E23K, a common polymorphism in the pore-forming subunit K_IR6.2 of pancreatic β-cell ATP-sensitive K⁺ (K_ATP) channels, is functionally relevant and thus might play a major role in the pathophysiology of common type 2 diabetes. In this study, we show that in the simultaneous presence of activatory and inhibitory nucleotides, the polymorphism exerts opposite effects on the potencies of these modulators: channel opening through nucleoside diphosphates is facilitated, whereas sensitivity toward inhibition through ATP is slightly decreased. The results support the conclusion that E23K predisposes to type 2 diabetes by changing the channel’s response to physiological variation of cytosolic nucleotides, resulting in K_ATP overactivity and discrete inhibition of insulin release.