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Purpose: Adenine nucleotides control cardiovascular function via P2X and P2Y receptors activation. The P2X4 is the most expressed P2X receptor subtype in human atria. Up-regulation of P2X4 receptor (P2X4R)-mediated inotropism has been observed in animal models of heart failure. This receptor is considered a putative pharmacological target for ischemia/reperfusion. Despite the effects of the P2X4R on cardiovascular pathophysiology, no studies addressed its role on the atrial pacemaker activity.Methods: Experiments were performed on isolated spontaneously beating right atria and on 4 Hz-paced right ventricle strips from Wistar rats superfused with gassed Tyrode's solution. Isometric muscle tension was monitored with a PowerLab data acquisition system.Results: ATP (100 μM) and its non-hydrolysable analogue, ATPΓS (100 μM), caused rapid and sustained negative chronotropic effects on spontaneously beating right atria. The negative chronotropic effect of ATP was potentiated by inhibiting ecto-NTPDases with POM-1 (100 μM). The P2X4R antagonist, 5'-BDBD (10 μM), prevented the negative chronotropic effect of ATP. ATP (100 μM) produced a mild decrease in the magnitude of paced ventricular tension, which was further potentiated, rather than attenuated, in the presence of 5'-BDBD (10 μM). Inhibition of the reverse mode of the Na+/Ca2+ exchanger (NCX) with KB-R7943 (3 μM) mimicked the effects of the P2X4R antagonist, 5'-BDBD (10 μM), both on spontaneously beating right atria and paced right ventricle.Conclusions: Data suggest that P2X4R activation decreases the SA node rate while increasing the ventricular contractile force. The mechanism underlying these dual effects of ATP in the rat heart may involve bolstering of the NCX function in the reverse mode. Na+ influx via P2X4 might inhibit/revert the electrogenic forward mode of the NCX decreasing chronotropy. Likewise, intracellular Ca2+ accumulation due to inhibition and/or reversal of the NCX might explain the positive inotropic effect attributed to P2X4R activation on paced right ventricular strips Thus, targeting the P2X4R activation or its underlying NCX-based mechanism may create novel well-tolerated heart-rate lowering drugs with potential benefits in patients with deteriorated ventricular function. [ABSTRACT FROM AUTHOR]

Purpose: Adenosine fine-tuning regulates cardiovascular function. The negative chronotropic effect of adenosine depends on the activation of the most abundant adenosine receptor in the heart, the A1AR, leading to GIRK/Kir3.1/3.4 channels opening in the SA node. The A1AR-mediated responses in the atrial muscle also depend on KCa2/SK channel function (unpublished observations). Since these channels are involved in atrial repolarization and in human rhythm disturbances, we aimed at studying the interplay between A1AR and SK channels both in situ and single-cell preparations of rat atria.Methods: Isometric contraction experiments were performed on isolated spontaneously beating atria from Wistar rats. For whole-cell voltage-clamp recordings, atrial myocytes were enzymatically isolated by retrograde perfusion. Immunolocalization studies (A1AR, GIRK1, KCa2.2 and KCa2.3) were evaluated by confocal microscopy. The SA node was identified as a connexin-43 low / neurofilament-160 high immunoreactive region.Results: The selective A1AR agonist R-PIA (0.001-1μM) decreased atrial chronotropism and inotropism in concentration-dependent manner. However, the negative chronotropic effect of R-PIA (IC50~0.03 μM) was evidenced at much lower (~30 times) concentrations than the negative inotropic action (IC50~1 μM). Selective blockade of A1AR and of GIRK/Kir3.1/3.4 channels, respectively with DPCPX (100 nM) and tertiapin Q (300 nM), significantly attenuated the effects R-PIA. Blockade of KCa2/SK channels with apamin (30 nM) potentiated the negative inotropic response of R-PIA, without affecting atrial rate. Apamin-induced atrial sensitization to R-PIA was not observed with other cardiodepressant agents, namely oxotremorine (0.01-3 μM). Voltage-clamp experiments demonstrated that adenosine, via A1AR, plays a dual role in atrial cardiomyocytes by activating inwardly rectifying GIRK/Kir3.1/3.4 and inactivating outward Ca2+-activated KCa2/SK currents upon cell depolarization. Immunolabelling of A1AR, KIR3.1, KCa2.2 and KCa2.3 was observed throughout the right atria.Conclusion: Data demonstrate for the first time that A1AR has opposing effects on distinct K+ currents operating atrial repolarization. While A1AR activation favors the opening of inwardly rectifying GIRK/KIR3.1/3.4 channels modulating atrial automatism, it may also inactivate outward KCa2/SK currents. Taking into consideration that KCa2/SK channel dysfunction has pro-arrhythmic effects drugs targeting the A1AR for supraventricular tachycardia must be used with caution as they might precipitate atrial arrhythmias. [ABSTRACT FROM PUBLISHER]