Imidazole binding reagent diethyl pyrocarbonate (DEPC) inhibits avian intrapulmonary chemoreceptor discharge in vivo

Data indicate that avian intrapulmonary chemoreceptors (IPC) transduce CO₂ stimuli by sensing the products of CO₂ hydration, [H⁺] and [HCO₃ ^-]. The alphastat regulation hypothesis of physiological pH sensitivity suggests that proteins sense [H ⁺] through changes in the ionization state of imidazole groups (alpha_Im). To test whether imidazole is involved with IPC CO ₂ sensitivity, we administered diethyl pyrocarbonate (DEPC) intravenously while recording from IPC exposed to varying levels of inspired CO₂. At physiological pH, DEPC converts pH sensitive imidazole groups to pH-insensitive N-carbethoxyhistidyl residues. Single cell extracellular neural recordings were made from vagal filaments in anesthetized, unidirectionally ventilated Anas platyrhynchos. Without DEPC, IPC discharge rate was inversely proportional to inspired CO₂ with characteristic dynamic responses to rapid CO₂ alterations (n = 10). After DEPC treatment (≥15 mM), mean sensitivity of IPC discharge to static inspired CO₂ levels was decreased 75% (P < 0.05), and mean peak dynamic IPC discharge rate was decreased 80% (P < 0.05). Additionally, we tested whether DEPC might alter IPC discharge by binding imidazole groups in the enzyme carbonic anhydrase (CA), but we found no effect on CA catalytic rate. We conclude that DEPC inhibits IPC CO₂ signal transduction by modifying imidazole groups on acid-sensitive proteins other than CA, possibly membrane acid-base exchangers or ion channels. These data support the alphastat regulation hypothesis in IPC CO₂ respiratory chemoreception and suggests a more direct link between H⁺ and membrane excitability.