Direct interaction of Na-azide with the KATP channel.
1. The effects of the metabolic inhibitor sodium azide were tested on excised macropatches from Xenopus oocytes expressing cloned ATP-sensitive potassium (KATP) channels of the Kir6.2/SUR1 type. 2. In inside-out patches from oocytes expressing Kir6.2 delta C36 (a truncated form of Kir6.2 that expresses in the absence of SUR), intracellular Na-azide inhibited macroscopic currents with an IC50 of 11 mM. The inhibitory effect of Na-azide was mimicked by the same concentration of NaCl, but not by 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 both in the presence or absence of intracellular Mg2+. 4. 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 activatory effect of Na-azide conferred by SUR. 5. We conclude that, in addition to its well-established inhibitory effect on cellular metabolism, which leads to activation of KATP channels in intact cells, intracellular Na-azide has direct effects on the KATP channel. Inhibition is intrinsic to Kir6.2, is mediated by Na+, and is modulated by SUR. There is also a small, ATP-dependent, stimulatory effect of Na-azide mediated by the SUR subunit. The direct effects of 3 mM Na-azide on KATP channels are negligible in comparison to the metabolic activation produced by the same Na-azide concentration.
Trapp
S
S
University Laboratory of Physiology, Parks Road, Oxford OX1 3PT.
Ashcroft
F M
FM
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D016695:Glycosyltransferases; D008565:Membrane Proteins; D051379:Mice; D015221:Potassium Channels / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D011971:Receptors, Immunologic / Q000187:drug effects* / Q000502:physiology; D054340:Receptors, KIR; D012097:Repressor Proteins / Q000187:drug effects* / Q000502:physiology; D029701:Saccharomyces cerevisiae Proteins; D019810:Sodium Azide / Q000494:pharmacology*; D012965:Sodium Chloride / Q000494:pharmacology; D014982:Xenopus laevis
D004791:Enzyme Inhibitors; D008565:Membrane Proteins; D015221:Potassium Channels; D011971:Receptors, Immunologic; D054340:Receptors, KIR; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D012965:Sodium Chloride; D000255:Adenosine Triphosphate; D019810:Sodium Azide; D016695:Glycosyltransferases; C092031:SUR1 protein, S cerevisiae