An application of CO₂/HCO₃⁻-free solution (Zero-CO₂) did not increase intracellular pH (pH_i) in ciliated human nasal epithelial cells (c-hNECs), leading to no increase in frequency (CBF) or amplitude (CBA) of the ciliary beating. This study demonstrated that the pH_i of c-hNECs expressing carbonic anhydrase IV (CAIV) is high (7.64), while the pH_i of ciliated human bronchial epithelial cells (c-hBECs) expressing no CAIV is low (7.10). An extremely high pH_i of c-hNECs caused pH_i, CBF and CBA to decrease upon Zero-CO₂ application, while a low pH_i of c-hBECs caused them to increase. An extremely high pH_i was generated by a high rate of HCO₃⁻ influx via interactions between CAIV and Na⁺/HCO₃⁻ cotransport (NBC) in c-hNECs. An NBC inhibitor (S0859) decreased pH_i, CBF and CBA and increased CBF and CBA in c-hNECs upon Zero-CO₂ application. In conclusion, the interactions of CAIV and NBC maximize HCO₃⁻ influx to increase pH_i in c-hNECs. This novel mechanism causes pH_i to decrease, leading to no increase in CBF and CBA in c-hNECs upon Zero-CO₂ application, and appears to play a crucial role in maintaining pH_i, CBF and CBA in c-hNECs periodically exposed to air (0.04% CO₂) with respiration.

An application of CO2/HCO3--free solution (Zero-CO2) did not increase intracellular pH (pHi) in ciliated human nasal epithelial cells (c-hNECs), leading to no increase in the beating frequency (CBF) or bend distance (CBD, an index of amplitude). This study demonstrated that the pHi of the c-hNECs expressing carbonic anhydrase IV (CAIV) is high (7.64), while that of the ciliated human bronchial epithelial cells (c-hBECs) expressing no CAIV is low (7.10). Upon applying Zero-CO2, an extremely high level of pHi caused the pHi, CBF and CBD to decrease, while a low level of pHi caused them to increase. The high pHi was generated by a high rate of HCO3- influx via interac-tions between CAIV and Na+/HCO3- cotransport (NBC) in c-hNECs. An NBC inhibitor (S0859) decreased the pHi and CBF, and Zero-CO2 application increased the CBF in S0859-treated c-hNECs. In conclusion, the interactions of CAIV and NBC maximize the HCO3- influx to increase the pHi in c-hNECs. This novel mechanism causes the pHi to decrease, leading to no increase in CBF and CBD in c-hNECs upon Zero-CO2 application, and appears to play a crucial role in maintaining pHi, CBF and CBD in c-hNECs periodically exposed to air (0.04% CO2) with respiration.