The term adrenoceptor was first introduced by AHLQUIST to explain the different physiological effects of catecholamines [69]. In 1948 Ahlquist [70] described the initial subclassification of adrenoceptors into α- and β-receptors on the basis of their pharmacological characteristics. For his study he used the agonists epinephrine, norepinephrine and isoprenaline. He showed that this substances always lead to a constriction of the smooth muscles of arterioles, the uterus and M. dilatator pupillae in the same order of effectiveness: L(–)-epinephrine ≥ L(–)-norepinephrine ≫ isoprenaline (α-receptors). The vasodilatatory effect and the positive chronotropic effects (i.e., rise of heart rate) follow the order isoprenaline > L(–)-epinephrine ≥ L(–)-norepinephrine (β-receptors).
He postulated that the effects of these sympathomimetics are mediated by different receptors:
These types of receptors were further splitted into several subgroups. Table 1.4 summarises their pharmacological differentiation and physiological effects.
The existence of these receptor subtypes has been proven by radioligand assays. It has been shown, that a number of tissues may contain both β-receptors. This distribution is often unbalanced, as table 1.3 shows.
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© 2001 Alexander Binder