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

Hypoglycaemic effects of the novel antidiabetic agent repaglinide in rats and dogs

1. Repaglinide, a novel compound with a nonsulphonylurea structure, is currently being clinically tested as a therapeutic agent. In the present study, the hypoglycaemic effects of repaglinide in rats and dogs were investigated. 2. Whereas the R-enantiomer, AG-EE 624 ZW, showed only weak hypoglycaemic activity, the S-enantiomer, repaglinide, turned out to be a potent hypoglycaemic compound in rats after oral as well as after intravenous administration. Only 50% of the dose of repaglinide was needed to be equieffective with the racemic mixture AG-EE 388 ZW. The corresponding ED50 values calculated for the effects after 120 min p.a. (intravenous administration) were 3.4 micrograms kg-1 (repaglinide) and 6 micrograms kg-1 (AG-EE 388 ZW). 3. When compared to glimepiride or glibenclamide, repaglinide displayed a 18 to 25 times higher potency in fasted rats. The ED50 values calculated for the effects after 120 min p.a. (oral administration) were 10 micrograms kg-1 (repaglinide), 182 micrograms kg-1 (glimepiride) and 255 micrograms kg-1 (glibenclamide). 4. In glucose loaded rats (0.5, 1.0, 2.0 and 3.0 g kg-1 glucose, p.o.) repaglinide exerted a very strong antihyperglycaemic activity which was even more pronounced than under normoglycaemic conditions. So for a reduction in blood glucose of 1 mmol l-1, 10.3, 9.3, 7.0 8.4 and 7.2 micrograms kg-1 repaglinide were needed after glucose loads of 0.0, 0.5, 1.0, 2.0 and 3.0 g kg-1, respectively. 5. In beagle dogs repaglinide again showed a pronounced hypoglycaemic effect (ED50 28.3 micrograms kg-1) which lasted for up to 24 h. However, insulin levels were only transiently increased. 6. The in vivo data presented are well supported by recently published in vitro findings. From its activity profile, repaglinide appears to be a promising new therapeutic agent.

Characterization of the binding sites for [3H]glibenclamide in rat liver membranes

The specific binding sites for sulfonylureas in the rat liver membrane fraction were demonstrated and characterized. [3H]Glibenclamide binding to the liver membrane was specific, time- and temperature-dependent, and reversible. Scatchard analysis showed a single class binding site. The dissociation constant (Kd) for glibenclamide was 1.1 microM and the binding capacity (Bmax) was 50 pmol/mg protein. [3H]Glibenclamide binding could be displaced by other sulfonylureas. Half-maximal inhibition of binding (IC50) for glimepiride, gliclazide, acetohexamide, tolbutamide and chlorpropamide was 4.2 microM, 74 microM, 0.33 mM, 0.60 mM, 1.2 mM, respectively. Each value is close to the reported blood concentration when a therapeutic dose of each drug is administered orally. The order of IC50 values is coincident with the order of potency of the clinical hypoglycemic effect of these drugs. We had shown that these concentrations of sulfonylureas stimulate 6-phosphofructo-2-kinase in the liver or hepatocytes and inhibit phosphoenolpyruvate carboxykinase in the hepatoma cells. The specific binding sites demonstrated here may play some roles when sulfonylureas affect carbohydrate metabolism in the liver.

Effect of the hypoglycaemic drug (-)-AZ-DF-265 on ATP-sensitive potassium channels in rat pancreatic beta-cells

1. It has previously been shown that the hypoglycaemic drug (-)-AZ-DF-265 stimulates insulin release from mouse islets; in the presence of 3 mM glucose it also inhibits 86Rb-efflux from 86Rb-loaded islets. Based on these data we tested the hypothesis that (-)-AZ-DF-265 inhibits ATP-sensitive potassium channels. 2. We voltage-clamped the plasma membrane of single rat pancreatic beta-cells in the whole-cell configuration and measured the current flowing through ATP-sensitive potassium channels. (-)-AZ-DF-265 was applied to the outside of the cell; it inhibited the current half-maximally at a concentration of 1.2 +/- 0.2 nM with a Hill coefficient of 0.7 +/- 0.1. The inhibition was reversible on washing. 3. In intact RINm5F cells, the sulphonylurea [3H]-glibenclamide bound with an affinity of 0.24 +/- 0.01 nM and a Hill coefficient of 2.1 +/- 0.4. Treatment with (-)-AZ-DF-265 led to the displacement of [3H]-glibenclamide; this effect was half-maximal at 8.6 +/- 1.7 nM and displayed a Hill coefficient of 0.53 +/- 0.01. Meglitinide and tolbutamide, which represent, respectively, the benzamido and sulphonylurea moieties of glibenclamide, showed Hill coefficients of 0.8 +/- 0.1 and 0.9 +/- 0.1, respectively. 4. At pH 7.4 and with phosphate/borate buffer, (-)-AZ-DF-265 had an apparent octanol/water partition coefficient of about 53, which is intermediate between the partition coefficients of glibenclamide and glipizide. Nevertheless, (-)-AZ-DF-265 bound only to a minor degree to glass and plastic.5. In conclusion, (-)-AZ-DF-265 inhibits ATP-sensitive potassium channels and displaces [3H]-glibenclamide from the sulphonylurea receptor.