Uroplakins Do Not Restrict CO2 Transport through Urothelium

Abstract Lipid bilayers and biological membranes are freely permeable to CO2, and yet partial CO2 pressure in the urine is 3–4-fold higher than in blood. We hypothesized that the responsible permeability barrier to CO2 resides in the umbrella cell apical membrane of the bladder with its dense array of uroplakin complexes. We found that disrupting the uroplakin layer of the urothelium resulted in water and urea permeabilities (P) that were 7- to 8-fold higher than in wild type mice with intact urothelium. However, these interventions had no impact on bladder PCO2 (∼1.6 × 10−4 cm/s). To test whether the observed permeability barrier to CO2 was due to an unstirred layer effect or due to kinetics of CO2 hydration, we first measured the carbonic anhydrase (CA) activity of the bladder epithelium. Finding none, we reduced the experimental system to an epithelial monolayer, Madin-Darby canine kidney cells. With CA present inside and outside the cells, we showed that PCO2 was unstirred layer limited (∼7 × 10−3 cm/s). However, in the total absence of CA activity PCO2 decreased 14-fold (∼ 5.1 × 10−4 cm/s), indicating that now CO2 transport is limited by the kinetics of CO2 hydration. Expression of aquaporin-1 did not alter PCO2 (and thus the limiting transport step), which confirmed the conclusion that in the urinary bladder, low PCO2 is due to the lack of CA. The observed dependence of PCO2 on CA activity suggests that the tightness of biological membranes to CO2 may uniquely be regulated via CA expression.