Mulhern S A, Fishman P H, Spiegel S
Division of Nutrition, Food and Drug Administration, Washington, D.C. 20204.
J Membr Biol. 1989 Jul;109(1):21-8. doi: 10.1007/BF01870787.
The fluorescent anionic dye, bisoxonol, and flow cytometry have been used to monitor changes in the membrane potential of rat thymocytes exposed to the B subunit of cholera toxin. The B subunit induced a rapid hyperpolarization, which was due to activation of a Ca2+-sensitive K+ channel. Reduction of extracellular Ca2+ to less than 1 microM by the addition of [ethylene-bis(oxyethylenenitrilo)]tetraacetic acid immediately abolished the hyperpolarization caused by the B subunit. Cells treated with quinine and tetraethylammonium lost their ability to respond to the B subunit, whereas 4-aminopyridine did not have any effect. Thus, calcium-sensitive and not voltage-gated K+ channels appeared to be responsible for the hyperpolarization. The results of ion substitution experiments indicated that extracellular Na+ was not essential for changes in membrane potential. Further studies with ouabain, amiloride and furosemide demonstrated that electrogenic Na+/K+ ATPase, Na+/H+ antiporter and Na+/K+/Cl- cotransporter, respectively, were not involved in the hyperpolarization process induced by the B subunit. Thus, crosslinking of several molecules of ganglioside GM1 on the cell surface of rat thymocytes by the pentavalent B subunit of cholera toxin modulated plasma membrane permeability to K+ by triggering the opening of Ca2+-sensitive K+ channels. A role for gangliosides in regulating ion permeability would have important implications for the function of gangliosides in various cellular phenomena.
荧光阴离子染料双羟萘酚以及流式细胞术已被用于监测暴露于霍乱毒素B亚基的大鼠胸腺细胞膜电位的变化。B亚基诱导了快速的超极化，这是由于Ca2+敏感的K+通道被激活。通过添加[乙二胺双(氧乙基腈)]四乙酸将细胞外Ca2+浓度降低至小于1微摩尔，立即消除了B亚基引起的超极化。用奎宁和四乙铵处理的细胞失去了对B亚基的反应能力，而4-氨基吡啶则没有任何影响。因此，似乎是钙敏感而非电压门控的K+通道导致了超极化。离子替代实验结果表明，细胞外Na+对于膜电位的变化并非必不可少。用哇巴因、氨氯吡脒和呋塞米进行的进一步研究表明，分别是生电Na+/K+ ATP酶、Na+/H+反向转运体和Na+/K+/Cl-协同转运体不参与B亚基诱导的超极化过程。因此，霍乱毒素的五价B亚基使大鼠胸腺细胞表面的几分子神经节苷脂GM1交联，通过触发Ca2+敏感的K+通道开放来调节质膜对K+的通透性。神经节苷脂在调节离子通透性方面的作用对于其在各种细胞现象中的功能具有重要意义。
