Analysis of the differential modulation of sulphonylurea block of beta-cell and cardiac ATP-sensitive K+ (K(ATP)) channels by Mg-nucleotides.
Sulphonylureas stimulate insulin secretion by binding with high-affinity to the sulphonylurea receptor (SUR) subunit of the ATP-sensitive potassium (K(ATP)) channel and thereby closing the channel pore (formed by four Kir6.2 subunits). In the absence of added nucleotides, the maximal block is around 60-80 %, indicating that sulphonylureas act as partial antagonists. Intracellular MgADP modulated sulphonylurea block, enhancing inhibition of Kir6.2/SUR1 (beta-cell type) and decreasing that of Kir6.2/SUR2A (cardiac-type) channels. We examined the molecular basis of the different response of channels containing SUR1 and SUR2A, by recording currents from inside-out patches excised from Xenopus oocytes heterologously expressing wild-type or chimeric channels. We used the benzamido derivative meglitinide as this drug blocks Kir6.2/SUR1 and Kir6.2/SUR2A currents, reversibly and with similar potency. Our results indicate that transfer of the region containing transmembrane helices (TMs) 8-11 and the following 65 residues of SUR1 into SUR2A largely confers a SUR1-like response to MgADP and meglitinide, whereas the reverse chimera (SUR128) largely endows SUR1 with a SUR2A-type response. This effect was not specific for meglitinide, as tolbutamide was also unable to prevent MgADP activation of Kir6.2/SUR128 currents. The data favour the idea that meglitinide binding to SUR1 impairs either MgADP binding or the transduction pathway between the NBDs and Kir6.2, and that TMs 8-11 are involved in this modulatory response. The results provide a basis for understanding how beta-cell K(ATP) channels show enhanced sulphonylurea inhibition under physiological conditions, whereas cardiac K(ATP) channels exhibit reduced block in intact cells, especially during metabolic inhibition.
Reimann
Frank
F
University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK.
Dabrowski
Michael
M
Jones
Phillippa
P
Gribble
Fiona M
FM
Ashcroft
Frances M
FM
D018528:ATP-Binding Cassette Transporters; D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D001549:Benzamides / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D007004:Hypoglycemic Agents / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D016296:Mutagenesis / Q000502:physiology; D009206:Myocardium / Q000378:metabolism; D009865:Oocytes / Q000502:physiology; D015221:Potassium Channels / Q000235:genetics / Q000378:metabolism; D024661:Potassium Channels, Inwardly Rectifying / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D011955:Receptors, Drug / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D064233:Sulfonylurea Receptors; D014044:Tolbutamide / Q000494:pharmacology; D014982:Xenopus laevis
D018528:ATP-Binding Cassette Transporters; C577508:Abcc8 protein, mouse; C577509:Abcc8 protein, rat; C577511:Abcc9 protein, mouse; D001549:Benzamides; D007004:Hypoglycemic Agents; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D011955:Receptors, Drug; D011993:Recombinant Fusion Proteins; D064233:Sulfonylurea Receptors; D000255:Adenosine Triphosphate; C030516:meglitinide; D014044:Tolbutamide