Beta-Receptor Blocking Potencies of the Three Newly Synthesized β-Adrenergic Antagonists (S-596, K-351, N-696) as Assessed with the Radioligand Binding Assay Method in Rat Cardiac Muscle Membrane Treated with Neuraminidase

Abstract

To assess the β-blocking potencies of three newly synthesized β-blockers, i.e., S-596: dl-2-(3′-t-butylamino-2′-hydroxypropylthio)-4-(5′-carbamoyl-2′-thienyl)-thiazole hydrochloride, K-351: 3.4-dihydro-8-(2-hydroxy-3-isopropyl-aminopropoxy)-3-nitroxy-2H-1-benzopyran, and N-696: 4,3-(tert-butylamino)-2-hydroxypropoxy)-N-methylisocarbostyril hydrochloride, values of Ki (inhibition constant) were obtained by the radioligand binding assay method using rat heart membrane treated with neuraminidase. The pA2 values of these blockers as antagonists against the positive chronotropic and inotropic actions of isoproterenol were also determined by pharmacological methods using guinea pig atria. To assess the selectivity of these β-blockers, pA2 values were also obtained in the isolated trachea using isoproterenol as an agonist. The order of potencies as assessed by Ki and pA2 values were S-596>K-351 >N-696. When the Ki values were plotted against the pA2 values together with the Ki and pA2 values of our previous study (Horii et al., 1974; Nakazawa et al., 1978; Nakagawa et al., 1980; Nakagawa et al., 1983; Nagatomo et al., 1983), straight lines were obtained with r values of 0.93 (P<0.001) (positive chronotropic action) and 0.91 (P<0.001) (positive inotropic action), respectively. Thus, the validity of using the β-adrenoceptor binding assay method developed in this laboratory using cardiac membrane treated with neuraminidase for assessment of the potencies of β-adrenoceptor blockers in cardiac muscle was further substantiated.