Bennett M R, Farnell L, Gibson W G, Karunanithi S
Department of Physiology, University of Sydney, N.S.W., Australia.
Biophys J. 1995 Mar;68(3):925-35. doi: 10.1016/S0006-3495(95)80268-7.
The time course of most quantal currents recorded with a small diameter electrode placed over visualized varicosities of sympathetic nerve terminals that secrete ATP was determined: these had a time to reach 90% of peak of 1.3-1.8 ms and a time constant of decay of 12-18 ms; they were unaffected by blocking ectoenzymes or the uptake of adenosine. Monte Carlo methods were used to analyze the stochastic interaction between ATP, released in a packet from a varicosity, and the underlying patch of purinoceptors, to reconstitute the time course of the quantal current. This leads to certain restrictions on the possible number of ATP molecules in a quantum (about 1000) and the density of purinoceptors at the junctions (about 1000 microns-1), given the known geometry of the junction and the kinetics of ATP action. The observed quantal current has a relatively small variability (coefficient of variation < 0.1), and this stochastic property is reproduced for a given quantum of ATP. Potentiation effects (of about 12%) occur if two quanta are released from the same varicosity because the receptor patch is not saturated even by the release of two quanta. The simulations show that quantal currents have a characteristically distinct shape for varicosities with different junctional cleft widths (50-200 nm). Finally, incorporation of an ectoenzyme with the known kinetics of ATPase into the junctional cleft allows for a quantal current of the observed time course, provided the number of ATP molecules in a quantum is increased over the number in the absence of the ATPase.
通过放置在分泌ATP的交感神经末梢可见膨体上的小直径电极记录的大多数量子电流的时间进程被确定：这些电流达到峰值90%的时间为1.3 - 1.8毫秒，衰减时间常数为12 - 18毫秒；它们不受外切酶阻断或腺苷摄取的影响。使用蒙特卡罗方法分析从膨体成包释放的ATP与潜在嘌呤能受体片之间的随机相互作用，以重建量子电流的时间进程。鉴于已知的连接几何结构和ATP作用动力学，这对量子中可能的ATP分子数量（约1000个）和连接处嘌呤能受体的密度（约1000微米-1）产生了一定限制。观察到的量子电流具有相对较小的变异性（变异系数<0.1），并且对于给定数量的ATP，这种随机特性会重现。如果从同一膨体释放两个量子，会出现增强效应（约12%），因为即使释放两个量子，受体片也未饱和。模拟表明，对于具有不同连接间隙宽度（50 - 200纳米）的膨体，量子电流具有特征性的明显形状。最后，将具有已知ATP酶动力学的外切酶纳入连接间隙中，只要量子中的ATP分子数量比不存在ATP酶时增加，就可以产生观察到的时间进程的量子电流。
