Manthey A A
Department of Physiology, University of Tennessee, Memphis, 62 South Dunlap, Memphis, TN 38163, USA.
Pflugers Arch. 1998 May;435(6):818-26. doi: 10.1007/s004240050589.
The possibility that increases in agonist concentration beyond threshold levels may force changes in the character of high-conductance open states of skeletal muscle nicotinic acetylcholine receptor channels (nAChR) was examined by seeing whether differences in several critical ionic properties of nAChR currents could be detected with changes in agonist level. Single- and bi-ionic whole-cell currents of Na+ and Li+ in voltage-clamped frog (Rana pipiens) muscle fibers were measured during local superfusion of endplates with carbamylcholine (carb) at concentrations of 54 microm (low-carb) and 270 microM (high-carb). Three ionic properties that would be affected by changes in the open-state configuration of channel subunits were tested. First, ion-saturation characteristics. Peak Na+ and Li+ currents in low-carb trials showed sublinear dependence on ion concentrations from 0 to 60 mM with Km values of 78 (Na+) and 49 (Li+) mM and a power function slope of 0. 75 on double-log plot. In contrast, the concentration dependence of Na+ and Li+ currents in high-carb tests was linear through the origin with a power function slope of 1.02. Second, Na+/Li+ selectivity. The ratio of peak Na+ and Li+ currents in low-carb tests varied from 1.86 to 2.28 for ion concentrations of from 20 to 60 mM [mean = 2.02 +/- 0.06 (SEM)] whereas the ratio for high-carb trials ranged from only 1.29 to 1.52 [mean = 1.42 +/- 0.40 (SEM)]. Third, competitive interactions of Na+ and Li+ currents. Equimolar mixtures of Na+ and Li+ in low-carb tests produced bi-ionic inward currents which were never larger than the single-ion Na+ current alone, but bi-ionic currents at the high-carb level were always greater than the single-ion Na+ current, approximating the sum of the single-ion Na+ and Li+ currents in most cases. The results are consistent with a decrease in ion-channel binding at the high-carb level and support the possibility of agonist-induced changes in the high-conductance open-state configuration of nAChR subunits which result in a weakening of constraints on cation movements through the channel.
通过观察烟碱型乙酰胆碱受体通道（nAChR）电流的几个关键离子特性是否会随着激动剂水平的变化而被检测到，研究了激动剂浓度超过阈值水平可能会迫使骨骼肌nAChR通道高电导开放状态的特征发生变化的可能性。在以54微摩尔（低碳酰胆碱）和270微摩尔（高碳酰胆碱）的浓度对终板进行局部灌注氨甲酰胆碱（carb）期间，测量了电压钳制的青蛙（豹蛙）肌肉纤维中Na+和Li+的单离子和双离子全细胞电流。测试了三个会受到通道亚基开放状态构型变化影响的离子特性。首先，离子饱和特性。低碳酰胆碱试验中的峰值Na+和Li+电流显示，在0至60毫摩尔的离子浓度下，与离子浓度呈次线性关系，Na+的Km值为７８毫摩尔，Li+的Km值为49毫摩尔，在双对数图上的幂函数斜率为0.75。相比之下，高碳酰胆碱试验中Na+和Li+电流的浓度依赖性是通过原点的线性关系，幂函数斜率为1.02。其次，Na+/Li+选择性。在低碳酰胆碱试验中，对于20至60毫摩尔的离子浓度，峰值Na+和Li+电流的比值在1.86至2.28之间变化[平均值 = 2.02 ± 0.06（标准误）]，而高碳酰胆碱试验的比值仅在1.29至1.52之间[平均值 = 1.42 ± 0.40（标准误）]。第三，Na+和Li+电流的竞争性相互作用。低碳酰胆碱试验中Na+和Li+的等摩尔混合物产生的双离子内向电流从未大于单独的单离子Na+电流，但高碳酰胆碱水平下的双离子电流总是大于单离子Na+电流，在大多数情况下接近单离子Na+和Li+电流之和。这些结果与高碳酰胆碱水平下离子通道结合的减少一致，并支持激动剂诱导nAChR亚基高电导开放状态构型发生变化的可能性，这导致通过通道的阳离子移动的约束减弱。
