Alveolar hypercapnia augments pulmonary C-fiber responses to chemical stimulants: role of hydrogen ion.

To determine whether the excitabilities of pulmonary C fibers to chemical and mechanical stimuli are altered by CO(2)-induced acidosis, single-unit pulmonary C-fiber activity was recorded in anesthetized, open-chest rats. Transient alveolar hypercapnia (HPC) was induced by administering CO(2)-enriched gas mixture (15% CO(2), balance air) via the respirator inlet for 30 s, which rapidly lowered the arterial blood pH from a baseline of 7.40 +/- 0.01 to 7.17 +/- 0.02. Alveolar HPC markedly increased the responses of these C-fiber afferents to several chemical stimulants. For example, the C-fiber response to right atrial injection of the same dose of capsaicin (0.25-1.0 microg/kg) was significantly increased from 3.07 +/- 0.70 impulses/s at control to 8.48 +/- 1.52 impulses/s during HPC (n = 27; P < 0.05), and this enhanced response returned to control within approximately 10 min after termination of HPC. Similarly, alveolar HPC also induced significant increases in the C-fiber responses to right atrial injections of phenylbiguanide (4-8 microg/kg) and adenosine (0.2 mg/kg). In contrast, HPC did not change the response of pulmonary C fibers to lung inflation. Furthermore, the peak response of these C fibers to capsaicin during HPC was greatly attenuated when the HPC-induced acidosis was buffered by infusion of bicarbonate (1.36-1.82 mmol. kg(-1). min(-1) for 35 s). In conclusion, alveolar HPC augments the responses of these afferents to various chemical stimulants, and this potentiating effect of CO(2) is mediated through the action of hydrogen ions on the C-fiber sensory terminals.