Portela A, de Xamar Oro J R, Stewart P A, Gimeno A L, Guardado I, Brennan M, Llobera O H, Pérez J C
Acta Physiol Lat Am. 1977;27(4):157-76.
The effects of high doses of X-radiation (100 kilorads) on the neuromuscular transmission of isolated sciatic nerve-sartorius muscle preparations of the frog, as evaluated by bioelectrical characteristics, were explored. Intracellular microelectrode recordings after X-irradiation showed that the resting, excitation and action potentials of nerve fibers approaching the synaptic terminal region of the motor end-plate became lessened, and also presented a slower velocity of impulse propagation, earlier than that observed in muscle cells. After forty minutes following the irradiation period, the neuromuscular transmission became blocked, although muscle fibers still responded to direct electrical stimulation. Records taken at the motor end-plate region of muscle cells, demonstrated the presence of postsynaptic miniature end-plate potentials (m.e.p.p.'s), the sequence of which fits closely into a random Poisson distribution. X-irradiation elicited an increase of the rate of m.e.p.p.'s and induced membrane changes over fine terminal nerve branches, leading into a failure to initiate and propagate action potentials. Only as time progressed, this nerve bioelectrical impairment was accompanied by a similar one in muscle cells, associated to the inability to develop contractile tension. The increase of m.e.p.p.'s frequency due to depolarization by a high K+ concentration, of presynaptic nerve membranes of control and irradiated preparations, was reversed by a high Mg+--Ca2+--free media. However, a concentration of Mg2+, which normally reduced quantal release of acetylcholine (ACh), without altering presynaptic nerve membrane potentials, failed to modify the spontaneous basal frequency of m.e.p.p.'s, both in irradiated and control preparations. The findings of the present study suggest that the presynaptic ACh synthesis, storage, and availability for ACh liberation are not early affected by X-rays, i.e. at a time when transmission from nerve to muscle had already failed. On the contrary, the most precocius membrane changes following X-irradiation, seem related to the mechanisms underlying nerve resting, excitation, and action potentials. The smallest presynaptic nerve terminal fibers close to the motor end-plate, are the most radiosensitive portion of the myoneural junction. Indeed, their bioelectric characteristics are early affected by X-rays, resulting in a failure to generate and propagate nerve impulses which lead to a blockade of neuromuscular transmission.
研究了高剂量X射线（100千拉德）对青蛙离体坐骨神经-缝匠肌标本神经肌肉传递的影响，通过生物电特性进行评估。X射线照射后的细胞内微电极记录显示，靠近运动终板突触终末区域的神经纤维的静息电位、兴奋电位和动作电位减小，并且冲动传播速度也变慢，比在肌肉细胞中观察到的情况更早出现。照射后40分钟，神经肌肉传递被阻断，尽管肌肉纤维仍对直接电刺激有反应。在肌肉细胞运动终板区域的记录表明存在突触后微小终板电位（m.e.p.p.），其序列紧密符合随机泊松分布。X射线照射导致m.e.p.p.频率增加，并在细小的终末神经分支上引起膜变化，导致无法引发和传播动作电位。只有随着时间的推移，这种神经生物电损伤才伴随着肌肉细胞中类似的损伤，这与无法产生收缩张力有关。高钾浓度使对照和照射标本的突触前神经膜去极化，从而导致m.e.p.p.频率增加，而高镁无钙培养基可逆转这种增加。然而，正常情况下能减少乙酰胆碱（ACh）量子释放而不改变突触前神经膜电位的镁离子浓度，在照射和对照标本中均未能改变m.e.p.p.的自发基础频率。本研究结果表明，突触前ACh的合成、储存以及用于ACh释放的可用性在早期不受X射线影响，即在神经到肌肉的传递已经失败的时候。相反，X射线照射后最早出现的膜变化似乎与神经静息、兴奋和动作电位的潜在机制有关。靠近运动终板的最小突触前神经终末纤维是肌神经接头中对辐射最敏感的部分。实际上，它们的生物电特性在早期就受到X射线的影响，导致无法产生和传播神经冲动，从而导致神经肌肉传递阻断。
