Connor E A, Fiekers J F, Neel D S, Parsons R L, Schnitzler R M
J Physiol. 1984 Jun;351:657-74. doi: 10.1113/jphysiol.1984.sp015269.
Characteristics of receptor-channel activation and desensitization have been compared at voltage-clamped snake slow and twitch fibre end-plates maintained in an isotonic potassium propionate solution. Miniature end-plate current (m.e.p.c.) decay was slower and less voltage dependent at slow fibre end-plates than at twitch fibre end-plates. The peak m.e.p.c. amplitude versus voltage relationship and reversal potential were similar at the two end-plate types. Acetylcholine-induced noise and m.e.p.c.s were recorded at slow fibre end-plates. At most slow fibres the spectral density was not adequately fitted by a single Lorentzian function. Rather, the observed spectral density was greater at high frequencies than the values predicted using the m.e.p.c. decay rate. The noise could be well described by the sum of two Lorentzian functions, one of which corresponded to a single Lorentzian function with the corner frequency determined by the m.e.p.c. decay rate. The shape of the carbachol concentration-peak end-plate current relationship was similar at both slow and twitch fibre end-plates. However, for all concentrations tested, the peak carbachol-induced end-plate current (e.p.c.carb.) value was markedly less at slow fibre end-plates than at twitch fibre end-plates. The onset of desensitization was determined using two methods. The first concerned analysis of the time course of decay of the e.p.c.carb. from a peak value during the sustained application of agonist. The second involved a double-perfusion technique in which a 'desensitizing' dose was applied for varying intervals before the application of a second 'test' dose of carbachol. With both methods the development of desensitization at both end-plate types was dependent on carbachol concentration and duration of exposure. At each end-plate type the time course of desensitization onset often exhibited two components; one with a time constant of seconds and a slower component having time constants in the range of tens to hundreds of seconds. The slope of the relationship between carbachol concentration and equilibrium desensitization at slow and twitch fibre end-plates was close to two, suggesting that two molecules of agonist are probably bound during the development of desensitization. However, for all concentrations tested, desensitization developed more rapidly and to a greater extent at twitch fibre end-plates than at slow fibre end-plates. The voltage dependence of the 3 min steady-state desensitization produced by 108 microM-carbachol was very similar (approximately -0.0250 mV-1) at both fibre types.(ABSTRACT TRUNCATED AT 400 WORDS)
在等渗丙酸钾溶液中保持电压钳制的蛇慢肌纤维和快肌纤维终板上，比较了受体通道激活和脱敏的特征。与快肌纤维终板相比，慢肌纤维终板上的微小终板电流（m.e.p.c.）衰减较慢且电压依赖性较小。两种终板类型的m.e.p.c.峰值幅度与电压关系以及反转电位相似。在慢肌纤维终板上记录了乙酰胆碱诱导的噪声和m.e.p.c.。在大多数慢肌纤维中，光谱密度不能用单一的洛伦兹函数充分拟合。相反，在高频下观察到的光谱密度大于使用m.e.p.c.衰减率预测的值。噪声可以用两个洛伦兹函数的和很好地描述，其中一个对应于由m.e.p.c.衰减率确定角频率的单一洛伦兹函数。在慢肌纤维和快肌纤维终板上，卡巴胆碱浓度与峰值终板电流关系的形状相似。然而，对于所有测试浓度，慢肌纤维终板上卡巴胆碱诱导的峰值终板电流（e.p.c.carb.）值明显低于快肌纤维终板。使用两种方法确定脱敏的起始。第一种方法涉及分析在持续应用激动剂期间e.p.c.carb.从峰值衰减的时间过程。第二种方法涉及双灌注技术，其中在应用第二个“测试”剂量的卡巴胆碱之前，以不同的间隔应用“脱敏”剂量。两种方法都表明，两种终板类型的脱敏发展都取决于卡巴胆碱浓度和暴露持续时间。在每种终板类型中，脱敏起始的时间过程通常表现出两个成分；一个时间常数为秒，另一个较慢的成分时间常数在几十到几百秒范围内。慢肌纤维和快肌纤维终板上卡巴胆碱浓度与平衡脱敏之间关系的斜率接近2，这表明在脱敏发展过程中可能结合了两个激动剂分子。然而，对于所有测试浓度，快肌纤维终板上的脱敏比慢肌纤维终板上发展得更快且程度更大。由108 microM - 卡巴胆碱产生的3分钟稳态脱敏的电压依赖性在两种纤维类型中非常相似（约 -0.0250 mV-1）。（摘要截于400字）
