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

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. Intralaryngeal CO2 reflexly enhances the activity of upper airway dilating muscles and decreases ventilation (Boushey & Richardson, 1973; Lee et al. 1986; Nolan et al. 1990; Bartlett et al. 1992), effects that can also be elicited reflexly by laryngeal negative pressure (Mathew et al. 1982; Mathew & Farber, 1983; Mathew, 1984; Van Lunteren et al. 1984; Sant'Ambrogio et al. 1985; Leiter & Daubenspeck, 1990; Horner et al. 1991; Mezzanotte et al. 1992; Zhang & Mathew, 1992). Topical anaesthesia of the laryngeal mucosa or section of the superior laryngeal nerves (SLNs) abolishes these reflexes, thus indicating that the receptors responsible are superficially located and that their afferent fibres travel in the SLNs. It has been proposed that laryngeal CO2 and negative pressure may work synergistically to promote upper airway patency (Anderson et al. 1990; Nolan et al. 1990). Indeed, intralaryngeal CO2 has been shown to affect the activities of laryngeal pressure-sensitive receptors as recorded from afferent units of the SLN (Anderson et al. 1990; Bartlett & Knuth, 1992; Bradford et al. 1993, 1998; Ghosh & Mathew, 1994). In particular, receptors stimulated by negative pressure, which have a low discharge rate when the larynx is unventilated, tend to be excited by increased levels of intralaryngeal CO2, whereas pressure-sensitive receptors of other categories are mainly inhibited (Bradford et al. 1993, 1998; Ghosh & Mathew, 1994). To elucidate the mechanisms of CO2-induced effects on laryngeal receptor activity, an isolated, in situ, luminally perfused laryngeal preparation was developed in which the superficially located laryngeal receptors were subjected to different solutions of known compositions, and to sinusoidal fluctuations of pressure simulating a breathing cycle. To test the hypothesis that CO2 alters receptor activity by acidifying the receptor's microenvironment, the effects of changes in PCO2 and pH of the perfusing media on the activities of laryngeal pressure-sensitive receptors were studied. To do this, four different solutions were used: (1) neutral acapnic solution with a pH of 7.4 or 7.0, equilibrated with air, (2) acidic hypercapnic solution with a pH of 6.8 or 6.3, equilibrated with 9 % CO2, (3) neutral hypercapnic solution with a pH of 7.4 or 7.0, equilibrated with 9 % CO2, and (4) acidic acapnic solution with a pH of 6.8 or 6.3, equilibrated with air.