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

Aims. - In human atrial myocytes (HuAM) two beta-adrenergic receptors (beta-AR) and four splicing-variants of the serotonin 5-HT4 receptor are present. Multiple coupling with G stimulatory (G,) and G inhibitory (G) proteins has been proposed for both beta(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, could play a role in triggering human atrial arrhythmias. This study evaluated the different G protein coupling of beta(1)-AR, beta(2)-AR and 5-HT4 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. - beta(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 (Delta V-h), P < 0.01. beta(2)-AR stimulation (ISO 1 mu M + CGP 20712A) also significantly shifted V-h (P < 0.0001), but with Delta V-h[beta(2)-AR] significantly smaller than the effect caused by 1 mu M beta(1)-AR stimulation (P < 0.05). Pre-treatment of HuAM with PTX did not alter the effect of beta(1)-AR stimulation (both 0.1 and 1 mu M) and 1 mu M 5-HT on I-f, but significantly increased the effect in response to beta(2)-AR stimulation and 0. 1 mu M 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 beta(1)-AR, beta(2)-AR and 5-HT4 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. (c) 2005 Elsevier Ltd. All rights reserved.