Airway Exposure to 1,3-Beta-d-Glucan Induces Airway Hyperresponsiveness in Guinea Pigs

Abstract

1,3-Beta-d-glucan (β-glucan) is a component of mold cell walls and is frequently found in fungi and house dust mites. The studies of β-glucan are inconsistent, although it has been implicated in airway adverse responses. This study was carried out to determine whether airway hyperresponsiveness was seen 24 h after airway exposure to β-glucan in guinea pigs. Two matching guinea pigs were exposed intratracheally to either β-glucan or its vehicle. Twenty-four hours after intratracheal instillation, there was no difference between these two groups in the baseline of the total pulmonary resistance (R _L), dynamic lung compliance (C _dyn), arterial blood pressure, and heart rate. In contrast, the responses of R _L to capsaicin injection were significantly increased in β-glucan animals; capsaicin at the same dose of 3.2 μg/kg increased R _L by 184% in vehicle animals and by 400% in β-glucan animals. The effective dose 200% to capsaicin injection was lower in the β-glucan animals. Furthermore, the increases in R _L were partially reduced after transient lung hyperinflation to recruit the occluding airways; however, the R _L induced by capsaicin injection after lung hyperinflation was significantly larger than the baseline in β-glucan animals; also, the lung wet-to-dry ratio in capsaicin-injected animals was augmented in the β-glucan group. Moreover, the airway hyperresponsiveness was accompanied by increases in neutrophils in the bronchoalveolar lavage fluid in the β-glucan animals. Furthermore, the levels of substance P and the calcitonin gene-related peptide in the bronchoalveolar lavage fluid collected after capsaicin injection were increased in β-glucan animals. We provide definitive evidence that β-glucan can induce airway hyperresponsiveness in guinea pigs, and the neuropeptide releases play an important role in this airway hyperresponsiveness.

Figure 1

Effect of β-glucan exposure on the baseline of airway reactivity and cardiovascular parameters. Baseline data of total pulmonary resistance (R_L; cmH₂O/mL/s), dynamic lung compliance (C_dyn; mL/cmH₂O), arterial blood pressure (ABP; mmHg), and heart rate (HR; beats/min) were calculated as the mean over 10-breath interval before capsaicin injection. Data represent mean ± SEM of 8 guinea pigs in each group. No statistical significance was found between any two groups.

Figure 2

Experimental record illustrating the responses of transpulmonary pressure (P_tp), respiratory flow (V̇), and ABP to accumulating doses of capsaicin injections (0.8, 1.6, and 3.2 μg/kg were injected intravenously at 2 min intervals, as indicated by arrows). The responses were recorded 24 h after vehicle (upper) or β-glucan (lower) intratracheal instillation in two guinea pigs (vehicle: 380 g body weight; β-glucan: 360 g body weight).

Figure 3

Dose responses of R_L and C_dyn to intravenous injections of capsaicin (Cap) in vehicle and β-glucan-treated guinea pigs. Peak responses to each dose of capsaicin in each guinea pig were averaged over the three consecutive breaths that occurred. ○: responses in vehicle animals; ■: responses in β-glucan-treated animals. Data were mean ± SEM of eight guinea pigs in each group. *P < 0.05, ***P < 0.001, significant difference comparing corresponding data between the vehicle and β-glucan-treated animals.

Figure 4

Effective dose 200% (ED200) to capsaicin injections 24 h after the vehicle (open bar) or β-glucan (hatched bar) instillation in guinea pigs. ED200 is the dose of the stimulant (capsaicin was chosen in the present study) that produced a doubling of the baseline response of R_L. Data were mean ± SEM of eight guinea pigs in each group. *P < 0.05, a significant difference between the vehicle and β-glucan animals.

Figure 5

Effect of lung hyperinflation (HI) on the R_L changes induced by capsaicin (Cap) injection in the vehicle (open bars) and β-glucan (hatched bars) animals. Lung hyperinflation was done by blocking the airflow outlet of the ventilator for two respiratory cycles (3 × V_T). Data were mean ± SEM of eight guinea pigs in each group. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.

Figure 6

Comparison of the lung wet-to-dry ratio without or with capsaicin (Cap) injection in the vehicle (open bars) and β-glucan (hatched bars) animals. Data were mean ± SEM of eight guinea pigs in each group. **P < 0.01, ***P < 0.001, ****P < 0.0001.

Figure 7

Systemic impact of intratracheal instillation of the vehicle (round) or β-glucan (square) in the complete blood count. Data were mean ± SEM of six guinea pigs in each group. *P < 0.05, **P < 0.01.

Figure 8

Effect of the intratracheal instillation of the vehicle (round) or β-glucan (square) in cell numbers in the BALF. Data were mean ± SEM of six guinea pigs in each group. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.

Figure 9

Level of CGRP and substance P (SP) in BALF obtained from the vehicle (round) and β-glucan (square) animals. Data were means ± SEM of six guinea pigs in each group. **P < 0.01, ***P < 0.001, ****P < 0.0001.