Nutter T J, Adams D J
Department of Molecular and Cellular Pharmacology, University of Miami School of Medicine, Florida 33101, USA.
J Gen Physiol. 1995 Jun;105(6):701-23. doi: 10.1085/jgp.105.6.701.
Acetylcholine-evoked currents mediated by activation of nicotinic receptors in rat parasympathetic neurons were examined using whole-cell voltage clamp. The relative permeability of the neuronal nicotinic acetylcholine (nACh) receptor channel to monovalent and divalent inorganic and organic cations was determined from reversal potential measurements. The channel exhibited weak selectivity among the alkali metals with a selectivity sequence of Cs+ > K+ > Rb+ > Na+ > Li+, and permeability ratios relative to Na+ (Px/PNa) ranging from 1.27 to 0.75. The selectivity of the alkaline earths was also weak, with the sequence of Mg2+ > Sr2+ > Ba2+ > Ca2+, and relative permeabilities of 1.10 to 0.65. The relative Ca2+ permeability (PCa/PNa) of the neuronal nACh receptor channel is approximately fivefold higher than that of the motor endplate channel (Adams, D. J., T. M. Dwyer, and B. Hille. 1980. Journal of General Physiology. 75:493-510). The transition metal cation, Mn2+ was permeant (Px/PNa = 0.67), whereas Ni2+, Zn2+, and Cd2+ blocked ACh-evoked currents with half-maximal inhibition (IC50) occurring at approximately 500 microM, 5 microM and 1 mM, respectively. In contrast to the muscle endplate AChR channel, that at least 56 organic cations which are permeable to (Dwyer et al., 1980), the majority of organic cations tested were found to completely inhibit ACh-evoked currents in rat parasympathetic neurons. Concentration-response curves for guanidinium, ethylammonium, diethanolammonium and arginine inhibition of ACh-evoked currents yielded IC50's of approximately 2.5-6.0 mM. The organic cations, hydrazinium, methylammonium, ethanolammonium and Tris, were measureably permeant, and permeability ratios varied inversely with the molecular size of the cation. Modeling suggests that the pore has a minimum diameter of 7.6 A. Thus, there are substantial differences in ion permeation and block between the nACh receptor channels of mammalian parasympathetic neurons and amphibian skeletal muscle which represent functional consequences of differences in the primary structure of the subunits of the ACh receptor channel.
利用全细胞膜片钳技术检测了大鼠副交感神经元中烟碱样受体激活介导的乙酰胆碱诱发电流。通过测量反转电位，确定了神经元烟碱型乙酰胆碱（nACh）受体通道对单价和二价无机及有机阳离子的相对通透性。该通道对碱金属离子的选择性较弱，选择性顺序为Cs+ > K+ > Rb+ > Na+ > Li+，相对于Na+的通透率（Px/PNa）范围为1.27至0.75。碱土金属离子的选择性也较弱，顺序为Mg2+ > Sr2+ > Ba2+ > Ca2+，相对通透性为1.10至0.65。神经元nACh受体通道的相对Ca2+通透性（PCa/PNa）比运动终板通道高约五倍（亚当斯，D. J.，T. M. 德怀尔，和B. 希勒。1980年。《普通生理学杂志》。75:493 - 510）。过渡金属阳离子Mn2+是可通透的（Px/PNa = 0.67），而Ni2+、Zn2+和Cd2+可阻断乙酰胆碱诱发电流，半数最大抑制浓度（IC50）分别约为500 microM、5 microM和1 mM。与肌肉终板乙酰胆碱受体通道不同，后者至少有56种可通透的有机阳离子（德怀尔等人，1980年），在所测试的大多数有机阳离子中，发现它们能完全抑制大鼠副交感神经元中乙酰胆碱诱发的电流。胍盐、乙铵、二乙醇铵和精氨酸对乙酰胆碱诱发电流的浓度 - 反应曲线得出的IC50约为2.5 - 6.0 mM。有机阳离子肼盐、甲铵、乙醇铵和Tris是可测量通透的，通透率与阳离子的分子大小成反比。模型表明，孔的最小直径为7.6 Å。因此，哺乳动物副交感神经元和两栖动物骨骼肌的nACh受体通道在离子通透和阻断方面存在显著差异，这代表了乙酰胆碱受体通道亚基一级结构差异的功能后果。
