ABSTRACT
Drugs whose occupancy of the adrenoceptor results in a tissue response are agonists and these sympathomimetic amines were described in Chapter 4. Drugs which occupy adrenoceptors but generate no or negligible responses are the adrenoceptor antagonists. The occupancy of the receptor by these substances prevents the access to the receptor of the neurotransmitter, noradrenaline, and circulating adrenaline and exerts inhibitory effects upon sympathetic function. They also antagonize the effect of exogenously administered sympathomimetic amines and their pharmacological properties are the subject of this chapter. Most of these drugs are competitive in that they compete with the agonist for receptor binding and may be displaced or overcome by excessive levels of agonist. The characteristics of the blockade have been discussed in Chapter 3, together with the methods of calculating the affinities of competitive antagonists by Schild analysis. An important feature of competitive blockade is the parallel displacement of agonist dose-response curves in the presence of the antagonist, with no depression of maximum, indicating the surmountable nature of the antagonism. A few irreversible antagonists that bind covalently to adrenoceptors are also described.
