Beta-adrenoceptor activation plays a role in the reverse rate-dependency of effective refractory period lengthening by dofetilide in the guinea-pig atrium, in vitro

Abstract

1. Blockers of the rapid component of the delayed rectifier potassium current (I(Kr)) prolong cardiac action potential duration (APD) and effective refractory period (ERP) in a reverse rate-dependent manner. Since activation of beta-adrenoceptors attenuates prolongation of APD evoked by I(Kr) blockers, rate-dependent neuronal noradrenaline liberation in the myocardium may contribute to the reverse rate-dependent nature of the effects of I(Kr) blockers. In order to test this hypothesis, we studied the effects of dofetilide, a pure I(Kr) blocker, on ERP after activation or blockade of beta-adrenoceptors and after catecholamine depletion in guinea-pig left atrial myocardium paced at 3, 2 and 1 Hz, in vitro. 2. Dofetilide (100 nM) lengthened ERP in a reverse rate-dependent manner in the left atrial myocardium of guinea-pigs. Strong activation of beta-adrenoceptors using 10 nM isoproterenol abolished the dofetilide-induced lengthening of ERP at all pacing rates. 3. Blockade of the beta-adrenoceptors with metoprolol (1 micro M), atenolol (3 micro M) or propranolol (300 nM) increased the dofetilide-evoked prolongation of ERP at 3 and 2 Hz, but not at 1 Hz. As a consequence, metoprolol attenuated while propranolol and atenolol fully eliminated the reverse rate-dependent nature of the dofetilide-induced ERP lengthening. In catecholamine-depleted atrial preparations of the guinea-pig (24 h pretreatment with 5 mg kg(-1) reserpine i.p.), the effect of dofetilide on ERP was not frequency dependent, and propranolol did not alter the effects of dofetilide. 4. In contrast to results obtained in guinea-pig atrial preparations, propranolol failed to change the reverse rate-dependent effect of dofetilide on ERP in the right ventricular papillary muscles of rabbits and guinea-pigs. 5. As an indication of the functional consequences of rate-dependent noradrenaline liberation, propranolol decreased twitch tension at 3 and 2 Hz but not at 1 Hz in the atrial myocardium of control guinea-pigs, whereas no such effect was detected in catecholamine-depleted atrial preparations. Propranolol failed to change contractility of ventricular myocardium in guinea-pigs and rabbits. 6. It is concluded that rate-dependent noradrenaline release and the ensuing beta-adrenoceptor activation contributed to the reverse rate-dependent nature of ERP prolongation caused by I(Kr) blockers in isolated guinea-pig atrial myocardium.

Figure 1

Frequency-dependent effects of dofetilide in the presence of metoprolol (a), propranolol (b) or atenolol (c) on the effective refractory period in guinea-pig left atrial myocardium. After measuring baseline values, separate groups of preparations were treated with either metoprolol (1 μM), dofetilide (100 nM), dofetilide+metoprolol (Dof+Met) or were used as control in (a). In panels (b and c) propranolol (300 nM) or atenolol (3 μM) was used, respectively. The incubation period lasted for 30 min. Values are means±s.e.m., n indicates the number of experiments. Data are expressed as percentage changes in comparison with the baseline values (*P<0.05, ***P<0.001 vs 3 Hz in the same group; °P<0.05, °°P<0.01, °°°P<0.001 vs dofetilide at the same frequency).

Figure 2

Frequency-dependent effects of dofetilide and propranolol on the effective refractory period in atria obtained from reserpinised guinea-pigs. Reserpine (5 mg kg⁻¹ i.p.) was administered 24 h before the experiments. After measuring baseline values, separate groups of preparations were treated with propranolol (300 nM), dofetilide (100 nM), dofetilide+propranolol (Dof+Prop) or were used as control. Each drug was applied for 30 min. Values are means±s.e.m., n indicates the number of experiments. Data are expressed as percentage changes in comparison with the baseline values. No significant changes were observed between the dofetilide- and the dofetilide+propranolol-treated groups.

Figure 3

Frequency-dependent effects of dofetilide and propranolol on twitch tension in left atria obtained from untreated (a) and reserpinised (b) guinea-pigs. Reserpine (5 mg kg⁻¹ i.p.) was administered 24 h before the experiments. After measuring baseline values, separate groups of preparations were treated with propranolol (300 nM), dofetilide (100 nM), dofetilide+propranolol (Dof+Prop) or were used as control. Each incubation period lasted for 30 min. Values are means±s.e.m., n indicates the number of experiments. (*P<0.05, **P<0.01, ***P<0.001 vs control at the same frequency).

Figure 4

Frequency-dependent effects of dofetilide and propranolol on the effective refractory period in the right ventricular papillary muscles of rabbits (a) and guinea-pigs (b). After measuring baseline values, separate groups of preparations were treated with pro-pranolol (300 nM), dofetilide (100 nM), dofetilide+propranolol (Dof+Prop) or were used as control. Each incubation period lasted for 30 min. Values are means±s.e.m., n indicates the number of experiments. Data are expressed as percentage changes in comparison with the baseline values (*P<0.05, ***P<0.001 vs 3 Hz in the same group. °P<0.05 vs dofetilide at the same frequency).

Figure 5

Frequency-dependent effects of dofetilide and propranolol on twitch tension in right ventricular papillary muscles of rabbits (a) and guinea-pigs (b). After measuring baseline values, separate groups of tissues were treated with propranolol (300 nM), dofetilide (100 nM), dofetilide+propranolol (Dof+Prop) or were used as control. Each incubation period lasted for 30 min. Values are means±s.e.m., n indicates the number of experiments. Asterisks (***P<0.001) denote significant differences vs control at the same frequency.