Interaction of the antiarrhythmic agents SR 33589 and amiodarone with the β‐adrenoceptor and adenylate cyclase in rat heart

Abstract

1 The effects of SR 33589 and amiodarone on the cardiac β-adrenoceptor were studied in vitro and after chronic treatment by means of [¹²⁵I]-(−)−iodocyanopindolol ([¹²⁵I]-(−)−CYP) binding and measurement of adenylate cyclase activity. 2 Binding of [¹²⁵I]-(−)−CYP was inhibited in a dose-dependent manner by SR 33589 (IC₅₀ = 1.8 ± 0.4 μM, n_H = 0.93 ± 0.06) and amiodarone (IC₅₀ = 8.7 ± 2.0 μM, n_H = 0.92 ± 0.03). Saturation binding experiments indicated a non-competitive interaction such that SR 33589 (1 and 3 μm) and amiodarone (5 and 10 μm) reduced the B_max of [¹²⁵I]-(−)−CYP binding without any effect on the K_D. Kinetic studies showed that the rate of association of [¹²⁵I]-(−)−CYP was unchanged while the rate of dissociation was increased both in the presence of SR 33589 (10 μm) and amiodarone (30 μM). 3 Under the same conditions, the receptor stimulated adenylate cyclase activity was inhibited in a dose-dependent, but non-competitive manner, by SR 33589 (isoprenaline-, glucagon- and secretin-stimulated enzyme inhibited 50% at 6.8 ± 0.6 μM, 31 ± 10 μm and 12 ± 3 μM, respectively) while the basal, GTP- and Gpp(NH)p-stimulated enzyme was inhibited by 5–10% and the NaF and forskolin-stimulated enzyme by 50% at 500 μM. Amiodarone exhibited a similar pattern of inhibition. 4 After chronic oral treatment (50, 100, 150 mg kg⁻¹ per day, 14 days), both SR 33589 and amiodarone produced a dose-dependent decrease in B_max without any effect on K_D as determined from [¹²⁵I]-(−)−CYP saturation experiments and a decrease of the isoprenaline- and glucagon-stimulated adenylate cyclase activity without any effect on basal enzyme activity or activity when stimulated by agents acting directly on regulatory catalytic units. 5 Unlike amiodarone, SR 33589 does not contain iodine substituents. Plasma levels of T₃, T₄ and rT₃ were unchanged after SR 33589 treatment except a decrease in T₄ level at the highest dose whilst the T₄ T₃ ratio and the level of rT₃ were dose-dependently increased by amiodarone treatment. 6 In vitro, SR 33589 and amiodarone were characterized as non-competitive β-adrenoceptor antagonists. Chronic treatment led to a down-regulation of the β-adrenoceptor; the down-regulation cannot be attributed to an indirect effect mediated by the thyroid hormones. To reconcile these opposing observations, we propose that SR 33589 and amiodarone interact with the β-adrenoceptor at a site close to the intracellular loops which are involved in the coupling with G_s and contain the phosphorylable sites.