Adaptation of airway stretch receptors in newborn and adult dogs.

We studied the adaptive properties of tracheal and bronchial slowly adapting mechanoreceptors as an index of their dynamic responsiveness in newborn and adult dogs. The receptors studied exhibited a marked rate dependency in their adaptive characteristics: faster inflations leading to a higher adaptation, a behavior typically found in other mechanoreceptors. During inflations of comparable rates airway slowly adapting mechanoreceptors of newborn dogs reach initial firing rates (Fi) that are lower than those of adult dogs. These increments in activity appear to be reduced in proportion to their lower static discharge (Fs). Therefore when adaptation is represented as adaptation index (A.I. = (Fi - Fs)/Fi X 100), i.e., in relative terms, it does not differ between age groups. Tracheal and bronchial slowly adapting mechanoreceptors do not differ in their adaptation for either of the age groups. In conclusion, since the dynamic sensitivity is lower in newborns, it it cannot be a factor contributing to their higher breathing rate. Nevertheless, since newborns breathe faster than adults, the activation of their stretch receptors should be greater than it would have otherwise been, perhaps contributing to their faster rate of breathing.