Chem. Senses 28: 595-601,
2003
© Oxford University Press 2003
Real-time Monitoring of Nasal Mucosal pH During Carbon Dioxide Stimulation: Implications for Stimulus Dynamics
1 Division of Occupational and Environmental Medicine 2 Division of Allergy and Immunology, University of California, San Francisco, CA, USA
Correspondence to be sent to: Dennis Shusterman, Upper Airway Biology Laboratory, University of California, San Francisco, 1301 S. 46th Street, Bldg. 112, Richmond, CA 94804, USA. e-mail: dennis{at}itsa.ucsf.edu
Carbon dioxide is a commonly employed irritant test compound in nasal chemesthetic studies because it is essentially free of olfactory stimulus properties. CO2 is thought to act via hydration to H2CO3 and dissociation to H+ in nasal mucus, with resulting activation of acid sensors. However, transient changes in nasal mucosal pH have not been documented during CO2 stimulation in humans. We placed a small pH probe on the floor of the right anterior nasal cavity during CO2 stimulation in eight human subjects with historically high (>30%) and low (
20%) CO2 detection thresholds. Three second pulses of CO2 (15–45% v/v) paired with air in random order (12–15 s inter-stimulus interval; 60 s inter-trial interval) were administered by nasal cannula at 5 l/min. in an ascending series. For each subject, both a CO2 detection threshold and suprathreshold psychophysical ratings [
; labeled magnitude scale] were generated. All subjects showed phasic drops in pH associated with CO2 stimulation (
pH). For all subjects combined, a positive correlation was apparent between applied [CO2] and both pH and , as well as between pH and themselves (P < 0.0001 for each comparison). Subjects with historically low CO2 thresholds showed steeper dose-response curves for as a function of both applied [CO2] and pH, but not for pH as a function of applied [CO2]. For the six of eight subjects with measurable pH changes at threshold, pH was positively related to log [CO2 threshold] (P < 0.01). These data imply that variability in CO2 detection thresholds and suprathreshold rating may derive from intrinsic differences in neural sensitivity, rather than differences in stimulus activation to hydrogen ion.
Key words: carbon dioxide, nasal mucosal pH, psychophysical testing, sensory irritation
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