We characterized changes in the vagal chronotropic response during 3-min trains of vagal stimulation at 3, 5, and 8 Hz in anesthetized, chemically sympathectomized (6-hydroxydopamine) newborn canines (<15 d of age). In response to vagal stimulation, the sinus cycle length gradually increased (within 30 s) to a maximum value that was dependent upon the stimulation frequency (p < 0.001). The chronotropic response then attenuated over the remainder of the vagal train. However, unlike in adult dogs, the degree of attenuation of the vagal chronotropic response (fade) was also highly dependent upon the frequency of vagal stimulation in the range 3–8 Hz (p < 0.002). We then compared the maximum change in sinus cycle length and fade in a group of neonates while stimulating the vagus at 3 Hz before and after the administration of physostigmine (0.2 mg/kg i.v.). Physostigmine resulted in a significant increase not only in the maximum percent change in sinus cycle length but in the magnitude of attenuation of the vagal response as well. Therefore, unlike the adult, in the newborn the magnitude of both the vagal response and fade are dependent upon concentrations of acetylcholine released in response to modest levels of vagal stimulation (≤8 Hz). This may be the result of differences between the newborn and the adult in the prestreceptor mechanisms of fade or in the susceptibility of the muscarinic receptor to desensitization by the neurotransmitter acetylcholine.
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机译：我们表征了麻醉，化学交感神经（6-羟基多巴胺）新生犬（年龄小于15天）在3、5和8 Hz迷走神经刺激的3分钟训练过程中迷走神经变时反应的变化。响应迷走神经刺激，窦周期长度逐渐增加（在30 s内）至取决于刺激频率的最大值（p <0.001）。然后在迷走神经序列的其余部分上，变时响应减弱。但是，与成年犬不同，迷走神经变时性反应（衰落）的衰减程度也高度取决于迷走神经刺激频率（3-8 Hz）（p <0.002）。然后，我们比较了在使用毒扁豆碱（0.2mg / kg i.v.）给药前后以3Hz刺激迷走神经的一组新生儿的窦周期长度和衰落的最大变化。毒扁豆碱不仅导致窦周期长度的最大百分比变化，而且导致迷走神经反应的衰减幅度也显着增加。因此，与成年人不同，在新生儿中，迷走神经反应和褪色的强度均取决于对迷走神经刺激的适度水平（≤8Hz）释放的乙酰胆碱的浓度。这可能是新生儿和成年人在衰老的前体感受器机制或毒蕈碱受体对神经递质乙酰胆碱脱敏敏感性方面差异的结果。
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