Pharmacological modulation of the hyperpolarization-activated current ( If) in human atrial myocytes: focus on G protein-coupled receptors

Abstract

Aims. – In human atrial myocytes (HuAM) two β-adrenergic receptors (β-AR) and four splicing-variants of the serotonin 5-HT 4 receptor are present. Multiple coupling with G stimulatory (G s) and G inhibitory (G i) proteins has been proposed for both β 2-AR and 5-HT (4b) subtypes, but no functional data exist in HuAM. Serotonin (5-HT) and catecholamines are able to trigger arrhythmias in human atrium, but the underlying cellular mechanisms are not completely understood. The pacemaker current ( I f) is an inward Na +/K + current, constitutively present in HuAM and directly modulated by cAMP; I f could play a role in triggering human atrial arrhythmias. This study evaluated the different G protein coupling of β 1-AR, β 2-AR and 5-HT 4 receptors by assessing the modulation of I f by selective stimuli. Methods. – HuAM were isolated from right atrial appendages and utilized for patch-clamp recording. The coupling of receptor subtypes with G i proteins was tested by incubating HuAM in pertussis toxin (PTX). Results. – β 1-AR stimulation (Isoprenaline [ISO] + ICI 118,551), and 5-HT caused a concentration-dependent significant shift of the half activation potential of I f activation curve (Δ V h), P < 0.01. β 2-AR stimulation (ISO 1 μM + CGP 20712A) also significantly shifted V h ( P < 0.0001), but with Δ V h[β 2-AR] significantly smaller than the effect caused by 1 μM β 1-AR stimulation ( P < 0.05). Pre-treatment of HuAM with PTX did not alter the effect of β 1-AR stimulation (both 0.1 and 1 μM) and 1 μM 5-HT on I f, but significantly increased the effect in response to β 2-AR stimulation and 0.1 μM 5-HT ( P < 0.05 for both), thus suggesting a G i protein coupling of these receptors. Conclusions. – Our results provide the first functional evidence of the different G protein coupling of β 1-AR, β 2-AR and 5-HT 4 receptors in HuAM. Further they support the view that I f current might play an important role in triggering catecholamines and serotonin-induced atrial arrhythmias.