Adrenaline-mediated biased agonism at the B2 adrenoceptor in an in vivo model of takutsubo cardiomyopathy
Author(s)
Paur, Helen Elizabeth
Type
Thesis
Abstract
Stress (Takotsubo) cardiomyopathy is a severe form of acute heart failure (HF) with
rapid onset, characterized by hypocontraction of the heart from the mid-left ventricle to
apex. It is precipitated by extreme stress and plasma levels of the catecholamines
adrenaline and noradrenaline are significantly elevated. This thesis describes the
development of a new model of Takotsubo cardiomyopathy through rapid intravenous
injection of a supraphysiological dose of adrenaline into anaesthetised male Sprague-
Dawley (SD) rats and real-time imaging of the heart using either 2D-echocardiography or
cardiac magnetic resonance (CMR). This in vivo rat model was used, in conjunction with
in vitro IonOptix video-edge detection of cardiomyocyte contractility, to firstly
demonstrate that this syndrome results from an adrenaline-induced trafficking-switch of
the coupling of the pleiotropic β2-adrenceptor (AR) from Gs-adenylyl cyclase (AC)-cyclic
adenosine monophosphate (cAMP) cardiostimulant to Gi-activated cardiodepressant
pathways. Cardiomyocyte β2AR:β1AR population ratios, determined by radioligand
binding, suggested that an apical-basal gradient in β2AR expression accounted for the in
vivo apical hypokinesis. Certain β-blockers can act as biased agonists of a Gi-coupled
isoform of the β2AR in animal and human cardiomyocytes via a pathway that involves
p38 MAPK. Administration of the clinical β-blockers propranolol and carvedilol 15
minutes after adrenaline either enhanced or failed to reverse apical hypocontractility. In
vitro analysis demonstrated that β2AR-Gi signalling involves p38 MAPK activation.
However, pre-treatment with either the p38 MAPK inhibitor SB203580 or the β2AR
blocker ICI-118,551 prior to adrenaline resulted in significant mortality, which suggested
that the β2AR-Gs/Gi coupling switch is a cardioprotective strategy. Gi-protein is
upregulated in chronic HF, while β1ARs are down-regulated. In contractility studies of
chronically failing human/rat cardiomyocytes, β2AR-Gs responses were present with
minimal active Gi-protein component. This suggested that while in acute HF β2AR-Gs
positively inotropic responses are impaired and β1ARs are preserved, the opposite may
be true in chronic HF.
rapid onset, characterized by hypocontraction of the heart from the mid-left ventricle to
apex. It is precipitated by extreme stress and plasma levels of the catecholamines
adrenaline and noradrenaline are significantly elevated. This thesis describes the
development of a new model of Takotsubo cardiomyopathy through rapid intravenous
injection of a supraphysiological dose of adrenaline into anaesthetised male Sprague-
Dawley (SD) rats and real-time imaging of the heart using either 2D-echocardiography or
cardiac magnetic resonance (CMR). This in vivo rat model was used, in conjunction with
in vitro IonOptix video-edge detection of cardiomyocyte contractility, to firstly
demonstrate that this syndrome results from an adrenaline-induced trafficking-switch of
the coupling of the pleiotropic β2-adrenceptor (AR) from Gs-adenylyl cyclase (AC)-cyclic
adenosine monophosphate (cAMP) cardiostimulant to Gi-activated cardiodepressant
pathways. Cardiomyocyte β2AR:β1AR population ratios, determined by radioligand
binding, suggested that an apical-basal gradient in β2AR expression accounted for the in
vivo apical hypokinesis. Certain β-blockers can act as biased agonists of a Gi-coupled
isoform of the β2AR in animal and human cardiomyocytes via a pathway that involves
p38 MAPK. Administration of the clinical β-blockers propranolol and carvedilol 15
minutes after adrenaline either enhanced or failed to reverse apical hypocontractility. In
vitro analysis demonstrated that β2AR-Gi signalling involves p38 MAPK activation.
However, pre-treatment with either the p38 MAPK inhibitor SB203580 or the β2AR
blocker ICI-118,551 prior to adrenaline resulted in significant mortality, which suggested
that the β2AR-Gs/Gi coupling switch is a cardioprotective strategy. Gi-protein is
upregulated in chronic HF, while β1ARs are down-regulated. In contractility studies of
chronically failing human/rat cardiomyocytes, β2AR-Gs responses were present with
minimal active Gi-protein component. This suggested that while in acute HF β2AR-Gs
positively inotropic responses are impaired and β1ARs are preserved, the opposite may
be true in chronic HF.
Date Issued
2012
Date Awarded
2012-04
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Lyon, Alexander
Gorelik, Julia
Harding, Sian
Publisher Department
National Heart and Lung Institute
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)