PERMEABILITY PROPERTIES OF THE INTACT MAMMALIAN BLADDER EPITHELIUM

Because the mammalian bladder must store urine of composition which di ffers markedly fi om that of plasma for prolonged periods, the bladder permeability barrier must maintain extremely low permeabilities to su bstances which normally cross membranes relatively rapidly, such as wa ter, protons, and small nonelectrolytes like urea and ammonia. In the present studies, permeabilities of the apical membrane of dissected ra bbit bladder epithelium to water, urea, ammonia, and protons were meas ured in Ussing chambers and averaged (in cm/s) for water, 5.15 +/- 0.4 3 x 10(-5) for urea, 4.51 +/- 0.67 x 10(-6); for ammonia, 5.14 +/- 0.6 2 x 10(-4): and for protons, 2.98 +/- 1.87 x 10(-3), respectively. The se permeability values are exceptionally low and are expected to resul t in minimal to no leakage of these normally permeable substances acro ss the epithelium. Water permeabilities in intact whole rabbit bladder s were indistinguishable from those obtained in the dissected epitheli al preparation. Moreover, addition of nystatin to the apical solution of dissected epithelia rapidly increased water permeability in conjunc tion with loss of epithelial resistance. These results confirm that th e apical membrane of the bladder epithelial cells represents the bladd er permeability barrier. In addition, they establish a model system th at will permit examination of how membrane structure reduces permeabil ity and how epithelial injury compromises barrier function.