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

葡萄糖诱导的胰岛素分泌的选择性障碍已被认为是非胰岛素依赖型糖尿病(NIDDM)发病的主要特征之一。我们最近报道，在NIDDM大鼠模型中，细胞内钙离子对葡萄糖的反应选择性地受损。为了阐明这种损伤的分子机制，我们用膜片钳技术直接研究了ATP敏感的K~(++)通道和电压依赖性的Ca~(2+)通道(VDCC)的活动，这两种通道在葡萄糖刺激后细胞内钙水平的升高中起着重要的作用。在NIDDM遗传模型GK大鼠的β细胞中，葡萄糖对K&lt；ATP&Gt；通道活性的抑制作用减弱，而通道的ATP敏感性保持不变。这清楚地表明NIDDMβ细胞的细胞内葡萄糖代谢受损。而葡萄糖代谢中间产物甘油醛或酮基异己酸酯对通道的抑制作用与对照组相似。然而，在NIDDMβ细胞中，甘油醛-3-磷酸的异构体二羟丙酮(DHA)-磷酸对通道活动的抑制作用减弱。由于DHA-磷酸可以进入磷酸甘油穿梭，该穿梭被认为是糖酵解和线粒体氧化之间代谢途径的直接联系，因此推测葡萄糖代谢受损的负责部位位于这个穿梭中。此外，在GK大鼠的β细胞中，穿孔贴片记录也发现通过葡萄糖代谢直接增强VDCC活性的作用被降低。这些事实被认为与葡萄糖诱导的NIDDMβ细胞胰岛素分泌的选择性损害密切相关。

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.