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 β-adrenergic receptors (β-AR) and four splicing-variants of the serotonin 5-HT4 receptor are present. Multiple coupling with G stimulatory (Gs) and G inhibitory (Gi) proteins has been proposed for both β2-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 (If) is an inward Na+/K+ current, constitutively present in HuAM and directly modulated by cAMP; If could play a role in triggering human atrial arrhythmias. This study evaluated the different G protein coupling of β1-AR, β2-AR and 5-HT4 receptors by assessing the modulation of If by selective stimuli. Methods. - HuAM were isolated from right atrial appendages and utilized for patch-clamp recording. The coupling of receptor subtypes with Gi proteins was tested by incubating HuAM in pertussis toxin (PTX). Results. - β1-AR stimulation (Isoprenaline [ISO] + ICI 118,551), and 5-HT caused a concentration-dependent significant shift of the half activation potential of If activation curve (ΔVh), P < 0.01. β2-AR stimulation (ISO 1 μM + CGP 20712A) also significantly shifted Vh (P < 0.0001), but with ΔVh[β 2-AR] significantly smaller than the effect caused by 1 μM β1-AR stimulation (P < 0.05). Pre-treatment of HuAM with PTX did not alter the effect of β1-AR stimulation (both 0.1 and 1 μM) and 1 μM 5-HT on If, but significantly increased the effect in response to β2-AR stimulation and 0.1 μM 5-HT (P < 0.05 for both), thus suggesting a Gi protein coupling of these receptors. Conclusions. - Our results provide the first functional evidence of the different G protein coupling of β1-AR, β2-AR and 5-HT4 receptors in HuAM. Further they support the view that If current might play an important role in triggering catecholamines and serotonin-induced atrial arrhythmias In human atrial myocytes (HuAM) two beta-adrenergic receptors (beta-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 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(f) 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-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: 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 (DeltaV(h)), P < 0.01. beta(2)-AR stimulation (ISO 1 microM + CGP 20712A) also significantly shifted V(h) (P < 0.0001), but with DeltaV(h)[beta(2)-AR] significantly smaller than the effect caused by 1 microM 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 microM) and 1 microM 5-HT on I(f), but significantly increased the effect in response to beta(2)-AR stimulation and 0.1 microM 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-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.