human Ether-à-go-go Related Gene (hERG) encodes the α-subunit of potassium channels that mediate the cardiac rapid delayed rectifier K+ current (IKr). The gating kinetics of hERG channels mean that in addition to regulating ventricular action potential duration, they exert a protective role early during diastole, generating rapid outward transient currents in response to unwanted premature depolarizing stimuli (Lu et al., 2001). Hyperkalemia and hypokalemia are known to modulate hERG/IKr, the T wave of the electrocardiogram and also to influence susceptibility to ventricular arrhythmia (El-Sherif & Turitto, 2011). However, whilst hERG current (IhERG) is known to exhibit a paradoxical dependence on extracellular K+ concentration ([K+]e) (Sanguinetti et al., 1995), the effect of [K+]e on the channel’s protective response to premature stimuli has not been reported. Therefore, we have studied the effects of altering [K+]e on IhERG during application of premature depolarizing stimuli. IhERG was recorded at 37oC from hERG-expressing HEK-293 cells expressing hERG 1a channels or co-expressing hERG 1a and 1b channels. Whole-cell recordings were made using a standard K+-based pipette solution and an external Tyrode’s solution in which [K+]e was varied between 1 and 10 mM. Data are expressed as mean ± S.E.M values. With a standard depolarizing voltage step to +20 mV followed by repolarization to -40 mV, lowering [K+]e from 4 to 1 mM induced, for hERG1a, a reduction of ≈31.5± 1.0 % and ≈21.8 ± 1.6 % of step and tail IhERG respectively (n=31 cells). On the other hand, increasing [K+]e from 4 to 10 mM resulted in an increase of ≈33.7±7.5% of the step IhERG and a decrease of ≈38.9±3.8% of the IhERG tail (n=15 cells). To assess the effect of different [K+]e on the protective role of hERG against premature stimuli, a voltage protocol was used that was comprised of paired human ventricular action potentials (APs) separated by differing inter-AP intervals (cf Lu et al., 2001; Du et al., 2010). Increasing [K+]e from 4 to 10 mM produced an increase of ≈24.9±5.6% (n=7 cells) in maximal IhERG transient amplitude, whilst reducing [K+]e to 1mM reduced IhERG transients, with a reduction in the maximal WT IhERG transient of ≈31.5±2.3% (n=7 cells). Similar effects were seen for co-expressed hERG 1a/1b. In particular, during application of 1mM [K+]e, IhERG transients elicited by premature AP stimuli were markedly suppressed for hERG1a/1b (with a reduction of ≈32.1±4.1% of the maximal IhERG transient; n=8 cells). Collectively, the results of these experiments indicate that alterations to [K+]e are likely to alter IhERG/IKr early during diastole; in particular hypokalemia is likely to impair the channel’s protective effect against unwanted premature stimuli and this may contribute to ventricular arrhythmia susceptibility in hypokalemia.