Long-term facilitation of upper airway muscle activity induced by episodic upper airway negative pressure and hypoxia in spontaneously breathing anaesthetized rats

Abstract

Obstruction of the upper airway (UA) is associated with episodes of hypoxia and upper airway negative pressure (UANP). In the companion paper it is shown that episodic hypoxia elicits long-term facilitation (LTF) of tongue protrudor, retractor and respiratory pump muscle activity. However, whether repeated exposure to UANP also induces LTF is unknown. We hypothesized that repetitive exposure to UANP would induce LTF of UA and respiratory pump muscle activity and when coupled with hypoxia, as occurs when the UA obstructs, would lead to an even greater facilitation of muscle activity and the response to UANP. Experiments were performed in 24 anaesthetized, spontaneously breathing rats with intact vagi. To induce LTF, UANP stimuli (-10 cmH(2)O) of 5 s duration were delivered every 30 s for 3 min (+/- hypoxia). This was repeated eight times over 1 h, each 3 min episode separated by 5 min of normoxia. Genioglossus (GG), hyoglossus (HG) and diaphragm (Dia) muscle activity was recorded before, during and for 1 h following the last exposure to episodic UANP alone (n = 8), UANP and hypoxia together (n = 8) or normoxia alone (n = 8). During the final hour, single pulses of UANP were applied at 1 min and every 10 min thereafter to determine whether LTF of the response to UANP had been induced. Our results show that LTF of GG muscle activity and its response to UANP was induced following exposure to episodic UANP stimuli alone and UANP applied during hypoxia. However, there was no significant difference between these responses. Episodic UANP alone also induced LTF of HG muscle activity but this effect did not manifest until 40 min following the last episode of repeated UANP stimulation. In the presence of hypoxia, no LTF of HG muscle response to UANP was found. In conclusion, episodic UANP stimulation induces LTF of UA dilator and retractor tongue muscles, but no further facilitation occurs when coupled with hypoxia. This response may serve as an important protective mechanism of respiratory homeostasis during sleep, particularly in patients who suffer from obstructive sleep apnoea.

Figure 2. Effect of episodic upper airway negative pressure (UANP) and hypoxia together on upper airway and respiratory pump muscle activities in anaesthetized spontaneously breathing rat

Original record shows genioglossus (GG), hyoglossus (HG), and diaphragm (Dia) muscle responses to UANP alone under baseline conditions, on the first (no. 1) and last (no. 8) of eight repeated episodes of UANP and hypoxia stimulation together, and in response to UANP alone at individual time points over the subsequent hour following repeated exposure to UANP and hypoxia. The horizontal line in each muscle recording represents zero activity; amplification settings were similar for all EMG recordings. P_ua, upper airway pressure.

Figure 1. Effect of episodic upper airway negative pressure (UANP) on upper airway and respiratory pump muscle activities in anaesthetized spontaneously breathing rat

Original record shows genioglossus (GG), hyoglossus (HG), and diaphragm (Dia) muscle activities and their responses to UANP under baseline conditions, during the first (no. 1) and last (no. 8) of eight 3 min repeated episodes of UANP stimulation alone and at individual time points over the subsequent hour. The horizontal line in each muscle recording represents zero activity; amplification settings were similar for all EMG recordings. P_ua, upper airway pressure.

Figure 3. Group mean (±s.e.m., n= 8) effect of repeated exposure to upper airway negative pressure (UANP) and/or hypoxia on peak genioglossus muscle activity in anaesthetized spontaneously breathing rats

The mean change in pre-stimulus (open symbols) and first-breath (filled symbols) peak genioglossus (GG) muscle activity expressed as a percentage of baseline in response to UANP stimulation alone under control conditions (average of 5 UANP stimuli), during the first (no. 1) and last (no. 8) of eight repeated episodes of UANP alone (circles), UANP together with hypoxia (squares) and normoxia alone (triangles, no pressure applied), and to single pulses of UANP alone delivered at separate time points over the subsequent hour. *Significant change in peak GG activity from control pressure responses (P < 0.05). ‡Significant difference from pre-stimulus breath activity. LTF, long-term facilitation.

Figure 4. Group mean (±s.e.m., n= 8) effect of repeated exposure to upper airway negative pressure (UANP) and/or hypoxia on peak hyoglossus muscle activity in anaesthetized spontaneously breathing rats

The mean change in pre-stimulus (open symbols) and first-breath (filled symbols) peak hyoglossus (HG) muscle activity expressed as a percentage of baseline in response to UANP stimulation alone under control conditions (average of 5 UANP stimuli), during the first (no. 1) and last (no. 8) of eight repeated episodes of UANP alone (circles), UANP together with hypoxia (squares) and normoxia alone (triangles, no pressure applied), and to single pulses of UANP alone delivered at separate time points over the subsequent hour. *Significant change in peak HG activity from control pressure responses (P < 0.05). ‡Significant difference from pre-stimulus breath activity. LTF, long-term facilitation.

Figure 5. Long-term facilitation of peak diaphragm muscle activity following exposure to repeated episodes of upper airway negative pressure (UANP) and/or hypoxia in anaesthetized spontaneously breathing rats

Group mean (±s.e.m., n= 8) change in pre-stimulus breath peak diaphragm (Dia) muscle activity expressed as a percentage of baseline following exposure to episodic UANP alone (circles), UANP together with hypoxia (squares) and normoxia alone (triangles). ‡Significant change in peak Dia activity from corresponding time points during normoxia alone (P < 0.05). N.S., no significant difference at any time point measured across the hour.

Figure 7. Long-term facilitation (LTF) of diaphragm (Dia) muscle total respiratory time and breathing frequency (group mean ±s.e.m.) in anaesthetized spontaneously breathing rats

Breathing frequency measured at different time points following exposure to upper airway negative pressure (UANP) stimulation alone (circles) and UANP together with hypoxia (squares). Open symbols represent breathing frequency on breaths immediately preceding and filled symbols, frequency on the first breath during UANP stimulation. *Significant difference from baseline (P < 0.05). B, baseline activity.

Figure 6. Long-term facilitation (LTF) of inspiratory and expiratory diaphragm (Dia) muscle activity (group mean ±s.e.m.) in anaesthetized spontaneously breathing rats

A, inspiratory time (T_I) measured during control pressure stimuli (Con, average 5 stimuli) and at different time points following repeated exposure to upper airway negative pressure (UANP) stimulation alone (circles) and UANP together with hypoxia (squares). Open symbols represent T_I on breaths immediately preceding and filled symbols T_I on the first breath of UANP stimulation. B, expiratory time (T_E) measured during control pressure stimuli (Con, average 5 stimuli) and at different time points following repeated exposure to upper airway negative pressure (UANP) stimulation alone (circles) and UANP together with hypoxia (squares). Open symbols represent T_E on breaths immediately preceding and filled symbols T_E on the first breath during UANP stimulation. *Significant difference from control (P < 0.05). ‡Significant difference at all corresponding time points. B, baseline activity.