Dexmedetomidine's inhibitory effects on acetylcholine release from cholinergic nerves in guinea pig trachea: a mechanism that accounts for its clinical benefit during airway irritation

Abstract

Background: Airway instrumentation can evoke upper airway reflexes including bronchoconstriction and cough which can cause serious complications including airway trauma, laryngospasm or bronchospasm which may in turn lead to difficulty with ventilation and hypoxemia. These airway events are mediated in part by irritant-induced neuronal modulation of airway tone and cough responses. We investigated whether the commonly used anesthetic agents dexmedetomidine, lidocaine or remifentanil attenuated neuronal and airway smooth muscle responses in the upper airways of guinea pigs.

Methods: The ability of dexmedetomidine, lidocaine or remifentanil to attenuate direct cholinergic nerve stimulation, C-fiber stimulation or direct smooth muscle contraction were studied using isolated tracheal rings from male guinea pigs under four paradigms; (1) the magnitude of contractile force elicited by cholinergic electrical field stimulation (EFS); (2) the amount of acetylcholine released during cholinergic EFS; (3) the direct airway smooth muscle relaxation of a sustained acetylcholine-induced contraction and (4) the magnitude of C-fiber mediated contraction.

Results: Dexmedetomidine (1-100 μM) and lidocaine (1 mM) attenuated cholinergic 30Hz EFS-induced tracheal ring contraction while remifentanil (10 μM) had no effect. Dexmedetomidine at 10 μM (p = 0.0047) and 100 μM (p = 0.01) reduced cholinergic EFS-induced acetylcholine release while lidocaine (10 μM-1 mM) and remifentanil (0.1-10 μM) did not. Tracheal ring muscle force induced by the exogenous addition of the contractile agonist acetylcholine or by a prototypical C-fiber analogue of capsaicin were also attenuated by 100 μM dexmedetomidine (p = 0.0061 and p = 0.01, respectively). The actual tracheal tissue concentrations of dexmedetomidine achieved (0.54-26 nM) following buffer application of 1-100 μM of dexmedetomidine were within the range of clinically achieved plasma concentrations (12 nM).

Conclusions: The α2 adrenoceptor agonist dexmedetomidine reduced cholinergic EFS-induced contractions and acetylcholine release consistent with the presence of inhibitory α2 adrenoceptors on the prejunctional side of the postganglionic cholinergic nerve-smooth muscle junction. Dexmedetomidine also attenuated both exogenous acetylcholine-induced contraction and C-fiber mediated contraction, suggesting a direct airway smooth muscle effect and an underlying mechanism for cough suppression, respectively.

Keywords: Airway Management; Anesthetic Agents; Bronchodilation; Muscle Relaxation; Smooth Muscle; α2 adrenoceptor agonist.

Fig. 1

Guinea pig tracheal ring muscle force generated in response to cholinergic electrical field stimulation (EFS). a Following stable EFS-induced contractions, 100 μM of dexmedetomidine, 10 μM remifentanil or 1 mM lidocaine or vehicle (0.1% DMSO) were added to buffer solution. Dexmedetomidine and lidocaine inhibited EFS-induced contractions. b, c Dexmedetomidine attenuates guinea pig tracheal ring muscle force generated in response to cholinergic EFS contraction. Representative trace of EFS-induced muscle force over time with increasing concentrations of dexmedetomidine (b) and expressed as percent change from the initial baseline EFS-induced contraction (c). Control tracheal rings treated with the vehicle DMSO (0.1%) serves as a time control. Mean ± SEM (n = 4). *p < 0.05

Fig. 2

Effect of dexmedetomidine (a), remifentanil (b), lidocaine (c) and tetrodotoxin (d) on acetylcholine (Ach) release from guinea pig tracheal rings in response to cholinergic EFS. Data were normalized to an initial measurement of Ach release obtained before addition of drugs. Mean ± SEM (n = 7). *p < 0.05, **p < 0.01 significant difference from control

Fig. 3

Relaxant effect of dexmedetomidine and lidocaine on exogenous acetylcholine (Ach)-induced contraction of guinea pig tracheal rings. a Representative trace of Ach-induced contraction and subsequent relaxation or sustained contraction. b, c, d Following stable Ach-induced contractions, 0.1–100 μM of dexmedetomidine, 2 nM–10 μM of remifentanil, 10–1000 μM of lidocaine or vehicle control (DMSO or water) was added to the buffer solutions and muscle force change was measured. Mean ± SEM (n = 5). **p < 0.01

Fig. 4

Reduction of capsaicin-induced contraction in guinea pig tracheal rings. Tracheal rings were pretreated with 1–100 μM dexmedetomidine, 0.1–10 μM remifentanil, 10–1000 μM lidocaine, 200 μM bupivacaine or 1 μM tetrodotoxin for 20 min before contraction with 10 μM capsaicin. Dexmedetomidine at 100 μM significantly reduced this contraction (a) but remifentanil had no effect (b). Local anesthetics lidocaine and bupivacaine (Bupi) attenuated this C-fiber mediated contraction, which are mediated by tetrodotoxin (TTX) –resistant sodium channels. Mean ± SEM (n = 6). *p < 0.05, **p < 0.01

Fig. 5

Schematic diagram of dexmedetomidine’s effect on airway smooth muscle (ASM) contractile tone. Sensory information from the upper airway (e.g. irritation by endotracheal tubes or suction catheters) reaches the brain medulla (nucleus tractus solitarius (NTS)) via afferent sensory pathways. Signaling in the NTS is mediated by neurotransmitters including γ-aminobutyric acid (GABA), glutamate and neurokinins, and sends projections to the airway-related vagal preganglionic neurons (AVPN). These neurons are the origin of the parasympathetic nerves, which, after synapsing at peripheral parasympathetic ganglia, release acetylcholine (Ach) from post-ganglionic cholinergic nerves. Released Ach binds to muscarinic receptors (M3 on ASM) and causes ASM contraction. The ganglionic M1 muscarinic receptor is known to facilitate neurotransmission, and neuronal presynaptic M2 muscarinic receptors are known to inhibit Ach release. C-fiber efferents express TRPV1 (transient receptor potential family vanilloid 1) which is a receptor for capsaicin, which induces the release of substance P which binds to the neurokinin (NK) receptors and also facilitates ASM contraction. In the current study, we demonstrated that: (1) dexmedetomidine inhibits Ach release from postganglionic cholinergic nerves; (2) dexmedetomidine directly relaxes an Ach-induced ASM contraction; and (3) dexmedetomidine attenuates C-fiber mediated contraction