Direct interaction of Na-azide with the K-ATP channel

1 The effects of the metabolic inhibitor sodium azide were tested on excise d macropatches from Xenopus oocytes expressing cloned ATP-sensitive potassi um (K-ATP) channels of the Kir6.2/SUR1 type. 2 In inside-out patches from oocytes expressing Kir6.2 Delta C36 (a truncat ed form of Kir6.2 that expresses in the absence of SUR), intracellular Na-a zide inhibited macroscopic currents with an IC50 of ii mM. The inhibitory e ffect of Na-azide was mimicked by the same concentration of NaCl, but not b y sucrose. 3 Na-azide and NaCl blocked Kir6.2/SUR1 currents with IC50 Of 36 mM and 19 mM, respectively. Inhibition was abolished in the absence of intracellular Mg2+. In contrast, Kir6.2 Delta C36 currents were inhibited by Na-azide bot h in the presence or absence of intracellular Mg24 Kir6.2/SUR1 currents were less sensitive to 3 mM Na-azide in the presence of MgATP. This apparent reduction in sensitivity is caused by a small acti vatory effect of Na-azide conferred by SUR. 5 We conclude that, in addition to its well-established inhibitory effect o n cellular metabolism, which leads to activation of K-ATP channels in intac t cells, intracellular Na-azide has direct effects on the K-ATP channel. In hibition is intrinsic to Kir6.2, is mediated by Na+, and is modulated by SU R. There is also a small, ATP-dependent, stimulatory effect of Na-azide med iated by the SUR subunit. The direct effects of 3 mM Na-azide on K-ATP chan nels are negligible in comparison to the metabolic activation produced by t he same Na-azide concentration.