title: Mg(2+)-dependent inhibition of KATP by sulphonylureas in CRI-G1 insulin-secreting cells.
abstract: 1 Patch-clamp recording techniques were used to examine the effects of tolbutamide, glibenclamide, meglitinide and thiopentone on KATP in CRI-GI insulin-secreting cells in the presence and absence of Mg2+. 2 In the absence of Mg2+ in the intracellular bathing solution, tolbutamide was significantly less effective when applied either to the intracellular or to the extracellular surfaces of cell-free patches. Removal of extracellular Mg2+ did not alter the effectiveness of tolbutamide provided that Mg2+ was present at the intracellular surface of the patch. 3 Tolbutamide was also significantly less effective when applied to the intracellular surface of cell-free patches when Mn2+ was used as a replacement for Mg2+. 4 Both the sulphonylurea, glibenclamide and the non-sulphonylurea derivative, meglitinide also showed Mg2+ dependent inhibitory effects in cell-free patches. In contrast, the barbiturate thiopentone inhibited KATP in a Mg(2+)-independent manner. 5 Whole-cell IK(ATP) were used to quantify the effects of tolbutamide and glibenclamide in the presence and absence of intracellular Mg2+. Concentration-inhibition curves, in the presence of intracellular Mg2+, resulted in IC50 values of 12.1 microM and 2.1 nM for tolbutamide and glibenclamide, respectively. In the absence of intracellular Mg2+, the corresponding IC50 values were 25.3 mM and 3.6 microM, respectively. The values of IC50 for thiopentone in the presence and absence of intracellular Mg2+ were 69.4 microM and 69.2 microM, respectively. 6 With respect to the high affinity binding sites for [3H]-glibenclamide in CRI-G1 membranes, no significant differences were found between the dissociation constants for, or the maximal binding capacities of, [3H]-glibenclamide in the presence or absence of Mg2+. 7. In the CRI-G1 insulin-secreting cell line, it is concluded that intracellular Mg2+ does not influence the affinity of the sulphonylureas for the sulphonylurea receptor but that this ion is critically important for the interaction between the sulphonylurea receptor and KATP.
authors.affiliation: Department of Pharmacology, University of Cambridge.
authors.forename: K
authors.initials: K
authors.lastname: Lee
authors.forename: S E
authors.initials: SE
authors.lastname: Ozanne
authors.forename: C N
authors.initials: CN
authors.lastname: Hales
authors.forename: M L
authors.initials: ML
authors.lastname: Ashford
mesh_terms: D000255:Adenosine Triphosphate / Q000502:physiology*; D000818:Animals; D001549:Benzamides / Q000494:pharmacology; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004594:Electrophysiology; D005905:Glyburide / Q000493:pharmacokinetics / Q000494:pharmacology; D007004:Hypoglycemic Agents / Q000494:pharmacology; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D008024:Ligands; D008274:Magnesium / Q000502:physiology*; D015221:Potassium Channels / Q000187:drug effects*; D051381:Rats; D013453:Sulfonylurea Compounds / Q000494:pharmacology*; D013874:Thiopental / Q000494:pharmacology; D014044:Tolbutamide / Q000494:pharmacology
chemical_list: D001549:Benzamides; D007004:Hypoglycemic Agents; D008024:Ligands; D015221:Potassium Channels; D013453:Sulfonylurea Compounds; D000255:Adenosine Triphosphate; C030516:meglitinide; D014044:Tolbutamide; D008274:Magnesium; D013874:Thiopental; D005905:Glyburide