Sheridan R E, Lester H A
Proc Natl Acad Sci U S A. 1975 Sep;72(9):3496-500. doi: 10.1073/pnas.72.9.3496.
In Electrophorus electroplaques, the agonist-induced postsynaptic conductance depends on membrane potential. During steady exposure to agonists, after a voltage step the conductance relaxes on a millisecond time scale, exponentially approaching a new equilibrium value. The relaxation rate constant k is an instantaneous function of voltage, insensitive to the past or present conductance. Two components sum to form k. A concentration-sensitive component increases linearly with agonist concentration and decreases during desensitization or exposure to curare. Thus this component reflects the average frequency at which acetylcholine receptors are opening. The voltage-sensitive component, obtained by extrapolating k to zero agonist concentration, increases at more positive potentials. For acetylcholine, the voltage-sensitive component equals the rate constant for the exponential decay of postsynaptic currents; it thus seems to be the closing rate for active receptors. The voltage-sensitive component has the relative amplitudes acetylcholine less than carbamoylcholine less than decamethonium, and for each agonist equals the closing rate determined from "noise" measurements at neuromuscular junctions. The kinetic data explain several aspects of the steady-state conductance induced by agonists, but shed no light on apparent cooperative effects.
在电鳗的电板中，激动剂诱导的突触后电导取决于膜电位。在持续暴露于激动剂期间，电压阶跃后，电导在毫秒时间尺度上松弛，呈指数方式趋近新的平衡值。松弛速率常数k是电压的瞬时函数，对过去或当前的电导不敏感。k由两个成分相加而成。一个浓度敏感成分随激动剂浓度线性增加，在脱敏或暴露于箭毒期间降低。因此，该成分反映了乙酰胆碱受体开放的平均频率。通过将k外推至零激动剂浓度获得的电压敏感成分，在更正的电位时增加。对于乙酰胆碱，电压敏感成分等于突触后电流指数衰减的速率常数；因此它似乎是活性受体的关闭速率。电压敏感成分的相对幅度为乙酰胆碱小于氨甲酰胆碱小于十烃季铵，并且对于每种激动剂，其等于从神经肌肉接头处的“噪声”测量确定的关闭速率。动力学数据解释了激动剂诱导的稳态电导的几个方面，但未阐明明显的协同效应。
