Many members of the two-pore-domain potassium (K⁺) channel family have been detected in the mammalian heart but the endogenous correlates of these channels still have to be identified. We investigated whether I_KAA, a background K⁺ current activated by negative pressure (stretch) and by arachidonic acid (AA) and sensitive to intracellular acidification, could be the native correlate of TREK-1 in adult rat atrial cells. Using the inside-out configuration of the patch-clamp technique, we found that I_KAA, like TREK-1, was outwardly rectifying in physiological K⁺ conditions, with a conductance of 41 pS at +50 mV. Like TREK-1, I_KAA was reversibly activated by clinical concentrations of volatile anesthetics (in mmol/L, chloroform 0.18, halothane 0.11, and isoflurane 0.69). In cell-attached experiments, I_KAA was inhibited by chlorophenylthio-cAMP (500 μmol/L) and also by stimulation of β-adrenergic receptors with isoproterenol (1 μmol/L). In addition, TREK-1 mRNAs were detected in all cardiac tissues, and the TREK-1 protein was immunolocalized in isolated atrial myocytes. Such a background potassium channel might contribute to the positive inotropic effects produced by β-adrenergic stimulation of the heart. It might also be involved in the regulation of the atrial natriuretic peptide secretion.