ined the role of cardiac b1- and b2adrenoceptor subtypes in the pathogenesis of Doxcardiotoxicity and only one study, at late-onset Dox-CM, assessed b3-AR subtype, which is recently described as a new target for some b-blockers such as nebivolol. Despite this lack in experimental data, some clinical studies investigated bblocker therapies in Dox-CM. Kalay et al., demonstrated that left ventricular diameters remained constant and diastolic function was better preserved after Dox-treatment in patients receiving carvedilol, compared to placebo. However, Georgakopoulos et al., demonstrated that metoprolol, a b-blocker without antioxidative properties, failed to give cardioprotection in lymphoma-treated doxorubicin patients. Although the exact mechanism is still poorly understood, the authors highlighted the importance of an early diagnosis, because a delayed treatment is inefficient. The aim of the present study was to identify a putative target involved at the early phase of a chronic Dox-CM in a rat model. This study demonstrated for the first time that b2-AR expression was increased from 35 days after the first Dox-injection, this effect was maintained until 70 days after the first Dox-injection, resulting in an increase of b2-AR-induced contractility. In addition, b1-AR function was preserved, in spite of decreased b1-AR protein expression. This discrepancy could be explained by an increase of adenylyl cyclase expression and/ or activity as illustrated by an increased forskolin-induced response in Dox-CM rats. Methods Ethics Statement All experiments were performed in accordance with the 1996 Guide for the Care and Use of Laboratory Animals published by the U.S. National Institute of Health. The protocol was approved by the Direction Departementale de la Protection des Populations and all efforts were made to minimize LOXO 101 suffering. Animal Model One hundred and fifty eight male Sprague-Dawley rats were purchased from Janvier and were housed under standard conditions of room temperature, humidity and 12 h light/dark cycle. At d70, rats were used to perform echocardiography-Doppler 19329329 and biochemical studies. 2D speckle-tracking method on every medial myocardial segment. Left Ventricle Catheterization LV catheterization was performed, at d35, by a 2F microtip pressure catheter. Anaesthesia maintenance on spontaneously breathing rats was performed with an inhalational anaesthesia system for small animal. Isoflurane was delivered through a nose mask at a concentration of 2% volume and 1 L.min21 O2 flow to limit hemodynamic repercussion. Body temperature was monitored by rectal probe and maintained constant by warming-blanket. The right carotid artery was isolated, ligated at the proximal part and the pressure catheter was inserted in. Signals were recorded using an Analogic/Digital converter, stored and displayed on a computer by the IOX1.5.7 Software System. Data were analysed using Datanalyst software. The following parameters were obtained: LV end diastolic pressure, LV contraction and relaxation velocities, the index of LV relaxation constant. The animals were thereafter sacrificed by injection of a pentobarbital overdose. Echocardiography-Doppler Transthoracic echocardiography was performed using a commercially available ultrasound system equipped with 
a 10 MHz sectorial probe. Rats were anaesthetized with a gas-mixture of 1% isoflurane in O2. The chest was shaved and the animal was positioned on a heating pad in a supine position. 26028783 All record