Sulfonylurea receptors and ATP-sensitive K+ channels in clonal pancreatic alpha cells. Evidence for two high affinity sulfonylurea receptors

We tested for the presence of sulfonylurea receptors in pancreatic alpha cells. Two high affinity sulfonylurea receptors were identified in clonal pancreatic alpha cells (alpha TC-6): a 140-kDa species observed previously in clonal pancreatic beta cells (HIT) and a second 150-kDa protein. The dissociation constant (Kd) for both receptors is approximately 3.5 nM for an iodinated glyburide analog, 5-iodo-2-hydroxyglyburide. The estimated number of receptors (Bmax) increases approximately 2-fold, from 3.1 to 6.8 pmol/mg of membrane protein as the pH of the binding buffer is reduced from 7.5 to 6. Consistent with the notion that high affinity sulfonylurea receptors are integral components of the ATP-sensitive K+ channel, we demonstrated the presence of ATP-sensitive K+ channels in inside-out patches of alpha TC-6 cells. Whole cell K+ currents that activated with time showed inward rectification at positive potentials (above 0 mV) and were almost completely suppressed by 5 nM glyburide. Likewise, glyburide blocked 86Rb+ efflux from ATP-depleted alpha TC-6 cells, an effect that was reversed by 400 microM diazoxide. The presence of sulfonylurea receptors provides a mechanism by which sulfonylureas can directly modulate alpha cell function. The properties of the 150-kDa receptor and the role of ATP-sensitive K+ channels in alpha cells remain to be elucidated, but as in beta cells, ATP-sensitive K+ channels may be involved in metabolic regulation of alpha cells by glucose.