Long-term effects of hyperpolarization-activated inward current blockade on cardiac parasympathetic activity
Authors: Scridon A , Halatiu VB , Balan AI , Cozac DA , Moldovan GV , Banescu C , Perian M , Serban RC 
Published: May 2021

Abstract

Funding Acknowledgements
Type of funding sources: Public grant(s) – National budget only. Main funding source(s): This work was supported by a grant of the Romanian Ministry of Education and Research, CNCS - UEFISCDI

Background
The autonomic control of the pacemaker current, If, and the molecular mechanisms underlying parasympathetic If modulation are well understood. Conversely, the effects of chronic If blockade on the parasympathetic nervous system and on the heart rate (HR) response to acute parasympathetic changes are still largely unknown. Such interactions could significantly influence the course of patients undergoing chronic therapy with the If blocker ivabradine.

Purpose
We aimed to assess the effects of long-term If blockade using ivabradine on cardiac autonomic modulation and on the cardiovascular response to acute in vivo and in vitro parasympathetic stimulation.

Methods
Radiotelemetry ECG transmitters were implanted in 6 Control and 10 ivabradine-treated male Wistar rats (IVA; 3 weeks, 10 mg/kg/day); sympathetic and parasympathetic heart rate variability parameters were assessed. At the end of the study, the right atrium was removed and right atrial HCN(1-4) RNA expression levels were analyzed. The HR and systolic blood pressure (SBP) responses to in vivo electrical stimulation of the right vagus nerve (2–20 Hz) and the spontaneous sinus node discharge rate (SNDR) response to in vitro cholinergic receptors stimulation using carbamylcholine (10-9–10-6 mol/L) were assessed in 6 additional Control and 10 IVA rats.

Results
At the end of the study, mean 24-h HR was significantly lower in the IVA compared with the Control rats (301.3 ± 7.5 bpm vs. 341.5 ± 8.3 bpm; p&lt; 0.01). Ivabradine administration led to a significant increase in vagal tone and shifted the sympatho-vagal balance towards vagal dominance (awake, asleep, and over 24-h; all p&lt; 0.05). In the Control rats, in vivo vagus nerve stimulation induced a progressive decrease in both the SBP (p = 0.0001) and the HR (p&lt; 0.0001). Meanwhile, in the IVA rats, vagal stimulation had no effect on the HR (p = 0.16) and induced a significantly lower drop in SBP (p&lt; 0.05). Ivabradine-treated rats also presented a significantly lower SNDR drop in response to carbamylcholine (p&lt; 0.01) and significantly higher HCN4 expression (p = 0.02).

Conclusion
Long-term If blockade using ivabradine caused a significant increase in vagal tone and shifted the autonomic balance towards vagal dominance in rats. Given the highly proarrhythmic effects of vagal activation at the atrial level, these findings could provide an explanation for the increased risk of atrial fibrillation associated with ivabradine use in clinical trials. In addition, ivabradine reduced the HR response to direct muscarinic receptors stimulation, canceled the cardioinhibitory response and blunted the hemodynamic response to in vivo vagal stimulation, and led to significant sinus node HCN4 up-regulation. These data suggest that ivabradine-induced HCN4 and the consequent If up-regulation could render the sinus node less sensitive to acute vagal inputs and could thus protect against excessive bradycardia induced by acute vagal activation.