This report describes a hitherto unreported anionic background current from human atrial cardiomyocytes. Under whole-cell patch-clamp with anion-selective conditions, an outwardly rectifying anion current (I-ANION) was observed, which was larger with iodide than nitrate, and with nitrate(than chloride as charge carrier. In contrast with a previously identified background anionic current from small mammal cardiomyocytes, I-ANION was not augmented by the pyrethroid tefluthrin (10 mu M); neither was it inhibited by hyperosmolar external solution nor by DIDS (200 mu M); thus I-ANION was not due to basal activity of volume-sensitive anion channels. I-ANION was partially inhibited by the Cl- channel blockers NPPB (50 mu M) and Gly H-101 (30 mu M). Incorporation Of IANION into a human atrial action potential (AP) simulation led to depression of the AP plateau, accompanied by alterations to plateau inward calcium current, and to AP shortening at 50% but not 90% of complete repolarization, demonstrating that I-ANION can influence the human atrial AP profile. (c) 2007 Elsevier Inc. All rights reserved.