We have shown previously that overexpression of Kir3.4 using adenoviral gene transfer in adult atrial myocytes results in formation of homotetrameric channel complexes with weaker inward rectification than endogenous ACh-activated current (I_K(ACh))¹. In the present study, properties of current carried by homomeric Kir3.4 channels were investigated and compared with endogenous I_K(ACh) using whole cell voltage clamp. In Kir3.4 overexpressing myocytes intracellular Na⁺ ≥ 15 mM, supplied via the patch pipette, resulted in concentration-dependent activation of a background inward-rectifying current (I_bir) without saturation up to 60 mM. In contrast to reports based on oocyte studies, endogenous (Kir3.1/Kir3.4) heteromeric channels were completely insensitive to [Na⁺]_i in this range of concentrations. Myocytes expressing a mutant Kir3.4 subunit (D223N), which lacks the putative Na⁺ binding aspartate residue, were devoid of Na⁺-dependent activation of I_bir. [Na⁺]_i-dependent background inward-rectifying current was neither affected by Pertussis toxin nor by GDP-β-S, suggesting its activation to be independent of the G protein cycle. Using a protocol to induce PIP₂ depletion via endogenous PLC-coupled α₂ adrenergic receptors, native Kir3.1/3.4 channel currents were inhibited by about 75%. In contrast, inhibition of I_bir amounted to < 20 %. Kir3.x channel currents can be selectively blocked by Tertiapin-Q (T-Q). Inhibition of I_K(ACh) by T-Q can be described using an EC₅₀ of 12 nM, in line with published data. Sensitivity of I_bir to the blocking venom was almost 20 fold higher (EC₅₀: 0.7 nM). These data clearly suggest, that [Na⁺]_i-dependent gating has no relevance to endogenous Kir3.1/Kir3.4 channels in cardiac myocytes. Furthermore, our results identify I_bir as a homotetrameric Kir3.4 channel current with novel properties of regulation and pharmacology. I_bir shares some properties in common with a background current recently described in atrial myocytes from an animal model of atrial fibrillation and human AF patients^2,3.