The ROMK potassium channel is present in mammalian urinary tract epithelia and muscle

Abstract

There is increasing evidence that mammalian urinary tract epithelial cells utilize membrane channels and transporters to transport solutes across their apical (luminal) and basalateral membranes to modify solute concentrations in both cell and urine. This study investigates the expression, localization, and regulation of the ROMK (Kir 1.1) potassium channels in rat and dog ureter and bladder tissues. Immunoblots of homogenates of whole ureter, whole bladder, bladder epithelial cells, and bladder smooth muscle tissues in both rat and dog identified ϳ45-to 50-kDa bands characteristic of ROMK in all tissues. RT-PCR identified ROMK mRNA in these same tissues in both animal species. ROMK protein localized by immunocytochemistry was strongly expressed in the apical membranes of the large umbrella cells lining the bladder lumen and to a lesser extent in the cytoplasm of epithelial cells and smooth muscle cells in the rat bladder. ROMK protein and mRNA were also discovered in cardiac, striated, and smooth muscle in diverse organs. There was no difference in immunoblot expression of ROMK abundance in bladder homogenates (whole bladder, epithelial cell, or muscle cell) or ureteral homogenates between groups of rats fed high-or low-potassium diets. Although the functional role of ROMK in urinary tract epithelia and smooth muscle is unknown, ROMK may participate in the regulation of epithelial and smooth muscle cell volume and osmolality, in the dissipation of potassium leaked or diffused from urine across the epithelial cell apical membranes or tight junctions, and in net or bidirectional potassium transport across urinary tract epithelia. epithelial sodium channel; calponin MAMMALIAN URINARY TRACT EPITHELIA (urothelia) has long been held to serve solely as a storage conduct for urine made by the kidney and, in particular, to prevent modification of urine by inhibiting net flux of water and solutes across the epithelial cell luminal membrane (7). The barrier(s) preventing transepithelial flux of urine constituents have been shown to include: the lumenal membrane (containing unique uroplakin proteins) of the large ("umbrella") cells lining the urinary tract lumen (12), tight junctions adjoining the umbrella cells, and a glycosaminoglycans layer overlying and adjacent to the lumenal membrane (13, reviewed in Ref. 17). Evidence for the effectiveness of these barriers, which have been thought to be passive in nature and not to utilize or require membrane transporters, comes from Ussing chamber studies documenting very low urothelial permeability to water, ions, and nonelectrolytes and very high transepithelial cell resistance in stretched bladders and apical membrane endosomes obtained from rabbits under basal conditions (2, 24).However, although the permeabilities of urothelia for water and various urinary solutes may be low under basal conditions, urothelial permeabilities in animals subjected to dietary, physiological (e.g., water loading/water restricted), or other stress could be altered. Furthermore, urothelial cells are...

Volumetric Deformation of Live Cells Induced by Pressure-Activated Cross-Membrane Ion Transport

“…Whether this is so is an important question to answer. Recent studies have in fact shown that cross-membrane ion transport in bladder epithelial cells [10] or chondrocytes [11] can be triggered by pressure shocks or compressions. Unfortunately, most experiments of this kind were conducted on the tissue level and, hence, are unable to reveal the activation mechanisms behind.…”

Dietary protein affects urea transport across rat urothelia

“…In contrast, the percentage of instilled urea reabsorbed was greater in the two dietary groups receiving the lowest protein (26 and 23%) than in those receiving higher protein (11 and 9%), suggesting the possibility that a bladder/urothelial factor, also affected by dietary protein, may have altered bladder permeability. These findings demonstrate significant regulated urea transport across the urothelium, resulting in alteration of urine excreted by the kidneys, and add to the growing evidence that the lower urinary tract may play an unappreciated role in mammalian solute homeostasis.solute transport across bladder epithelia; urothelial creatinine transport; regulation of urothelial urea transport ALTHOUGH MAMMALIAN LOWER URINARY tract functions primarily as a short-term transit and storage vehicle for urine made by the kidneys (13), recent data demonstrate that mammalian urothelia (the epithelial cell lining of the urinary tract from renal pelvis to proximal urethra) is a surprisingly dynamic and complex tissue that may play an unappreciated role in water and solute homeostasis (35)(36)(37)(38)(39) reviewed in Ref. 22).…”

“…solute transport across bladder epithelia; urothelial creatinine transport; regulation of urothelial urea transport ALTHOUGH MAMMALIAN LOWER URINARY tract functions primarily as a short-term transit and storage vehicle for urine made by the kidneys (13), recent data demonstrate that mammalian urothelia (the epithelial cell lining of the urinary tract from renal pelvis to proximal urethra) is a surprisingly dynamic and complex tissue that may play an unappreciated role in water and solute homeostasis (35)(36)(37)(38)(39) reviewed in Ref. 22).…”

“…Furthermore, water and solute channels and transporters known to facilitate and regulate vectoral water and solute transport across renal epithelia have recently been described in urothelial cells (34,38,39). Thus several labs have reported on the presence, regulation, and localization of aquaporins 1-3 (36), ROMK (37), epithelial Na ϩ channel (ENaC) (34), and urea transporter (UT)-B (26,38) in rat urothelia using standard RT-PCR, Western blot, and immunocytochemical techniques. In particular, UT-B was localized to epithelial cell cytoplasm and to all epithelial cell membranes except the apical luminal membrane (26,38).…”

Properties of the Urothelium that Establish the Blood–Urine Barrier and Their Implications for Drug Delivery

“…The apical plasma membrane of superficial UCs also expresses an amiloride-insensitive, oxytocin-stimulated cation channel for Ca 2+ , as well as a voltage-sensitive channel for K + . The renal outer medullary potassium channels are localized to the apical plasma membrane of superficial UCs and could regulate the transmembrane electrical potential, sensory transduction, composition of extracellular and intracellular K + concentrations, and thus cell volume and osmolality (Spector et al 2008). Na + conductance is also found in the basolateral membrane of superficial UCs that expresses a barium-sensitive, voltage-sensitive sodium channel as well as a lidocaine-sensitive potassium channel (Frings et al 1990).…”