Fisher J T
Department of Physiology, Queen's University, Kingston, Ont., Canada.
Can J Physiol Pharmacol. 1992 Apr;70(4):590-6. doi: 10.1139/y92-075.
It is clear from the literature that considerable postnatal development occurs in the contractile properties of skeletal and cardiac muscle. Nevertheless, few studies have focused on developmental changes in airway smooth muscle or on the functional capabilities of airway innervation in the newborn. Conclusions about force generation, based on measurements of pulmonary mechanics during stimulation of the vagus nerves, suggest that the newborn possesses a reduced capability to narrow airway diameter relative to the adult. This reduced in vivo response is accompanied by a reduction in maximal force generating capabilities when compared on the basis of force per unit tissue cross-sectional area (stress) in vitro. However, studies of porcine airways suggest that such a finding may simply reflect a reduction in the relative amount of contractile protein (myosin heavy chain) as seen in fetal or preterm smooth muscle. Thus, comparisons based on force normalized per cross-sectional area of myosin alter conclusions from one in which fetal tracheal smooth muscle generates less maximal force than the adult, to one in which the fetal trachea has greater contractile capabilities. Interestingly, comparisons of maximal isometric force in bronchial smooth muscle between different age groups remain unaffected when myosin heavy chain normalization is applied. Finally, there appears to be an age at which maximal force is significantly greater than at any other age, independent of the amount of smooth muscle (determined morphologically), smooth muscle myosin content, or myosin isoform. Whether this enhanced in vitro response is reflected in vivo, or is counteracted by other physiological mechanisms, remains to be seen.
从文献中可以清楚地看出，骨骼肌和心肌的收缩特性在出生后会发生相当大的发育变化。然而，很少有研究关注气道平滑肌的发育变化或新生儿气道神经支配的功能能力。基于迷走神经刺激期间肺力学测量得出的关于力产生的结论表明，相对于成年人，新生儿使气道直径变窄的能力降低。当在体外以单位组织横截面积（应力）为基础进行比较时，这种体内反应的降低伴随着最大力产生能力的降低。然而，对猪气道的研究表明，这样的发现可能仅仅反映了在胎儿或早产平滑肌中所见的收缩蛋白（肌球蛋白重链）相对量的减少。因此，基于肌球蛋白每横截面积标准化力的比较改变了结论，从胎儿气管平滑肌产生的最大力比成年人小的结论，变为胎儿气管具有更大收缩能力的结论。有趣的是，当应用肌球蛋白重链标准化时，不同年龄组支气管平滑肌最大等长力的比较不受影响。最后，似乎存在一个年龄，此时最大力明显大于其他任何年龄，这与平滑肌的量（通过形态学确定）、平滑肌肌球蛋白含量或肌球蛋白同工型无关。这种体外反应增强是否在体内得到体现，或者是否被其他生理机制抵消，仍有待观察。
