Laryngeal-receptor responses to phasic CO2 in anesthetized cats

Presence of task-1 channel in the laryngeal mucosa in the newborn lamb

Nearly 40 potassium channels have been described in respiratory epithelial cells. Of these are found several members of the 4-transmembrane domain, 2-pore K(+) channel family (K2P family), namely Twik-1 and -2, Trek-1 and -2, Task-2, -3, and -4, Thik-1, and KCNK7. The aim of this study was to verify whether the Twik-related acid-sensitive K(+) channel, subtype 1 (Task-1) (also known as KCNK3), is present in the laryngeal mucosa in the newborn lamb. Through the use of immunohistochemistry and nested polymerase chain reaction (PCR) amplification, results indicate that Task-1 protein and mRNA are present in the laryngeal mucosa, in both the ciliated, pseudostratified columnar (respiratory) epithelium and the nonkeratinized, stratified squamous epithelium. The complete ovine Task-1 protein sequence showed high homology levels with previously reported mouse, bovine, and human Task-1 sequences. This includes a complete homology for the C-terminal amino acid sequence, which is mandatory for protein trafficking to the cell membrane. These results represent the first demonstration that Task-1, a pH-sensitive channel responsible for setting membrane potential, is present in the laryngeal mucosa of a newborn mammal.

The reflex effects on the respiratory regulation of the CO2 at the different flow rate and concentration

PURPOSE

The purpose of this study was to investigate the activation of the respiratory centers during insufflation of the larynx with CO2 at different flow rates and concentrations.

MATERIALS AND METHODS

The experiments were carried out in spontaneous air breathing rabbits, anesthetized with thiopental sodium (25 mg kg(-1) i.v.). The larynx was separated from the oropharyngeal cavity and the trachea. The tidal volume (VT) and respiratory frequency (f min(-1)) were recorded from the lower tracheal cannula. The respiratory minute volume (VE) was calculated, the action potentials from the right phrenic nerve were recorded and the inspiratory (TI) and expiratory (TE) periods and the mean inspiratory flow rate (VT/TI) were calculated. The larynx was insufflated at flow rates of 500 mL min(-1) and 750 mL min(-1), with 7 and 12% CO2-Air by means of a respiratory pump.

RESULTS

Insufflation of the larynx, with both gas mixtures, decreased the f and VT significantly. The TI and TE were found to increase significantly due to the decreasing in f. There was a significant decrease in VT/TI ratio. Following bilateral midcervical vagotomy, on the passing of both gas mixtures, significant decreases were observed in the VT, and the responses of f, TI and TE were abolished. After cutting the superior laryngeal nerve, the responses of the VT to both gas mixtures were abolished.

CONCLUSION

In conclusion, the results of this study purpose that the stimulation of the laryngeal mechanoreceptors by the effect of hypercapnia decreases the activation of the respiratory center.

Effects of hypoxia and hypercapnia on nonnutritive swallowing in newborn lambs

The aim of the present study was to investigate the effect of hypercapnia and hypoxia on apnea and nonnutritive swallowing (NNS) frequency, as well as on the coordination between NNS and phases of the respiratory cycle in newborn lambs, while taking into account the potential effects of states of alertness. Six lambs were chronically instrumented for recording electroencephalogram, eye movements, diaphragm and thyroarytenoid muscle (a glottal adductor) activity, nasal airflow, and electrocardiogram. Polysomnographic recordings were performed in nonsedated lambs exposed to air (control), 10% O(2), and 5% CO(2) in a random order at 3, 4, and 5 days of age. Although hypercapnia decreased apnea frequency in wakefulness and active sleep (P = 0.002 vs. air and hypoxia), hypoxia had no significant effect on apnea. In addition, although hypercapnia increased NNS frequency during wakefulness and quiet sleep (P < 0.005 vs. air and hypoxia), hypoxia tended to decrease NNS frequency. Finally, only hypercapnia altered NNS-breathing coordination by increasing NNS at the transition from inspiration to expiration (ie-type NNS; P < 0.001 vs. air and hypoxia). In conclusion, whereas hypercapnia increases overall NNS frequency by specifically increasing ie-type NNS, hypoxia has the inverse tendency. Results were identical in all three states of alertness.

Distribution of pH regulators in the rat laryngeal nerve: the spatial relationship between Na+/HCO3− cotransporters and Na+/H+ exchanger type 3

We studied the distribution of Na(+)/HCO(3)(-) cotransporters (NBC) and Na(+)/H(+) exchanger type 3 (NHE3) in the laryngeal nerve by immunohistochemistry to elucidate the spatial relationship of pH regulation system in the peripheral nerves. The nervous components, i.e., the nerve cells in the nodose and local ganglia and nerve fibers, were immunoreactive for NBC. Glial components such as Schwann cells and satellite cells surrounding nerve fibers and nerve cell bodies were also immunoreactive for NBC in most cases, while the cells comprising the perineurium and endoneurium were immunoreactive for NHE3. These results suggest that NBC-dependent pH regulation systems are present in the laryngeal nerve. Whereas, NHE3 may regulate extracellular pH rather than intracellular pH. In conclusion, spatial relationship of NBC and NHE3 in the laryngeal nerve would be important for pH regulation. Perineural lymph may have key role for acid-induced modulation of axons and Schwann cells.

Contribution of free nerve endings in the laryngeal epithelium to CO2 reception in rats

The superior laryngeal nerve (SLN) contains CO2-sensitive fibers. In the laryngeal epithelium, two candidates for CO2 reception have been identified, namely the intraepithelial free nerve endings and the taste buds. To elucidate the contribution of free nerve endings to CO2 reception, electrophysiological activities were recorded during various stages of regeneration of nerve endings following SLN-crush in rats. The left SLN was crushed surgically and maintained from 4 to 40 days for regeneration of nerve endings. Laryngeal sections were processed for immunohistochemical staining of protein gene product 9.5 to observe regeneration of free nerve endings and taste buds in the epithelium. By day 4 after SLN-crush, both the free nerve endings and taste buds had disappeared. Regeneration of the free nerve endings was recognized from day 8, while that of the taste buds started at day 16. On day 40, the number of taste buds on SLN-crush side was similar to that on the untreated side. Electrophysiological recording of SLN throughout the regeneration period (excluding day 4), showed response to intralaryngeal 9% CO2 (stimulation or inhibition) whether or not taste buds were present. Our results showed intralaryngeal CO2 reception without taste bud involvement, indicating that the free nerve endings in the laryngeal epithelium are receptive to intralaryngeal CO2.

Effects of C fiber blockade on cardiorespiratory responses to laryngeal stimulation in concious lambs

The primary aim of the study was to explore cardiorespiratory reflexes originating from laryngeal C fiber endings in the neonatal period. Seventeen lambs were instrumented for recording glottal adductor and diaphragm EMG, heart rate, systemic arterial pressure and respiratory movements. C fiber blockade was induced in eight lambs by 30 mg/kg capsaicin, the remaining nine lambs serving as controls. Cardiorespiratory reflexes were induced in non-sedated lambs by flowing air, menthol or 13% CO2, or by injecting water or 50 microg capsaicin in the laryngeal inlet through an endoscope. Responses to all stimuli but capsaicin were similar between the two groups. While cardiorespiratory responses were induced by capsaicin in control lambs, the responses were significantly inhibited in lambs with C fiber blockade. We conclude that laryngeal C fiber endings are functional and responsible for laryngeal chemoreflexes in newborn lambs.

Airway receptors

There are many types of afferent receptor in the airways; at least five in the larynx: pressure, drive, cold, irritant and C-fibre; and at least four in the trachea and bronchi: slowly and rapidly adapting stretch receptors (SARs and RARs), C-fibre receptors, and those in neuroepithelial bodies (NEBs). Histologically enough sensory structures have been identified to account for the various patterns of afferent activity, but most correlations are poor. For the larynx, four or more sensory structures have not definitively been identified with afferent discharges and reflex responses. For the trachea and bronchi, only SARs have been clearly identified morphologically and physiologically. The reflexes and afferent discharges from RARs and C-fibre receptors are fairly clear, some at least of the sensory terminals lie in the epithelium, but receptor complexes have not been mapped out. Nerves in NEBs have been identified, but not their local and central reflex actions.

Effects of CO2 and H+ on laryngeal receptor activity in the perfused larynx in anaesthetized cats

1. Intralaryngeal CO2 reflexly decreases ventilation and increases upper airway muscle activity. Topical anaesthesia of the laryngeal mucosa or cutting the superior laryngeal nerves (SLNs) abolishes these reflexes, indicating that the receptors responsible are superficially located and that their afferent fibres are in the SLN. Intralaryngeal CO2 affects the activity of receptors recorded from the SLN. 2. An isolated, luminally perfused laryngeal preparation was developed in anaesthetized, paralysed cats in order to compare the effects of solutions with varying levels of pH and PCO2 on pressure-sensitive laryngeal receptor activity. Since the pH of tracheal surface fluid is reported to be approximately 7.0, two neutral (pH 7.4 and 7.0) and two acidic (pH 6.8 and 6.3) solutions were used. 3. Compared with neutral acapnic control solutions, neutral hypercapnic (PCO2 64 mmHg) solutions either excited or inhibited the discharge of 113 out of 211 pressure-sensitive SLN afferents. In 24 receptors, the effects of hypercapnic solutions with either neutral or acidic pH were similar in both direction and magnitude. In 50 receptors affected by neutral hypercapnic solutions, acidic acapnic solutions had no effect on 66 % of units and significantly smaller effects in the remaining units. In 17 receptors, the effects of neutral solutions with a PCO2 of 35 mmHg were significantly less than for neutral solution with a PCO2 of 64 mmHg. 4. These results show that the effects of CO2 on laryngeal pressure-sensitive receptors are independent of the pH of the perfusing media, and suggest that acidification of the receptor cell or its microenvironment is the main mechanism of CO2 chemoreception.