3. Airway sensory nerves stimulated by acid
It is well documented that increasing acidity in the extracellular fluid activated nociceptive nerve endings in various somatic and visceral tissues, and evoked pain sensation [26], [27], [28] and [29]. In the respiratory tract, indirect evidence of an activation of C-fiber sensory nerves by proton was first reported in an isolated perfused guinea-pig lung preparation. Constant perfusion of pulmonary arteries with the acid buffer at pH of 5.0 caused the release of tachykinins and CGRP from these sensory nerves, which could be blocked by capsazepine, a selective antagonist of the “capsaicin receptor”, suggesting that “capsaicin receptors” were activated during acid stimulation [30] and [31]. Direct evidence of acid stimulation of C-fiber sensory nerves was established by Fox and coworkers in an isolated airway-nerve preparation [32]. They demonstrated that C-fiber afferents innervating the guinea pig trachea, but not Aδ fibers, were stimulated when the pH of the perfusing buffer was reduced to 5.0. Their study further showed that the stimulatory effect of proton is mediated through activation of the capsaicin receptor because it could be abolished by capsazepine [32].