Ewald D A
J Membr Biol. 1976 Oct 20;29(1-2):47-65. doi: 10.1007/BF01868951.
The cholinergic sensitivity of rat diaphragm muscle, me-sured as the magnitude of depolarization responses to repetitive, iontophoretic pulses of acetylcholine (ACh) onto neuromuscular endplates, is increased by addition of ATP to the perfusion medium. Depolarization responses begin to increase within the first min after addition of 10 mM ATP and plateau at 60% above control levels (mean value) after 4 to 6 min. Neither the magnitude nor the time course of the potentiations corresponds to changes in resting potential or membrane resistance. Other nucleotides are equally or less effective at the same concentration: ATP=ADP greater than UTP greater than AMP=GTP (=no added nucleotide control) The duration of the individual ACh responses does not increase during continuous exposure to the active nucleotides for up to 15 min except when the muscle is pretreated with eserine. Mild enzymatic predigestion of the muscle with collagenase and then protease, increasing the availability of the postjunctional membrane to bath-applied drugs, decreases the variability and increases the magnitude of the potentiation to a given dose of ATP. The dose-response curve for ATP is then more than half-maximal at 1 mM and the ranking of the other nucleotides relative to ATP is the same as without predigestion. There is an optimum Ca++ concentration for the potentiation between zero and 2 mM: potentiation is enhanced in Ca++ -free medium, partially blocked in twice-normal Ca++ medium, and totally blocked in Ca++ -free medium 10 min after a 5 min exposure to 2.5 mM EGTA. The similar Ca++ dependence of ACh receptor activation in the absence of added nucleotide suggests that ATP directly facilitates receptor activation by ACh. This facilitory action could be one of the physiological roles for the ATP released from stimulated phrenic nerve.
以对重复离子电泳施加于神经肌肉终板的乙酰胆碱（ACh）脉冲的去极化反应幅度来衡量，大鼠膈肌的胆碱能敏感性会因向灌注介质中添加ATP而增加。添加10 mM ATP后，去极化反应在第一分钟内开始增加，并在4至6分钟后达到比对照水平（平均值）高60%的稳定状态。增强作用的幅度和时间进程均与静息电位或膜电阻的变化无关。在相同浓度下，其他核苷酸的效果相同或更差：ATP = ADP＞UTP＞AMP = GTP（=未添加核苷酸的对照）。在持续暴露于活性核苷酸长达15分钟的过程中，除肌肉用毒扁豆碱预处理外，单个ACh反应的持续时间不会增加。用胶原酶然后蛋白酶对肌肉进行轻度酶促预消化，增加了接头后膜对浴中应用药物的可及性，降低了变异性，并增加了对给定剂量ATP的增强幅度。然后，ATP的剂量反应曲线在1 mM时超过半数最大反应，其他核苷酸相对于ATP的排名与未预消化时相同。在零至2 mM之间存在增强作用的最佳Ca++浓度：在无Ca++的介质中增强作用增强，在两倍正常Ca++的介质中部分受阻，在暴露于2.5 mM EGTA 5分钟后10分钟的无Ca++介质中完全受阻。在未添加核苷酸的情况下，ACh受体激活对Ca++的类似依赖性表明，ATP直接促进ACh对受体的激活。这种促进作用可能是受刺激的膈神经释放的ATP的生理作用之一。
