Arkhammar P, Nilsson T, Rorsman P, Berggren P O
J Biol Chem. 1987 Apr 25;262(12):5448-54.
The effects of glucose, diazoxide, K+, and tolbutamide on the activity of K+ channels, membrane potential, and cytoplasmic free Ca2+ concentration were investigated in beta-cells from the Uppsala colony of obese hyperglycemic mice. With [K+]e = [K+]i = 146 mM, it was demonstrated that the dominating channel at the resting potential is a K+ channel with a single-channel conductance of about 65 picosiemens and a reversal potential of about +70 mV (pipette potential). This channel is characterized by complex kinetics with openings grouped in bursts. The channel was completely inhibited by 20 mM glucose in intact cells or by intracellularly applied Mg-ATP (1 mM). The number of active channels was markedly reduced already by 5 mM glucose. However, the single channel current of the channels remaining active was unaffected, indicating no major depolarization. To evoke a substantial depolarization of the membrane and thereby action potentials, a total block in channel activity was necessary. This could be achieved either by increasing the concentration of glucose to 20 mM or by combining 5 mM glucose with 100 microM tolbutamide. In both cases, the effect was counteracted by the hyperglycemic sulfonamide diazoxide. The effects on single channel activity were paralleled by changes in membrane potential and cytoplasmic free Ca2+ concentration, also when the latter measurements were performed at room temperature. The transient increase in the number of active channels and the resulting hyperpolarization observed after raising the glucose concentration to 20 mM probably reflected a drop in cytoplasmic ATP concentration. It is suggested that ATP works as a key regulator of the beta-cell membrane potential and thereby the opening of voltage-activated Ca2+ channels.
在来自乌普萨拉肥胖高血糖小鼠群体的β细胞中，研究了葡萄糖、二氮嗪、钾离子和甲苯磺丁脲对钾离子通道活性、膜电位和细胞质游离钙离子浓度的影响。当细胞外钾离子浓度（[K⁺]e）=细胞内钾离子浓度（[K⁺]i）= 146 mM时，研究表明，静息电位下占主导的通道是一种钾离子通道，其单通道电导约为65皮西门子，反转电位约为 +70 mV（移液管电位）。该通道具有复杂的动力学特征，开放呈簇状。在完整细胞中，该通道被20 mM葡萄糖或细胞内施加的1 mM Mg - ATP完全抑制。5 mM葡萄糖就已显著减少了活性通道的数量。然而，剩余活性通道的单通道电流未受影响，表明没有明显的去极化。为了引起膜的显著去极化从而引发动作电位，通道活性必须完全被阻断。这可以通过将葡萄糖浓度增加到20 mM或通过将5 mM葡萄糖与100 μM甲苯磺丁脲联合使用来实现。在这两种情况下，高血糖磺酰胺二氮嗪都能抵消这种作用。对单通道活性的影响与膜电位和细胞质游离钙离子浓度的变化平行，即使后者的测量是在室温下进行的。将葡萄糖浓度提高到20 mM后观察到的活性通道数量的短暂增加以及由此产生的超极化可能反映了细胞质ATP浓度的下降。有人提出，ATP作为β细胞膜电位的关键调节因子，从而也是电压激活钙离子通道开放的关键调节因子。
