A Paradoxical Response of Pulmonary Slowly Adapting Receptor Units During Constant Pressure Lung Inflation

It is known that activity of pulmonary slowly adapting receptors (SARs) decreases slowly, due to adaptation, during lung inflation with constant pressure. However, it is not uncommon to observe that some SAR units increase their activity instead during the course of lung inflation (we call it creeping). In the current studies, we seek to characterize this phenomenon retrospectively by analyzing previously collected data. From 52 anesthetized, open‐chest and mechanically ventilated rabbits, we examined activities of SAR units from the cervical vagus nerve during different levels of lung inflation with a constant pressure (10, 20, and 30 cmH 2 O) for 4 seconds. We found that creeping occurred in 46.0% units (64 of 139 units). The increased activity took 3 forms. It could be gradual, abrupt, or a combination of the two (i.e., the unit increased activity gradually first and then abruptly). The creeping occurred more frequently and earlier as the lung inflation pressure increased. For example, creeping occurred in 1.4% of the units and activity started to increase at 3.0 sec at 10 cmH 2 O (n=2), occurred in 24.5% at 2.2 sec at 20 cmH 2 O (n=34), and in 41.7% at 1.6 sec at 30 cmH 2 O (n=58). In many cases SAR activity decreased or stopped completely following the creeping, i.e., the unit deactivated. During a careful literature review, creeping can be identified in different species (opossums, dogs, cats, guinea pigs, and turtles) from the figures reported. However, only one paper discussed this issue as a side finding (1). Farber et al inflated the lung with a constant pressure of 10 cmH 2 O to examine the adaptation rate of SARs in opossums and found that 9 out of 40 SAR units (22.5%) increased their activity. Undoubtedly, with higher pressure and longer inflation protocols, much more significant portion of the units will exhibit creeping. Clearly, creeping is a common behavior of the mechanosensory unit across species, which has been largely neglected by the scientific community. We speculate that during lung inflation, sudden increases in SAR input will provide a breaking effect (negative feedback) to counter‐react the lung inflation. Certainly, such a mechanism is more beneficial at high pressure levels. Fortunately, such a propencity of activation at higher inflation levels is a built in mechanism of the pulmonary sensory system. Elucidation of underlying mechanisms of creeping will certainly improve our understanding of the mechanosensory function. Support or Funding Information Supported by a Merit Review Award (PULM‐024‐17S) from Veteran Administration of the United States(1) Farber JP , Fisher JT and Sant’Ambrogio G. Distribution and discharge properties of airway receptors in the opossum, Didelphis marsupialis . Am J Physiol 245 : R209 – R214 , 1983 .