Ectopic activity in the rat pulmonary vein can arise from simultaneous activation of alpha1- and beta1-adrenoceptors

Abstract

Background and purpose: Atrial fibrillation (AF) is the most common electrical cardiac disorder in clinical practice. The major trigger for AF is focal ectopic activity of unknown origin in sleeves of cardiac muscle that extend into the pulmonary veins. We examined the role of noradrenaline in the genesis of ectopic activity in the pulmonary vein.

Experimental approach: Mechanical activity of strips of pulmonary vein isolated from male Wistar rats was recorded via an isometric tension meter. Twitch contractions of cardiac myocytes were evoked by electrical field stimulation in a tissue bath through which flowed Krebs-Heinseleit solution warmed to 36-37 degrees C and gassed with 95% O(2) 5% CO(2).

Key results: The superfusion of noradrenaline induced ectopic contractions in 71 of 76 different isolated pulmonary veins. Ectopic contractions in the pulmonary vein were not associated with electrically evoked twitch contractions. The effect of noradrenaline on the pulmonary vein could be replicated by the simultaneous, but not separate, application of the alpha adrenoceptor agonist phenylephrine and the beta adrenoceptor agonist isoprenaline. The use of selective agonists and antagonists for adrenoceptor subtypes showed that ectopic activity in the pulmonary vein arose from the simultaneous stimulation of alpha(1) and beta(1) adrenoceptors. The application of noradrenaline to isolated strips of left atrium did not induce ectopic contractions (n=10). conclusions: These findings suggest an origin for ectopic activity in the pulmonary vein that requires activation of both alpha and beta adrenoceptors. They also open new perspectives towards our understanding of the triggering of AF.

Figure 1

NA-induced ectopic contractions in the pulmonary vein. (a) A continuous recording of tension in an isolated pulmonary vein. NA was superfused across the preparation for the period indicated by the bar above the trace. The horizontal line under the recording indicates the region that is shown upon an expanded timescale in (b). (c–f) Analysis of the ectopic contractions, which were recorded during the burst indicated by the line under the trace in (b). (c) Contraction amplitude. (d) Contraction frequency. (e) Time to peak contraction. (f) Contraction duration measured as the width at 50% of the amplitude.

Figure 2

Dose-dependent induction of ectopic activity by NA. The numbers in parentheses represent the number of individual preparations.

Figure 3

The effect of α- and β-adrenoceptor antagonists on ectopic contractions induced by NA. Traces represent segments of otherwise continuous recordings of tension obtained from isolated pulmonary veins. Ectopic contractions were induced by superfusion of 10⁻⁵ M NA. (a) Antagonists of α-adrenoceptors. For the periods indicated by the bars above the traces, the solution superfusing the preparation also contained either the α₂-receptor antagonist yohimbine (10⁻⁷ M) or the α₁-receptor antagonist prazosin (5 × 10⁻⁶ M). Similar results were obtained in six different preparations. (b) Antagonists of β-adrenoceptors. For the periods indicated by the bars, the solution also contained either the β₁-receptor antagonist atenolol (5 × 10⁻⁶ M), the β₂-receptor antagonist ICI118551 (10⁻⁷ M) or the β₃-receptor antagonist SR59230A (5 × 10⁻⁷ M). Similar results were obtained in five different preparations.

Figure 4

Ectopic contractions in the pulmonary vein require activation of both α₁- and β₁-adrenoceptors. Traces represent segments of otherwise continuous recordings of tension from different isolated pulmonary veins. Ectopic contractions were induced by the superfusion of the preparations with solutions that contained PE (5 × 10⁻⁶ M) and ISO (10⁻⁷ M). (a) Ectopic contractions require the simultaneous application of PE and ISO. The periods for which the preparation was exposed to the different agonists are indicated by bars above the trace. Similar results were obtained in six different preparations. (b) Agonists of α-adrenoceptors. For the periods indicated by the bars, PE was replaced first by the α₂-receptor agonist UK14304 (5 × 10⁻⁶ M) and then by the α₁-receptor agonist cirazoline (10⁻⁶ M). Similar results were obtained in five different preparations for UK14304 and six different preparations for cirazoline. (c) Agonists of β-adrenoceptors. For the periods indicated by the bars, ISO was replaced by either the β₃-receptor agonist BRL37344 (10⁻⁷ M), the β₁-receptor agonist denopamine (5 × 10⁻⁷ M), or the β₂-receptor agonist formoterol (10⁻⁸ M). Similar results were obtained in six different preparations for denopamine and five each for BRL37344 and formoterol.

Figure 5

Effects of agonist concentration upon ectopic contractions in the pulmonary vein. (a) The effects of the concentration of ISO upon the incidence of ectopic activity recorded in the presence of either 10⁻⁶ (open columns) or 5 × 10⁻⁶ M (filled columns) PE. The numbers in parentheses represent n different preparations. (b) Induction and suppression of ectopic activity by ISO. These traces represent segments of an otherwise continuous recording of tension in an isolated pulmonary vein. The periods for which the preparation was exposed to PE and different concentrations of ISO are indicated by the bars above the traces.

Figure 6

A comparison of the responses of the pulmonary vein (a and c) and the left atrium (b and d) to adrenoceptor stimulation. Each preparation was subjected to 0.1 Hz FS. The periods for which preparations were superfused with solutions, which contained either NA (a and b) or PE (c and d) are indicated by the bars above the traces.