Arrhythmogenic Interaction Between Sympathetic Tone and Mechanical Stretch in Rat Pulmonary Vein Myocardium

Abstract

Rapid firing from pulmonary veins (PVs) frequently initiates atrial fibrillation, which is a common comorbidity associated with hypertension, heart failure, and valvular disease, i.e., conditions that pathologically increase cardiomyocyte stretch. Autonomic tone plays a crucial role in PV arrhythmogenesis, while its interplay with myocardium stretch remains uncertain. Two-microelectrode technique was used to characterize electrophysiological response of Wistar rat PV to adrenaline at baseline and under mild (150 mg of applied weight that corresponds to a pulmonary venous pressure of 1 mmHg) and moderate (10 g, ∼26 mmHg) stretch. Low concentrations of adrenaline (25-100 nmol/L) depolarized the resting membrane potential selectively within distal PV (by 26 ± 2 mV at baseline, by 18 ± 1 mV at 150 mg, P < 0.001, and by 5.9 ± 1.1 mV at 10 g, P < 0.01) suppressing action potential amplitude and resulting in intra-PV conduction dissociation and rare episodes of spontaneous activity (arrhythmia index of 0.4 ± 0.2, NS vs. no activity at baseline). In contrast, 1-10 μmol/L of adrenaline recovered intra-PV propagation. While mild stretch did not affect PV electrophysiology at baseline, moderate stretch depolarized the resting potential within distal PV (-56 ± 2 mV vs. -82 ± 1 mV at baseline, P < 0.01), facilitated the triggering of rapid PV firing by adrenaline (arrhythmia index: 4.4 ± 0.2 vs. 1.3 ± 0.4 in unstretched, P < 0.001, and 1.7 ± 0.8 in mildly stretched preparations, P < 0.005, at 10 μmol/L adrenaline) and induced frequent episodes of potentially arrhythmogenic atrial "echo" extra beats. Our findings demonstrate complex interactions between the sympathetic tone and mechanical stretch in the development of arrhythmogenic activity within PVs that may impact an increased atrial fibrillation vulnerability in patients with elevated blood pressure.

Keywords: adrenalin; arrhythmia; mechano-electrical response; pulmonary veins; stretch.

FIGURE 1

Complex interactions between sympathetic tone and mechanical stretch in rat pulmonary vein (PV) myocardium. Top: Experimental protocol for testing different levels of sympathetic stimulation in unstretched and stretched PV preparations. Physiological status of PV_dis inexcitability induced be either low concentrations of adrenaline (25–100 nmol/L), or pathological stretch (10.5 g) was tested by a brief application of acetylcholine (ACh), which was administered to hyperpolarize the RP and recover PV_dis excitability and then washed out to continue experimental protocol. All measurements were performed during constant left atrial pacing, except brief (5–10 min) periods when pacing was stopped (no pacing, NP), and spontaneous rhythm was recorded if present. Below, changes in the resting membrane potential (middle panels) and action potential amplitude (low panels) are shown for distal (red traces) and ostium (blue traces) PV regions at baseline (no adrenaline applied) and under low (25–100 nmol/L) and high (1–10 μmol/L) concentrations of adrenaline measured in unstretched and stretched (150 mg and 10 g) preparations. Changes are shown for individual rats (light blue and light red lines) as well as mean ± SEM (solid lines). Red dashed box indicates the presence of PV_dis inexcitability. N = 6–7 rats. *P < 0.05, **P < 0.01, ***P < 0.001 for PV_dis vs. PV_ost; ^#P < 0.05, ^##P < 0.01, ^###P < 0.001 within the same group vs. baseline by repeated measurements two-way ANOVA with Bonferroni correction.

FIGURE 2

Biphasic effect of adrenaline on intra-pulmonary vein (PV) conduction. (A) Application of 50 nmol/L adrenaline results in the depolarization of resting membrane potential (RP) exclusively in PV_dis leading to intra-PV conduction dissociation. Two simultaneous microelectrode recordings from PV_dis (top recording in red) and PV_ost (bottom recording in blue) are shown. Below, selected time windows (gray rectangles) are shown enlarged before (baseline) and during PV_dis inexcitability under 50 nmol/L adrenaline. Left atria (LA) was constantly paced with S1S1 = 300 ms. (B) Application of 1 μmol/L adrenaline recovered intra-PV conduction suppressed by 50 nmol/L adrenaline. Below, selected time windows (gray rectangles) are shown enlarged before and after the recovery of excitability of PV_dis.

FIGURE 3

Arrhythmic events induced within the pulmonary vein (PV) myocardium by low (25–100 nmol/L) and high (1–10 μmol/L) concentrations of adrenaline in unstretched and stretched (150 mg and 10.5 g) preparations. (A) Arrhythmia score calculated as described in the section “Materials and Methods.” N = 6–7 rats. P values are calculated by repeated measurements two-way ANOVA with Bonferroni correction. (B–D) Representative examples of arrhythmic events recorded in PV preparations under different conditions. Two simultaneous microelectrode recordings from PV_dis (top recording in red) and PV_ost (bottom recording in blue) are shown for each event. (B) A brief application of 1 μmol/L acetylcholine (ACh) in 150 mg stretched preparation hyperpolarized the RP in PV_dis back to a baseline level and suppressed spontaneous PV activity (no pacing, NP) under 1 μmol/L adrenaline (Adr) application. After ACh washout, spontaneous slow PV rhythm (arrhythmia score is 3) was suppressed by atrial pacing (S1S1 = 300 ms). (C,D) Stable slow rhythm (arrhythmia score is 4, C) and fast regular rhythm (arrhythmia score is 5, D) induced by 10 μmol/L adrenaline in unstretched (C) and 150 mg stretched (D) PV preparations.

FIGURE 4

Complex electrical activity recorded in pulmonary vein (PV) preparations under 10 μmol/L adrenaline applied during 10 g stretch. Two simultaneous microelectrode recordings from PV distal (top recording in red) and PV ostium (bottom recording in blue) are shown. (A) Recovery of intra-PV conduction with arrhythmogenic activity after application of 10 μmol/L adrenaline in 10 g stretched PV. Left panel shows a continuous 1-min recording of adrenaline application, while right panel represents an enlarged section of the left recording indicated by a gray rectangle. Spontaneous activity was induced in distal PV and not suppressed by left atrial (LA) pacing. Instead, a faster rhythm in distal PV interfered with a slower rate of electrical pacing resulting in frequent atrial extra beats (shown as red action potentials in PV ostium recordings; red arrows indicate propagating PV distal beats). (B) Burst of fast electrical activity triggered in distal PV by atrial extra beat interfered with intrinsic PV rhythm.