Validity of (-)-[3H]-CGP 12177A as a radioligand for the 'putative beta4-adrenoceptor' in rat atrium

1. We have recently suggested the existence in the heart of a 'putative beta4-adrenoceptor' based on the cardiostimulant effects of non-conventional partial agonists, compounds that cause cardiostimulant effects at greater concentrations than those required to block beta1- and beta2-adrenoceptors. We sought to obtain further evidence by establishing and validating a radioligand binding assay for this receptor with (-)-[3H]-CGP 12177A ([-]-4-(3-tertiarybutylamino-2-hydroxypropoxy) benzimidazol-2-one) in rat atrium. We investigated (-)-[3H]-CGP 12177A for this purpose for two reasons, because it is a non-conventional partial agonist and also because it is a hydrophilic radioligand. 2. Increasing concentrations of (-)-[3H]-CGP 12177A, in the absence or presence of 20 microM (-)-CGP 12177A to define non-specific binding, resulted in a biphasic saturation isotherm. Low concentrations bound to beta1- and beta2-adrenoceptors (pKD 9.4+/-0.1, Bmax 26.9+/-3.1 fmol mg(-1) protein) and higher concentrations bound to the 'putative beta4-adrenoceptor' (pKD 7.5+/-0.1, Bmax 47.7+/-4.9 fmol mg(-1) protein). In other experiments designed to exclude beta1- and beta2-adrenoceptors, (-)-[3H]-CGP 12177A (1-200 nM) binding in the presence of 500 nM (-)-propranolol was also saturable (pKD 7.6+/-0.1, Bmax 50.8+/-7.4 fmol mg(-1) protein). 3. The non-conventional partial agonists (-)-CGP 12177A (pKi 7.3+/-0.2), (+/-)-cyanopindolol (pKi 7.6+/-0.2), [-]-pindolol (pK1 6.6+/-0.1) and (+/-)-carazolol (pKi 7.2+/-0.2) and the antagonist (-)-bupranolol (pKi 6.6+/-0.2), all competed for (-)-[3H]-CGP 12177A binding in the presence of 500 nM (-)-propranolol at the 'putative beta4-adrenoceptor', with affinities closely similar to potencies and affinities determined in organ bath studies. 4. The catecholamines competed with (-)-[3H]-CGP 12177A at the 'putative beta 4-adrenoceptor' in a stereoselective manner, (-)-noradrenaline (pKiH 6.3+/-0.3, pKiL 3.5+/-0.1), (-)-adrenaline (pKiH 6.5+/-0.2, pKiL 2.9+/-0.1), (-)-isoprenaline (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1), (+)-isoprenaline (pKi< 1.7), (-)-RO363 ((-)-(1-(3,4-dimethoxyphenethylamino)-3-(3,4-dihydroxyphenoxy++ +)-2-propranol)oxalate, pKi 5.5+/-0.1). 5. The inclusion of guanosine 5-triphosphate (GTP 0.1 mM) had no effect on binding of (-)-CGP 12177A or (-)-isoprenaline to the 'putative beta4-adrenoceptor'. In competition binding studies, (-)-CGP 12177A competed with (-)-[3H]-CGP 12177A for one receptor state in the absence (pKi 7.3+/-0.2) or presence of GTP (pKi 7.3+/-0.2). (-)-Isoprenaline competed with (-)-[3H]-CGP 12177A for two states in the absence (pKiH 6.6+/-0.3, pKiL 3.5+/-0.1; % H 25+/-7) or presence of GTP (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1; % H 37+/-6). In contrast, at beta1-adrenoceptors, GTP stabilized the low affinity state of the receptor for (-)-isoprenaline. 6. The specificity of binding to the 'putative beta 4-adrenoceptor' was tested with compounds active at other receptors. High concentrations of the beta 3-adrenoceptor agonists, BRL 37344 ((RR+SS)[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]phenoxy]acetic acid, 6 microM), SR 58611A (ethyl{(7S)-7-[(2R)-2-(3-chlorophenyl)-2-hydroxyethylamino]-5,6,7,8-tetrahydronaphtyl2-yloxy} acetate hydrochloride, 6 microM), ZD 2079 ((+/-)-1-phenyl-2-(2-4-carboxymethylphenoxy)-ethylamino)-ethan-1-ol, 60 microM), CL 316243 (disodium (R,R)-5-[2-[2-(3-chlorophenyl)-2-hydroxyethyl-amino]propyl]- 1,3-benzodioxole-2,2-dicarboxylate, 60 microM) and antagonist SR 59230A (3-(2-ethylphenoxy)-1-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-2S-2-propanol oxalate, 6 microM) caused less than 22% inhibition of (-)[3H]-CGP 12177A binding in the presence of 500 nM (-)propranolol. Histamine (1mM), atropine (1 microM), phentolamine (10 microM), 5-HT (100 microM) and the 5-HT4 receptor antagonist SB 207710 ((1-butyl-4-piperidinyl)-methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate, 10nM) caused less than 26% inhibition of binding. 7.Non-conventional partial agonists, the antagonist (-)bupranolol and catecholamines all competed for (-)[3H]-CGP 12177A binding in the absence of (-)propranolol at beta1-adrenoceptors, with affinities (pKi) ranging from 1.6-3.6 log orders greater than at the 'putative beta 4-adrenoceptor'. 8.We have established and validated a radioligand binding assay in rat atrium for the 'putative beta 4-adrenoceptor' which is distinct from beta1-, beta2- and beta 3-adrenoceptors. The stereoselective interaction with the catecholamines provides further support for the classification of the receptor as 'putative beta 4-adrenoceptor'.