Identification of vagal sensory receptors in the rat lung: are there subtypes of slowly adapting receptors?

Abstract

SUMMARY1. We studied the characteristics of pulmonary sensory receptors whose afferent fibres are in the left vagus nerve of opened-chest rats. The activity of these receptors was recorded during mechanical ventilation approximating eupnoea, as well as during deflation, stepwise inflations and constant-pressure inflations of the lungs. Data were also collected from closed-chest rats and analysed separately.2. Ninety-four per cent of receptors were located in the ipsilateral lung or airways with the remainder in the contralateral lung.3. Not only were slowly adapting receptors (SARs) the most abundant pulmonary receptors but 21 % of them were either exclusively or predominantly active during the deflationary phase of the ventilatory cycle. Deflationary units were found in opened-and closed-chest rats. The average conduction velocity for all fibres innervating SARs averaged 29-7 m s-'. 4. We found rapidly adapting receptors (RARs) to be extremely rare in the rat. Their activity was sparse and irregular. The conduction velocities of fibres innervating RARs averaged 12 3 m s-'.5. Far more abundant than RARs in the remaining population of pulmonary fibres were C fibres. They were observed to have an average conduction velocity of 2-1 m s-1, base-level activity which was irregular and a high pressure threshold of activation and were stimulated by intravenous capsaicin injection.6. Notable differences exist between pulmonary receptors in rats and those reported in other species. The variations include the abundant existence of intrapulmonary SARs with exclusively deflationary modulation and the rarity of RARs. We also encountered C fibres which have not previously been described systematically in the rat.

Excitatory Mechanism of Deflationary Slowly Adapting Pulmonary Stretch Receptors in the Rat Lung

“…Similar types of afferent units of the vagus nerve in the same species have been reported by Bergren and Peterson (1993); they suggested that three categories of receptors, as suggested by Tsubone (1986), belong to slowly adapting pulmonary stretch receptors (SARs). Based on the difference between phasic firing patterns of SARs in both closed and opened chest rats, Bergren and Peterson (1993) classified four subtypes of SARs: inflationary, most inflationary, deflationary, and most deflationary. Wei and Shen (1985) also demonstrated four different types of SARs in rabbits, cats, and monkeys (i.e., phasic and tonic inspiratory units and phasic and tonic expiratory units).…”

“…The deflationary SARs are specific types of myelinated vagal afferent fibers responding to lung deflation on airway collapse. The thickening of the airway smooth muscle is considered to act as an effective stimulus for deflationary SARs oriented in a perpendicular manner to the axis of the smooth muscle (Bergren and Peterson, 1993). The mechanism mediating the mechanical deformation of deflationary SARs has not yet been determined, but those receptors may have mechanosensitive channels on the endings.…”

“…On the other hand, the decrease in deflationary SAR activity induced by lung collapse is slow, and the average values for deflationary SAR adaptation index (AI) are 20%, indicating a slowly adapting fashion (Tsubone, 1986). Bergren and Peterson (1993) found that the deflationary SAR AI was elevated above inflationary SARs at positive 20 cm of H 2 O. From these observations, it is possible that the AI for deflationary SARs reflects faithfully the dynamic sensitivity of these receptors, which basically depends on the viscoelasticity of the structures containing deflationary SARs.…”

Hypersensitivity of lung vagal C fibers induced by acute intermittent hypoxia in rats: role of reactive oxygen species and TRPA1

“…In series 4, suppressive effects of pretreatment with dimethylthiourea, N-acetyl-L-cysteine, 1.5 mg/kg HC-030031, 3.0 mg/kg HC-030031, AP18, vehicle 1, or vehicle 2 (the vehicle of HC-030031 or AP18) (each treatment: n ϭ 8) on 0% O 2 AIH-induced potentiation of the LVCF responses to injection of capsaicin were investigated in seven groups of rats. Additionally, the suppressive effects of pretreatment with dimethylthiourea, N-acetyl-L-cysteine, 3.0 mg/kg HC-030031, vehicle 1, or vehicle 2 (each treatment: n ϭ 8) on this potentiating response of AIH to ␣,␤-methylene-ATP was also evaluated in another five groups of rats. The LVCF responses to these chemical stimulants were measured at time points as described in series 1.…”

“…Only fibers that responded to capsaicin within 2 s after the injection were studied (12). Finally, the conduction velocity of the afferent fibers was measured, as described previously (3). Before the end of each experiment, the general locations of C fibers studied were identified within the lung structure by gently probing the tissues with use of a polyethylene rod (diameter 2 mm).…”