Slowly Adapting Sensory Units Have More Receptors in Large Airways than in Small Airways in Rabbits

Abstract

Sensory units of pulmonary slowly adapting receptors (SARs) are more active in large airways than in small airways. However, there is no explanation for this phenomenon. Although sensory structures in large airways resemble those in small airways, they are bigger and more complex. Possibly, a larger receptor provides greater surface area for depolarization, and thus has a lower activating threshold and/or a higher sensitivity to stretch, leading to more nerve electrical activities. Recently, a single sensory unit has been reported to contain multiple receptors. Therefore, sensory units in large airways may contain more SARs, which may contribute to high activities. To test this hypothesis, we used a double staining technique to identify sensory receptor sizes. We labeled the sensory structure with Na⁺/K⁺-ATPase antibodies and the myelin sheath with myelin basic protein (MBP) antibodies. A SAR can be defined as the end formation beyond MBP labeling. Thus, we are able to compare sizes of sensory structures and SARs in large (trachea and bronchi) vs. small (bronchioles <500 μm in diameter) airways in the rabbit. We found that even though the sensory structure was bigger in large airways than in small airways (3340 ± 223 vs. 1168 ± 103 μm²; P < 0.0001), there was no difference in receptor sizes (349 ± 14 vs. 326 ± 16 μm²; > 0.05). However, the sensory structure contains more SARs in large airways than in small airways (9.6 ± 0.6 vs. 3.6 ± 0.3; P < 0.0001). Thus, our data support the hypothesis that greater numbers of SARs in sensory units of large airways may contribute to higher activities.

Keywords: airway receptor; airway sensors; lung afferents; sensory receptor; sensory receptor cells; sensory unit; vagus nerve.

Figure 1

A double staining approach to illustrate SAR sensory structures identified in rabbit airways. Na⁺/K⁺-ATPase stains all structures in the sensory unit (red), whereas myelin basic protein (MBP) stains the myelin sheath (green) and shows yellow (co-staining) in the composite figure (top-right and bottom parts). Clearly, the axon demyelinated before it reaches the end formation. Thus, the receptor can be identified (pure red portions without co-stain with MBP). Top (small airway, 300 μm in diameter): The parent axon of the sensory structure is running from the bottom up. It gives off three branches, indicated by white arrows 1, 2, and 3 in the top-left panel. Its first branch is at the bottom of the figure, the second one in the middle part, and the third one at the upper part. Six receptors can be identified in this microscopic view (1 in the first branch, 3 in the second and 2 in the third). They are showing red on double stain. Bottom (trachea): two parent axons (one starts at up-right and one at low left, indicated by white arrows 1 and 2) can be identified. The up-right sensory structure (1) has 9 receptors and the low-left one (2) has 13 receptors. Insets are enlarged to illustrate the sensory receptors.

Figure 2

Comparison of airway receptors in small (A,A′) and large (B,B′) airways. The airway sensory structure was stained with Na⁺-K⁺-ATPase antibody (red); myelin sheath was stained with MBP antibody (green). The structure is bigger in (B) than in (A). However, the average sizes of receptors (end formation after nerve demyelination) are the same in small (229.9 μm²) and large (229.5 μm²) airways (C). There are two receptors in (A′) and eight receptors in (B′).

Figure 3

Group dada for sizes of sensory structures and receptors. The sensory structure is bigger in large airways (n = 16) than in small airways (n = 36) (^**** denotes P < 0.0001). However, the receptor sizes were the same in large (n = 153) and small (n = 129) airways (P > 0.05).

Figure 4

Size distribution of airway sensory structures (projection area). Sizes of 900 and 1500 represent areas of 600–1199 and 1200–1799 μm², respectively. Distribution peaks are 900 and 2700 μm² in small and large airways, respectively. The structure was significantly bigger in large airways (3340 ± 223 μm², n = 16) than in small airways (1168 ± 103 μm², n = 36; P < 0.0001).

Figure 5

Distribution of airway receptor size (projection area). Sizes of 150 and 270 represent areas of 120–179 and 240–299 μm², respectively. The distribution patterns of receptors in small (n = 129) and large (n = 153) airways are similar, so that the data are pooled together to show the distribution pattern of total receptors (n = 282). The distribution shows 4 peaks at about 200, 400, 600, and 800 μm² (Please see footnote² for details).

Figure 6

A receptor (end formation after nerve demyelination) can be singlet (A), doublet (B) or triplet (C). Airway receptor was stained with Na⁺-K⁺-ATPase antibody (red); myelin sheath was stained with MBP antibody (green). Receptor sizes are: 257 μm² in A, 505 (238 and 267) μm² in B, and 719 (278, 222, and 219) μm² in (C). Arrows show the point of nerve demyelination; arrow heads show a single end formation.

Figure 7

Illustration of graphical identification of four peaks. The upper panel is adapted from Figure 5. The lower panel shows the first three peaks. In the upper panel, subtracting the black portions (columns 270, 330, and 390 μm² according to columns 150, 90 and 30 μm², and 450 and 510 μm² according to 690 and 630 μm²) results in 2 normal distribution peaks roughly at 200 and 600 μm² (the first and third peaks in the lower panel). The black portion in columns 270, 330, and 390 is obtained by subtracting columns 150, 90 and 30 from 270, 330 and 390, respectively. The five black columns may represent another group of receptors with a peak at 400 μm² (the second peak in the lower panel).