title: Pharmacological modulation of the hyperpolarization-activated current (I f) in human atrial myocytes: focus on G protein-coupled receptors.
abstract: OBJECTIVE
  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.
authors.affiliation: Department of Preclinical and Clinical Pharmacology, University of Florence, Viale Pieraccini 6, 50139 Florence, Italy.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Lonardo
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Cerbai
authors.forename: Simona
authors.initials: S
authors.lastname: Casini
authors.forename: Gabriele
authors.initials: G
authors.lastname: Giunti
authors.forename: Massimo
authors.initials: M
authors.lastname: Bonacchi
authors.forename: Francesco
authors.initials: F
authors.lastname: Battaglia
authors.forename: Brenno
authors.initials: B
authors.lastname: Fiorani
authors.forename: Pier Luigi
authors.initials: PL
authors.lastname: Stefano
authors.forename: Guido
authors.initials: G
authors.lastname: Sani
authors.forename: Alessandro
authors.initials: A
authors.lastname: Mugelli
mesh_terms: D000368:Aged; D001145:Arrhythmias, Cardiac / Q000209:etiology / Q000378:metabolism; D005260:Female; D006325:Heart Atria / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007093:Imidazoles / Q000494:pharmacology; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials; D008875:Middle Aged; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011412:Propanolamines / Q000494:pharmacology; D018342:Receptors, Adrenergic, beta-1 / Q000378:metabolism; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism*; D044422:Receptors, Serotonin, 5-HT4 / Q000378:metabolism; D012701:Serotonin / Q000494:pharmacology
chemical_list: D007093:Imidazoles; D011412:Propanolamines; D018342:Receptors, Adrenergic, beta-1; D018343:Receptors, Adrenergic, beta-2; D043562:Receptors, G-Protein-Coupled; D044422:Receptors, Serotonin, 5-HT4; D012701:Serotonin; C026777:ICI 118551; C049763:CGP 20712A; D007545:Isoproterenol