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 ...

It has been recently shown that the physiological processing of glucagon into its C-terminal (19-29) fragment, miniglucagon, by cardiac cells was essential for the contractile positive inotropic effect of the hormone. However, the mechanisms underlying the effects of miniglucagon remained undetermined. In the present study, we assessed the effects of miniglucagon on Ca2+ homeostasis in embryonic chick ventricular myocytes. In quiescent cells, short-term applications of 0.1 nmol/L miniglucagon markedly increased the accumulation of 45Ca into intracellular compartments resistant to digitonin lysis and sensitive to caffeine. Ca2+ accumulation into the sarcoplasmic reticular (SR) store was further attested by fura 2 imaging studies on quiescent or prestimulated cells: miniglucagon potentiated Ca2+ release from the SR compartment triggered by caffeine and evoked a rise in cytosolic Ca2+ when applied on cells pretreated with 1 mumol/L thapsigargin, a specific inhibitor of the SR Ca2+ pump. Glucagon alone produced a small cytosolic Ca2+ signal that was considerably amplified by miniglucagon. The action of glucagon was mimicked by 8-bromo-cAMP and was blocked by isradipine, suggesting that it relied on the activation of L-type Ca2+ channels, via phosphorylation. We conclude that the combined actions of miniglucagon and glucagon on Ca2+ accumulation into SR stores and Ca2+ release from the same stores are likely to support the positive inotropic effect elicited in vivo by glucagon on heart contraction.

We conclude that AA drives miniglucagon action in the heart and that the positive inotropic effect of glucagon on heart contraction relies on both second messengers, cAMP and AA.

No abstract

Xanthine, a major purine by-product of ATP, accumulates during myocardial ischemia. In the present study, we show that xanthine (0.5-1 mM) impaired the occurrence of cytosolic Ca2+ concentration ([Ca2+]i) transients, visualized in fura 2-loaded cells, and twitches of contraction in ventricular cardiocytes in response to electrical stimulation. This effect of xanthine was independent of superoxide anion production. That it was a result of decreased membrane excitability was supported by the following: 1) it was reversed by increasing either the amplitude of the stimulus voltage required to stimulate cardiocytes or the extracellular concentration of NaCl; and 2) xanthine reversed the depolarization following electrical stimulation in cardiocytes loaded with the voltage-sensitive dye bis-oxonol. P2 purinergic-agonists, including ATP (10 microM), but not P1 purinergic agonists reproduced the effects seen with xanthine. In addition, a lack of additivity between xanthine and ATP at maximal concentrations was observed. We conclude that xanthine, through activation of a P2 purinoceptor, may contribute to myocardial arrhythmia occurring during ischemia-reperfusion injury.