Activation of epithelial Na(+) channel activity in the rabbit urinary bladder by cAMP

The urothelium: a multi-faceted barrier against a harsh environment

All mucosal surfaces must deal with the challenge of exposure to the outside world. The urothelium is a highly specialized layer of stratified epithelial cells lining the inner surface of the urinary bladder, a gruelling environment involving significant stretch forces, osmotic and hydrostatic pressures, toxic substances, and microbial invasion. The urinary bladder plays an important barrier role and allows the accommodation and expulsion of large volumes of urine without permitting urine components to diffuse across. The urothelium is made up of three cell types, basal, intermediate, and umbrella cells, whose specialized functions aid in the bladder's mission. In this review, we summarize the recent insights into urothelial structure, function, development, regeneration, and in particular the role of umbrella cells in barrier formation and maintenance. We briefly review diseases which involve the bladder and discuss current human urothelial in vitro models as a complement to traditional animal studies.

Functional anatomy of the female lower urinary tract and pelvic floor

Stress continence depends upon three factors: proximal urethral support, vesical neck closure, and urethral contractility. The position of the vesical neck is not static but mobile and under voluntary control. Its support depends upon connections of the urethrovaginal endopelvic fascia to the medial aspect of the levator ani. In addition, these fasciae are attached to the arcus tendineus fasciae pelvis which supports the urethra during levator relaxation, and probably during stress. Levator contraction supports the proximal urethra and also pulls the vesical neck anteriorly against a band of endopelvic fascia which is suspended between the arcus tendinei, compressing it closed. Relaxation of the muscles allows the vesical neck to descend, and facilitates its opening. The connective tissue and smooth muscle of the trigonal ring encircles the vesical neck's lumen, and may contribute to closure of this area. The striated urogenital sphincter muscle can contract to assist in maintaining continence in continent women whose vesical neck is not competent. It has a circular sphincteric portion from 20 to 60% of urethral length. From 60 to 80% it has a considerable bulk of muscle which forms an arch at the perineal membrane that would compress the urethra from above.

A biomimetic tissue from cultured normal human urothelial cells: analysis of physiological function

The urinary bladder and associated tract is lined by the urothelium. Once considered as just an impermeable epithelium, it is becoming evident that the urothelium not only functions as a volume-accommodating urinary barrier but has additional roles, including sensory signaling. Lack of access to normal human urothelium has hampered physiological investigation, and although cell culture systems have been developed, there has been a failure to demonstrate that normal human urothelial (NHU) cells grown in vitro retain the capacity to form a functional differentiated urothelium. The aim of this study was to develop a biomimetic human urothelium from NHU cell cultures. Urothelial cells isolated from normal human urothelium and serially propagated as monolayers in serum-free culture were homogeneous and adopted a proliferative, nondifferentiated phenotype. In the presence of serum and physiological concentrations of calcium, these cells could be reproducibly induced to form stratified urothelia consisting of basal, intermediate, and superficial cells, with differential expression of cytokeratins and superficial tight junctions. Functionally, the neotissues showed characteristics of native urothelium, including high transepithelial electrical resistance of >3,000 Ω·cm2, apical membrane-restricted amiloride-sensitive sodium ion channels, basal expression of Na+-K+-ATPase, and low diffusive permeability to urea, water, and dextran. This model represents major progress in developing a biomimetic human urothelial culture model to explore molecular and functional relationships in normal and dysfunctional bladder physiology.

Expression of neuronal nitric oxide synthase (nNOS) and nitric-oxide-induced changes in cGMP in the urothelial layer of the guinea pig bladder

The urothelium plays a sensory role responding to deformation of the bladder wall; this involves the release of adenosine trisphosphate (ATP) and nitric oxide (NO), which affect afferent nerve discharge and bladder sensation. The urothelial cells responsible for producing ATP and NO and the cellular targets, other than afferent nerves, for ATP and NO remain largely unexplored. Sub-urothelial interstitial cells (SU-ICs) lie immediately below the urothelium and respond to NO with a rise in cGMP. To determine which cells might target SU-ICs by producing NO, areas of dome, lateral wall and base wall were treated with isobutyl-methyl-xanthine, exposed to the NO donor diethylamino NONOate and then fixed for immunohistochemistry. Surface urothelial cells (SUCs) in the base and dome expressed neuronal nitric oxide synthase (nNOS), whereas those in the lateral wall did not. Distinct populations of SUCs were present in the bladder base. SUCs with significant amounts of nNOS lay adjacent to cells with low levels of nNOS. In specific base regions, the few SUCs present contained nNOS within discrete intracellular particles. In the basal urothelial cell (BUC) layer of the lateral wall, nNOS-positive (NOS(+)) BUCs neither showed an elevation in cGMP in response to NO, nor expressed the beta1 sub-unit of soluble guanylate cyclase, protein kinase I or protein kinase II. Thus, they produced but did not respond to NO. The BUC layer also stained for the stem cell factor c-Kit suggesting its involvement in urothelial cell development. No NOS(+) BUCs were present in the SUC-sparse region in the bladder base. Exogenous NO produced an elevation in cGMP in SUCs and SU-ICs. The distribution and proportion of these target cells varied between the dome, lateral wall and base. cGMP(+) SU-ICs were present as a dense layer in the bladder base but were rarely seen in the lateral wall, which contained nNOS(+) BUCs. No nNOS(+) BUCs and cGMP(+) SU-ICs were apparent in the dome. The degree of complexity in nNOS distribution and NO target cells is therefore greater than has previously been described and may reflect distinct physiological functions that have yet to be identified.

Regulation of Na+ channel density at the apical surface of rabbit urinary bladder epithelium

We have investigated the effects of various manipulations on Na(+) transport across the rabbit urinary bladder epithelium. After bladders were mounted in Ussing chambers there was a spontaneous and significant (>4-fold) increase in amiloride-sensitive short-circuit current (equivalent to net Na(+) transport) over a 6-h period. The increase in current was almost abolished by brefeldin A, an inhibitor of anterograde vesicular transport, and reduced after a 3-h delay by cycloheximide, an inhibitor of protein synthesis. The spontaneous increase in short-circuit current was potentiated by treatment of bladders with either forskolin, which causes an elevation in cAMP levels, or aldosterone. Acting together, these two agents produced a significant synergistic effect on short-circuit current. The short-circuit current recovered rapidly after reduction in intracellular Na(+) levels, achieved either by lowering the extracellular Na(+) concentration or blockade of epithelial Na(+) channels with the sulphydryl modifying reagent p-chloromercuribenzenesulphonic acid (PCMBS). Recovery after PCMBS treatment was partially sensitive to brefeldin A. Short-circuit current saturated as the extracellular Na(+) concentration was increased (EC(50) = 51 mM). Saturation occurred over a range of Na(+) concentrations in which single channel permeability is known to remain constant, indicating that it depends on a reduction in epithelial Na(+) channel density at the apical plasma membrane. Exposure of bladders to a high Na(+) concentration caused an increase in endocytotic activity, detected through an increase in the uptake of the fluid-phase marker fluorescein isothiocyanate (FITC)-dextran into vesicles located beneath the apical plasma membrane. We conclude that the urinary bladder epithelium is able to respond rapidly and efficiently to changes in its environment by regulating the density of epithelial Na(+) channels in its apical surface.