Airway Rapidly Adapting Receptors

Abstract

By the turn of the millennium, only three major types of receptors were recognized in the airways [C‐fiber receptors (CFRs), rapidly adapting receptors (RARs), and slowly adapting receptors (SARs)]. RARs and SARs are mechanical receptors with myelinated afferents. RARs were thought to be sensitive to a wide variety of mechanical (inflation and deflation) and chemical stimuli, and display a spectrum of responses. It is perplex because such characteristics make the RAR reflex system non‐specific. Careful review of literature shows RARs are not strictly defined. Many investigators ascribe observed sensory properties and reflex effects to RARs when SARs and CFRs were ruled out. Thus, RARs are credited more.In this report, we did a series experiments to characterize ‘RARs' in rabbits. We found that some ‘RARs' share many characteristics of CFRs, including low resting discharge frequency, chemosensitivity, and insensitivity to mechanical stimulation. These fibers have conduction velocities of 4–15 m/s, belong to the Aδ afferent fibers, and are termed high threshold Ad receptors (HTARs). Using histochemical staining [antibodies against Na+/K+‐ATPase and myelin basic protein (MBP)], we found that many mechanical receptor structures share a single axon, indicating that what we assessed is not a sensory receptor but a sensory unit that contains many receptors. By recording sensory unit activities, we found that many units had multiple receptive fields. Blocking a field with lidocaine, we found that each field contained at least a receptor. Moreover, both SARs and RARs can respond to lung inflation and deflation. Unit responses to lung inflation and deflation can be selectively blocked by injection of lidocaine into a field. This indicates that lung deflation response is elicited from deflation activated receptors (DARs). Furthermore, a single sensory unit may contain both RARs and SARs, each can be selectively blocked by injection of lidocaine into a field. Based on these characteristics of airway mechanical sensory units, we conclude that a sensory unit consists of many different receptors for sensing different mechanical forces. Three major types of mechanical sensors operate in the lung: SARs, RARs, and DARs. Many SAR and RAR units also respond to lung deflation because they contain DARs. Pure DARs, which respond to lung deflation only, are rare in large animals but are easily identified in small animals. Actually, some chemosensitive responses ascribed to RARs may instead belong to HTARs, while some mechanical sensitive responses belong to DARs.In summary, with identification of DARs and HTARs, afferent properties and reflex functions of RARs should be redefined. RARs, by definition, are mechanoreceptors. They are stimulated by lung inflation but not deflation. Their afferent properties may not include chemosensitivities, which are belong to HTARs. Therefore, lung deflation and chemical stimulation induced reflex effects also should be ascribed to those sensors.Support or Funding InformationSupported by a VA Merit Review Award PULM‐029‐10SThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.