceptor subtype, noradrenaline seems to be the natural agonist for both receptors. Evidence is presented for an intrasynaptic location of the α1-adrenoceptor and an extrasynaptic location of the α2-adrenoceptor and also for a functional presynaptic α2-adrenoceptor in the human forearm circulation. Selective hyperresponsiveness to α1- or α2-adrenoceptor stimulation in hypertensive patients could not be established. Calcium entry blockers were shown to attenuate the vasoconstriction induced by selective α2-adrenoceptor stimulation but not by selective α1-adrenoceptor stimulation. These data support the physiological and pharmacological relevance of the new concepts on α-adrenoceptors for the situation in man.