Carbon monoxide (CO), hydrogen sulphide (H2S) and nitric oxide (NO) are now established as enzyme-derived endogenous signalling molecules of widespread physiological importance, providing a range of beneficial effects in the heart and circulation1. CO is a product of heme oxygenase-1, and 2 (HO-1, HO-2) and accounts for many effects of HO-12. H2S is produced via cystathionine γ-lyase, cystathionine β-synthetase and 3-mercaptopyruvate sulfurtransferase. H2S modulates diverse cardiovascular functions3. Many redox-sensitive proteins have been implicated in atrial fibrillation (AF), including the voltage-gated K+ channel Kv1.5, encoded by KCNA5. Kv1.5 activates rapidly but inactivates slowly and incompletely4. In the heart, expression is largely confined to the atria where it is responsible for the repolarizing ultrarapid outward current, IKur, active throughout phases 1-3 of the atrial action potential (AP)5. Changes in Kv1.5 activity may predispose to AF-associated arrhythmia and it represents an attractive pharmacological target for AF treatment. The aim of this study was to evaluate oxidative modulation of Kv1.5 and how this is regulated by exogenous gasotransmitters. Electrophysiological recordings were made from HEK293 expressing recombinant hKv1.5. CO was applied via the CO releasing molecule CORM-2 (30 µM), H2S (300 µM) was applied via NaHS and NO via an NO donor SNAP (100 µM). All values are mean ± S.E.M. CO reduced Kv1.5 by 55.2 ± 3.78%, n=19 measured at 50 mV (P< 0.05, Student’s paired t test). This inhibition was reversible and dose-dependent. Application of the inactive form (iCORM) had no significant effect. H2S (300 µM) irreversibly inhibited Kv1.5 by 31.5 ± 4.7%, n=8, P< 0.05. When CO and H2S were tested in combination, H2S inhibited Kv1.5 and prevented further inhibition by CO, if H2S was applied before CO. In contrast, if CO was applied before H2S, a further inhibition occurred; the degree of inhibition caused by NaHS was similar to that in the absence of CORM-2 (35.9 ±6.7%, n=5, P< 0.05). SNAP (100 µM) inhibited Kv1.5 by 15.77 ±5.2%, n=4, P< 0.05; this effect was irreversible after a 5 min wash. Pre-treatment of cells with a nitric oxide synthase inhibitor (L-NAME, 1 mM, 1hr, 37°C) significantly reduced the inhibitory effects of CORM-2 or H2S on Kv1.5 by 81.8 ±7.45%, n=6, and 91.3 ± 3.2% n=8, respectively, indicating involvement of NO formation. These data indicate that the redox-sensitive Kv1.5 channel is modulated by the gasotransmitters CO and H2S, and that both gases inhibit Kv1.5 activity via NO formation. Further studies are required to identify the underlying mechanisms in more detail and establish the potential for related therapeutic interventions that target AF.