Uroplakins do not restrict CO2 transport through urothelium.
Lipid bilayers and biological membranes are freely permeable to CO(2), and yet partial CO(2) pressure in the urine is 3-4-fold higher than in blood. We hypothesized that the responsible permeability barrier to CO(2) 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 P(CO2) (∼1.6 × 10(-4) cm/s). To test whether the observed permeability barrier to CO(2) was due to an unstirred layer effect or due to kinetics of CO(2) 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 P(CO2) was unstirred layer limited (∼7 × 10(-3) cm/s). However, in the total absence of CA activity P(CO2) decreased 14-fold (∼ 5.1 × 10(-4) cm/s), indicating that now CO(2) transport is limited by the kinetics of CO(2) hydration. Expression of aquaporin-1 did not alter P(CO2) (and thus the limiting transport step), which confirmed the conclusion that in the urinary bladder, low P(CO2) is due to the lack of CA. The observed dependence of P(CO2) on CA activity suggests that the tightness of biological membranes to CO(2) may uniquely be regulated via CA expression.
Institut für Biophysik, Johannes Kepler Universität, 4020 Linz, Austria.
Florian
F
Zocher
Mark L
ML
Zeidel
Andreas
A
Missner
Tung-Tien
TT
Sun
Ge
G
Zhou
Yi
Y
Liao
Maximilian
M
von Bodungen
Warren G
WG
Hill
Susan
S
Meyers
Peter
P
Pohl
John C
JC
Mathai
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002245:Carbon Dioxide / Q000378:metabolism*; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D002256:Carbonic Anhydrases / Q000378:metabolism; D002460:Cell Line; D004285:Dogs; D055786:Gene Knockout Techniques; D051379:Mice; D010539:Permeability / Q000187:drug effects; D060170:Uroplakin II / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D060171:Uroplakin III / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D019459:Urothelium / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*
D002257:Carbonic Anhydrase Inhibitors; D060170:Uroplakin II; D060171:Uroplakin III; D002245:Carbon Dioxide; D002256:Carbonic Anhydrases