Sulfonylurea and K⁺-Channel Opener Sensitivity of K_ATP Channels: Functional Coupling of Kir6.2 and Sur1 Subunits

The sensitivity of K_ATP channels to high-affinity block by sulfonylureas and to stimulation by K⁺ channel openers and MgADP (PCOs) is conferred by the regulatory sulfonylurea receptor (SUR) subunit, whereas ATP inhibits the channel through interaction with the inward rectifier (Kir6.2) subunit. Phosphatidylinositol 4,5-bisphosphate (PIP₂) profoundly antagonized ATP inhibition of K_ATP channels expressed from cloned Kir6.2+SUR1 subunits, but also abolished high affinity tolbutamide sensitivity. By stabilizing the open state of the channel, PIP₂ drives the channel away from closed state(s) that are preferentially affected by high affinity tolbutamide binding, thereby producing an apparent loss of high affinity tolbutamide inhibition. Mutant K_ATP channels (Kir6.2[ΔN30] or Kir6.2[L164A], coexpressed with SUR1) also displayed an “uncoupled” phenotype with no high affinity tolbutamide block and with intrinsically higher open state stability. Conversely, Kir6.2[R176A]+SUR1 channels, which have an intrinsically lower open state stability, displayed a greater high affinity fraction of tolbutamide block. In addition to antagonizing high-affinity block by tolbutamide, PIP₂ also altered the stimulatory action of the PCOs, diazoxide and MgADP. With time after PIP₂ application, PCO stimulation first increased, and then subsequently decreased, probably reflecting a common pathway for activation of the channel by stimulatory PCOs and PIP₂. The net effect of increasing open state stability, either by PIP₂ or mutagenesis, is an apparent “uncoupling” of the Kir6.2 subunit from the regulatory input of SUR1, an action that can be partially reversed by screening negative charges on the membrane with poly-l-lysine.