The effect of reducing extracellular calcium concentration ([Ca2+]o) on vagal afferent excitability was analyzed in a guinea pig isolated vagally innervated trachea-bronchus preparation. Afferent fibers were characterized as either having low-threshold, rapidly adapting mechanosensors (Aδ fibers) or nociceptive-like phenotypes (Aδ and C fibers). The nociceptors were derived from neurons within the jugular ganglia, whereas the low-threshold mechanosensors were derived from neurons within the nodose ganglia. Reducing [Ca2+]o did not affect the excitability of the low-threshold mechanosensors in the airway. By contrast, reducing [Ca2+]o selectively increased the excitability of airway nociceptors as manifested by a substantive increase in action potential discharge in response to mechanical stimulation, and in a subset of fibers, by overtly evoking action potential discharge. This increase in the excitability of nociceptors was not mimicked by a combination of ω-conotoxin and nifedipine or tetraethylammonium. Whole cell patch recordings from airway-labeled and unlabeled neurons in the vagal jugular ganglia support the hypothesis that [Ca2+]o. inhibits a nonselective cation conductance in vagal nociceptors that may serve to regulate excitability of the nerve terminals within the airways.
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