INTRODUCTION

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PULMONARY AFFERENTS can be categorized as nonmyelinated fibers (C-fibers) and myelinated fibers (4, 11, 16, 18). According to the adaptation rate of afferent activity during constant pressure (or constant volume) inflation of the lung, the myelinated afferents are further divided into those innervating rapidly adapting receptors (RARs) and slowly adapting receptors (SARs). The inputs from the SARs are known to influence breathing patterns by altering inspiratory and expiratory times (23). SARs display a variety of discharge patterns and can be further classified. For example, SARs are classified as low or high threshold according to their resting discharge pattern (12). Low-threshold SARs fire continuously throughout the ventilatory cycle, whereas the high-threshold SARs fire discontinuously with a clear pause during the deflation phase of ventilatory cycles. SARs are also suggested to be classified into type I and type II according to their response to constant pressure inflation of the lungs (16). Type I receptors tend to saturate in their response when inflation pressure is above 10 cmH₂O and are believed to be mainly in central airways, whereas type II receptors have a relatively linear response to lung inflation, increasing activity with increasing pressure and are believed to be more in peripheral airways. The nonuniform discharge pattern of the SARs is of interest because full understanding of their behavior is necessary to understand their physiological roles in control of breathing. Therefore, many investigators are interested in the relationship between the discharge pattern and receptor location (2, 5, 6, 9, 10, 12, 15, 20, 22). Even though SARs are the most throughly characterized pulmonary receptors, we still do not fully understand their behavior and its relation to pulmonary reflexes.

During recording of pulmonary afferent activity of myelinated fibers, some SARs with an intermediate adaptation rate are encountered (3). As described by Coleridge and Coleridge, "Receptors showing pronounced adaptation cannot be distinguished from RARs by their adaptation rate alone but they can be identified as slowly adapting stretch receptors by the remarkable regularity of interspike interval characterizing their steady-state discharge." The question of whether these intermediate receptors are the same as the other SARs in their afferent properties and in their reflex functions has never been addressed. The present study is an attempt to answer the first part of this question by investigating responses of SARs to mechanical changes, such as to passive deflation [positive-end-expiratory pressure (PEEP) removal] and to different levels of constant pressure inflation. It was found that SARs are heterogeneous and can be further categorized, some being typical SARs, whereas the others being RAR-like.

	METHODS

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General. Twenty rabbits were anesthetized with pentobarbital sodium (30 mg/kg iv). Then doses of anesthetics (6 mg iv) were given hourly. Adequate anesthesia was maintained by absence of an active corneal reflex and pedal reflex to toe pinching. Additional small doses (2 mg iv) were given whenever necessary. The trachea was cannulated low in the neck, and the chest was opened widely in the midline. The lungs were ventilated by a Harvard ventilator (model 683); PEEP was maintained by placing the expiratory outlet under 3-4 cmH₂O.

Recording of afferent activity. Afferent activities were recorded according to the conventional method (21, 22). The vagus nerve (either right or left) was separated from the carotid sheath, placed on a dissecting platform, and covered by mineral oil. A small slip was cut from the vagus nerve, and the peripheral end was placed on recording electrodes; the main trunk of the vagus nerve was left intact. The electrodes were connected to a high-impedance probe (Grass HIP5) from which the output was led to an amplifier (Grass P511). After suitable amplification, action potentials from single fiber strands of the vagus nerve were displayed on an oscilloscope and monitored by a loudspeaker. In addition, a voltage analog of impulse frequency was produced by a ratemeter (Frederick Haer, Brunswick, ME) at a binwidth of 0.1 s. The single unit is ensured by a uniformity in amplitude and contour of action potentials displayed in the oscilloscope. Action potentials and its analog frequency along with blood pressure and airway pressure were recorded by a thermorecorder (Astro-Med; Dash IV). The approximate location of the receptors was determined by gently exploring the external surface of the lungs with a cotton tip. No attempt was made to locate endings more precisely by exploring the airway lumen or dissecting the airways.

Protocols and data analysis. After a receptor was identified, the lungs were inflated by three tidal volumes to standardize the lung mechanics. Adaptation rate was assessed by adaptation index, which was first defined by Knowlton and Larrabee (7) and then elaborated by Widdicombe (19). Receptors with an adaptation index less than 65% were included in the study as SARs. Adaptation indexes were determined at three different levels of constant pressure of lung inflation, i.e., 10, 15, and 20 cmH₂O. The inflation pressures were applied randomly. Adaptation index was calculated by the formula
The response of the receptor to PEEP removal was examined. Receptor activity was recorded as impulses per second (imp/s), calculated from a mean of three respiratory cycles. The results for each test were expressed as percent of pretest value. Data for groups are reported as means ± SE. One-way analysis of variance (ANOVA) was used to test the difference among three groups of data. Differences were considered statistically significant at P < 0.05.

	RESULTS

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One hundred and four receptors, which have adaptation indexes less than 65%, were identified as pulmonary slowly adapting stretch receptors. Their resting discharge patterns fit into the category of SARs, i.e., having a remarkable regularity of interspike interval (3). The receptors with the similar basal firing pattern, but with a higher adaptation index (>70%), were excluded from the group data because their adaptation index falls into the RAR range. An example of one such receptor is illustrated (see Fig. 5).

These 104 SARs were divided into two groups according to whether their activity increased (RAR-like) or decreased (typical) during PEEP removal. In RAR-like SARs (n = 11), mean activity increased from 31 ± 6 to 46 ± 7 imp/s, and peak activity increased from 70 ± 8 to 100 ± 14 imp/s during PEEP removal (Fig. 1); in "typical" SARs (n = 93), mean discharge decreased from 44 ± 2 to 25 ± 1 imp/s and peak activity decreased from 95 ± 5 to 61 ± 3 imp/s during the same PEEP reduction (Fig. 2). The receptor field was identified in 82 of the 104, and all appeared to be located in lobar bronchi or more distally in the lung. Eleven of the 82 receptors were identified as RAR-like and the other 71 as typical. The afferent fibers were not always running ipsilaterally. For example, 11 of the 82 SARs (accounting for 13.4%; 2 belong to RAR-like group and 9 belong to typical group) were identified in the contralateral lung.

View larger version (70K): [in this window] [in a new window]   Fig. 1.   Discharge of a rapidly adapting receptor (RAR)-like slowly adapting receptor (SAR) in an anesthetized, open-chest, and artificially ventilated rabbit. Traces are IMP, receptor activity; imp/s, receptor activity counted at a binwidth of 0.1 s; Paw, airway pressure. This receptor has a regular firing pattern, typical of SARs at normal functional residual capacity. However, it responds to positive-end-expiratory pressure (PEEP) removal with increased discharge and firing in the deflation phase (A and B, which are continuous recordings). Note that during deflation receptor exhibited cardiac rhythm. Adaptation index of this receptor was 37%.

View larger version (57K): [in this window] [in a new window]   Fig. 2.   A typical SAR. In contrast to the receptor in Fig. 1, both peak discharge and mean discharge decreased during PEEP removal. Note that there was no phase shifting in SAR discharge during PEEP removal (A); the adaptation index was similar during constant pressure inflation of the lungs to 10, 15, and 20 cmH2O (B-D), although the firing rate was higher at higher inflation pressure.

In general, receptor activity in both groups is positively correlated with the inflation pressure (Table 1 and Fig. 2). Activity increased as inflation pressure increased, although the increments in receptor activity were higher in RAR-like receptors than the typical SARs (Table 1). A point worth noting is that afferent activity in the RAR-like group always has a phase shift during PEEP removal, i.e., peak discharge shifted from inflation phase to deflation phase of the lungs. During PEEP removal, the activity became either confined to deflation or bimodal, present in both deflation and inflation. In bimodal type it seemed that inflation phase behavior was similar to that of typical SARs, i.e., the activity decreased during PEEP removal. The real difference is that RAR-like fire in deflation.

The adaptation rate of RAR-like receptors is higher than that of the typical SARs, although there was some overlap in adaptation rate between these two groups of SARs. When the lungs were inflated with a constant pressure of 20 cmH₂O, the adaptation indexes were 47 ± 3%, ranging from 19 to 64% for RAR-like SARs, and 17 ± 1%, ranging from 5 to 42% for typical SARs, respectively. The difference in adaptation indexes was statistically significant (P < 0.001). Distribution of adaptation index of the two groups is illustrated in Fig. 3.

View larger version (21K): [in this window] [in a new window]   Fig. 3.   Distribution of adaptation index. Indexes of 15 and 55% represent receptors that have indexes from 10 to 19 and 50 to 59%, respectively. A: RAR-like SARs (n = 11). B: typical SARs (n = 81). Occurrence is expressed as percentage of the total receptors tested in that group. Note that there was some overlap between the 2 groups.

Another difference between these two groups lies in the dependency of adaptation index on the inflation pressure. Adaptation index of the typical SARs was independent of inflation pressure (Figs. 2 and 4), whereas that of RAR-like SARs was pressure dependent. It increased as the inflation pressure increased (Fig. 4). Figure 5 illustrates a pulmonary RAR with an adaptation index of 75%. Like RAR-like SARs, this receptor has high-basal activity with a regular discharge pattern and has a very pronounced dependency of adaptation rate on the inflation pressure.

View larger version (18K): [in this window] [in a new window]   Fig. 4.   Adaptation indexes of typical (A) and RAR-like (B) SARs during constant pressure inflation of the lung to 10, 15, and 20 cmH2O. The adaptation indexes at different inflation pressure are the same in typical SARs (n = 81; P = 0.516 in one-way ANOVA) but significantly different in RAR-like SARs (n = 10; P < 0.005 in one-way ANOVA).

View larger version (79K): [in this window] [in a new window]   Fig. 5.   Dependence of adaptation index of an RAR on inflation pressure during constant pressure inflation to 15 (A and E), 12 (B and D), and 7 (C) cmH2O. The higher the inflation pressure, the greater the adaptation index. This RAR has a regular basal firing pattern and is very active as all the RAR-like SARs reported in this study. Note that response at a given pressure is reproducible and the dependence on inflation pressure is very pronounced. Adaptation index of this receptor was 75%.

There is no particular association of high or low threshold receptors with the two groups. Neither type I nor type II receptors were restricted to either of the two groups. The distributions of RAR-like and typical SARs into high and low thresholds and into type I and type II response are very similar (Table 2).

	DISCUSSION

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The rabbit is a species in which pulmonary afferents are well characterized (1, 8, 10, 13, 14). Afferent properties of SARs and RARs in the rabbit are essentially the same as those found in the cat (21, 22). Resting discharge patterns of the afferents reported in the present study are the same as those of the SARs reported in the previous studies and are indistinguishable from the SARs described in the literature (3, 18), having a clear respiratory modulation with regular spike intervals. However, the present results demonstrate that SARs can be subdivided into two groups, based on their response to PEEP removal. RAR-like SARs increased their activity during PEEP removal, whereas typical SARs decreased their activity during PEEP removal. There is no clear distinction between the two groups of SARs in terms of their gross location, firing threshold (low vs. high thresholds), firing patterns (regularity in spike intervals), and their response saturation to lung inflation (type I or type II). However, comparing the two groups of SARs, it is clear that one group is RAR-like and the other group is typical of classical SARs. RARs and SARs are different, at least in the following four aspects: 1) basal discharge, 2) adaptation rate, 3) response to lung deflation, and 4) inflation pressure dependent in adaptation index. The differences between RAR-like and typical SARs are discussed as follows in these four aspects.

RAR-like SARs have lower basal activity than typical SARs. RARs and SARs differ in their discharge pattern at normal tidal volume under resting condition. RARs are fairly inactive with an average activity about 1 imp/s (21). Their discharge is scattered irregularly throughout the respiratory cycle, although more activity may occur during the inflation phase, especially when lung compliance is decreased. On the contrary, SARs are very active with an average activity about 37.3 imp/s (22), firing regularly during each ventilator cycle (3). Their activity is clearly modulated by the cyclic changes in airway pressure. SARs activity increases during inflation and decreases during deflation of the lung. Although the basal discharge pattern of RAR-like SARs is indistinguishable from that of typical SARs and there was a substantial overlap in their discharge frequency, the resting discharge frequency of RAR-like SARs (31 ± 6 imp/s) is relatively lower than that of the typical SARs (44 ± 2 imp/s). In other words, RAR-like SARs behavior is intermediate between that of RARs and of typical SARs.

RAR-like SARs have higher adaptation rates than typical SARs (see RESULTS). RAR-like SARs had an adaptation index of 47 ± 3% and the typical ones had an adaptation index of 17 ± 1%. The adaptation index of the typical SARs (19% at the inflation pressure of 10 cmH₂O) in the present study is very close to that (22% at the inflation pressure of 10 cmH₂O) reported by Bartlett and St. John (1). It has been recognized for a long time that some myelinated afferents have a rate of adaptation intermediate between the rapid adaptation of RARs and the slow adaptation of SARs (19). In an earlier study (21), some receptors with an intermediate adaptation rate and with an irregular discharge pattern were observed. These receptors should be classified into RARs. On the other hand, some other receptors with similarly intermediate adaptation rate but with a remarkable regular interspike interval might be considered as SARs (3). It is clear that the RAR-like SARs described in the present study belong to this group.

RAR-like SARs are stimulated by deflation of the lungs below functional residual capacity (PEEP removal) and have a phase shift in peak activity during PEEP removal (Fig. 1). Typically, SAR activity decreases as transpulmonary pressure decreases (8), such as during PEEP removal (22). In contrast, RAR-like SARs are stimulated by PEEP removal, which is characteristic of RARs (17, 21). Furthermore, RAR-like SARs, resembling RAR behavior, exhibited a phase shift in their peak activity during PEEP removal, i.e., peak discharge shifted from the inflation to the deflation phase when PEEP is removed (21). It is reported that some myelinated afferents are quiet during inspiration but display an expiratory activity. These afferents are believed to be responsible for the Hering-Breuer deflation reflex (10). However, the vagal afferents with expiratory discharges shown by Luck (10) are not RAR-like SARs, because the latter increase their activity during inflation from functional residual capacity.

Another difference between RAR-like and typical SARs is the dependence of the adaptation index of the former on transpulmonary pressure, that is, the higher the transpulmonary pressure, the greater the adaptation index (Figs. 4 and 5). Similarly, the adaptation index of some RARs decreases at low constant inflation pressure (9-10 cmH₂O) (21). On the other hand, the adaptation index in typical SARs is pressure independent (Figs. 2 and 4) (1). Recognizing the heterogeneity of SARs suggests a reinterpretation of the data of Bartlett and St. John (1). They reported that although the adaptation index of SARs tended to be greater at 10 cmH₂O than at 5 cmH₂O of constant pressure inflation, there was no difference statistically. Bartlett and St. John did not distinguish the typical SARs from RAR-like SARs. The typical SARs, which account for a majority of the population, are pressure independent. Therefore, the difference in adaptation index between the two inflation pressures was not statistically significant. On the other hand, the smaller population of the RAR-like SARs, which is pressure dependent, produced the trend of a greater adaptation index at higher pressure.

In summary, there is a spectrum of behavior among the myelinated pulmonary vagal afferents. Some are rapidly adapting with irregular discharge (typical RARs), some are rapidly adapting with regular discharge (Fig. 5), some are intermediately adapting with irregular discharge (classified as RARs, see Ref. 21), some are intermediately adapting with regular discharge (classified as SARs but they are RAR-like, see present results and Ref. 3), whereas the others are slowly adapting with regular discharge (typical SARs). Typical RARs and typical SARs are two ends of the spectrum. It seems reasonable to define a typical RAR as a myelinated afferent with a rapidly adapting rate (which is inflation-pressure dependent), low basal activity, irregular discharge pattern, and increased activity during lung deflation; a typical SAR as a myelinated afferent with a slowly adapting rate (which is inflation-pressure independent), high basal activity, regular discharge pattern and decreased activity during lung deflation. Although we do not know the reason(s) that myelinated afferents behave differently, the local environment and the property of the sensory endings undoubtedly account for much of the variability. It remains to be determined whether myelinated fibers exhibit a spectrum in function? If so, do the differences in function correspond to the differences in afferent behavior?"

Perspectives