Desensitization and functional antagonism by β‐adrenoceptor and muscarinic receptor agonists, respectively: a comparison with receptor alkylation for calculation of apparent agonist affinity

1 Apparent affinity constants ( K D ) for prenalterol, an agonist of low intrinsic efficacy at β 1 ‐adrenoceptors in rat left atria, have been determined by use of receptor desensitization and functional antagonism induced by isoprenaline and carbachol, respectively. The values obtained have been compared to those values estimated with the irreversible β‐adrenoceptor antagonist, bromoacetylalprenololmenthane (BAAM). 2 The −log K D values for prenalterol estimated by desensitization or irreversible antagonism ranged from 6.8–7.1 and 6.2–7.1, respectively. 3 Carbachol produced functional antagonism of the response to prenalterol even though it was removed before addition of prenalterol. This effect was mediated by M 2 ‐muscarinic receptors. Pretreatment of animals with pertussis toxin did not affect the functional antagonism elicited by carbachol. The apparent K D value obtained after pre‐exposure to carbachol (6.8) was similar to those estimated by use of either alkylation with BAAM or desensitization with isoprenaline (see above). 4 It is concluded that acute desensitization or functional antagonism of responses to agonists of low intrinsic efficacy provides a means to estimate apparent K D constants. This approach could be useful to characterize receptors for which an irreversible antagonist may not be available.