Inhibitory effect of sulfur dioxide inhalation on Hering-Breuer inflation reflex in mice: role of voltage-gated potassium channels

Abstract

Slowly adapting receptors (SARs), vagal mechanosensitive receptors located in the lung, play an important role in regulating the breathing pattern and Hering-Breuer inflation reflex (HBIR). Inhalation of high concentration of sulfur dioxide (SO₂), a common environmental and occupational air pollutant, has been shown to selectively block the SAR activity in rabbits, but the mechanism underlying this inhibitory effect remained a mystery. We carried out this study to determine if inhalation of SO₂ can inhibit the HBIR and change the eupneic breathing pattern, and to investigate further a possible involvement of voltage-gated K⁺ channels in the inhibitory effect of SO₂ on these vagal reflex-mediated responses. Our results showed 1) inhalation of SO₂ (600 ppm; 8 min) consistently abolished both the phasic activity of SARs and their response to lung inflation in anesthetized, artificially ventilated mice, 2) inhalation of SO₂ generated a distinct inhibitory effect on the HBIR and induced slow deep breathing in anesthetized, spontaneously breathing mice, and these effects were reversible and reproducible in the same animals, 3) This inhibitory effect of SO₂ was blocked by pretreatment with 4-aminopyridine (4-AP), a nonselective blocker of voltage-gated K⁺ channel, and unaffected by pretreatment with its vehicle. In conclusion, this study suggests that this inhibitory effect on the baseline breathing pattern and the HBIR response was primarily mediated through the SO₂-induced activation of voltage-gated K⁺ channels located in the vagal bronchopulmonary SAR neurons.NEW & NOTEWORTHY This study demonstrated that inhaled sulfur dioxide completely and reversibly abolished the activity of vagal bronchopulmonary slowly adapting receptors, significantly inhibited the apneic response to lung inflation, and induced slow deep breathing in anesthetized mice. More importantly, our results further suggested that this inhibitory effect was mediated through an action of sulfur dioxide and its derivatives on the voltage-gated potassium channels expressed in the slowly adapting receptor sensory neurons innervating the lung.

Keywords: 4-aminopyridine; air pollutant; breathing pattern; lung; slowly adapting receptor; vagus nerve.

Graphical abstract

Figure 1.

Experimental records illustrating the effect of SO₂ inhalation on SAR responses to lung inflation in an anesthetized, open chest, and artificially ventilated mouse (28.5 g). Notice that this SAR discharged synchronously with each inspiratory cycle of the respirator; the receptor activity increased intensely and sustained for the entire duration of the constant-pressure lung inflation (30 cmH₂O for 8 s) before the SO₂ inhalation challenge (600 ppm, 8 min) (A). However, both the baseline phasic activity and the increased discharge during lung inflation were completely abolished at 20 min (B) and only partially recovered at 60 min (C) after termination of the SO₂ inhalation. This receptor was located in the right lung. ABP, arterial blood pressure; AP, action potential; P_tr, tracheal pressure; SAR, slowly adapting receptor.

Figure 2.

Experimental records illustrating the effect of SO₂ inhalation on the HBIR apneic response to lung inflation after pretreatments with 4-AP and vehicle in an anesthetized, spontaneously breathing mouse (32 g). The HBIR responses were elicited by lung inflation with a constant pressure (16 cmH₂O); the end of apnea was detected by the first inspiratory effort that was identified by the negative spike of esophageal pressure (P_es). A, B, and C were HBIR responses before (control), at 1 min and 30 min after termination of the SO₂ inhalation (600 ppm, 8 min), respectively. Left and right panels are the responses obtained after pretreatments with vehicle (isotonic saline) and 4-AP (2 µg/g), respectively, in the same animal. ABP, arterial blood pressure; HBIR, Hering–Breuer inflation reflex; P_tr, tracheal pressure; 4-AP, 4-aminopyridine.

Figure 3.

Effect of pretreatment with 4-AP on SO₂-induced inhibition of HBIR apneic responses in anesthetized, spontaneously breathing mice (n = 7; male; age = 12.9 ± 0.3 wk; weight = 32.2 ± 0.7 g). Left and right panels: group data of apneic responses to the lung inflation maintained at two different constant tracheal pressure (8 cmH₂O and 16 cmH₂O, respectively) before (control), at 1 min and 30 min after termination of the SO₂ inhalation (600 ppm, 8 min). Effects of both vehicle (isotonic saline; open bars) and 4-AP (2 µg/g; closed bars) pretreatments were tested in each animal, and the sequence of pretreatments was alternated between animals. The horizontal dashed lines depict an apneic ratio of 1 (i.e., no apnea). Data are represented as means ± SE (n = 7). Statistical analysis was performed using two-way ANOVA and Fisher’s least significant difference. #Significantly (P < 0.05) different from the corresponding data obtained during control (before SO₂ inhalation). *Significantly (P < 0.05) different from the corresponding data obtained after the pretreatment with vehicle. HBIR, Hering–Breuer inflation reflex; 4-AP, 4-aminopyridine.

Figure 4.

Experimental records illustrating the effect of SO₂ inhalation on the breathing pattern after pretreatments with 4-AP and vehicle in an anesthetized, spontaneously breathing mouse (33 g). A, B, and C: baseline breathing patterns before (control), at 1 min and 30 min after termination of SO₂ inhalation (600 ppm, 8 min), respectively. Left and right panels are the responses obtained after pretreatments with vehicle (isotonic saline) and 4-AP (2 µg/g), respectively, in the same animal. ABP, arterial blood pressure; $\dot{\text{V}}$, airflow rate; V_T, tidal volume; 4-AP, 4-aminopyridine.

Figure 5.

Pretreatment with 4-AP reversed SO₂-induced slow deep breathing in anesthetized, spontaneously breathing mice (n = 7; male; age = 12.9 ± 0.3 wk; weight = 32.2 ± 0.7 g). A and B: group data of respiratory frequency and tidal volume, respectively, obtained before (control), at 1 min, and 30 min after termination of the SO₂ inhalation (600 ppm, 8 min) in mice pretreated with vehicle (isotonic saline; open bars) and 4-AP (2 µg/g; closed bars); effects of both pretreatments were tested in each animal, and the sequence of these pretreatments was alternated between animals. Data are represented as means ± SE (n = 7). Statistical analysis was performed using two-way ANOVA and Fisher’s least significant difference. #Significantly (P < 0.05) different from the corresponding data obtained during control (before SO₂ inhalation). *Significantly (P < 0.05) different from the corresponding data obtained after the pretreatment with vehicle.