Gibb A J, Marshall I G
J Physiol. 1984 Jun;351:275-97. doi: 10.1113/jphysiol.1984.sp015245.
The effects of tubocurarine and trimetaphan have been examined at voltage-clamped rat diaphragm neuromuscular junctions during (a) single and repetitive stimulation of the phrenic nerve in cut muscles and (b) repetitive ionophoretic application of acetylcholine (ACh). Tubocurarine (2.5 X 10(-7)-10(-6)M) produced a concentration-dependent reduction in the amplitude of neurally evoked end-plate currents (e.p.c.s). It also reduced their time constant of decay (tau e.p.c.) in a manner that was independent of membrane potential, and not markedly dependent on the tubocurarine concentration. Likewise the snake alpha-neurotoxin, erabutoxin b, reduced the e.p.c. amplitude and produced a voltage-independent shortening of tau e.p.c. Estimates of mean channel lifetime (tau noise) from ACh-induced e.p.c. fluctuations revealed that (a) tau noise was 46.4 +/- 3.7% shorter than tau e.p.c. measured at the same end-plate. At these same end-plates in the presence of tubocurarine (2.5 X 10(-7)M) tau e.p.c. was 32.6 +/- 1.0% shorter than the control tau e.p.c. but tubocurarine did not change tau noise, (b) trimetaphan (2.5 X 10(-5)-2 X 10(-4)M) produced a concentration-dependent and voltage-dependent reduction of tau e.p.c., and a concentration-dependent reduction of peak e.p.c. amplitude. Trimetaphan (2.5 X 10(-5)M) produced a 50% reduction of tau noise. (a) Both tubocurarine and trimetaphan produced concentration-dependent increases in the run-down of trains of neurally evoked e.p.c.s (50 Hz, 0.4 s). This effect did not vary with membrane potential in tubocurarine, but was voltage dependent when induced by trimetaphan. (b) Erabutoxin b reduced the e.p.c. amplitude but did not produce any increase in the run-down of trains of neurally evoked e.p.c.s. During 50 Hz repetitive ionophoretic application of ACh, tubocurarine (2.5 X 10(-7)M) reduced the amplitude of each current in the train without inducing any run-down of the current amplitudes. This effect was not dependent on the membrane potential. In contrast trimetaphan (2.5 X 10(-5)M) induced a voltage-dependent run-down of trains of ionophoretically evoked e.p.c.s. We conclude that tubocurarine and erabutoxin b reduce the e.p.c. amplitude by blocking the post-junctional ACh receptor. Tubocurarine produces tetanic rundown of e.p.c.s. by a prejunctional mechanism, whereas the effects of trimetaphan during single and repetitive stimulation are at least partly due to block of the open ion channel associated with the ACh receptor.
在电压钳制的大鼠膈神经肌肉接头处，研究了筒箭毒碱和阿方那特的作用，实验条件如下：(a) 切断肌肉后对膈神经进行单次和重复刺激；(b) 对乙酰胆碱（ACh）进行重复离子电泳给药。筒箭毒碱（2.5×10⁻⁷ - 10⁻⁶M）使神经诱发的终板电流（e.p.c.s）幅度呈浓度依赖性降低。它还以与膜电位无关且不太依赖筒箭毒碱浓度的方式降低了其衰减时间常数（tau e.p.c.）。同样，蛇α-神经毒素—— erabutoxin b，降低了e.p.c.幅度，并使tau e.p.c.出现与电压无关的缩短。根据ACh诱导的e.p.c.波动估算的平均通道寿命（tau noise）表明：(a) tau noise比在同一终板处测得的tau e.p.c.短46.4±3.7%。在存在筒箭毒碱（2.5×10⁻⁷M）的这些相同终板处，tau e.p.c.比对照tau e.p.c.短32.6±1.0%，但筒箭毒碱未改变tau noise；(b) 阿方那特（2.5×10⁻⁵ - 2×10⁻⁴M）使tau e.p.c.呈浓度依赖性和电压依赖性降低，使峰值e.p.c.幅度呈浓度依赖性降低。阿方那特（2.5×10⁻⁵M）使tau noise降低了50%。(a) 筒箭毒碱和阿方那特均使神经诱发的e.p.c.s串（50Hz，0.4s）的衰减呈浓度依赖性增加。在筒箭毒碱作用下，这种效应不随膜电位变化，但由阿方那特诱发时则与电压有关。(b) erabutoxin b降低了e.p.c.幅度，但未使神经诱发的e.p.c.s串的衰减增加。在50Hz重复离子电泳给予ACh期间，筒箭毒碱（2.5×10⁻⁷M）降低了串中每个电流的幅度，而未诱发电流幅度的任何衰减。这种效应不依赖于膜电位。相反，阿方那特（2.5×10⁻⁵M）诱发了离子电泳诱发的e.p.c.s串的电压依赖性衰减。我们得出结论，筒箭毒碱和erabutoxin b通过阻断接头后ACh受体来降低e.p.c.幅度。筒箭毒碱通过接头前机制使e.p.c.s出现强直衰减，而阿方那特在单次和重复刺激期间的作用至少部分是由于阻断了与ACh受体相关的开放离子通道。
