Evidence for more than one binding site for sulfonylureas in insulin-secreting cells

Specific binding of both [3H]glibenclamide and [3H]gliquidone has been observed in a particulate fraction of insulin-secreting rat tumour (RIN m5F) cells. The binding of both the labels was time-dependent, of high affinity (including a low affinity binding site), saturable and reversible. The rank order of inhibition of [3H]glibenclamide binding was glibenclamide greater than gliquidone greater than AG-EE 388 = AG-EE 86 = AG-EE 319 greater than AG-EE 436 (AG coded drugs are benzoic acid derivatives which lack the sulfonylurea moiety of sulfonylureas). The Kds of high affinity binding for glibenclamide and gliquidone were 0.08 and 1.3 nM, respectively. When [3H]gliquidone was used as the labelled compound this rank order of binding and the affinities of drugs were different, e.g. glibenclamide was less potent than gliquidone. The Kd values of high affinity binding to the [3H]gliquidone binding site were 810 and 79 nM with respect to glibenclamide and gliquidone. The binding site labelled by [3H]gliquidone, in contrast to that labelled by [3H]glibenclamide, was not able to discriminate between the two enantiomers AG-EE 319 and AG-EE 436. The data indicate that there are different binding sites for glibenclamide and gliquidone in RIN m5F cells. In extension to data of other groups it is speculated that there exists more than one specific binding site for sulfonylureas and other related compounds, e.g. benzoic acid derivatives and that sulfonylureas behave differently not only in quantitative but in qualitative terms as well.