Studies on multimolecular modulation of ATP-sensitive K^+ channels in pancreatic β-cells

ATP敏感性K^+通道由磺酰脲受体单元和内向整流K^+通道的孔单元的异源多聚体组成。发现细胞内Ca^2+抑制磺酰脲类药物与其受体结合和通道关闭之间的功能性转导。氧自由基对细胞内代谢的抑制导致磺酰脲类药物对通道的抑制效力降低。细胞内ATP也是这些亚基之间信号转导的调节剂之一，但需要降低至<10 μM才能抑制磺酰脲诱导的通道关闭。细胞骨架蛋白肌动蛋白维持磺酰脲类药物对通道的抑制作用，而质膜磷脂PIP_2通过拮抗细胞内Ca^2+来稳定肌动蛋白效应，据报道，Ca ^2+可使肌动蛋白丝从F-肌动蛋白降解为G-肌动蛋白。PBP_2还通过降低ATP敏感性增加通道活性。此外，我们还发现，细胞外ATP刺激P2 Y受体，通过降低膜PIP_2水平，导致心脏ATP敏感性K^+通道活性降低。这些结果表明，膜PIP_2的水平可能是通道的基础活动的决定因素。我们认为，通道是由周围的分子：膜磷脂，Ca^<2+>，肌动蛋白丝和ATP调节。

ATP-sensitive K^+ channels are composed of heteromultimers of sulfonylurea-receptor unit and pore unit of inwardly rectifying K^+ channels. Intracellular Ca^<2+> was found to inhibit functional transduction between binding of sulfonylureas to their receptors and closure of channels. Inhibition of intracellular metabolism by oxygen-free radicals resulted in a reduction of inhibitory efficacy of channels by sulfonylureas. Intracellular ATP was also one of modulators of signal transduction between these subunits, but was required to be decreased to concentrations <10 μM for inhibition of sulfonylurea-induced closure of the channel. Actin, cytoskeletal proteins, maintained the inhibitory effect of sulfonylureas on the channel, and PIP_2, plasma membrane phospholipids, stabilizes the actin effect by antagonizing intracellular Ca^<2+> that is reported to depolymerize actin filaments from F-actin to G-actin. PBP_2 also increased activity of the channel by reducing ATP sensitivity. Furthermore, we found that stimulation of P2Y-receptors by extracellularly applied ATP resulted in reduction of activity of cardiac ATP-sensitive K^+ channels via reduction of membrane PIP_2 levels. These results suggest that levels of membrane PIP_2 may be a determinant of basal activity of the channels. We propose that the channel is modulated by surrounding molecules: membrane phospholipids, Ca^<2+>, actin filaments and ATP.