CO₂ transduction mechanisms in avian intrapulmonary chemoreceptors: Experiments and models

Intrapulmonary chemoreceptors (IPC) are neurons that sense tonic and phasic CO₂ stimuli in the lungs of birds and diapsid reptiles. IPC are different from most other vertebrate respiratory CO₂ receptors because: (1) they are stimulated by low P_CO2 and inhibited by high P_CO2, (2) they have extremely rapid response characteristics, (3) their CO₂ sensitivity is nearly abolished by intracellular inhibitors of carbonic anhydrase, and (4) their CO₂ sensitivity is strongly depressed by inhibiting Na⁺/H⁺ antiport exchange. Experimental evidence suggests that IPC respond to intracellular pH, not CO ₂ directly, and that intracellular pH and IPC discharge are determined by a kinetic balance between CO₂ hydration/dehydration rates, transmembrane acid/base exchange rates, and intracellular buffering. We review experimental evidence for and against various mechanisms of IPC CO ₂ chemotransduction, present a conceptual and mathematical model of the proposed mechanisms, and compare this model to CO₂ transduction in other respiratory chemoreceptors.