No abstract

Protease‐activated receptor‐2 (PAR2) is involved in airway inflammation and airway hyperresponsiveness, the prominent features of asthma. Airway acidification has been consistently observed in airway inflammatory conditions, and is known to cause cardiorespiratory symptoms that are at least in part mediated through the activation of bronchopulmonary C‐fibers and the subsequent reflexes. This study was carried out to investigate the effect of PAR2 activation on the acid signaling in vagal pulmonary C‐fibers that may occur during airway inflammation. Our in‐vivo study showed that, in anesthetized and spontaneously breathing rats, intratracheal instillation of trypsin (1 mg/ml, 0.1 ml), an activator of PAR2, significantly enhanced the chemoreflex responses to right‐atrial injection of lactic acid. In isolated pulmonary C‐neurons, pre‐incubation of SLIGRL‐NH2 (100 μM, 2 min), a specific PAR2 activating peptide, markedly potentiated the whole‐cell inward currents evoked by extracellular acidification. In addition, a similar sensitizing effect was observed when the neurons were pretreated with 2‐furoyl‐LIGRL‐NH2 (100 μM, 2 min), a newly reported potent and selective PAR2 agonist. In conclusion, activation of PAR2 modulates the acid signaling in pulmonary C‐fiber sensory nerves, and the interaction may contribute to the pathogenesis of asthma and other airway inflammatory diseases. (Supported by grants from NIH, AHA and Parker B. Francis Fellowship in Pulmonary Research)

Chronic exposure to sulfur dioxide (SO2), an air pollutant, can cause airway injury and lung diseases. Acute exposure to SO2 triggers cough and reflex bronchoconstriction, indicating a stimulatory effect of SO2 on airway sensory nerves. Indeed, a recent study in our lab has demonstrated that vagal bronchopulmonary C‐fibers are the primary target of inhaled SO2. This study was carried out to investigate the underlying mechanism of this stimulatory effect of SO2 on bronchopulmonary C‐fibers. Our results showed: 1) Inhalation of SO2 (1,000–2,000 ppm, 10 breaths) evoked a pronounced and reproducible stimulatory effect on pulmonary C‐fibers in anesthetized rats. 2) Pretreatment with an intravenous infusion of sodium bicarbonate (120 μmol/kg) raised the baseline arterial pH by 0.08 units, which profoundly diminished the stimulatory effect of SO2 on pulmonary C‐fibers by 83.9%. 3) This stimulatory effect of SO2 was also significantly attenuated by a pretreatment with amiloride (a blocker of acid‐sensing ion channels, ASICs; 11 μmol/kg) alone (Δ = 70.7%); AMG9810 (an antagonist of transient receptor potential vanilloid type‐1, TRPV1; 10 μmol/kg) alone (Δ = 76.7%); and by a combination of amiloride and AMG9810 (Δ = 91.4%). 4) To further investigate if this stimulatory effect is generated by a direct action of SO2 on sensory nerves, the change in intracellular Ca2+ concentration, [Ca2+]i, was measured in isolated rat vagal pulmonary sensory neurons. Perfusion with extracellular fluid saturated with SO2 (1,000 and 2,000 ppm) evoked a significant increase in [Ca2+]i in a concentration‐dependent manner in these neurons, and these responses to SO2 were markedly suppressed by a combined pretreatment with amiloride and AMG9810. 5) In addition, inhalation of SO2 (300 and 600 ppm for 5 min) evoked cough reflex responses consistently in awake mice in a concentration‐dependent manner; the cough responses were almost completely prevented by a pretreatment with amiloride aerosol inhalation (5 mM for 4 min) in TRPV1‐knockout mice. In conclusion, this study suggested that inhalation of SO2 lowered the pH in airway/lung tissues, which exerted a stimulatory effect on vagal bronchopulmonary C‐fibers by activating both ASICs and TRPV1 channels.Support or Funding InformationNIH grants AI123832 & UL1TR001998This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

A recent study demonstrated that the responses of rat pulmonary sensory neurons to TRPV1 activators were enhanced by increasing temperature (Ni and Lee, FASEB J., 2007), but the role of the TPRV1 channel in the potentiating effect could not be determined. In the present study, we used whole cell perforated patch‐clamp technique to study the effect of increasing temperature on the response of isolated nodose/jugular sensory neurons to non‐selective TRPV1 channel activators in TRPV1‐null (TRPV1−/−) and wild type (WT) mice. Our results showed that, in voltage‐clamp mode, the inward current evoked by 2‐aminoethoxydiphenyl borate (2‐APB), a common activator of TRPV1‐3 channels, was greatly potentiated by increasing temperature from 36 to 40.5°C in WT neurons (n = 9; P < 0.01), but was not affected in TRPV1−/− neurons (n = 9; P = 0.54). Similarly, the inward current evoked by acid (pH 5.5), an activator of both TRPV1 channel and the acid sensing ion channel, was enhanced by increasing temperature (n = 7; P < 0.05), and this effect was not observed in TRPV1−/ − neurons (n = 13; P = 0.11). These results demonstrated that disruption of the TRPV1 channel eliminates the potentiating effect of increasing temperature on the response of nodose/jugular neurons to non‐selective TRPV1 channel activators, suggested a possible interaction between these chemical activators and increasing temperature at the TRPV1 channel.

Tumor necrosis factor alpha (TNFα), a pro‐inflammatory cytokine, plays a significant role in the pathogenesis of asthma. We recently reported that TNFα pretreatment elevated the sensitivities of vagal bronchopulmonary C‐fibers and silent rapidly adapting rectors (RARs) to capsaicin (Front. Physiol. 2017). In this study, we investigated the effect of TNFα on cough response to sulfur dioxide (SO2), a common irritant gas, and the relative contributions of these vagal afferents. Our results showed: 1) Inhalation of SO2 (300 and 600 ppm; 8 min each) consistently elicited coughs in a concentration‐dependent manner in awake mice moving freely in a recording chamber; coughs were recorded via telemetry sensors implanted in the intrapleural space. The cough frequencies during and immediately after the SO2 inhalation challenges increased markedly in the same animals after TNFα (10 μg/ml; 0.03 ml) was administered into the lung by intratracheal instillation, and this increase in cough sensitivity sustained for >7 days after the TNFα pretreatment. In contrast, the cough response to SO2 inhalation challenges did not change after pretreatment with vehicle (0.03 ml of PBS) in a matching group of control mice. 2) In single‐fiber recording experiments in anesthetized mice, the same SO2 inhalation challenges (for 8 min) evoked a pronounced stimulatory effect on C‐fiber afferents, reaching a peak in 1–4 min. The increased fiber activity was concentration dependent and lingered for several more minutes after the termination of SO2 challenge. This stimulatory effect of SO2 was significantly elevated in the TNFα‐treated mice. 3) The same SO2 inhalation challenges, either 300 or 600 ppm, did not activate any of the silent RARs in vehicle‐treated mice, and only 600 ppm SO2 caused a very weak stimulation in 2 out of 7 of these silent RARs in TNFα‐treated mice. 4) Inhalation of SO2 (300 or 600 ppm) consistently induced an inhibitory effect on both slowly adapting receptors and phasic RARs; these afferents ceased to discharge completely in 1–5 min after the onset of SO2 (600 ppm) inhalation challenge, and a majority of them slowly returned to controls after resuming air breathing for >1 hr. No detectable difference in this inhibitory effect was found between TNFα‐ and vehicle‐treated mice. In conclusion, the cough response to SO2 inhalation challenge is enhanced by the TNFα pretreatment in wake mice, and an increase in the bronchopulmonary C‐fiber sensitivity to SO2 is primarily responsible.Support or Funding InformationNIH grant AI123832This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.