title: Mechanisms of enhanced beta-adrenergic reserve from cardiac resynchronization therapy.
abstract: BACKGROUND
  Cardiac resynchronization therapy (CRT) is the first clinical heart failure treatment that improves chamber systolic function in both the short-term and long-term yet also reduces mortality. The mechanical impact of CRT is immediate and well documented, yet its long-term influences on myocyte function and adrenergic modulation that may contribute to its sustained benefits are largely unknown.
  
  
  RESULTS
  We used a canine model of dyssynchronous heart failure (DHF; left bundle ablation, atrial tachypacing for 6 weeks) and CRT (DHF for 3 weeks, biventricular tachypacing for subsequent 3 weeks), contrasting both to nonfailing controls. CRT restored contractile synchrony and improved systolic function compared with DHF. Myocyte sarcomere shortening and calcium transients were markedly depressed at rest and after isoproterenol stimulation in DHF (both anterior and lateral walls), and CRT substantially improved both. In addition, beta(1) and beta(2) stimulation was enhanced, coupled to increased beta(1) receptor abundance but no change in binding affinity. CRT also augmented adenylate cyclase activity over DHF. Inhibitory G-protein (Galpha(i)) suppression of beta-adrenergic stimulation was greater in DHF and reversed by CRT. Galpha(i) expression itself was unaltered; however, expression of negative regulators of Galpha(i) signaling (particularly RGS3) rose uniquely with CRT over DHF and controls. CRT blunted elevated myocardial catecholamines in DHF, restoring levels toward control.
  
  
  CONCLUSIONS
  CRT improves rest and beta-adrenergic-stimulated myocyte function and calcium handling, upregulating beta(1) receptors and adenylate cyclase activity and suppressing G(i)-coupled signaling associated with novel RGS upregulation. The result is greater rest and sympathetic reserve despite reduced myocardial neurostimulation as components underlying its net benefit.
authors.affiliation: Department of Medicine, Johns Hopkins Medical Institutions, Baltimore, MD, USA.
authors.forename: Khalid
authors.initials: K
authors.lastname: Chakir
authors.forename: Samantapudi K
authors.initials: SK
authors.lastname: Daya
authors.forename: Takeshi
authors.initials: T
authors.lastname: Aiba
authors.forename: Richard S
authors.initials: RS
authors.lastname: Tunin
authors.forename: Veronica L
authors.initials: VL
authors.lastname: Dimaano
authors.forename: Theodore P
authors.initials: TP
authors.lastname: Abraham
authors.forename: Kathryn M
authors.initials: KM
authors.lastname: Jaques-Robinson
authors.forename: Kathryn
authors.initials: K
authors.lastname: Jacques
authors.forename: Edwin W
authors.initials: EW
authors.lastname: Lai
authors.forename: Karel
authors.initials: K
authors.lastname: Pacak
authors.forename: Wei-Zhong
authors.initials: WZ
authors.lastname: Zhu
authors.forename: Rui-ping
authors.initials: RP
authors.lastname: Xiao
authors.forename: Gordon F
authors.initials: GF
authors.lastname: Tomaselli
authors.forename: David A
authors.initials: DA
authors.lastname: Kass
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D002395:Catecholamines / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D004285:Dogs; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism; D006333:Heart Failure / Q000378:metabolism / Q000503:physiopathology* / Q000628:therapy*; D009200:Myocardial Contraction / Q000502:physiology; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000502:physiology; D010138:Pacemaker, Artificial; D020710:RGS Proteins / Q000502:physiology; D011869:Radioligand Assay; D018342:Receptors, Adrenergic, beta-1 / Q000502:physiology*; D018343:Receptors, Adrenergic, beta-2 / Q000502:physiology*; D012518:Sarcomeres / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology
chemical_list: D002395:Catecholamines; D020710:RGS Proteins; D018342:Receptors, Adrenergic, beta-1; D018343:Receptors, Adrenergic, beta-2; D005576:Colforsin; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D000262:Adenylyl Cyclases