The signaling pathway mediating the contractile effect of ␤ 2 -adrenergic receptors (␤ 2 -AR) in the heart is still matter of debate. By using embryonic chick ventricular cardiomyocytes that express both functional ␤ 1 -and ␤ 2 -ARs, we show here that the specific ␤ 2 -AR agonist, zinterol, increases the amplitude of Ca 2؉ transients and cell contraction of electrically stimulated cells. Zinterol, up to 10 M, did not stimulate adenylyl cyclase activity, and its effect on Ca ␤ 1 -and ␤ 2 -Adrenergic receptors (␤ 1 -and ␤ 2 -ARs) 1 coexist in the hearts of various animal species, including humans. However, their relative amount and their respective participation in the positive chronotropic and inotropic effects of adrenaline and noradrenaline vary depending on the cardiac tissue, the animal species, and/or the pathophysiological state (1, 2). In the non-failing human left ventricle, ␤ 1 -ARs represent 80% of the total ␤-ARs but mediate about 60% only of ␤-adrenergicinduced ventricular contractility (3). In the human failing heart, the ␤ 1 /␤ 2 -AR ratio decreases, and the contribution of ␤ 2 -AR to the contractile responses becomes predominant over that of ␤ 1 -AR, in particular at low adrenaline concentrations (3, 4). For these reasons, the potential role of ␤ 2 -AR for improving cardiac performance has received considerable attention. In fact, the myocardial-targeted overexpression of ␤ 2 -ARs in transgenic mice significantly enhanced myocardial left ventricular contractility (5).It is well documented that ␤ 1 -AR and ␤ 2 -AR subtypes are coupled to adenylyl cyclase activation and that stimulation of both receptors generally leads to an increase in cellular cAMP (4, 6, 7). In human healthy heart, ␤ 2 -ARs are more efficiently coupled to adenylyl cyclase than ␤ 1 -ARs (6 -10). However, during cardiac failure, ␤ 2 -AR subtypes are partially uncoupled from adenylyl cyclase (6, 7), whereas their contribution to the positive inotropic effects of adrenaline and noradrenaline is increased to 63% (7). In addition, studies in the rat heart (11, 12) and in the non-failing and failing canine heart (13) have demonstrated a dissociation between the inotropic effect of ␤ 2 -AR and cellular cAMP increase. Based on those observations, Xiao et al. (12) proposed that unidentified signal transduction pathway(s), other than adenylyl cyclase and cAMP, could be involved in the cardiac inotropic response to ␤ 2 -AR stimulation.Angiotensin II (14,15), bradykinin (16,17), and endothelin (15, 18), which exert positive inotropic responses, evoke AA release in heart. Furthermore, in a recent study, we have demonstrated that glucagon action relies not only on cAMP but also on the synergistic support of AA, by activation of the cPLA 2 which hydrolyzes the sn-2 fatty acyl ester bonds of membranous phospholipids (15).The aim of the present study was to investigate the respective role of cAMP and AA in the cardiac response to ␤-adrenergic agonists. We used the model of embryonic chick ventricular cardiomyocytes that has been widely exploited ...

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We have recently established that enhancement of intracellular calcium cycling and contraction in response to ␤ 2 -adrenergic receptor (␤ 2 -AR) stimulation exclusively relies on the activation of the cytosolic phospholipase A 2 (cPLA 2 ) and arachidonic acid production, via a pertussis toxin-sensitive G protein (possibly Gi), in embryonic chick cardiomyocytes. We aimed to investigate the relevance of the ␤ 2 -AR/Gi/cPLA 2 pathway in the human myocardium. In left ventricular biopsies obtained from explanted hearts, ␤ 2 -AR stimulation exerted either an inhibition of cPLA 2 that was insensitive to pertussis toxin (PTX) treatment, or an activation of cPLA 2 , sensitive to PTX treatment. In right atrial appendages from patients who were undergoing open heart surgery, we demonstrated that ␤ 2 -AR-induced activation of cPLA 2 was favored in situations of altered ␤ 1 -AR and/or ␤ 2 -AR/adenylyl cyclase (AC) stimulations. Alterations were characterized by an increase in EC 50 value of norepinephrine and a decrease in the maximal AC activation in response to zinterol, respectively. Quantitative reverse transcription-polymerase chain reaction analyses highlighted a positive correlation between the expression of AC5 and AC6 mRNAs in human cardiac atria, which suggested that functional alterations in AC responses were unlikely to be related to changes in the AC5/ AC6 mRNA ratio. In addition, the shift from the cyclic AMP to the arachidonic acid pathway was not supported at the transcriptional level by opposite regulation of AC and cPLA 2 mRNAs expression. This study gives the first evidence of the recruitment of cPLA 2 by ␤ 2 -ARs in the human heart and suggests that the Gi/cPLA 2 pathway could substitute for a deficient Gs/AC pathway in mediating ␤ 2 -AR responses.

Le rôle messager de l'acide arachidonique dans le cardiomyocyte L'acide arachidonique est un constituant structural majeur des membranes et un précurseur essentiel de nombreuses molécules bioactives (eicosanoïdes). C'est aussi un second messager intracellulaire. Sous l'effet des phospholipases A2 (PLA2), C ou D, l'acide arachidonique est libéré des phospholipides membranaires en réponse à plusieurs hormones et dans certaines conditions pathologiques, l'ischémie en particulier. Son rôle de second messager dans les cardiomyocytes est à l'ordre du jour : il pourrait être le médiateur de l'apoptose déclenchée par le TNFα, comme cela a été décrit dans les cellules leucémiques. Produit sous l'action de la PLA2, il active la sphingomyélinase qui hydrolyse la sphingomyéline et libère le céramide. L'AA est aussi le second messager du mini-glucagon, fragment carboxy-terminal du glucagon : le mini-glucagon stimule la PLA2 et entraîne une libération graduée de l'AA membranaire ; l'AA, comme le mini-glucagon, agit en synergie avec le glucagon et l'AMP cyclique pour augmenter l'amplitude des oscillations calciques et la contraction des cellules stimulées électriquement (action inotrope positive). Les cibles de l'AA pourraient être la protéine-kinase C et les canaux ioniques.