Incorporation of cytoplasmic vesicles into apical membrane of mammalian urinary bladder epithelium.

Abstract

The mammalian urinary bladder is a distensible organ which undergoes a series of slow fillings and rapid emptyings (micturition), during a 24 h period. Recent evidence suggests that the bladder epithelium undergoes a three-phase, structural change during the expansion stage of this cycle1,2. First, macroscopic multicellular folds are smoothed then microscopic folds are stretched out, resulting in a flattening of the urine-facing (apical) membrane. The last and most speculative phase is an incorporation or fusion of cytoplasmic vesicles with the apical membrane, resulting in an alteration of the cell shape from cuboidal to squamous. Such fusion of cytoplasmic vesicles is interesting not only from the point of view of the bladder as a storage reservoir for the discharge of the kidneys, but also from the perspective of hormonal regulation of the ion reabsorptive processes in this epithelium1,3. Recently, the determination of the electrical properties of the apical membrane of mammalian urinary bladder has revealed that aldosterone (a Na+-conserving hormone) increases both Na+ conductance and selectivity4. The origin of such highly Na+ selective channels is unknown. Here we report that mammalian urinary bladder epithelium accommodates stretching by incorporating into the apical membrane vesicles from a cytoplasmic pool, which are withdrawn to effect recovery from stretch. The vesicle membrane contains Na+-selective channels—this has implications for the regulation of membrane transport.