Inhibition of a TREK-like K+ channel current by noradrenaline requires both β1- and β2-adrenoceptors in rat atrial myocytes

Aims Noradrenaline plays an important role in the modulation of atrial electrophysiology. However, the identity of the modulated channels, their mechanisms of modulation, and their role in the action potential remain unclear. This study aimed to investigate the noradrenergic modulation of an atrial steady-state outward current (I-Kss). Methods and results Rat atrial myocyte whole-cell currents were recorded at 36 degrees C. Noradrenaline potently inhibited I-Kss (IC50 = 0.90 nM, 42.1 +/- 4.3% at 1 mu M, n = 7) and potentiated the L-type Ca2+ current (I-CaL, EC50 = 136 nM, 205 +/- 40% at 1 mu M, n = 6). Noradrenaline-sensitive I-Kss was weakly voltage-dependent, time-independent, and potentiated by the arachidonic acid analogue, 5,8,11,14-eicosatetraynoic acid (EYTA; 10 mu m), or by osmotically induced membrane stretch. Noise analysis revealed a unitary conductance of 8.4 +/- 0.42 pS (n = 8). The biophysical/pharmacological properties of I-Kss indicate a TREK-like K+ channel. The effect of noradrenaline on I-Kss was abolished by combined beta(1)-/beta(2)-adrenoceptor antagonism (1 mu M propranolol or 10 mu M beta(1)-selective atenolol and 100 nM beta(2)-selective ICI-118,551 in combination), but not by beta 1- or 2-antagonist alone. The action of noradrenaline could be mimicked by beta(2)-agonists (zinterol and fenoterol) in the presence of beta(1)-antagonist. The action of noradrenaline on I-Kss, but not on I-CaL, was abolished by pertussis toxin (PTX) treatment. The action of noradrenaline on I-CaL was mediated by beta(1)-adrenoceptors via a PTX-insensitive pathway. Noradrenaline prolonged APD(30) by 52 +/- 19% (n = 5; P < 0.05), and this effect was abolished by combined beta(1)-/beta(2)-antagonism, but not by atenolol alone. Conclusion Noradrenaline inhibits a rat atrial TREK-like K+ channel current via a PTX-sensitive mechanism involving co-operativity of beta(1)-/beta(2)-adrenoceptors that contributes to atrial APD prolongation.