Mechanism Of Stimulative Effect Of Volatile Anesthetics On Airway Nociceptors

BackgroundInhaled anesthetics are a diverse group of volatile anesthesia gas, their commoncharacteristic is to suppress the central nervous system (CNS) activity. In contrast to theireffects in the CNS, some anesthetics can excite peripheral nociceptive receptors and areperceived as pungent. At present, from clinical observation, inhalation of isoflurane anddesflurane induces airway irritation in patients such as cough and even laryngospasm, butrespiratory complications seldom occur in patients with sevoflurane. The mechanism ofstimulative effect of general anesthetics on airway is not clear, but its clinical featuressuggest they may activate airway nociceptors.Our study has found that certain inhaled anesthetics can sensitize vagal C-fiberswhich are important nociceptors in lungs, inducing the release of substance P. Combinedwith present domestic and foreign researches, we assume that the volatile anesthetics elicitpronounced respiratory symptoms, such as cough and laryngospasm, by activating thevagal C-fibers and transient receptor potential cation channels (TRPs channel) in theafferent nerve endings, which are mediated through both central reflex pathways and localaxon-reflex.Objective1. To discusses the effect of volatile anesthetics on vagal C-fibers in lungs, andexamine C-fibers responsiveness to other chemical stimulator such as capsaicin underadministration of volatile anesthetics;2. To observe the sensitivity of bronchopulmonay nociceptors to inhaled anestheticsmediated by transient receptor potential cation channels subtypes (e.g.TRPA1and TRPV1)in an in vitro mice lung preparation;3. To investigate the responsiveness of transient receptor potential cation channelssubtypes (e.g.TRPA1and TRPV1) expressed in primary vagal afferent neurons; 4. The administration of volatile anesthetics in cell and invitro expetiments is special,volatile anesthetics solution is required. In this part of study, we discussesed the volatileanesthetics solution preparation and the changes of anesthetic potencies.Methods1. To examine the impulse activity of single pulmonary C-fiber to differentconcentrations of isoflurane and sevoflurane (0.5MAC,1MAC,2MAC) in rats. Once aC-fiber was activated by isflurane, it was tested for its responsiveness to capsaicin underisoflurane administration;2. To examine the impulse activity of single pulmonary C-fiber in ex vivo mice lungpreparation, identify sensitive small bronchioles (receptive fields) to chemical stimulator toinvestigate. We observed the impulse activity of pulmonary C-fibers to different clinicallyrelevant concentrations of isoflurane, desflurane and sevoflurane (0.5MAC,1MAC,2MAC). To further identify the role of TRPA1in airway nociceptor to general anesthetics,we examined whether TRPA1antagonist HC-030031inhibited the activity of C-fiber toisoflurane and desflurane. Likewise, we tested the activity of C-fiber to isoflurane anddesflurane in TRPV1null mice to investigate the role of TRPV1;3. The fluorescent tracer DiI was instilled into the lungs via a30gauge needleinserted into the lumen of the trachea; the incision was then closed. All mice wre killed forcell culture of pulmonary sensory neurons after7-10days. The response of TRPV1andTRPA1expressed in vagal afferent neurons to isoflurane were measured by calciumimaging. We used capsaicin and TRPA1antagonist to observe the change of intracellularcalcium concentration;4. Three excess liquid anesthetics were added into Krebs solution in glass with Teflonstoppers and allowed to equilibrate at room temperature overnight. Anestheticconcertrations in stock or diluted solution were measured using gas chromatography/massspectrometry. The diluted solution were opened for15min, then measured repeatedly toobserver the volatility; In order to determine losses in the delivery system, samples in thebath were measured. Results1. A total of30C-fibers were recorded in the vagal nerve truck in23rats.18inisoflurane group,12in sevoflurane group. The baseline discharge frequency was0.55±0.07impulse/s in resting state, The increase of C-fibers to capsaicin was4.55±0.51impulse/s, with a mean onset latency of1.62±0.14s.C-fibers (0.95±0.1impulse/s) were not activated by0.5MAC isflurane (p>0.1).When rats were inhaled1MAC isoflurane, C-fibers discharge frequency (2.2±0.25impulse/s) was significantly higher than the baseline (p <0.01). When isofluraneconcentration increased to2MAC, C-fiber discharge frequency further increased (4.18±0.31impulse/s), above the baseline (p <0.01), and also higher than0.5MAC and1MACgroup (p <0.01).Three concentration of sevoflurane could not activate C-fibers (p>0.1). C-fiberdischarge frequency by1MAC and2MAC isoflurane was also significantly higher thanthe same concentration of sevoflurane (p <0.01).Although1.5%isoflurane could activate C-fibers, discharge frequency was lower thancapsaicin (p <0.01). In the state of isoflurane inhalation, of internal jugular vein give equaldoses of capsaicin, C-fiber discharge frequency to capsaicin in the state of isofluraneinhalation was significantly higher than without isoflurane (p <0.01).2.124/226(54.9%) were cC-fibers in mice lung preparation. Discharge frequencywere higher than baseline through injection of three concentrations of isoflurane anddesflurane (p <0.01). The activity of C-fibers to2MAC isoflurane and desflurane washigher than0.5MAC group (p <0.05). Sevoflurane group had no change compared withbaseline (p>0.05).At the same MAC concentrations, isoflurane and desflurane group were higher thansevoflurane group (p <0.01), isoflurane group and desflurane group had no difference (p>0.05).30μM HC-030031markedly attenuated isoflurane and desflurane-evoked activationof C-fibers. A total of30C-fibers were recorded in TRPV1knockout mice, When1MAC and2MAC isoflurane and desflurane were injected onto receptive field, C-fiber were stillactivated, but the discharge frequency of two concentrations of isoflurane was lower thanthe wild type mice (p <0.05), HC-030031could inhibit the activity (p <0.01).3.20%~30%neurons were pulmonary vagal afferent neurons.2MAC isoflurane and0.3mM CA could make intracellular calcium concentration increased; HC-030031couldinhibit the activity.2MAC isoflurane sensitized the activation of TRPV1to capsaicin, intracellularcalcium concentration was significantly high; Isoflurane+capsaicin could obviouslyincrease the intracellular calcium concentration (p <0.01).4. The concentration (mM) of three anesthetics saturated solution respectively is:Isoflurane15.7±0.5; Desflurane20.2±0.8; Sevoflurane6.0±0.3. Stock solution Rest for15min did not significantly decrease, desflurane and isoflurane concentration even risenslightly instead, but it had no statistically difference compared with sealed group (p>0.05).The measured value of Sample taken from the diluted solution was always less thanthe calculated value, this part of the loss represented the anesthetic preparation stage,accounting for2-13%. The concentration of isoflurane rose rapidly to steady levels, thebath concentration was always lower than the calculated value, this part represented theanesthetic delivery system loss, accounting for about10-40%. From the stock dilution toperfusion completed, the whole process of loss was10-50%. The losses of highconcentration were higher than low concentration during the hole perfusion.Conclusion1. Pungent anesthetics activate vagal C-fiber to produce cough, shortness of breath,hypersecretion, laryngospasm and other respiratory complications; Isoflurane sensitizeC-fibers to capsaicin.2. Pungent anesthetics directly activate TRPA1expressed in nodose ganglia and vagalnerve endings to stimulate C-fibers. HC-030031can block the activity of TRPA1. 3. Although pungent anesthetics cannot directly activate TRPV1, they can enhanceTPRV1sensitivity to the chemical stimulation. C-fibers activity to chemical or mechanicalstimulation will decrease if TRPV1was null. This study revealed TRPA1is an importantreceptor in anesthetics-evoked respiratory complications. TRPV1has synergy role. Bothco-existence in airway are important factors to defense the external stimuli. Sevofluranecannot activate TRPA1and TRPV1, is highly suitable for induction and maintainance ofanesthesia.