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

Abstract

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. 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. 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. 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.

Effects of Carbonic Anhydrase Inhibition on the Responsiveness of Laryngeal Receptors in Cats to CO2

“…The preparation was similar to that described previously (Wang et al 1999). A cannula was inserted rostrally via a higher tracheostomy and its tip positioned just below the cricoid cartilage.…”

“…Test solutions had the same pH as control solutions but were equilibrated with 9 % CO 2 (see Wang et al 1999) and had a bicarbonate concentration of 39 mM. The isotonicity of the solutions was maintained by varying the amount of NaCI.…”

“…It has been shown in cats and dogs that intralaryngeal CO 2 reflexly enhances the activity of dilating muscles in the upper airway and decreases ventilation, effects which involve afferent information travelling in the superior laryngeal nerves (SLNs; Boushey & Richardson, 1973;Lee et al 1986;Nolan et al 1990;). Also, the intralaryngeal level of CO 2 modulates the activities of laryngeal receptors (including pressure receptors) in the SLNs (Boushey et al 1974;Anderson et al 1990b); Bradford et al 1993;Ghosh & Mathew, 1994;Wang et al 1999). In particular, negative pressure receptors which have a low discharge rate when the larynx is unventilated are mainly excited by CO 2 , while pressure receptors of other categories are mainly inhibited (Bradford et al 1993;Ghosh & Mathew, 1994;Wang et al 1999).…”

“…Also, the intralaryngeal level of CO 2 modulates the activities of laryngeal receptors (including pressure receptors) in the SLNs (Boushey et al 1974;Anderson et al 1990b); Bradford et al 1993;Ghosh & Mathew, 1994;Wang et al 1999). In particular, negative pressure receptors which have a low discharge rate when the larynx is unventilated are mainly excited by CO 2 , while pressure receptors of other categories are mainly inhibited (Bradford et al 1993;Ghosh & Mathew, 1994;Wang et al 1999). It has been suggested that CO 2 and negative pressure perceived by laryngeal pressure receptors may act synergistically in maintaining and/or restoring upper airway patency (Nolan et al 1990;Bradford et al 1993;Ghosh & Mathew, 1994;Wang et al 1999).…”

“…In particular, negative pressure receptors which have a low discharge rate when the larynx is unventilated are mainly excited by CO 2 , while pressure receptors of other categories are mainly inhibited (Bradford et al 1993;Ghosh & Mathew, 1994;Wang et al 1999). It has been suggested that CO 2 and negative pressure perceived by laryngeal pressure receptors may act synergistically in maintaining and/or restoring upper airway patency (Nolan et al 1990;Bradford et al 1993;Ghosh & Mathew, 1994;Wang et al 1999). There is evidence that inhibition of carbonic anhydrase (CA) by acetazolamide or methazolamide significantly delays and diminishes the responses of chemoreceptors to CO 2 in the carotid body (Black et al 1971;Travis, 1971;Iturriaga et al 1991Iturriaga et al , 1993.…”