Classification and Function of Adrenoceptors
The classification of adrenoceptors is based primarily on investigations of the effects of various pharmacological agents on functional parameters. In principle, potency orders of agonists and inhibitory effects of specific antagonists have provided the basis for the classification of α and β and the further subclassification into α1/ α2 and β1/β2. During the last few years, direct receptor binding studies have been used for receptor characterization.
Both functional studies and receptor binding studies indicate that both α1 and α2 and β1 and β2may coexist in the same target cell mediating the same response. The relative density of the two subtypes of α- and β-receptors may vary from organ to organ.
Receptor binding studies have made it possible to determine the total β-receptor density as well as the relative density of the β-receptor subtypes. Such studies have also provided information about receptor occupancy of β-adrenoceptor agonists at different functional effect levels.
Variation in the total receptor pool seems at least partly to be responsible for the variation in potency of the full agonist isoprenaline and the maximal response to the partial agonist prenalterol observed in the rabbit papillary muscle.
One week of treatment of cats with isoprenaline induced a reduction of the potency of isoprenaline and the maximal effect of prenalterol in the isolated papillary muscle. These changes were related to a decrease in the β-adrenoceptor density in the cardiac muscle.
KeywordsPermeability Turkey Noradrenaline Progesterone Adrenaline
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