α₁‐Adrenoceptor antagonist properties of CGP 12177A and other β‐adrenoceptor ligands: evidence against β₃‐ or atypical β‐adrenoceptors in rat aorta

Abstract

1 The a 1 -adrenoceptor antagonist properties of the b-adrenoceptor nonconventional partial agonist, CGP 12177A, was investigated in functional assays in rat aorta and in radioligand binding assays in rat cerebral cortical membranes. In addition, binding affinities of other b-adrenoceptor ligands were measured to investigate any correlation between a 1 -adrenoceptor affinity and relaxant potency in phenylephrine-constricted rings. 2 In functional studies, CGP 12177A produced parallel rightward shifts of the phenylephrine CRC with no reduction in the maximum responses. Schild regression analysis gave a straight line with a slope of 0.95 (95% CL: 0.87-1.04), suggesting reversible competitive antagonism, and gave a pK B value of 5.26. In contrast, CGP 12177A (p300 mM) had no effect on contraction induced by the thromboxane-mimetic, U46619. Abbreviations: B max , total number of receptor binding sites; CL, confidence limits; CRC, concentration-response curve; DMSO, dimethyl sulphoxide; E max , maximum response; IC 50 , concentration (M) of competing ligand that inhibits binding of radioligand by 50%; pEC 50 , negative logarithm of the concentration (M) of relaxant that produces 50% of its maximum response; pK B , negative logarithm of the equilibrium dissociation constant, obtained from functional experiments; pK D , negative logarithm of the equilibrium dissociation constant, obtained from saturation experiments; pK i , negative logarithm of the equilibrium dissociation constant, obtained from competition experiments; s.e.m., standard error of the mean

Tools to study β3-adrenoceptors

“…Firstly, it can also activate β 1 -adrenoceptors under some conditions by acting on a second ligand recognition site on this subtype (Joseph et al 2004a,b). Secondly, similarly to BRL 37,344, direct antagonism of α 1 -adrenoceptors has also been described for CGP 12,177 (Brahmadevara et al 2004;Briones et al 2005). Accordingly, effects of high concentrations of CGP 12,177 in guinea pig heart were not sensitive to any β-adrenoceptor antagonist, including the β 3 -selective L-748,337 (Kozlovski et al 2003).…”

“…Moreover, SR 59,230 can exhibit agonist rather than antagonist properties and, in some systems, even behave as a full agonist (Horinouchi and Koike 2001;Hutchinson et al 2005). Even more importantly, SR 59,230 appears to inhibit not only other β-adrenoceptor subtypes but also α 1 -adrenoceptors and may have rather similar affinity for the two adrenoceptor subfamilies (Brahmadevara et al 2004;Briones et al 2005;Leblais et al 2005).…”

“…However, in competition binding studies with human β-adrenoceptor subtypes, bupranolol has lower affinity for β 3 -adrenoceptors than for β 1 -and β 2 -adrenoceptors (Baker 2005;Candelore et al 1999). Moreover, bupranolol, which is chemically related to SR 59,230, shares its α 1 -adrenoceptor antagonist effects (Brahmadevara et al 2004;Leblais et al 2005).…”

“…Nevertheless, its congener cyanopindolol has been used as an antagonist to demonstrate involvement of a β 3 -adrenoceptor in certain functional responses (de Groot et al 2003). Finally, it should be considered that pindolol can also act on some subtypes of serotonin receptors (Guan et al 1992) and may even have some α 1 -adrenoceptor effects (Brahmadevara et al 2004). L-748,337 [(S)-N-[4-[2-[[3-[3(acetamidomethyl)phenoxy]-2-hydroxypropyl]amino]ethyl]phenylbenzenesulfonamide] was reported to have an affinity of 4 nM at the cloned human β 3 -adrenoceptor compared with 390 and 204 nM at the β 1 -and β 2 -adrenoceptors, respectively, and to be practically devoid of agonist activity at all three subtypes (Candelore et al 1999).…”

“…Thus, this compound is an antagonist of muscarinic acetylcholine receptors (Kubota et al 2002), a feature that may cause problems in functional experiments with the urinary bladder where precontraction is frequently induced by a muscarinic receptor agonist (see above). Moreover, BRL 37,344 was also reported to be an α 1 -adrenoceptor antagonist in low micromolar concentrations (Brahmadevara et al 2003(Brahmadevara et al , 2004Briones et al 2005;Leblais et al 2005). The poor subtype selectivity and β-adrenoceptor-independent effects of BRL 37,344 seriously limit its usefulness in pharmacological studies.…”