INTERACTION OF THE ANTIARRHYTHMIC AGENTS SR-33589 AND AMIODARONE WITHTHE BETA-ADRENOCEPTOR AND ADENYLATE-CYCLASE IN RAT-HEART

1 The effects of SR 33589 and amiodarone on the cardiac beta-adrenocep tor were studied in vitro and after chronic treatment by means of [I-1 25]-(-)-iodocyanopindolol ([I-125]-(-)-CYP) binding and measurement of adenylate cyclase activity. 2 Binding of [I-125]-(-)-CYP was inhibite d in a dose-dependent manner by SR 33589 (IC50 = 1.8 +/- 0.4 mu M, n(H ) = 0.93 +/- 0.06) and amiodarone (IC50 = 8.7 +/- 2.0 mu M, n(H) = 0.9 2 +/- 0.03). Saturation binding experiments indicated a non-competitiv e interaction such that SR 33589 (1 and 3 mu M) and amiodarone (5 and 10 mu M) reduced the B-max of [I-125]-(-)-CYP binding without any effe ct on the K-D. Kinetic studies showed that the rate of association of [I-125]-(-)-CYP was unchanged while the rate of dissociation was incre ased both in the presence of SR 33589 (10 mu M) and amiodarone (30 mu M). 3 Under the same conditions, the receptor stimulated adenylate cyc lase activity was inhibited in a dose-dependent, but non-competitive m anner, by SR 33589 (isoprenaline-, glucagon- and secretin-stimulated e nzyme inhibited 50% at 6.8 +/- 0.6 mu M, 31 +/- 10 mu M and 12 +/- 3 m u M, respectively) while the basal, GTP- and Gpp(NH)p-stimulated enzym e was inhibited by 5-10% and the NaF and forskolin-stimulated enzyme b y 50% at 500 mu M. Amiodarone exhibited a similar pattern of inhibitio n. 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 [I-125]-(-)-CYP saturation experiments and a decrease of the isoprenaline- and glucag on-stimulated adenylate cyclase activity without any effect on basal e nzyme activity or activity when stimulated by agents acting directly o n regulatory catalytic units. 5 Unlike amiodarone, SR 33589 does not c ontain iodine substituents. Plasma levels of T-3, T-4 and rT(3) were u nchanged after SR 33589 treatment except a decrease in T-4 level at th e highest dose Whilst the T-4 T-3 ratio and the level of rT(3) were do se-dependently increased by amiodarone treatment. 6 In vitro, SR 33589 and amiodarone were characterized as non-competitive beta-adrenocepto r antagonists. Chronic treatment led to a down-regulation of the beta- adrenoceptor; the down-regulation cannot be attributed to an indirect effect mediated by the thyroid hormones. To reconcile these opposing o bservations, we propose that SR 33589 and amiodarone interact with the beta-adrenoceptor at a site close to the intracellular loops which ar e involved in the coupling with G(s) and contain the phosphorylable si tes.