Stress Cardiomyopathy: a role for stimulus trafficking at the β2-adrenoceptor in cardiac depression?

Helen Paur, Sian Harding, Alexander Lyon, Catherine Mansfield, Edwin Garcia Castano, Viacheslav Nikolaev. 1Imperial College, London, United Kingdom, 2University of Wurzburg, Wurzburg, Germany.

Stress (Takotsubo) cardiomyopathy is a clinical condition characterised by acute left ventricular dysfunction that presents as akinetic apical and hyperkinetic basal myocardium. Surges in the levels of plasma catecholamines induce activation of cardiac β1 and β2 adrenoceptors (ARs), which regulate heart rate. Stimulus trafficking is defined as the ability of a receptor to couple to multiple G-protein signalling pathways. At physiological levels of adrenaline, the β2AR mediates a positive inotropic effect through coupling to the Gs-protein adenylyl cyclase protein kinase A (PKA) pathway. We hypothesise that higher concentrations of adrenaline induce a switch in β2AR coupling from Gs to Gi protein, through PKA-mediated receptor phosphorylation, leading to a negative inotropic effect and contributing to the phenotype of heart failure.

Previous echocardiography in rats has demonstrated that intravenously high concentrations of adrenaline (β2AR activation), but not noradrenaline (β1AR activation), elicit a negative inotropic effect at the mid-left ventricle (MLV) and apex but not the base of the hearts of normal rats. In-vivo over-expression of Gi protein within the apex of the rat heart mimics the apical dilatation and depressed contraction observed in patients with stress cardiomyopathy, and accelerates the response to adrenaline.

In this study, left ventricular cardiomyocytes were isolated from the excised hearts of Sprague-Dawley rats by ex-vivo enzymatic digestion. In-vitro measurement of myocyte contraction was performed using an IonOptix video motion detector setup. Radioligand binding assays indicated a larger population of β2ARs in myocytes from the apex of the heart compared to the base. This is consistent with a larger maximum positive β2AR contractile response (to 1μM isoprenaline in presence of 300nM CGP20712A) of apical cardiomyocytes in-vitro (96.7±20.6% increase over basal (apex) p<0.001, n=10; vs 60.0±12.6% (base) p<0.001, n=13). It was also noted that the in-vitro β2AR stimulation gave a transient positive inotropic effect, with a maximum response to isoprenaline of 50.1±5.26% dropping to 31.9±11.1% within 10 min (n=13 cells, max vs. min p<0.05). This was accompanied by phosphorylation of the β2AR PKA site. Gi overexpression in myocytes abolished the positive inotropic effect of β2AR stimulation.

These results suggest the presence of an apical-basal gradient in the expression of β2AR. The transient effect of β2AR stimulation, coupled with the adrenaline-mediated β2AR phosphorylation and the depressant effect of Gi are consistent with the hypothesis of a possible role for β2AR-Gi coupling in the apical akinesia observed in stress cardiomyopathy.