Urothelial function

Variation in urine composition in the human urinary tract: evidence of urothelial function in situ?

PURPOSE

An increased awareness of the concept that the urothelium has a significant transport function led us to question whether urine composition changes as it passes along the human lower urinary tract.

MATERIALS AND METHODS

Urine samples from the bladder and renal pelvis were collected from 30 adults who underwent percutaneous nephrolithotomy (27) or ureteral stent insertion before lithotripsy (3). Urine was obtained from the 2 renal pelves (operative and contralateral sides) in 6 patients (24%). Urine pH was measured using an ultra-thin glass pH electrode. Urinary osmolality, Na and K were measured by micro-osmometry and flame photometry, respectively. Comparison of data sets was achieved using conventional nonparametric statistical methods.

RESULTS

Median bladder urine pH in 30 patients, osmolality in 16, Na in 16 and K in 15 were significantly higher than in the renal pelvis at 6.76 (IQR 6.23 to 6.99), 469 mOsm. kg.1 (IQR 349 to 553), 132 (IQR 100 to 154) and 45 mM. (IQR 30 to 64) versus 6.08 (IQR 5.84 to 6.89), 308 mOsm. kg.1 (IQR 248 to 465), 90 (IQR 69 to 115) and 17 mM. (IQR 10 to 47), respectively (p < or = 0.05). There was no significant difference in these parameters in the urine of the paired renal pelves.

CONCLUSIONS

Bladder urine pH, osmolality, Na and K significantly differ from values in the renal pelvis in moderately hydrated humans. Our data show that urine composition is modified in the lower urinary tract, supporting the concept of a dynamic urothelium. We propose that urothelial-urinary interactions and urinalysis need reappraisal, particularly in investigations of urinary stone formation and sensory bladder function.

Pharmacologic perspective on the physiology of the lower urinary tract

Myogenic activity, distention of the detrusor, and signals from the urothelium may initiate voiding. In the bladder, afferent nerves have been identified not only in the detrusor, but also suburothelially, where they form a plexus that lies immediately beneath the epithelial lining. Extracellular adenosine triphosphate (ATP) has been found to mediate excitation of small-diameter sensory neurons via P2X3 receptors, and it has been shown that bladder distention causes release of ATP from the urothelium. In turn, ATP can activate P2X3 receptors on suburothelial afferent nerve terminals to evoke a neural discharge. However, most probably, not only ATP but also a cascade of inhibitory and stimulatory transmitters and mediators are involved in the transduction mechanisms underlying the activation of afferent fibers during bladder filling. These mechanisms may be targets for future drugs. The central nervous control of micturition involves many transmitter systems, which may be suitable targets for pharmacologic intervention. gamma-Aminobutyric acid, dopamine, enkephalin, serotonin, and noradrenaline receptors and mechanisms are known to influence micturition, and potentially, drugs that affect these systems could be developed for clinical use. However, a selective action on the lower urinary tract may be difficult to obtain. Most drugs currently used for treatment of detrusor overactivity have a peripheral site of action, mainly the efferent (cholinergic) neurotransmission and/or the detrusor muscle itself. In the normal bladder, muscarinic receptor stimulation produces the main part of detrusor contraction, but evidence is accumulating that in disease states, such as neurogenic bladders, outflow obstruction, idiopathic detrusor instability, and interstitial cystitis, as well as in the aging bladder, a noncholinergic activation via purinergic receptors may occur. If this component of activation is responsible not only for part of the bladder contractions, but also for the symptoms of the overactive bladder, it should be considered an important target for therapeutic interventions.

Activation and sensitisation of low and high threshold afferent fibres mediated by P2X receptors in the mouse urinary bladder

It has been proposed that extracellular ATP may be involved in visceral mechanosensory transduction by activating ligand-gated ion channels (P2X receptors). In this study, we have investigated the effects of the P2X(3) agonist alpha,beta-methylene ATP (alpha,beta-meATP) and antagonist 2',3'-O-trinitrophenyl-ATP (TNP-ATP) on pelvic afferents innervating the urinary bladder using an in vitro mouse bladder-pelvic nerve preparation. Intravesical application of alpha,beta-meATP (0.03-1 mM) increased multifibre discharges in a concentration-dependent manner. The agonist potentiated, whereas TNP-ATP (0.03 mM) attenuated, the multifibre responses to bladder distensions. Single-unit analysis revealed that both high threshold (HT) fibres (> 15 mmHg; known to be associated with nociception) and low threshold (LT) fibres (< 15 mmHg; probably associated with non-nociceptive events) could be induced to discharge by intravesical alpha,beta-meATP (1 mM, 0.1 ml). The response of the vast majority (21/22, 95.5 %) of HT fibres to bladder distensions was enhanced with a significantly reduced threshold and an increased peak response after exposure to the agonist. On the other hand, 59.7 % (46/77) of LT fibres showed a greater peak and a slightly reduced threshold for response to bladder distension in the presence of alpha,beta-meATP. An additional 11 'silent' fibres became mechanosensitive after exposure to alpha,beta-meATP. TNP-ATP (0.03 mM) did not affect the threshold of LT fibres, but it reduced the peak response of some (22/51, 43.1 %) LT fibres. Conversely, the antagonist resulted in a markedly elevated threshold and reduced peak activity in the majority (13/16, 81.3 %) of HT fibres. The results support the view that P2X(3) receptor-mediated mechanisms contribute to both nociceptive and non-nociceptive (physiological) mechanosensory transduction in the urinary bladder.

Purinergic signalling: ATP release

Adenosine triphosphate (ATP) has a fundamental intracellular role as the universal source of energy for all living cells. The demonstration of its release into the extracellular space and the identification and localisation of specific receptors on target cells have been essential in establishing, after considerable resistance, its extracellular physiological roles. It is now generally accepted that ATP is a genuine neurotransmitter both in the central and peripheral nervous systems. As such, there are numerous arguments which prove that the release of ATP by nerve terminals is by exocytosis. In some non-neuronal cells, however, recent evidence suggests that ATP release could also be carrier-mediated and would involve ATP-binding cassette proteins (ABC), an ubiquitous family of transport ATPases.

P2X3 knock-out mice reveal a major sensory role for urothelially released ATP

The present study explores the possible involvement of a purinergic mechanism in mechanosensory transduction in the bladder using P2X(3) receptor knock-out (P2X(3)-/-) and wild-type control (P2X(3)+/+) mice. Immunohistochemistry revealed abundant nerve fibers in a suburothelial plexus in the mouse bladder that are immunoreactive to anti-P2X(3). P2X(3)-positive staining was completely absent in the subepithelial plexus of the P2X(3)-/- mice, whereas staining for calcitonin gene-related peptide and vanilloid receptor 1 receptors remained. Using a novel superfused mouse bladder-pelvic nerve preparation, we detected a release of ATP proportional to the extent of bladder distension in both P2X(3)+/+ and P2X(3)-/- mice, although P2X(3)-/- bladder had an increased capacity compared with that of the P2X(3)+/+ bladder. The activity of multifiber pelvic nerve afferents increased progressively during gradual bladder distension (at a rate of 0.1 ml/min). However, the bladder afferents from P2X(3)-/- mice showed an attenuated response to bladder distension. Mouse bladder afferents of P2X(3)+/+, but not P2X(3)-/-, were rapidly activated by intravesical injections of P2X agonists (ATP or alpha,beta-methylene ATP) and subsequently showed an augmented response to bladder distension. By contrast, P2X antagonists [2',3'-O-(2,4,6-trinitrophenyl)-ATP and pyridoxal 5-phosphate 6-azophenyl-2',4'-disulfonic acid] and capsaicin attenuated distension-induced discharges in bladder afferents. These data strongly suggest a major sensory role for urothelially released ATP acting via P2X(3) receptors on a subpopulation of pelvic afferent fibers.

Non-bacterial cystitis

The concept of non-bacterial cystitis (NBC) combines sterile urine and cystitic symptoms as well as inflammatory changes, in particular in the mucosa and submucosa of the bladder. It includes a multiplicity of vicious circles along the entire continence reflex. An understanding of NBC presupposes knowledge of the origin of the normal urinary urge and its successful control. Against the background of the steadily increasing incidence of interstitial cystitis (often irreversible end-stage NBC), it is suggested here that in the face of a failure of first-line therapeutics (anticholinergics, cyclic antidepressants or oestrogens), one must consider without delay the possible presence of NBC.