Effect of extracellular calcium on excitability of guinea pig airway vagal afferent nerves

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.