Comparison of elastic properties and contractile responses of isolated airway segments from mature and immature rabbits.

Immature rabbits have greater maximal airway narrowing with bronchoconstriction in vivo compared with mature animals. As isolated immature lungs have a lower shear modulus, it is unclear whether the greater airway narrowing in the immature lung is secondary to less tethering between the airways and the lung parenchyma or to differences in the mechanical properties of the mature and immature airways. In the present study, we compared the mechanical properties of fluid-filled, isolated, intraparenchymal airway segments of the same generation from mature and immature rabbits. Stimulation with ACh resulted in greater airway narrowing in immature than mature bronchi. The immature bronchi were more compliant, had a lower resting airway volume, and were more collapsible compared with the mature bronchi. When the airways were contracted with ACh under isovolume conditions, the immature bronchi generated greater active pressure, and they were more sensitive to ACh than were mature bronchi. Our results suggest that maturational differences in the structure and function of the airways in the absence of the lung parenchyma can account for the greater maximal narrowing of immature than mature airways in vivo.