I'm growing MDCK II cells on Transwell Filters (from Corning, PET-membrane, 0.4µm pores) for 5 days to polarize them (Medium: DMEM, 10%FCS).
After that I do an immunostaining for apical (gp114 & gp135) and basolateral (Na-K-ATPase, E-cadherin) marker proteins and check their localization with a confocal microscope. What I find is, that the basolateral markers are correctly localized, but the apical markers are not. What can be the reason for that?

Further, if culturing confluent monolayers of my cells, I have to change medium every day, otherwise the pH drops (medium changes colour from red to yellow). Is that normal for MDCK II cells?

Would be great if someone could give me some hints!

Its been a while since I used them, but I always remember MDCK II and other epithelial lines like them always drop the pH fast for me as well.

Resistance - First is your meter working?

Next try following this
Strain 1 MDCK cells were used between passage 18 and 50. Cells were
grown in MEM, 10 mM Hepes, pH 7.3, and 5% FCS without antibiotics
as described (Balcarova-Stander et al., 1984). Cells were passed twice
weekly with a split ratio of 1:5. Transwell filter units (model No. 3415,
Costar, Cambridge, MA) contained a 0.33-cm 2 porous filter membrane
(3.0-gm pores) that had been treated for tissue culture. Filter units were incubated
in 24 well microtitre plates (Costar). These units were placed in
MEM containing 10% FCS, penicillin (100 U/ml) and streptomycin (100
p.g/ml) for 30 min before seeding. Preincubation medium was removed and
150 I.tl ofa trypsinized MDCK cell suspension (1.5 x 105 cells) was added
to each Transwell unit and placed in 1 ml fresh medium. Monolayers were
used after 4 d incubation (37°C, 5% CO2). After this time there were ,'03.5
x 105 MDCK cells/filter.

Ref. www.jcb.org/cgi/reprint/107/1/221.pdf

Finlay BB, Gumbiner B, Falkow S.
Free in PMC
Penetration of Salmonella through a polarized Madin-Darby canine kidney epithelial cell monolayer.
J Cell Biol. 1988 Jul;107(1):221-30.

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