Effect of age on the responses of rat bladder detrusor strips to adenosine triphosphate

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Changes in nerve-mediated contractility of the lower urinary tract in a mouse model of premature ageing

BACKGROUND AND PURPOSE

A high incidence of lower urinary tract disorders is associated with ageing. In the senescent-accelerated prone (SAMP8) mouse strain and the senescent-accelerated resistant (SAMR1) strain, we compared smooth muscle contractility in responses to intrinsic neurotransmitters, both in the bladder and urethra.

EXPERIMENTAL APPROACH

We analysed micturition frequency, the changes in muscle tension induced by electrical field stimulation or agonist administration, the density of nerves (adrenergic, cholinergic and nitrergic) and interstitial cells (ICs), as well as cGMP accumulation in bladder and urethral preparations.

KEY RESULTS

Senescent mice of the SAMP8 strain displayed increased micturition frequency and excitatory contractility of neurogenic origin in the bladder. While cholinergic nerve density remained unchanged, there was a mild sensitization to ACh in male mice. Potentiation in the detrusor may be also provoked by the stronger contribution of ATP, together with reduced adrenergic innervation in males and COX-derived prostanoid production in females. The greater excitatory contractility in the urethra was probably due to the sensitization to noradrenaline, in conjunction with attenuated nitrergic relaxation. There were also fewer neuronal NOS immunoreactive (ir) nerves and vimentin-positive ICs, although the sildenafil- and diethylamine-NONOate-induced relaxations and cGMP-ir remained unchanged.

CONCLUSIONS AND IMPLICATIONS

Premature senescent mice exhibit bladder and urethral hyperexcitability, coupled with reduced urethral relaxation of neurogenic origin, which could model the impaired urinary function in elderly humans. We propose that senescence-accelerated mice provide a useful tool to analyse the basic mechanisms of age-related changes in bladder and urethral function.

Contemporary concepts in the aetiopathogenesis of detrusor underactivity

Detrusor underactivity (DUA) is a poorly understood, yet common, bladder dysfunction, referred to as underactive bladder, which is observed in both men and women undergoing urodynamic studies. Despite its prevalence, no effective therapeutic approaches exist for DUA. Exactly how the contractile function of the detrusor muscle changes with ageing is unclear. Data from physiological studies in animal and human bladders are contradictory, as are the results of the limited number of clinical studies assessing changes in urodynamic parameters with ageing. The prevalence of DUA in different patient groups suggests that multiple aetiologies are involved in DUA pathogenesis. Traditional concepts focused on either efferent innervation or myogenic dysfunction. By contrast, contemporary views emphasize the importance of the neural control mechanisms, particularly the afferent system, which can fail to potentiate detrusor contraction, leading to premature termination of the voiding reflex. In conclusion, the contemporary understanding of the aetiology and pathophysiology of DUA is limited. Further elucidation of the underlying mechanisms is needed to enable the development of new and effective treatment approaches.

Purinergic signalling in the urinary tract in health and disease

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.

Aging differentially modifies agonist-evoked mouse detrusor contraction and calcium signals

Although aging-induced changes in urinary bladder neurotransmission have been studied in some detail, information regarding alterations in detrusor muscle is scanty and addresses only partial aspects of the myogenic response of detrusor. Rodent bladder aging shows several features similar to those reported in humans. The aim of this study was to characterize in aged mouse the alterations of detrusor muscle contraction and the putative underlying changes in Ca(2+) signals. We studied in vitro the myogenic contraction induced by agonists in detrusor strips from adult (3 months old) or aged (23-25 months old) mice. In addition, we determined the agonist-induced [Ca(2+)](i) signals by epifluorescence microscopy in fura-2 loaded isolated detrusor cells. Aging impaired the contractile response of bladder strips to cholinergic stimulation with bethanechol and to chemical depolarization with KCl-containing solutions. On the contrary, the response to purinergic stimulation (ATP) was enhanced. Aging also diminished the transient Ca(2+) signal evoked by bethanechol and the Ca(2+) influx induced by KCl in bladder strips. Treatments aimed to release calcium from intracellular stores (caffeine and a low level of ionomycin in Ca(2+)-free medium) showed that aging reduces the size of agonist-releasable stores. Similar to contraction, the mobilization of Ca(2+) by ATP was increased in aged cells. Therefore, the differential effects of aging on detrusor contraction are associated to alterations of [Ca(2+)](i) signals: the cholinergic inhibition is due to inhibition of voltage-operated Ca(2+) influx and reduction of the size of intracellular Ca(2+) stores, while the age-induced ATP response is accompanied by an enhanced Ca(2+) mobilization.

Role of Purinergic Signaling in Voiding Dysfunction

Purinergic signaling is a term that relates to adenosine triphosphate binding to its receptor (purinergic receptors such as P2X and P2Y subtypes). This pathway has been implicated in bladder functional disorders related to interstitial cystitis/painful bladder syndrome, neurogenic bladder secondary to spinal cord injury, lower urinary tract symptoms, diabetes, and aging. Purinergic signaling occurs at multiple sites, including the central nervous system, peripheral motor and sensory nerves, detrusor smooth muscle, and bladder urothelium. Future pharmacologic agents to treat bladder functional disorders may be able to target purinergic signaling at one or more of these sites.

Assessment and characterization of purinergic contractions and relaxations in the rat urinary bladder

The aim of the present study was to assess the purinoceptor functional responses of the urinary bladder by using isolated rat urinary bladder strip preparations. ATP elicited a transient bladder contraction followed by a sustained relaxation and ADP, UDP and UTP generated predominantly potent relaxations (relaxatory potencies: ADP = ATP > UDP = UTP). The ATP contractions were desensitized with the P2X(1/3) purinoceptor agonist/desensitizer alpha,beta-meATP and reduced by the P2 purinoceptor antagonist PPADS but unaffected by the P2 purinoceptor antagonist suramin. Electrical field stimulation (1-60 Hz) evoked frequency-dependent bladder contractions that were decreased by incubation with alpha,beta-meATP but not further decreased by PPADS. Suramin antagonized relaxations generated by UDP but not those by ADP, ATP or UTP. PPADS antagonized and tended to antagonize UTP and UDP relaxations, respectively, but did neither affect ADP nor ATP relaxations. ADP relaxations were insensitive to the P2Y(1) purinoceptor antagonist MRS 2179 and the ATP-sensitive potassium channel antagonist glibenclamide. The ATP relaxations were inhibited by the P1 purinoceptor antagonist 8-p-sulfophenyltheophylline but unaffected by the A2A adenosine receptor antagonist 8-(3-chlorostyryl)caffeine and glibenclamide. Adenosine evoked relaxations that were antagonized by the A2B adenosine receptor antagonist PSB 1115. Thus, in the rat urinary bladder purinergic contractions are elicited predominantly by stimulation of the P2X(1) purinoceptors, while UDP/UTP-sensitive P2Y purinoceptor(s) and P1 purinoceptors of the A2B adenosine receptor subtype are involved in bladder relaxation.

Effects of ageing and streptozotocin-induced diabetes on connexin43 and P2 purinoceptor expression in the rat corpora cavernosa and urinary bladder

OBJECTIVE

To investigate whether ageing and diabetes alter the expression of the gap junction protein connexin43 (Cx43) and of particular purinoceptor (P2R) subtypes in the corpus cavernosum and urinary bladder, and determine whether changes in expression of these proteins correlate with development of erectile and bladder dysfunction in diabetic and ageing rats.

MATERIALS AND METHODS

Erectile and bladder function of streptozotocin (STZ)-induced diabetic, insulin-treated and age-matched control Fischer-344 rats were evaluated 2, 4 and 8 months after diabetes induction by in vivo cystometry and cavernosometry. Corporal and bladder tissue were then isolated at each of these sample times and protein expression levels of Cx43 and of various P2R subtypes were determined by Western blotting.

RESULTS

In the corpora of control rats ageing was accompanied by a significant decrease in Cx43 and P2X(1)R, and increase in P2X(7)R expression. There was decreased Cx43 and increased P2Y(4)R expression in the ageing control rat bladder. There was a significant negative correlation between erectile capacity and P2X(1)R expression levels, and a positive correlation between bladder spontaneous activity and P2Y(4)R expression levels. There was already development of erectile dysfunction and bladder overactivity at 2 months after inducing diabetes, the earliest sample measured in the study. The development of these urogenital complications was accompanied by significant decreases in Cx43, P2Y(2)R, P2X(4)R and increase in P2X(1)R expression in the corpora, and by a doubling in Cx43 and P2Y(2)R, and significant increase in P2Y(4)R expression in the bladder. Changes in Cx43 and P2R expression were largely prevented by insulin therapy.

CONCLUSION

Ageing and diabetes mellitus markedly altered the expression of the gap junction protein Cx43 and of particular P2R subtypes in the rat penile corpora and urinary bladder. These changes in Cx43 and P2R expression provide the molecular substrate for altered gap junction and purinergic signalling in these tissues, and thus probably contribute to the early development of erectile dysfunction and higher detrusor activity in ageing and in diabetic rats.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Age-dependence of the spontaneous activity of the rat urinary bladder

Abnormal mechanical function of the bladder is manifested in a number of ways including higher frequency of involuntary detrusor contractions associated with reduced compliance of the bladder that is responsible for an increase in intraluminal pressure during filling. There are basically two ways to approach experimentally these problems: (1) by studying the neural control of the lower urinary tract function, and (2) by measuring the properties of smooth muscle cells in the bladder wall. Studies on smooth muscle function often do not take the origin of smooth muscle cells into account i.e., whether they were harvested from normal or overactive bladders. Although, this simplistic view may be beneficial to understanding the generation of the spontaneous activity of the bladder, however, it does not sufficiently explain the cell-to-cell propagation of the spontaneous smooth muscle activity. The spontaneous activity of smooth muscle is an important factor that works against the bladder compliance in the filling phase, and may inversely affect the neurally evoked response during micturition. The intensity of spontaneous activity is the age-dependent; it is high in neonatal bladders it is small or almost non-existent in adults and reemerges in older bladders. This review focuses on these age-dependent alterations of spontaneous bladder contractions and describes the possible mechanisms which may have important role in regulating the spontaneous contractions using the rat as an animal model.

Pharmacological effects of darifenacin on human isolated urinary bladder

Darifenacin [(S)-2--2,2-diphenylacetamide] is a novel antimuscarinic drug currently undergoing phase III trials for the treatment of overactive bladder. We investigated the functional antagonist potency of darifenacin, and the antimuscarinic drugs propiverine, oxybutynin and atropine, on human detrusor smooth muscle. Urinary bladder specimens were obtained from 20 patients who underwent total cystectomy for malignant bladder tumor. Using an organ-bath technique, the effects of the compounds on carbachol-, KCl-, CaCl(2)- or electrical field stimulation (EFS)-induced contractions of the tissues were evaluated. The order of antagonist potency (pA(2 )values) at the muscarinic M(3) receptors was: darifenacin (9.34) > atropine (9.26) > oxybutynin (7.74) > propiverine (7.68). Darifenacin and atropine, at concentrations up to 10(-6) mol/l, did not inhibit the KCl- and CaCl(2)-induced contractions (concentrations 80 and 5 mmol/l, respectively), while propiverine and oxybutynin (10(-5) mol/l) significantly inhibited these contractions. Pretreatment with darifenacin (10(-9)-10(-6) mol/l), propiverine (10(-8)- 10(-5) mol/l), oxybutynin (10(-8)-10(-5) mol/l) and atropine (10(-9)-10(-6) mol/l) significantly inhibited maximum EFS-induced contractions. Darifenacin inhibited contractions of human detrusor smooth muscle only through its antimuscarinic action, while propiverine and oxybutynin had both antimuscarinic and Ca(2+) channel antagonist actions. These findings indicate that darifenacin is a potent antagonist at the M(3) receptor and support its use as a treatment for overactive bladder.