title: RhoA/ROCK signaling and pleiotropic α1A-adrenergic receptor regulation of cardiac contractility.
abstract: OBJECTIVE
  To determine the mechanisms by which the α1A-adrenergic receptor (AR) regulates cardiac contractility.
  
  
  BACKGROUND
  We reported previously that transgenic mice with cardiac-restricted α1A-AR overexpression (α1A-TG) exhibit enhanced contractility but not hypertrophy, despite evidence implicating this Gαq/11-coupled receptor in hypertrophy.
  
  
  METHODS
  Contractility, calcium (Ca(2+)) kinetics and sensitivity, and contractile proteins were examined in cardiomyocytes, isolated hearts and skinned fibers from α1A-TG mice (170-fold overexpression) and their non-TG littermates (NTL) before and after α1A-AR agonist stimulation and blockade, angiotensin II (AngII), and Rho kinase (ROCK) inhibition.
  
  
  RESULTS
  Hypercontractility without hypertrophy with α1A-AR overexpression is shown to result from increased intracellular Ca(2+) release in response to agonist, augmenting the systolic amplitude of the intracellular Ca(2+) concentration [Ca(2+)]i transient without changing resting [Ca(2+)]i. In the absence of agonist, however, α1A-AR overexpression reduced contractility despite unchanged [Ca(2+)]i. This hypocontractility is not due to heterologous desensitization: the contractile response to AngII, acting via its Gαq/11-coupled receptor, was unaltered. Rather, the hypocontractility is a pleiotropic signaling effect of the α1A-AR in the absence of agonist, inhibiting RhoA/ROCK activity, resulting in hypophosphorylation of both myosin phosphatase targeting subunit 1 (MYPT1) and cardiac myosin light chain 2 (cMLC2), reducing the Ca(2+) sensitivity of the contractile machinery: all these effects were rapidly reversed by selective α1A-AR blockade. Critically, ROCK inhibition in normal hearts of NTLs without α1A-AR overexpression caused hypophosphorylation of both MYPT1 and cMLC2, and rapidly reduced basal contractility.
  
  
  CONCLUSIONS
  We report for the first time pleiotropic α1A-AR signaling and the physiological role of RhoA/ROCK signaling in maintaining contractility in the normal heart.
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia; Cardiology Department, St Vincent's Hospital, Darlinghurst, Australia; Faculty of Medicine, University of New South Wales, Sydney, Australia.
authors.forename: Ze-Yan
authors.initials: ZY
authors.lastname: Yu
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia.
authors.forename: Ju-Chiat
authors.initials: JC
authors.lastname: Tan
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia; Cardiology Department, St Vincent's Hospital, Darlinghurst, Australia.
authors.forename: Aisling C
authors.initials: AC
authors.lastname: McMahon
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia; Faculty of Medicine, University of New South Wales, Sydney, Australia.
authors.forename: Siiri E
authors.initials: SE
authors.lastname: Iismaa
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia.
authors.forename: Xiao-Hui
authors.initials: XH
authors.lastname: Xiao
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia.
authors.forename: Scott H
authors.initials: SH
authors.lastname: Kesteven
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia.
authors.forename: Melissa E
authors.initials: ME
authors.lastname: Reichelt
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia.
authors.forename: Marion C
authors.initials: MC
authors.lastname: Mohl
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia; Faculty of Medicine, University of New South Wales, Sydney, Australia.
authors.forename: Nicola J
authors.initials: NJ
authors.lastname: Smith
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia; Cardiology Department, St Vincent's Hospital, Darlinghurst, Australia; Faculty of Medicine, University of New South Wales, Sydney, Australia.
authors.forename: Diane
authors.initials: D
authors.lastname: Fatkin
authors.affiliation: Physiology Department, University of Sydney, Sydney, Australia.
authors.forename: David
authors.initials: D
authors.lastname: Allen
authors.affiliation: Physiology Department, University of New South Wales, Sydney, Australia.
authors.forename: Stewart I
authors.initials: SI
authors.lastname: Head
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia; Cardiology Department, St Vincent's Hospital, Darlinghurst, Australia; Faculty of Medicine, University of New South Wales, Sydney, Australia.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Graham
authors.affiliation: Victor Chang Cardiac Research Institute, Darlinghurst, Australia; Cardiology Department, St Vincent's Hospital, Darlinghurst, Australia; Faculty of Medicine, University of New South Wales, Sydney, Australia.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Feneley
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D009200:Myocardial Contraction / Q000502:physiology*; D010766:Phosphorylation; D018340:Receptors, Adrenergic, alpha-1 / Q000502:physiology*; D015398:Signal Transduction; D054460:rho-Associated Kinases / Q000378:metabolism*; D020742:rhoA GTP-Binding Protein / Q000378:metabolism*
chemical_list: D000316:Adrenergic alpha-Agonists; D018340:Receptors, Adrenergic, alpha-1; C515854:Rock1 protein, mouse; D054460:rho-Associated Kinases; D020742:rhoA GTP-Binding Protein; D002118:Calcium