INTERACTION OF FLUORESCEIN DERIVATIVES WITH SULFONYLUREA BINDING IN INSULIN-SECRETING CELLS

Recently evidence was presented that fluorescein derivatives (e.g. phl oxine B) inhibit glibenclamide binding by occupation of a nucleotide-b inding site at the ATP-sensitive potassium channel (K-ATP channel). Ho wever, this conclusion was inconsistent with the results of previous s tudies testing the effects of nucleotides on glibenclamide binding, To elucidate the interaction mode of fluorescein derivatives with sulfon ylurea binding, the effect of phloxine B on binding of [H-3]glibenclam ide to microsomes obtained from a pancreatic beta-cell line (HIT-T15) was examined. Phloxine B inhibited specific binding of glibenclamide h alf-maximally at 3.2 mu mol/l. The slope parameter for the displacemen t curve was close to one, suggesting a competitive interaction between both drugs, In accordance with this assumption 4 mu mol/l phloxine B did not show an effect on the number of high-affinity binding sites bu t increased the apparent dissociation constant for glibenclamide by 3. 1-fold and 30 mu mol/l phloxine B did not alter the rate of dissociati on of [H-3]glibenclamide. Moreover, MgATP (300 mu mol/l) significantly reduced the apparent affinity for binding of phloxine B to the sulfon ylurea receptor, This finding resembled the action of MgATP on binding of sulfonylureas to their receptor site. It is concluded that fluores cein derivatives inhibit glibenclamide binding due to competition for the same site at the sulfonylurea receptor.