Fitc sodium

hAEpCs were seeded on a chip as described above and cultured on the apical side for 2 days under static and submerged conditions. They were then placed at the air–liquid interface and cultured under static or dynamic conditions for 3 additional days. On day 5, the apical side was washed with PBS with calcium, which was subsequently exchanged with 80 μL of a permeability solution [1 µg mL⁻¹ FITC-sodium (0.4 kDa, Sigma-Aldrich) and 1.5 mg mL⁻¹ RITC-dextran (70 kDa, Sigma-Aldrich) in SAGM™ medium]. The chip was then transferred to the incubator and incubated in static conditions. After 2 h, the apical permeability solution was removed, the apical side was washed once with PBS with calcium, following which the PBS was removed, and the medium in the basal chamber exchanged. The supernatant solution was collected from the outlet well. A 50-µL aliquot of this solution was transferred to a 96-well flat-bottom plate and analyzed with a microplate reader (M1000 Infinite, Tecan) at 460 nm excitation/515 nm emission for FITC-sodium and 553 nm excitation/627 nm for RITC-dextran. The apparent permeability coefficient (Papp) was calculated according to equation (1), dQ/dt being the transport rate and C₀, the initial concentration of the permeability solution tested and A the surface area of the permeability barrier. $$P_{app}=\left(\frac{dQ}{dt}\right)⁎\frac{1}{C_0}⁎\frac{1}{A}$$