Characterization and significance of sulfonylurea receptors

This study describes and characterizes a putative sulfonylurea receptor. The radioligand used was [³H]glipizide (9 Ci/mmol). The β-cell plasma membranes were derived from a transplantable rat insulinoma generated by subcutaneous injection of RINm5F cells and purified by ultracentrifugation on a 15-55% sucrose gradient. Specific binding of [³H]glipizide to purified β-cell plasma membranes was determined to be maximal at temperatures of 4-23°C, pH 7.3, and an incubation of 2 h. Scatchard analysis indicated a single binding site with K_d = 7 nM and sulfonylurea binding of 0.93 pmol/mg membrane protein. Displacement of [³H]glipizide from the purified β-cell plasma membranes by various sulfonylureas and their analogues correlated well with their known hypoglycemic and insulin-releasing activities. Various agents, including nutrients, agents affecting Ca²⁺ flux, gastrointestinal hormones, and pancreatic hormones, had no effect on [³H]glipizide binding to the β-cell plasma membranes. Putative sulfonylurea receptors on β-cell and brain cell plasma membranes have been reported by several groups of investigators. Sulfonylurea binding to the β-cell is hypothesized to close an ATP-sensitive K⁺ channel, which leads to depolarization of the membrane and activation of a voltage-dependent Ca²⁺ channel.