Sodium in The Apical Solution Mediates Downregulation of Ion Transport in Cultured Pig Tracheal Epithelia

Abstract

Respiratory epithelium controls transepithelial ion and fluid transport for maintaining homeostasis of airway surface. Whether the respiratory epithelium could sense and respond to surface fluid changes is vastly unknown. To test this hypothesis, the apical surface of cultured and differentiated pig tracheal epithelia in Millicell inserts was gently washed with the Krebs solution for 3 times, followed by adding 40 μl the Krebs solution for 1 to 24 hr incubation. Our data demonstrate that the short-circuit currents (Isc) of epithelia at basal condition (Isc-Basal) and Isc changes by amiloride (ΔIsc-Amil) due to inhibition of the epithelial sodium channel (ENaC) both were gradually decreased at 4, 7 and 10 hr of incubation, compared to that of the control epithelia with no treatment. However, Isc-Basal and ΔIsc-Amil were recovered back to the control values after 24 hr of incubation. Moreover, the Isc changes by forskolin and IBMX or by Cl– transporter inhibitors GlyH-101, bumetanide and DPC all displayed similar time-dependent attenuation but with mild effects. These data suggest that ion transport of tracheal epithelia was reduced in response to wash and incubation of the apical Krebs solution. To further explore the mechanism leading to decreased Isc-Basal and ΔIsc-Amil, we either washed or directly incubated the apical side of tracheal epithelia with the Krebs solution or with the salt-free mannitol solution. The data indicate that after 7 hr, wash or incubation with the Krebs solutions all decreased Isc-Basal and ΔIsc-Amil, but with the mannitol solutions both treatments were without effect. By removal of individual Na+, Cl– or HCO3– in the Krebs solution, our data demonstrate that only the Na+-free solution did not decrease Isc-Basal and ΔIsc-Amil. These data suggest that the underlying mechanism requires apical Na+ and could be initiated quickly during the time of surface wash. Our further experiments found that pretreatment of cultured epithelia with the ERK inhibitor U0126 or the AMPK inhibitor compound C in the bottom medium largely attenuated the reductions in Isc-Basal and ΔIsc-Amil of tracheal epithelia by the apical Krebs solutions. In addition, the mRNA levels of ENaC-β and -γ subunits rather than other ion transporters were significantly reduced after 7 hr incubation and returned to normal at 24 hr, compared to that of the control with no treatment. Taken together, our data suggest that fluid challenge on the luminal surface of the tracheal epithelium may reduce transepithelial Na+ transport, which is mediated by Na+ in the apical solution resulting in activation of the ERK and AMPK signaling pathways and downregulation in the mRNA levels of ENaC-β and -γ subunits.