We have shown that overexpression of Kir3.4 in atrial myocytes results in formation of homotetrameric channel complexes that are activated by intracellular Na+ (>10 mM), whereas, in contrast to reports based on oocyte studies, endogenous (Kir3.1/3.4) channels are insensitive to [Na+ ]i. In the present study properties of [Na+ ]i-dependent background inward-rectifying current (Ibir ) were investigated and compared with ACh-activated current carried by endogenous Kir3.1/3.4 channels (IK(ACh) ). Ibir was neither affected by Pertussis toxin nor by GDP-b-S, suggesting its activation to be G-protein-independent. Using a protocol to induce PIP2 depletion via endogenous PLC-coupled a1 adrenergic receptors, endogenous Kir3.1/3.4 channel currents are inhibited by about 75%. In contrast, inhibition of Ibir amounts to < 20 %. Kir3.x channel currents can be selectively blocked by Tertiapin-Q (T-Q). Inhibition of IK(ACh) and Ibir can be described using IC50 / nH of 12 nM / 1.0 (IK(ACh) ) and 0.7 nM / 1.95 (Ibir ). These data clearly identify Ibir as a homotetrameric Kir3.4 channel current with novel properties of regulation and pharmacology. Ibir shares some properties with a background current recently described in atrial myocytes from an animal model of atrial fibrillation (Cha et al. Circ. Res. 113, 2006) and human AF patients (Dobrev et al. Circulation 112 , 2005). Supported by DFG