Physiological and molecular biological studies on functional alterations in ion channels of pancreatic beta cells in diabetes mellitus.

非胰岛素依赖型糖尿病（NIDDM）的主要发病特征之一是葡萄糖诱导的胰岛素分泌的选择性障碍。我们最近报道了在NIDDM大鼠模型中胰腺β细胞对葡萄糖的细胞内钙反应选择性受损。为了阐明这一损伤的分子机制，我们采用膜片钳技术直接研究了ATP敏感性K^+通道（K_<ATP>channel）和电压依赖性Ca^&lt;2+&gt;通道（VDCCs）的活性，这两种通道在葡萄糖刺激后细胞内钙水平升高中起重要作用。在<ATP>GK大鼠β细胞中，葡萄糖对K_2通道活性的抑制作用减弱，而通道对ATP的敏感性保持不变。这清楚地表明NIDDM β细胞中的细胞内葡萄糖代谢受损。另一方面，甘油醛或酮异己酸（通过葡萄糖代谢中间产物代谢）对通道的抑制作用在GK和对照大鼠之间相似。然而，二羟丙酮（DHA）-磷酸（甘油醛-3-磷酸的异构体）对通道活性的抑制在NIDDM β细胞中减少。由于DHA-磷酸可以进入甘油磷酸穿梭，这被认为是糖酵解和线粒体氧化之间的代谢途径中的直接联系，因此推测葡萄糖代谢受损的负责位点位于该穿梭中。此外，通过葡萄糖代谢直接增加VDCC活动也被发现减少GK大鼠β细胞中的穿孔斑记录。这些事实被认为与选择性损害葡萄糖诱导的NIDDM β细胞的胰岛素分泌密切相关。

The selective impairment of glucose-induced insulin secretion has been known as one of the major characteristics of pathogenetic aspects in non-insulin-dependent diabetes mellitus (NIDDM) . We have recently reported that the intracellular calcium responses to glucose are selectively impaired in pancreatic beta cells of NIDDM rat models. In order to clarify the molecular mechanism underlying this impairment, we investigated the activities of ATP sensitive K^+ channels (K_<ATP> channels) and voltage-dependent Ca^<2+> channels (VDCCs) directly using the patch-clamp technique, both of which are known to play an important role in the elevation of intracellular calcium levels after the glucose stimulation. The inhibition of K_<ATP> channels activities by glucose was reduced in beta cells of GK rats, a genetic model of NIDDM,whereas the ATP sensitivity of channels was intact. This clearly indicates that the intracellular glucose metabolism is impaired in NIDDM beta cells. On the other hand, the channels inhibition by glyceraldehyde or ketoisocaproate, which are matabolizad through the intermediates in glucose metabolism, was similar between GK and control rats. However, the inhibition of channel activities by dihydroxyacetone (DHA) -phosphate, an isomer of glyceraldehyde-3-phosphate, was reduced in NIDDM beta cells. Since DHA-phosphate can enter the glycerol phosphate shuttle which is thought to be a direct link in the metaolic pathway between glycolysis and mitochondrial oxidation, the responsible sites for impaired glucose metabolism is speculated to be located in this shuttle. In addition, the direct augmentation of VDCC activities through glucose metabolism was also found to be reduced by perforated-patch recording in beta cells of GK rats. These facts are though to be closely related to the selective impairment of glucose induced insulin secretion from NIDDM beta cells.