Gunst S J, Stropp J Q, Service J
Division of Thoracic Diseases and Internal Medicine, Mayo Clinic and Foundation, Rochester, Minnesota 55905.
J Appl Physiol (1985). 1990 May;68(5):2223-9. doi: 10.1152/jappl.1990.68.5.2223.
In normal humans and dogs, the airways do not constrict to closure even when maximally stimulated. However, airway closure can be produced in isolated canine lobes and bronchial segments that are stimulated with maximal concentrations of bronchoconstrictors. These observations suggest that under normal conditions, physiological mechanisms to limit bronchoconstriction exist in vivo. In this investigation, we evaluated how mechanical factors that influence airway smooth muscle contractility contribute to the modulation of the pressure-volume characteristics of contracted canine intraparenchymal airways in vitro. Our results demonstrated that maximal and even submaximal contractile stimuli can produce airway closure in bronchi that are allowed to contract under isobaric conditions. However, the effectiveness of bronchoconstrictors is significantly reduced when the airways are subjected to tidal volume oscillations during contraction. In addition, airways contracted isovolumetrically at low volumes exhibit a markedly reduced sensitivity to submaximal concentrations of acetylcholine. This may limit bronchoconstriction at low lung volumes and transpulmonary pressures where the effectiveness of parenchymal stress in keeping the airways open is reduced. Together these factors could provide a mechanism by which bronchoconstriction is limited to low levels of airway resistance under normal conditions in vivo.
在正常人和狗中，即使受到最大程度的刺激，气道也不会收缩至闭合。然而，在离体的犬肺叶和支气管节段中，用最大浓度的支气管收缩剂刺激时，可产生气道闭合。这些观察结果表明，在正常情况下，体内存在限制支气管收缩的生理机制。在本研究中，我们评估了影响气道平滑肌收缩性的机械因素如何在体外对收缩状态下的犬实质内气道的压力-容积特性调节产生影响。我们的结果表明，最大甚至次最大收缩刺激可使在等压条件下收缩的支气管发生气道闭合。然而，当气道在收缩过程中受到潮气量振荡时，支气管收缩剂的效力会显著降低。此外，在低容积下等容收缩的气道对次最大浓度的乙酰胆碱的敏感性明显降低。这可能会限制在低肺容积和跨肺压时的支气管收缩，此时实质应力维持气道开放的效力降低。这些因素共同作用可能提供一种机制，通过该机制在体内正常情况下支气管收缩被限制在低水平的气道阻力。
