Pulmonary Slowly Adapting Sensory Units Being More Active in Large Than in Small Airways May Result From More Sensory Receptors in the Units

It has been recognized for a long time that sensory units of pulmonary slowly adapting receptors (SARs) are more active in larger airways than in small airways. However, there is no explanation for this phenomenon. Structure is the basis for function. Sensory structures observed in peripheral airways resemble those found in central airways. However, in larger airways the sensory structures are bigger and more complex. Possibly, larger structures provide greater surface area for depolarization, and thus have a lower activating threshold and/or a higher sensitivity to stretch, leading to more activities. Recently, a single sensory unit has been reported to contain multiple receptors. Therefore, sensory structures in the large airway may contain more SARs, which may contribute to high activities. To test this hypothesis, we used a double staining approach to examine receptor structures in large (trachea and bronchi) vs small (bronchioles <500μm) airways in the rabbit. We labeled airway tissues with Na + /K + ‐ATPase antibodies to show all structures of the sensory unit and with myelin basic protein (MBP) antibodies to show the myelin sheath. We define an SAR as the end structure beyond MBP labeling. We found that even though the whole structure was bigger in large airways (5577±373 μm 2 ) than in small airways (2531±345 μm 2 ; P<0.0001), there was no difference in receptor sizes (583±23 μm 2 vs 627±37 μm 2 ; P>0.05). Furthermore, the receptor structure contains more SARs in the large airways (9.56±0.60) than the small airways (4.03±0.39; P<0.0001). Thus, we conclude that the higher activities in large sensory units may be due to a large number of receptors. (Supported by a VA Merit Review award PULM‐029‐10S)