Characterization and autoradiographic localization of [3H] alpha, beta-methylene adenosine 5'-triphosphate binding sites in human urinary bladder

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.

Purinoreceptor-mediated current in myocytes from renal resistance arteries

BACKGROUND AND PURPOSE

Ionotropic purinoreceptors (P2X) in renal vascular smooth muscle cells (RVSMCs) are involved in mediating the sympathetic control and paracrine regulation of renal blood flow (RBF). Activation of P2X receptors elevates [Ca(2+)](i) in RVSMCs triggering their contraction, leading to renal vasoconstriction and decrease of RBF. The goal of the present work was to characterize the P2X receptor-mediated ionic current (I(P2X)) and to identify the types of P2X receptors expressed in myocytes isolated from interlobar and arcuate arteries of rat kidney.

EXPERIMENTAL APPROACH

The expression of P2X receptors in isolated RVSMCs was analysed by reverse transcription (RT)-PCR. I(P2X) and membrane potential were recorded using the amphotericin B-perforated patch method.

KEY RESULTS

RT-PCR analysis on single RVSMCs showed the presence of genes encoding P2X1 and P2X4 receptors. Under voltage clamp conditions, the selective P2X receptor agonist alphabeta-methylene ATP (alphabeta-meATP) evoked I(P2X) similar to that induced by ATP. Under current clamp conditions, both ATP and alphabeta-meATP evoked a spike-like membrane depolarization followed by a sustained depolarization, linking P2X receptors in RVSMCs to sympathetic control of renal vascular tone. A selective antagonist of P2X1 receptors, NF279, reduced I(P2X) amplitude by approximately 65% concentration-dependently manner within the nanomolar to sub-micromolar range. The residual current was resistant to micromolar concentrations of NF279, but was inhibited by sub-millimolar to millimolar concentrations of NF279.

CONCLUSIONS AND IMPLICATIONS

Two types of functional P2X receptors, monomeric P2X1 and heteromeric P2X1/4 receptors, are expressed in RVSMCs. Our study has identified important targets for possible pharmacological intervention in the sympathetic control of renal circulation.

P2 purinoceptors: historical perspective and classification

This article presents an overview that gives some historical perspective to the detailed papers at the cutting edge of P2 purinoceptor research that follow. I consider the proposal, first put forward by Abbracchio & Burnstock (Pharmacol Ther 64:445-475, 1994), that P2 purinoceptors should be regarded as members of two main families: a P2X purinoceptor family consisting of ligand-gated ion channels, and a P2Y purinoceptor family consisting of G protein-coupled receptors. The latest subclasses of these two families (P2X1-4 and P2Y1-5), identified largely on the basis of molecular cloning and expression, are tabled. Finally, I suggest some future directions for P2 purinoceptor research, including studies of the long-term (trophic) actions of purines, the evolution and development of purinoceptors and therapeutic applications.

Frequency encoding of cholinergic- and purinergic-mediated signaling to mouse urinary bladder smooth muscle: modulation by BK channels

In the urinary bladder, contractions of the detrusor muscle and urine voiding are induced by the neurotransmitters ACh and ATP, released from parasympathetic nerves. Activation of K(+) channels, in particular the large-conductance Ca(2+)-activated K(+) (BK) channels, opposes increases in excitability and contractility of urinary bladder smooth muscle (UBSM). We have shown that deleting the gene mSlo1 in mice (Slo(-/-)), encoding the BK channel, leads to enhanced nerve-mediated and neurotransmitter-dependent contractility of UBSM (38). Here, we examine the location of the BK channel in urinary bladder strips from mouse. Immunohistochemical analysis revealed that the channel is expressed in UBSM but not in nerves that innervate the smooth muscle. The relationship between electrical field stimulation and force generation of the cholinergic and purinergic pathways was examined by applying blockers of the respective receptors in UBSM strips from wild-type and from Slo(-/-) (knockout) mice. In wild-type strips, the stimulation frequency required to obtain a half-maximal force was significantly lower for the purinergic (7.2 +/- 0.3 Hz) than the cholinergic pathway (19.1 +/- 1.5 Hz), whereas the maximum force was similar. Blocking BK channels with iberiotoxin or ablation of the Slo gene increased cholinergic- and purinergic-mediated force at low frequencies, i.e., significantly decreased the frequency for a half-maximal force. Our results indicate that the BK channel has a very significant role in reducing both cholinergic- and purinergic-induced contractility and suggest that alterations in BK channel expression or function could contribute to pathologies such as overactive detrusor.

Negative feedback regulation of nerve-mediated contractions by KCa channels in mouse urinary bladder smooth muscle

When the urinary bladder is full, activation of parasympathetic nerves causes release of neurotransmitters that induce forceful contraction of the detrusor muscle, leading to urine voiding. The roles of ion channels that regulate contractility of urinary bladder smooth muscle (UBSM) in response to activation of parasympathetic nerves are not well known. The present study was designed to characterize the role of large (BK)- and small-conductance (SK) Ca(2+)-activated K(+) (K(Ca)) channels in regulating UBSM contractility in response to physiological levels of nerve stimulation in UBSM strips from mice. Nerve-evoked contractions were induced by electric field stimulation (0.5-50 Hz) in isolated strips of UBSM. BK and SK channel inhibition substantially increased the amplitude of nerve-evoked contractions up to 2.45 +/- 0.12- and 2.99 +/- 0.25-fold, respectively. When both SK and BK channels were inhibited, the combined response was additive. Inhibition of L-type voltage-dependent Ca(2+) channels (VDCCs) in UBSM inhibited nerve-evoked contractions by 92.3 +/- 2.0%. These results suggest that SK and BK channels are part of two distinct negative feedback pathways that limit UBSM contractility in response to nerve stimulation by modulating the activity of VDCCs. Dysfunctional regulation of UBSM contractility by alterations in BK/SK channel expression or function may underlie pathologies such as overactive bladder.

Elementary purinergic Ca2+ transients evoked by nerve stimulation in rat urinary bladder smooth muscle

The translation of nerve transmission to Ca2+ signals in urinary bladder smooth muscle (UBSM) is incompletely understood. Thus, we sought to characterize Ca2+ signals in strips of UBSM loaded with the Ca2+-sensitive fluorescent dye, fluo-4, using laser scanning confocal microscopy. Two types of Ca2+ signals occurred spontaneously and could be evoked with field stimulation: large, rapid, global Ca2+ transients termed 'global Ca2+ flashes', and much smaller, localized Ca2+ transients. Global Ca2+ flashes were inhibited by the L-type voltage-dependent Ca2+ channel (VDCC) inhibitor, diltiazem and with P2X receptor blockade. Simultaneous intracellular recordings and Ca2+ measurements indicated that these events are caused by Ca2+ influx through VDCCs during action potentials. Small, local Ca2+ transients occurred spontaneously, and their frequency could be elevated with field stimulation. Atropine, an inhibitor of muscarinic receptors, did not affect these local Ca2+ transients. However, the desensitizing P2X receptor agonist alpha,beta-methylene ATP, and the purinergic antagonist, suramin, effectively inhibited the local Ca2+ transients. The frequency of these 'purinergic Ca2+ transients' was increased about 7-fold by a 10 s stimulus train (1 Hz). The amplitude, duration at one-half amplitude and the spatial spread of the evoked purinergic Ca2+ transients were F/F(o) = 2.4 +/- 0.13, 111.7 +/- 9.3 ms and 14.0 +/- 1.0 microm2, respectively. Tetrodotoxin inhibited evoked purinergic Ca2+ transients, indicating that they were dependent on nerve fibre activation. Purinergic Ca2+ transients were not dependent on VDCC activity. Neither 2-APB, an inhibitor of inositol 1,4,5-triphosphate (Ins(1,4,5)P3) (IP3)-induced Ca2+ release, nor ryanodine inhibited the purinergic Ca2+ transients. We have identified two novel Ca2+ signals in rat UBSM. Large, rapid, global Ca2+ flashes that represent Ca2+ influx through VDCCs during action potentials, and local, purinergic Ca2+ transients that represent Ca2+ entry through P2X receptors. Our results indicate that purinergic Ca2+ transients evoked by release of ATP from nerve varicosities are elementary signals in the process of nerve-smooth muscle communication.

Physiology of female micturition

Micturition is a dynamic physiologic process consisting of alternating storage and expulsion phases and is accomplished by complex interactions among innervation, smooth muscle, connective tissue, urothelium and supportive structures. Although our current understanding of the anatomy and physiology of the lower urinary tract is far from complete, intensive research over the last decade has dramatically improved our appreciation of the neural, biomechanical, biochemical, and morphologic properties of the bladder and urethra, as well as the hormonal influences and unique pelvic and perineal anatomy of women. Continued research related to the physiology of female micturition promises to offer new insights into the complex bladder-urethral interactions and to provide a basis for developing better management strategies for a variety of voiding dysfunctions in women.

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

OBJECTIVE

To assess age-related changes in bladder function using the contractile responses to ATP of detrusor strips from rats of various ages. Materials and methods Urinary bladders were obtained from male Wistar rats aged 9 weeks (young), 24 weeks (adult) and 24 months (aged). Contractions of urinary bladder muscle strips to ATP were measured isometrically. The size of the initial phasic response and the secondary contractile response that developed after washing out ATP (postwashout contraction) were measured. The magnitudes of the ATP-induced phasic and postwashout contraction were compared among the age groups. During the contractions, prostanoid concentrations in the organ-bath medium were measured using an enzyme immunoassay.

RESULTS

The ATP-induced postwashout contraction did not occur after stimulation with KCl or acetylcholine, but was induced by alpha,beta-methylene ATP. Both the phasic and postwashout contractions were concentration-dependent. Although the phasic contraction did not change progressively with age, the magnitude and duration of the postwashout contraction increased substantially with age. Nicardipine (a calcium antagonist) slightly inhibited both contractions. Suramin (a nonselective P2-receptor antagonist) did not significantly inhibit the phasic contraction, but reduced the postwashout contraction. PPADS (a selective P2X receptor antagonist) did not inhibit either contraction. Indomethacin (a prostaglandin synthesis inhibitor) had no effect on the phasic contraction but almost completely blocked the postwashout contraction when added before ATP stimulation, but was less effective when added after ATP. The prostaglandin E2 concentration in the organ bath increased during the postwashout contraction.

CONCLUSIONS

These findings suggest that the ATP-induced postwashout contraction is not directly mediated by P2x purinoceptors, but results from the synthesis of prostaglandins, especially E2, which is a sensory autacoid. The age-linked increase in postwashout contraction may be involved in the changes in sensory and voiding mechanisms seen in the aged urinary bladder.

Steady-state binding of [3H]ATP to rat liver plasma membranes and competition by various purinergic agonists and antagonists

Steady-state analysis of nucleotide-binding sites on rat liver plasma membranes was carried out using 3H-labelled ATP as radioligand under complete inhibition of ecto-ATPase activity by excess EDTA. Binding of [3H]ATP to the membranes is saturable, reversible and apparently involves one population of specific binding sites with Kd of about 90 nM and binding capacity (Bmax) of 15 pmol/mg protein. A broad spectrum of purinergic agonists and antagonists was examined as potential inhibitors of the measured binding. The displacement studies showed the following rank order of inhibitory potency for [3H]ATP-binding sites (pIC50 values in parentheses): ATPgammaS (7.49)>2-MeSATP (7.18)>ATP (6.91)>ADPbetaS (6.64)>/=ADP (6.56)>>RB2 (6.14)>>suramin (5.40)>>Ap4A (4. 57)>alpha,beta-MeATP (4.19)>/=beta,gamma-MeATP (3.97). AMP, adenosine, Ap5A, PPADS, beta-glycerophosphate as well as non-adenine nucleoside triphosphates GTP, UTP and CTP did not exert any effect on the measured binding at concentration ranges of 10-6-10-4 M. In order to ascertain whether ATP and its analogues are capable of interacting with the same binding domain, 2-MeSATP and ADP were treated as alternative ligands that could compete with unlabelled ATP for its binding sites. A 2-fold increase of Kd value for ATP-receptor interaction was observed in the presence of 2-MeSATP (60 nM) or ADP (250 nM) without any modulation of Bmax value, confirming that inhibitory effects of these compounds are competitive in nature. These studies demonstrate that ATP and its analogues are able to interact with a single binding domain on liver plasma membranes, which may be identified as ligand-binding component of P2 purinoceptors of the P2Y1 subtype.

The overactive bladder: pharmacologic basis of drug treatment

OBJECTIVES

To provide an overview of the basis for drug treatment of the overactive bladder.

METHODS

Published information is evaluated.

RESULTS

The causes of bladder overactivity are not known, but theoretically, increased afferent activity, decreased inhibitory control in the central nervous system (CNS) or peripheral ganglia, and increased sensitivity of the detrusor to efferent stimulation may be involved. Several CNS transmitters can modulate voiding, but few useful drugs with a defined CNS site of action have been developed. Drugs that stimulate gamma-aminobutyric acid receptors are used clinically. Potentially, drugs affecting opioid, 5-hydroxytryptamine, norepinephrine, dopamine, and glutamatergic receptors and mechanisms can be developed, but a selective action on the lower urinary tract may be difficult to obtain. Traditionally, drugs used for treatment of bladder overactivity have had a peripheral site of action, mainly efferent neurotransmission or the detrusor itself. Antimuscarinic drugs, beta-adrenoceptor agonists, alpha-adrenoceptor antagonists, drugs affecting membrane channels, prostaglandin synthetase inhibitors, and several other agents have been used with limited success. New information on the alpha-adrenoceptor and muscarinic receptor subtypes in the human detrusor has emerged and may be the basis for the development of new compounds with effects on bladder overactivity. Decreasing afferent activity seems an attractive therapeutic approach, and drugs affecting afferent nerves by causing release of tachykinins, such as capsaicin and analogs, as well as agents blocking tachykinin receptors, may be of therapeutic interest.

CONCLUSIONS

New drugs, specifically designed for the treatment of bladder overactivity, are desirable.

Characterization and autoradiographic localization of [3H] alpha,beta-methylene ATP binding sites in cat urinary bladder

1. The characteristics and distribution of [3H] alpha,beta-methylene ATP ([3H] alpha,beta-MeATP), a radioligand for P2X-purinoceptors, binding sites in cat urinary bladder detrusor were examined. 2. Saturation analysis revealed that, in cat bladder membrane preparations, only one population of binding sites with high affinity (Kd = 1.8 nM) was present, in contrast to other species where both high-and low-affinity binding sites are present. Another feature is that the density of the binding sites in the cat bladder (Bmax = 21.2 pmol/mg protein) is considerably higher (about 2-fold) than the high-affinity binding component in the rat bladder membrane preparations. 3. Displacement experiments with unlabelled purinoceptor ligands indicate that [3H] alpha,beta-MeATP mainly binds to P2X-purinoceptors. The order of binding displacement activity was: alpha,beta-methylene ATP, beta,gamma-methylene ATP > 2-methylthioATP > ATP > suramin and L-beta,gamma-methylene ATP > > adenosine. 4. Autoradiographic study demonstrated dense specific binding sites of [3H] alpha,beta-MeATP on detrusor smooth muscle of cat bladder. 5. The results of this study are consistent with pharmacological studies for the existence of P2X-purinoceptors in cat bladder.