Melatonin receptor blockade slows ventricular conduction by IK1 downregulation
Authors: Gonotkov M A , Durkina A V , Bernikova O G , Azarov J ,   Department of Cardiac Physiology, Institute of Physiology, Komi Science Centre, Ural Branch of Russian Academy of Sciences, Syktyvkar, Russia
Published: October 2021

Abstract

Background
Melatonin has been reported to prevent life-threatening arrhythmias in myocardial ischemia and reperfusion. The recent study from our group demonstrated that the antiarrhythmic action of melatonin was associated with enhanced recovery of resting membrane potential (RMP) and shortened activation time delay in the affected myocardium. This effect was independent of melatonin antioxidative properties, which raises a question about the involvement of melatonin receptor-dependent signaling in the electrophysiological action of melatonin. The aim of this study was to evaluate a role of MT1/MT2 melatonin receptors in the regulation of RMP and ventricular conduction velocity (CV).

Methods
Anesthetized male rats underwent coronary occlusion (5 min) and reperfusion (5 min). 5 min before ischemia induction the animals were intravenously injected with saline (control, n=7), melatonin (4 mg/kg, n=8), luzindole (MT1/MT2 receptor blocker, 0.4 mg/kg, n=6), and melatonin/luzindole combination (4 and 0.4 mg/kg, respectively, n=7). CV was measured in baseline and reperfusion by epicardial 64-lead mapping under epicardial pacing. In the enzymatically isolated ventricular myocytes (n=20 cells), voltage- and current-clamp modes of whole cell patch-clamp were used for measurements of IK1 and INa currents, and RMP, respectively. Melatonin (10 uM), luzindole (1 uM) and their combination were added to the bath solution.

Results
Luzindole application decreased baseline longitudinal CV as compared to placebo (52±3 vs 42±2 cm/s, p=0.028, respectively). Paired comparisons demonstrated that longitudinal CV decreased at reperfusion vs baseline in the control group (35±2 vs 53±6 cm/s, p=0.018) and in the luzindole group (33±4 vs 38±3 cm/s, p=0.039), whereas no significant differences were observed in the melatonin and melatonin/luzindole groups (40±5 vs 51±3 cm/s, p=0.114 and 39±3 vs 43±3 cm/s, p=0.498, respectively). Neither melatonin nor luzindole changed INa current characteristics (Figure, panels a and b). The average amplitude of IK1 current was 2.08±0.92 at −50 mV and −16.11±4.70 pA/pF at −120 mV in the normal Tyrode solution. Luzindole significantly reduced IK1 amplitude to 0.82±0.48 (p=0.028) and −5.6±1.6 (p=0.008) pA/pF at −50 and −120 mV, respectively. Addition of melatonin to the luzindole-containing solution partly abolished the luzindole effect (Figure, panel c). Luzindole also reduced RMP from −63±10 mV to −37±12 mV (p=0.013) and caused an increase in APD100 duration from 35±6 ms to 170±43 ms (p=0.003) (Figure, panel d).

Conclusion
Thus, MT1/MT2 receptor block with luzindole did not affect INa but led to the down-regulation of IK1 and depolarization of RMP. These effects can reduce accessibility of sodium channels and be the basis for the observed luzindole-dependent CV slowing. The obtained data support the significance of melatonin receptor-dependent signaling in the realization of antiarrhythmic effects of melatonin.

Funding Acknowledgement
Type of funding sources: Foundation. Main funding source(s): Russian Science Foundation