Isolated human bladder: evidence for an adenine dinucleotide acting on P2X-purinoceptors and for purinergic transmission

ATP as a cotransmitter in sympathetic and parasympathetic nerves - another Burnstock legacy

Geoff Burnstock created an outstanding scientific legacy that includes identification of adenosine 5'-triphosphate (ATP) as an inhibitory neurotransmitter in the gut, the discovery and characterisation of a large family of purine and uridine nucleotide-sensitive ionotropic P2X and metabotropic P2Y receptors and the demonstration that ATP is as an excitatory cotransmitter in autonomic nerves. The evidence for cotransmission includes that: 1) ATP is costored with noradrenaline in synaptic vesicles in postganglionic sympathetic nerves innervating smooth muscle tissues, including the vas deferens and most arteries. 2) When coreleased with noradrenaline, ATP acts at postjunctional P2X1 receptors to elicit depolarisation, Ca²⁺ influx, Ca²⁺ sensitisation and contraction. 3) ATP is also coreleased with acetylcholine from postganglionic parasympathetic nerves innervating the urinary bladder, where it stimulates postjunctional P2X1 receptors, and a second, as yet unidentified site to evoke contraction of detrusor smooth muscle. In both systems membrane-bound ecto-enzymes and soluble nucleotidases released from postganglionic nerves dephosphorylate ATP and so terminate its neurotransmitter actions. Currently, the most promising potential area of therapeutic application relating to cotransmission is treatment of dysfunctional urinary bladder. This family of disorders is associated with the appearance of a purinergic component of neurogenic contractions. This component is an attractive target for drug development and targeting it may be a rewarding area of research.

Purinergic signalling in the urinary bladder - When function becomes dysfunction

Knowledge of the participation of ATP and related purines in urinary tract physiology has been established over the last five decades through the work of many independent groups, inspired by, and building on the pioneering studies of Professor Geoffrey Burnstock and his coworkers. As part of a series of reviews in this tribute edition, the present article summarises our current understanding of purines and purinergic signalling in modulating and regulating urinary tract function. Purinergic mechanisms underlying the origin of bladder pain; sensations of bladder filling and urinary tract motility; and regulation of detrusor smooth muscle contraction are described, encompassing the relevant history of discovery and consolidation of knowledge as methodologies and pharmacological tools have developed. We consider normal physiology, including development and ageing and then move to pathophysiology, discussing the causal and consequential contribution of purinergic signalling mechanism and their constituent components (receptors, signal transduction, effector molecules) to bladder dysfunction.

Diadenosine tetraphosphate activates P2Y1 receptors that cause smooth muscle relaxation in the mouse colon

P2Y₁ receptors play an essential role in inhibitory neuromuscular transmission in the gastrointestinal tract. The signalling pathway involves the opening of small conductance calcium activated potassium-channels (K_ca2 family) that results in smooth muscle hyperpolarization and relaxation. Inorganic polyphosphates and dinucleotidic polyphosphates are putative neurotransmitters that potentially act on P2Y₁ receptors. A pharmacological approach using both orthosteric (MRS2500) and allosteric (BPTU) blockers of the P2Y₁ receptor and openers (CyPPA) and blockers (apamin) of K_ca2 channels was used to pharmacologically characterise the effect of these neurotransmitters. Organ bath and microelectrodes were used to evaluate the effect of P1,P4-Di (adenosine-5') tetraphosphate ammonium salt (Ap₄A), inorganic polyphosphates (PolyP) and CyPPA on spontaneous contractions and membrane potential of mouse colonic smooth muscle cells. PolyP neither modified contractions nor membrane potential. In contrast, Ap₄A caused a concentration-dependent inhibition of spontaneous contractions reaching a maximum effect at 100 μM Ap₄A response was antagonised by MRS2500 (1 μM), BPTU (3 μM) and apamin (1 μM). CyPPA (10 μM) inhibited spontaneous contractions and this response was antagonised by apamin but it was not affected by MRS2500 or BPTU. Both CyPPA and Ap₄A caused smooth muscle hyperpolarization that was blocked by apamin and MRS2500 respectively. We conclude that Ap₄A but not PolyP activates P2Y₁ receptors causing smooth muscle hyperpolarization and relaxation. Ap₄A signalling causes activation of K_ca2 channels through activation of P2Y₁ receptors. In contrast, CyPPA acts directly on K_ca2 channels. Further studies are needed to evaluate if dinucleotidic polyphosphates are released from inhibitory motor neurons.

ATP as a cotransmitter in the autonomic nervous system

The role of adenosine 5'-triphosphate (ATP) as a major intracellular energy source is well-established. In addition, ATP and related nucleotides have widespread extracellular actions via the ionotropic P2X (ligand-gated cation channels) and metabotropic P2Y (G protein-coupled) receptors. Numerous experimental techniques, including myography, electrophysiology and biochemical measurement of neurotransmitter release, have been used to show that ATP has several major roles as a neurotransmitter in peripheral nerves. When released from enteric nerves of the gastrointestinal tract it acts as an inhibitory neurotransmitter, mediating descending muscle relaxation during peristalsis. ATP is also an excitatory cotransmitter in autonomic nerves; 1) It is costored with noradrenaline in synaptic vesicles in postganglionic sympathetic nerves innervating smooth muscle preparations, such as the vas deferens and most arteries. When coreleased with noradrenaline, ATP acts at postjunctional P2X1 receptors to evoke depolarisation, Ca(2+) influx, Ca(2+) sensitisation and contraction. 2) ATP is also coreleased with acetylcholine from postganglionic parasympathetic nerves innervating the urinary bladder and again acts at postjunctional P2X1 receptors, and possibly also a P2X1+4 heteromer, to elicit smooth muscle contraction. In both cases the neurotransmitter actions of ATP are terminated by dephosphorylation by extracellular, membrane-bound enzymes and soluble nucleotidases released from postganglionic nerves. There are indications of an increased contribution of ATP to control of blood pressure in hypertension, but further research is needed to clarify this possibility. More promising is the upregulation of P2X receptors in dysfunctional bladder, including interstitial cystitis, idiopathic detrusor instability and overactive bladder syndrome. Consequently, these roles of ATP are of great therapeutic interest and are increasingly being targeted by pharmaceutical companies.

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.

Purinergic signalling in the lower urinary tract

The aim of this review is to describe the conceptual steps contributing to our current knowledge of purinergic signalling and to consider its involvement in the physiology and pathophysiology of the lower urinary tract. The voiding reflex involves ATP released as a cotransmitter with acetylcholine from parasympathetic nerves supplying the bladder and ATP released from urothelial cells during bladder distension to initiate the voiding reflex via P2X3 receptors on suburothelial low threshold sensory nerve fibres. This mechanosensory transduction pathway also participates, via high threshold sensory nerve fibres, in the initiation of pain in bladder and ureter. Treatment of prostate and bladder cancer with ATP is effective against the primary tumours in animal models and human cell lines, via P2X5 and P2X7 receptors, and also improves the systemic symptoms associated with advanced malignancy. Acupuncture is widely used for the treatment of urinary disorders, and a purinergic hypothesis is discussed for the underlying mechanism.

Therapeutic potential of purinergic signalling for diseases of the urinary tract

This review begins with background information about the discovery and conceptual steps contributing to our current knowledge of purinergic signalling. It then deals with several topics concerned with the physiology and pathophysiology of the lower urinary tract, including: the involvement in the voiding reflex of ATP released as a co-transmitter with acetylcholine from parasympathetic nerves supplying the bladder and ATP released from urothelial cells during bladder distension to initiate the voiding reflex via P2X₃ receptors on suburothelial low-threshold sensory nerve fibres; this latter mechanosensory transduction pathway is also involved via high-threshold fibres in the initiation of pain. Treatment of prostate and bladder cancer with ATP not only appears to be effective against the primary tumours, but also improves the systemic symptoms associated with advanced malignancy. There is dual control of the tone of blood vessels: constriction by ATP released as a co-transmitter from sympathetic nerves and vasodilatation via ATP released from endothelial cells during shear stress acting on endothelial P2 receptors to release nitric oxide. A purinergic hypothesis is discussed for the mechanism underlying acupuncture, widely used for the treatment of urinary disorders.

The role of the urothelium and ATP in mediating detrusor smooth muscle contractility

OBJECTIVES

To examine the contractility of urothelium-intact (+UE) and urothelium-denuded (-UE) rat detrusor strips under adenosine triphosphate (ATP) treatment. Purinergic signaling exists in the bladder but both the inhibitory effect of ATP on detrusor contractions and the function of urothelial ATP are not established.

METHODS

Detrusor strips were obtained from bladders of young adult rats. Isometric tension from both transverse and longitudinal contractions was measured using a myograph. The muscarinic agonist carbachol (CCh) was used to induce contractions, which were under the influences of different concentrations of ATP.

RESULTS

In both +UE and -UE strips, 1 mM ATP suppressed CCh-induced contractions. In longitudinal contractions, ATP added to the inhibitory effect of urothelium on CCh responses. Removal of the urothelium, but with exogenous ATP added, recovered the CCh responses to the same level as in +UE strips with no added ATP. Transverse contractions were less susceptible to ATP in the presence of urothelium.

CONCLUSIONS

We showed that the urothelium and ATP suppressed CCh-induced contractions to a similar extent. The findings suggest an inhibitory role of urothelial ATP in mediating detrusor smooth muscle contractility, which may be impaired in diseased bladders.

Spontaneous purinergic neurotransmission in the mouse urinary bladder

Spontaneous purinergic neurotransmission was characterized in the mouse urinary bladder, a model for the pathological or ageing human bladder. Intracellular electrophysiological recording from smooth muscle cells of the detrusor muscle revealed spontaneous depolarizations, distinguishable from spontaneous action potentials (sAPs) by their amplitude (< 40 mV) and insensitivity to the L-type Ca²⁺ channel blocker nifedipine (1 μm) (100 ± 29%). Spontaneous depolarizations were abolished by the P2X₁ receptor antagonist NF449 (10 μm) (frequency 8.5 ± 8.5% of controls), insensitive to the muscarinic acetylcholine receptor antagonist atropine (1 μm) (103.4 ± 3.0%), and became more frequent in latrotoxin (LTX; 1 nm) (438 ± 95%), suggesting that they are spontaneous excitatory junction potentials (sEJPs). Such sEJPs were correlated, in amplitude and timing, with focal Ca²⁺ transients in smooth muscle cells (measured using confocal microscopy), suggesting a common origin: ATP binding to P2X₁ receptors. sAPs were abolished by NF449, insensitive to atropine (126 ± 39%) and increased in frequency by LTX (930 ± 450%) suggesting a neurogenic, purinergic origin, in common with sEJPs. By comparing the kinetics of sAPs and sEJPs, we demonstrated that sAPs occur when sufficient cation influx through P2X₁ receptors triggers L-type Ca²⁺ channels; the first peak of the differentiated rising phase of depolarizations – attributed to the influx of cations through the P2X₁ receptor – is of larger amplitude for sAPs (2248 mV s⁻¹) than sEJPs (439 mV s⁻¹). Surprisingly, sAPs in the mouse urinary bladder, unlike those from other species, are triggered by stochastic ATP release from parasympathetic nerve terminals rather than being myogenic.

The elusive electromyogram in the overactive bladder: a spark of understanding

It has been said that a technique capable of recording a urinary bladder electromyogram could be useful in the clinical evaluation of the detrusor neuropathies and myopathies implicated in the generation of lower urinary tract symptoms. However, in contrast to electromyography of skeletal and cardiac muscle, detrusor smooth muscle electromyography has remained in its infancy despite 50 years of scientific effort. The principal problems appear to be isolation of the real signal from artefacts, and the doubtful existence of electromyographic activity during cholinergic muscle contraction. The discovery of purinergic neuromuscular transmission in the overactive human bladder has renewed interest in detrusor electromyography as, in contrast to cholinergic mechanisms, purinergic mechanisms can generate extracellular electrical activity. In this paper, the development and validation of a novel technique for recording electrical activity from neurologically intact guinea-pig and human detrusor in vitro is described. A purinergic electromyographic signal is characterised and it is shown that detrusor taken from overactive human bladders has a greater propensity to generate electromyographic activity than normal by virtue of an aberrant purinergic mechanism.

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.

Innervation of bladder and bowel

The autonomic neuromuscular junction is described and neurotransmission, co-transmission and neuromodulation are defined, as well as the 'chemical coding' of sympathetic, parasympathetic, sensory-motor and intrinsic neurons in the wall of the bladder and bowel. A detailed description of the patterns of innervation of smooth muscle of the bowel, bladder and urethra and of the urethral and anal sphincters by intramural and extrinsic autonomic nerves is presented, and the functional and pharmacological features of this innervation are summarized. Finally, changes in the pattern of innervation and expression of co-transmitters and receptors in the bladder and bowel that occur during development and old age and following trauma, surgery and disease are discussed.

Contractile activity of ATP and diadenosine tetraphosphate on urinary bladder in the rat: role of A1- and P2X-purinoceptors and nitric oxide

1. Both adenosine-5'-triphosphate (ATP) and diadenosine tetraphosphate (AP4A) produced a dose-dependent contraction of the isolated rat urinary bladder rings. AP(4)A dose-response curve was to the left of that of ATP, and maximum response was greater than that produced by ATP. 2. 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the A1-purinergic receptor blocker (0.01 mm) significantly inhibited the ATP- and AP4A-induced contractions at the whole dose range. The inhibition was between 31-41%, and 15-25% for ATP and AP4A respectively. 3. Pyridoxal phosphate 6-azophenyl-2',4'-disulphonic acid (PPADS), the P2X-purinoceptor antagonist (0.01 mm) potently inhibited the bladder contractions in response to ATP and AP4A by around 75-80%. 4. The nitric oxide (NO) precursor L-arginine reduced the bladder contractile response to ATP by about 22-41% and that of AP4A to a lesser extent by around 20-32%. 5. The nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 0.1 mM), did not produce any significant effect on ATP except for a weak inhibition of about 14% at the lowest dose of ATP. The contractions in response to AP4A were only slightly reduced by L-NAME by about 20%. 6. In conclusion, the contractile response of the bladder to ATP and to the dinucleotide AP4A is mediated mainly through P2X-purinoceptors and A1-purinergic receptors. In the detrusor muscle, NO donation possesses an inhibitory effect on ATP-mediated contractility more than that produced by the dinucleotide AP4A.

Age-related changes of P2X(1) receptor mRNA in the bladder detrusor from men with and without bladder outlet obstruction

The urinary bladder purinergic system is reported to change with age and with bladder dysfunction. Here, we examined the expression of purinergic P2X(1) receptors in detrusor and mucosa (urothelium+lamina propria) from male control bladder and investigated age-related P2X(1) receptor mRNA expression in control and obstructed detrusor. Biopsy specimens were obtained at cystoscopy from control patients (n=46, age range 30-86years) and patients diagnosed with outlet obstruction (n=29, 46-88years). Calponin expression (measured by RT-PCR) was similar in control and obstructed detrusor and did not change with age. Quantitative competitive RT-PCR was used to measure P2X(1) receptor and GAPDH mRNA in control and obstructed detrusor. P2X(1) receptor mRNA expression was 9-fold (p<0.0001) higher in the detrusor than in the mucosa. Expression of mRNA for the internal control GAPDH remained stable with age and across control and obstructed detrusor. No difference in P2X(1) receptor expression was observed between control and obstructed detrusor (p=0.35). However, an age-related decrease in P2X(1) mRNA expression was observed in control (n=27; p=0.0054; Spearman coefficient r=-0.520) but not obstructed detrusor (n=19; p=0.093; r=-0.396). Downregulation of P2X(1) mRNA expression might occur as a result of an increased component of neural ATP release in the aging bladder.

Nicotine potentiates the neurogenic contractile response of rabbit bladder tissue via nicotinic acetylcholine receptors: Nitric oxide and prostaglandins have no role in this process

Nicotine, a nicotinic acetylcholine receptors (nAChRs) agonist, has a role in modulation of the neurotransmitter release following nerve stimulation in both the central and peripheral nervous systems. The aim of this study was to determine whether electrical field stimulation (EFS)-evoked contractions are altered in rabbit bladder in the presence of nicotine and, if an alteration occurs, to investigate the effects of nitric oxide and prostaglandins on nicotine-induced alternation in isolated rabbit bladder. EFS-evoked contractile responses from rabbit bladder obtained were recorded with isometric force displacement transducers. Nicotine was added to preparations at various concentrations. The effects of hexamethonium, cadmium (Cd(2+)), indomethacin and N-nitro-L-arginine methyl ester (L-NAME) were tested on the EFS-evoked contractions in the presence of nicotine. Nicotine led to a dose-dependent increase in the amplitude of the EFS-evoked contractile responses. Cd(2+) and hexamethonium inhibited the nicotine-induced increase in EFS-evoked responses, whereas indomethacin and L-NAME had no effect. In conclusion, nicotine increased the EFS-evoked contractile responses possibly by facilitating release of neurotransmitters from nerve terminals by a mechanism dependent on the influx of Ca(2+) from voltage-gated Ca(2+) channels (VGCCs) via activation of nAChRs in isolated rabbit bladder. Nitric oxide and prostaglandins do not have a physiological role in the regulation of neurotransmitter release.

Identification of atropine- and P2X1 receptor antagonist-resistant, neurogenic contractions of the urinary bladder

Acetylcholine and ATP are excitatory cotransmitters in parasympathetic nerves. We used P2X1 receptor antagonists to further characterize the purinergic component of neurotransmission in isolated detrusor muscle of guinea pig urinary bladder. In the presence of atropine (1 microM) and prazosin (100 nM), pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (0.1-100 microM) and suramin (1-300 microM) inhibited contractions evoked by 4 Hz nerve stimulation in a concentration-dependent manner (IC50 of 6.9 and 13.4 microM, respectively). Maximum inhibition was 50-60%, which was unaffected by coadministration of the ectonucleotidase inhibitor ARL67156 (6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP) (100 microM). The remaining responses were abolished by tetrodotoxin (1 microM). PPADS and suramin also reduced contractions to exogenous ATP (300 microM) by 40-50%, but abolished those to the P2X1 agonist alpha,beta-methyleneATP (alpha,beta-meATP) (1 microM). The P2X1 antagonists reactive blue 2, NF279 (8,8'-[carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino)] bis-1,3,5-naphthalenetrisulfonic acid), MRS2159 (pyridoxal-alpha5-phosphate-6-phenylazo-4'-carboxylic acid) (100 microM), and NF449 [4,4',4,4-(carbonylbis(imino-5,1,3-benzenetriylbis(carbonylimino)))tetrakis-benzene-1,3-disulfonic acid] (3 microM) abolished contractions to alpha,beta-meATP (1 microM; n = 4-5), but only reduced contractions evoked by 4 Hz nerve stimulation by approximately 40-60% (n = 4-6) and ATP by 30-60% (n = 4-7). However, prolonged exposure to alpha,beta-meATP (50 microM) abolished contractions evoked by all three stimuli (n = 5-12). PPADS (100 microM) and suramin (300 microM) reduced the peak neurogenic contraction of the mouse urinary bladder to 30-40% of control. At the same concentrations, the P2X1 antagonists abolished the nonadrenergic, purinergic component of neurogenic contractions in the guinea pig vas deferens (n = 4-5). Thus, P2X1 receptor antagonists inhibit, but do not abolish, the noncholinergic component of neurogenic contractions of guinea pig and mouse urinary bladder, indicating a second mode of action of neuronally released ATP. This has important implications for treatment of dysfunctional urinary bladder, for which this atropine- and P2X1 antagonist-resistant site represents a novel therapeutic target.

Contractile activity of ATP and diadenosine tetraphosphate on urinary bladder in the rats: role of superoxide anion and urothelium

Both ATP and diadenosine tetraphosphate (AP(4)A) produced a dose-dependent contraction of rat isolated urinary bladder rings. The AP(4)A dose-response curve was to the left of that of ATP, and the maximum response was greater than that produced by ATP. Mechanical removal of the urothelium increased the contractile response to ATP by between 53% and 71%, and that to AP(4)A by 42% (at highest AP(4)A concentration) to 68% at lower concentration. Inhibition of Cu/Zn superoxide dismutase with diethylthiocarbamate (DETCA, 5 mm) significantly reduced the ATP-evoked contraction by 31% (at high ATP concentration) to 40% at low ATP concentration. Similarly, the AP(4)A-induced contractions were significantly decreased by 27% at low AP(4)A level to 38% at higher concentrations. Induction of exogenous superoxide anion stress by the use of the superoxide anion generator, pyrogallol (0.5 mm), significantly decreased both ATP- and AP(4)A-induced contractions of the rat urinary bladder over the whole dose range. Contractile responses to ATP decreased by 36-40%, and those to AP(4)A by 44-49%. In conclusion, the urinary bladder urothelium exerts an inhibitory control over the purinergic contractility produced by adenine mononucleotides and dinucleotides. Superoxide anion stress, whether endogenous or exogenous, attenuates the ATP-induced as well as AP(4)A-induced contractility.

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.

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

Urodynamic properties and neurotransmitter dependence of urinary bladder contractility in the BK channel deletion model of overactive bladder

Overactive bladder and incontinence are major medical issues, which lack effective therapy. Previously, we showed (Meredith AL, Thornloe KS, Werner ME, Nelson MT, and Aldrich RW. J Biol Chem 279: 36746-36752, 2004) that the gene mSlo1 encodes large-conductance Ca2+-activated K+ (BK) channels of urinary bladder smooth muscle (UBSM) and that ablation of mSlo1 leads to enhanced myogenic and nerve-mediated contractility and increased urination frequency. Here, we examine the in vivo urodynamic consequences and neurotransmitter dependence in the absence of the BK channel. The sensitivity of contractility to nerve stimulation was greatly enhanced in UBSM strips from Slo-/- mice. The stimulation frequency required to obtain a 50% maximal contraction was 8.3 +/- 0.9 and 19.1 +/- 1.8 Hz in Slo-/- and Slo+/+ mice, respectively. This enhancement is at least partially due to alterations in UBSM excitability, as muscarinic-induced Slo-/- contractility is elevated in the absence of neuronal activity. Muscarinic-induced Slo-/- contractility was mimicked by blocking BK channels with iberiotoxin (IBTX) in Slo+/+ strips, whereas IBTX had no effect on Slo-/- strips. IBTX also enhanced purinergic contractions of Slo+/+ UBSM but was without effect on purinergic contractions of Slo-/- strips. In vivo bladder pressure and urine output measurements (cystometry) were performed on conscious, freely moving mice. Slo-/- mice exhibited increased bladder pressures, pronounced pressure oscillations, and urine dripping. Our results indicate that the BK channel in UBSM has a very significant role in urinary function and dysfunction and as such likely represents an important therapeutic target.

Effect of neocuproine, a copper(i) chelator, on rat bladder function

The effects of a specific copper(I)-chelator, neocuproine (NC), and a selective copper(II)-chelator, cuprizone, on nonadrenergic-noncholinergic transmitter mechanisms in the rat urinary bladder were studied by measuring nerve-evoked contractions of bladder strips and voiding function under urethane anesthesia. After blocking cholinergic and adrenergic transmission with atropine and guanethidine, electrical field stimulation induced bimodal contractions of bladder strips. An initial, transient contraction that was blocked by the purinergic antagonist, suramin, was significantly enhanced by NC (20 and 200 microM applied sequentially) but not affected by cuprizone. The facilitating effect, which was blocked by suramin and reversible after washout of the drug, did not occur following administration of neocuproine-copper(I) complex (NC-Cu). NC (20 microM) significantly increased the second, more sustained contraction, whereas 200 microM decreased this response. These effects of NC on the sustained contractions were not elicited by NC-Cu and not blocked by suramin. The nitric oxide synthase inhibitor, l-nitroarginine, did not alter the responses to NC. NC (20 microM) elicited a marked increase in basal tone of the strips. This effect was less prominent after the second application of 200 microMNC or with NC-Cu treatment or in the presence of suramin. In anesthetized rats, during continuous infusion cystometry, intravesical infusion of 50 microM NC but not NC-Cu or cuprizone significantly decreased the intercontraction interval (ICI) without changing contraction amplitude. The ICI returned to normal after washout of NC. Suramin blocked this effect. These results indicate that NC enhances bladder activity by facilitating purinergic excitatory responses and that copper(I)-sensitive mechanisms tonically inhibit purinergic transmission in the bladder.

Pharmacology of the lower urinary tract: basis for current and future treatments of urinary incontinence

The lower urinary tract constitutes a functional unit controlled by a complex interplay between the central and peripheral nervous systems and local regulatory factors. In the adult, micturition is controlled by a spinobulbospinal reflex, which is under suprapontine control. Several central nervous system transmitters can modulate voiding, as well as, potentially, drugs affecting voiding; for example, noradrenaline, GABA, or dopamine receptors and mechanisms may be therapeutically useful. Peripherally, lower urinary tract function is dependent on the concerted action of the smooth and striated muscles of the urinary bladder, urethra, and periurethral region. Various neurotransmitters, including acetylcholine, noradrenaline, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in this neural regulation. Muscarinic receptors mediate normal bladder contraction as well as at least the main part of contraction in the overactive bladder. Disorders of micturition can roughly be classified as disturbances of storage or disturbances of emptying. Failure to store urine may lead to various forms of incontinence, the main forms of which are urge and stress incontinence. The etiology and pathophysiology of these disorders remain incompletely known, which is reflected in the fact that current drug treatment includes a relatively small number of more or less well-documented alternatives. Antimuscarinics are the main-stay of pharmacological treatment of the overactive bladder syndrome, which is characterized by urgency, frequency, and urge incontinence. Accepted drug treatments of stress incontinence are currently scarce, but new alternatives are emerging. New targets for control of micturition are being defined, but further research is needed to advance the pharmacological treatment of micturition disorders.

Transmitters contributing to the voiding contraction in female rats

OBJECTIVES

To assess in detail the contribution of acetylcholine and ATP to the different phases of the voiding contraction, urine flow and rhabdosphincter electromyographic (RB-EMG) activity in rats, using alpha,beta-methylene-ATP (desensitizing purinoceptors) and atropine (blocking muscarinic receptors). These agents and possibly other transmitters contribute to bladder emptying in rats, but how they contribute to the different phases of the micturition cycle, including the intraluminal pressure high-frequency oscillations (IPHFOs) is unclear.

MATERIALS AND METHODS

Adult anaesthetized female Sprague-Dawley rats were used; intravesical pressure, RB-EMG and urine flow from the distal urethra were recorded. After baseline recordings, alpha,beta-methylene-ATP (0.5 mg/kg), atropine (1 mg/kg), or both, were injected intravenously.

RESULTS

Alpha,beta-Methylene-ATP significantly decreased the maximum bladder pressure during the first micturition phase, whereas atropine had little effect; the maximum bladder pressure during the second phase was also reduced. IPHFOs were apparent after both treatments. Atropine significantly reduced the maximum bladder pressure during the third phase. The maximum urinary flow rate was reduced by both alpha,beta-methylene-ATP and atropine; after exposure to both agents together, urinary flow was markedly reduced or stopped, and overflow incontinence developed.

CONCLUSIONS

ATP contributes mainly to the initial and acetylcholine to the later phases of the voiding cycle in the rat. Neither agent abolished the IPHFOs; even after blocking the receptors for one transmitter and in the presence of IPHFOs, the bladder can still empty. However, if both receptors are blocked, overflow incontinence develops, suggesting that even if further transmitters are taking part in the voiding contraction, their physiological significance is questionable.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Comparative study of the contractile activity evoked by ATP and diadenosine tetraphosphate in isolated rat urinary bladder

The present study was designed to investigate the effect and possible mechanism(s) of action of ATP and diadenosine tetraphosphate (AP(4)A) on the isolated rat urinary bladder rings. ATP ( 0.1- 1 x 10(-3)M) or AP(4)A ( 0.01- 0.1 x 10(-3)M) produced contractions of the isolated bladder rings in a concentration-dependent manner. The contraction-induced by AP(4)A in the bladder rings was approximately ten times more potent than that produced by ATP. Addition of ATP prior to addition of AP(4)A or vice versa desensitized bladder tissue to the second agonist with great reduction in the contraction produced. Electrical field stimulation (EFS, 40 V, 0.5 ms, 2 Hz) produced contraction (79.8 +/-7.1 g tension x g(-1)tissue) in the bladder rings that can be greatly reduced by prior addition of ATP or AP(4)A. Theophylline, a P(1)-purinoceptor antagonist, significantly reduced the contraction-induced by AP(4)A and did altered that produced by ATP in bladder rings. Atropine, a non-selective muscarinic receptor antagonist, or indomethacin, a cyclo-oxygenase inhibitor, significantly suppressed the contractions of the bladder rings to ATP or AP(4)A. Similarly, nifedipine, an l -type Ca(2+)channel blocker, significantly attenuate the contractions induced by ATP and AP(4)A in the isolated rat urinary bladder rings. In conclusion, the results of the present study show that ATP, AP(4)A, and EFS evoked contractions in the rat urinary bladder rings and that the contractions induced by AP(4)A was more potent than that produced by ATP. Furthermore, the contractions evoked by ATP or AP(4)A were Ca(2+)-dependent and mediated at least in part through one of the cyclo-oxygenase products. Also, the present results suggested the involvement of the P(1)-purinoceptor in mediating the contractions evoked by AP(4)A but not ATP in the bladder rings.

Spontaneous phasic activity of the pig urinary bladder smooth muscle: characteristics and sensitivity to potassium channel modulators

A hallmark for unstable bladder contractions is hyperexcitability and changes in the nature of spontaneous phasic activity of the bladder smooth muscle. In this study, we have characterized the spontaneous activity of the urinary bladder smooth muscle from the pig, a widely used model for studying human bladder function. Our studies demonstrate that phasic activity of the pig detrusor is myogenic and is influenced by the presence of urothelium. Denuded strips exhibit robust spontaneous activity measured as mean area under the contraction curve (AUC=188.9+/-15.63 mNs) compared to intact strips (AUC=7.3+/-1.94 mNs). Spontaneous phasic activity, particularly the amplitude, is dependent on both calcium entry through voltage-dependent calcium channels and release from ryanodine receptors as shown by inhibition of spontaneous activity by nifedipine and ryanodine respectively. Inhibition of BK(Ca) channels by iberiotoxin (100 nM) resulted in an increase in contraction amplitude (89.1+/-20.4%) and frequency (92.5+/-31.0%). The SK(Ca) channel blocker apamin (100 nM) also increased contraction amplitude (69.1+/-24.3%) and frequency (53.5+/-13.6%) demonstrating that these mechanisms are critical to the regulation of phasic spontaneous activity. Inhibition of K(ATP) channels by glyburide (10 microM) did not significantly alter myogenic contractions (AUC=18.5+/-12.3%). However, K(ATP) channel openers (KCOs) showed an exquisite sensitivity for suppression of spontaneous myogenic activity. KCOs were generally 15 fold more potent in suppressing spontaneous activity compared to contractions evoked by electrical field-stimulation. These studies suggest that potassium channel modulation, particularly K(ATP) channels, may offer a unique mechanism for controlling spontaneous myogenic activity especially those associated with the hyperexcitability occurring in unstable bladders.

A robust method for evaluation of NANC transmission in human sigmoid colon muscle in vitro

INTRODUCTION

Human tissues are notoriously difficult to work with, giving results that are quantitatively variable within and between studies. Hence, previous investigations of nonadrenergic, noncholinergic (NANC) relaxation in human colon muscle report both partial and complete inhibitions of the NANC response by specific competitive inhibitors of nitric oxide (NO) production. We have established a robust and reproducible model to examine the contribution of NO during NANC relaxation assay in human sigmoid colon muscle strips.

METHODS

Complete control curves to long-train, stepwise, frequency-dependent, continuous electrical field stimulation (EFS) relaxation using vertical platinum electrodes connected to a biphasic pulse train stimulator generated NANC responses in fresh human sigmoid colon circular muscle strips set up in Bennett baths. A second complete curve was generated on the same strip in the presence of specific drugs to determine the contribution of NO to NANC relaxation. Responses to NO were also generated in muscle strips. Results were fitted to the Hill equation.

RESULTS

The first and second frequency-response curves without test drugs could be fitted to the Hill equation, resulting in similar midpoint locations ([f](50)), maximal asymptotes (alpha), and midpoint slope (n) parameters. L-N(G)-nitro-arginine (L-NOARG), TTX, and haemoglobin produced a tonic contraction in the muscle strips. NANC relaxations to EFS were inhibited by L-NOARG (30-37%), TTX (56-62%), and haemoglobin (48-90%). NO relaxations were concentration dependently inhibited by haemoglobin. Haemoglobin was equipotent in mediating tonic contraction and inhibiting NO relaxation.

DISCUSSION

We established reproducible assays for human colon muscle strips by the generation of two complete dose-response curves to long-train EFS, thus enabling a "within-preparations" study. The results suggest that NO contributes but is not the sole mediator of relaxations to long-train EFS in human sigmoid colon muscle. Moreover, a basal production of NO may serve to regulate tone of human colonic muscle.

The effect of nitric oxide on acetylcholine release in the rabbit bladder

We evaluated the effects of nitric oxide (NO) on acetylcholine release and the contractile response induced by electrical field stimulation in rabbit bladder smooth muscles using a muscle bath and high performance liquid chromatography coupled with microdialysis. Electrical field stimulation (supramaximum voltage, pulse duration 0.5 ms, frequency 5 and 20 Hz) was applied to a smooth muscle strip isolated from rabbit bladder. With low-frequency (5 Hz) stimulation, pretreatment with Nomega-nitro-L-arginine (L-NNA) (100 microM) significantly increased electrical field stimulation-induced acetylcholine release and contractile response, which were reduced by the addition of L-arginine. Pretreatment with sodium nitroprusside in the absence or presence of L-NNA significantly decreased electrical field stimulation-induced acetylcholine release and contractile response. In contrast, with high frequency (20 Hz) stimulation, pretreatment with L-NNA and sodium nitroprusside had no significant effect on either contractile response or acetylcholine release. Pretreatment with sodium nitroprusside caused no significant changes in carbachol and ATP-induced contractile responses. Sodium nitroprusside and L-NNA had no significant effects on the atropine-resistant part of the contraction induced by electrical field stimulation in rabbit bladder smooth muscles. The results suggest that there is a NO-mediated mechanism inhibiting acetylcholine release from cholinergic nerve endings in rabbit bladder, which may contribute to bladder function.

Age-related changes in cholinergic and purinergic neurotransmission in human isolated bladder smooth muscles

We evaluated the correlation among age, cholinergic and purinergic neurotransmissions in the electrical field stimulation-induced contractions in human isolated urinary bladder smooth muscles, using the muscle bath technique. Human bladder specimens were divided into three groups (G1, under 50years; G2, 51-70years; G3, over 70years old), and each muscle strip was suspended in a thermostatically controlled organ bath filled with oxygenated Krebs-Henseleit solution, connected to an isometric force displacement transducer, and an isometric tension development was recorded. The contractile responses induced by KCl, carbachol, adenosine triphosphate (ATP) and electrical field stimulation, and the effects of atropine and alpha, beta methylene ATP on electrical field stimulation-induced contractions were observed. The contractile response to KCl and concentration-response curves for carbachol and ATP, and frequency-response curves for electrical field stimulation were not significantly different among the three groups. The atropine sensitive and resistant parts of contraction induced by electrical field stimulation were decreased and increased with age, respectively. There are significant positive and negative correlations between age and the purinergic, and age and the cholinergic neurotransmissions in human isolated bladder smooth muscles, respectively. The age-related changes in neurotransmissions may contribute to the changes in bladder function in the elderly.

P2X receptor expression in mouse urinary bladder and the requirement of P2X(1) receptors for functional P2X receptor responses in the mouse urinary bladder smooth muscle

1. We have used subtype selective P2X receptor antibodies to determine the expression of P2X(1 - 7) receptor subunits in the mouse urinary bladder. In addition we have compared P2X receptor mediated responses in normal and P2X(1) receptor deficient mice to determine the contribution of the P2X(1) receptor to the mouse bladder smooth muscle P2X receptor phenotype. 2. P2X(1) receptor immunoreactivity was restricted to smooth muscle of the bladder and arteries and was predominantly associated with the extracellular membrane. Diffuse P2X(2) and P2X(4) receptor immunoreactivity not associated with the extracellular membrane was detected in the smooth muscle and epithelial layers. Immunoreactivity for the P2X(7) receptor was associated with the innermost epithelial layers and some diffuse staining was seen in the smooth muscle layer. P2X(3), P2X(5) and P2X(6) receptor immunoreactivity was not detected. 3. P2X receptor mediated inward currents and contractions were abolished in bladder smooth muscle from P2X(1) receptor deficient mice. In normal bladder nerve stimulation evoked contractions with P2X and muscarinic acetylcholine (mACh) receptor mediated components. In bladder from the P2X(1) receptor deficient mouse the contraction was mediated solely by mACh receptors. Contractions to carbachol were unaffected in P2X(1) receptor deficient mice demonstrating that there had been no compensatory effect on mACh receptors. 4. These results indicate that homomeric P2X(1) receptors underlie the bladder smooth muscle P2X receptor phenotype and suggest that mouse bladder from P2X(1) receptor deficient and normal animals may be models of human bladder function in normal and diseased states.

Pharmacological and molecular analysis of ATP-sensitive K(+) channels in the pig and human detrusor

The pharmacological and molecular properties of ATP-sensitive K(+) channels present in pig detrusor smooth muscle were investigated. In isolated pig detrusor strips, ATP-sensitive K(+) channel openers inhibited contractions elicited by low frequency field-stimulation in a concentration-dependent manner. The inhibitory effects of P1075 [N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine] were attenuated by glyburide with a pA(2) value of 7.38 (slope=1.08). The potency of the inhibitory effects of the K(+) channel openers on the field-stimulated contractions correlated well with those evoked by the muscarinic receptor agonist, carbachol (r=0.93) and furthermore, to relaxation of the pre-contracted (25 mM potassium chloride, KCl) human detrusor (r=0.95). Reverse transcriptase polymerase chain reaction (RT-PCR) analysis showed the presence of mRNA for sulfonylurea receptors SUR1 and SUR2B in both pig and human detrusor. Considering the similarities in the molecular and pharmacological profile of ATP-sensitive K(+) channels between the pig and the human detrusor, it is concluded that the pig detrusor may serve as a suitable in vitro model for the evaluation of novel K(+) channel openers with potential use in urological disorders in humans.

The effects of cyclophosphamide on neurotransmission in the urinary bladder of Suncus murinus, the house musk shrew

This study has shown that cyclophosphamide treatment of the insectivore Suncus murinus, causes a down regulation in both muscarinic and P2X receptors, together with a reduced responsiveness to exogenous histamine (0.3 mM) in the urinary bladder. Electrical field stimulation (70 V, 0.3 ms, 0.5-16 Hz, 10 s every 5 min) of bladders from both control and cyclophosphamide-treated animals showed identical responses. Since post-junctional alterations have been revealed by the reduced responsiveness to exogenous carbachol (0.1 microM-3 mM) and beta,gamma-methylene ATP (0.3-300 microM), it would appear that in the bladders of cyclophosphamide-treated animals there is also a pre-junctional effect, increased transmitter release compensating for the down regulation of the receptors. As the pattern of neurotransmission of the bladder of suncus more closely resembles that of human detrusor than other commonly studied laboratory animals, this insectivore appears to be a useful animal model for the study of bladder neurotransmission in pathophysiological conditions.

Nucleotide and dinucleotide effects on rates of paroxysmal depolarising bursts in rat hippocampus

Slices of rat hippocampus can be induces to generate spontaneous interictal-like bursts of action potentials when perfused with a with a medium containing no added magnesium and 4-aminopyridine (4AP). The frequency of these bursts is depressed by adenosine 5'triphosphate (ATP) and this effect can be prevented by cyclopentyltheophylline but not by adenosine deaminase. AMP (50 microM) had a similar action to reduce discharge rate. At 10 microM, adenosine, diadenosine tetraphosphate and diadenosine pentaphosphate all decreased the burst frequency. Adenosine deaminase (0.2 U ml-1) totally annulled the inhibition of epileptiform activity produced by 10 microM adenosine but reduced only the later components of the inhibition by 10 microM diadenosine tetraphosphate and diadenosine pentaphosphate. Cyclopentyltheophylline prevented the depression of burst discharges by diadenosine tetraphosphate. 5'-adenylic acid deaminase (AMPPase) did not significantly alter the discharge rate over the 10 min superfusion period used for drum application but did prevent the depressant effect of AMP and ATP. AMP deaminase did not prevent the inhibitory effects of diadenosine tetraphosphate. The results suggests that in the CA3 region of the hippocampus, diadenosine tertraphosphate and diadenosine pentaphosphate act partly by stimulating xanthine sensitive receptors directly and partly via metabolism to adenosine, and that AMP may be responsible for the inhibitory effects of ATP on epileptiform activity.

Inhibitory effects of propiverine, atropine and oxybutynin on bladder instability in rats with infravesical outlet obstruction

OBJECTIVE

To compare the inhibitory effects of propiverine HCl(BUP-4) with those of atropine and oxybutynin on the detrusor instability induced by partial obstruction of the bladder neck of female rats.

MATERIALS AND METHODS

Partial obstruction was created using partial ligation of the proximal urethra in 20 female Sprague-Dawley rats. Both the obstructed rats and a control group of 15 rats were evaluated cystometrically about 6 weeks later and the values compared both baseline and after injection with BUP-4, atropine or oxybutynin. During cystometry, the bladder capacity (BC), residual volume (RV), compliance and frequency of spontaneous activity (SA) were determined.

RESULTS

The BC, RV and frequency of SA were significantly increased, and compliance markedly decreased, in obstructed compared with normal rats. The micturition pressure was significantly decreased only after injection with BUP-4 in both normal and obstructed rats. For both, the BC was increased significantly after injection with atropine or BUP-4 (P < 0.05), with the increase greater after BUP-4 than after atropine in both groups (P < 0.01). After injecting BUP-4, the RV was significantly increased in both groups (P < 0.05); atropine increased the RV only in normal rats (P < 0.01) and oxybutynin had no significant effect on RV. Increases in compliance after the administration of each drug were significant only in obstructed rats (P < 0.01) and were markedly higher after oxybutynin (780%) than after the other drugs (180-250%). The frequencies and amplitude of SA after injection with each drug were significantly lower only in obstructed rats, but in these rats, there were no significant differences in this reduction after injecting oxybutynin or BUP-4, whereas they were significantly greater after injecting oxybutynin than after atropine.

CONCLUSION

Partial bladder outlet obstruction successfully created a hyperactive (unstable) bladder, typified by increased BC, RV, frequency of SA and a marked decrease in compliance. The greater BC, lower MP and frequency and amplitude of SA were prominent after the administration of BUP-4. Thus it is suggested that BUP-4 effectively inhibited bladder instability in rats induced by infravesical outlet obstruction and was more effective than oxybutynin in increasing BC.

The effects of adenine dinucleotides on epileptiform activity in the CA3 region of rat hippocampal slices

Alpha, omega-adenine dinucleotides (Ap(n)A) consist of two adenosine molecules linked at the 5' position by phosphate groups, the number of which is denoted by n and can range from 2 to 6. The aim of this study was to investigate the effect of Ap4A and Ap5A on the rate of epileptiform activity. Hippocampal slices (450 microm), when perfused with a medium containing no added magnesium and 4-aminopyridine (50 microM), generate epileptiform activity of an interictal nature. Ap4A and Ap5A at 1 microM depressed the discharge rate to a significant extent. At this concentration adenosine (1 microM) did not produce any effect. However at 10 microM adenosine, Ap4A and Ap5A all decreased the burst frequency. Adenosine deaminase (0.2 U/ml) totally annulled the inhibition of epileptiform activity produced by 10 microM adenosine or 1 microM Ap4A and Ap5A. Adenosine deaminase did not significantly change the maximum depression of activity produced by 10 microM Ap4A and Ap5A. 8-cyclopentyl-1,3-dimethylxanthine, an A1, receptor antagonist, increased the basal rate of epileptiform activity and prevented the depression of burst discharges by Ap4A. 5'-adenylic acid deaminase converts AMP into IMP which is inactive. 5'-adenylic acid deaminase did not prevent the inhibitory effects of Ap4A. The results suggests that in the CA3 region of the hippocampus, Ap4A and Ap5A act partly by stimulating xanthine-sensitive receptors directly and partly through the formation of the metabolite, adenosine.

Evidence for purinergic neurotransmission in the urinary bladder of pithed rats

The purpose of this study was to investigate the contribution of adenosine-5'-triphosphate (ATP) to segmental (L6-S2) spinal electrical stimulation evoked increases in intra-vesical pressure in pithed rats. Exogenous ATP and substance P produced dose-dependent increases in intra-vesical pressure (ED10 mmHg (dose required to elicit 10 mmHg increase in intra-vesical pressure)= 1.7 mg/kg and 1.1 microg/kg, i.v., respectively). Desensitisation (or antagonism) of P2x purinoceptors with alpha,beta-methylene ATP (alpha,beta-meATP; 30 microg/kg per min, i.v.) or pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 10 mg/kg, i.v.) significantly (p < 0.05) antagonized the intra-vesical pressure responses to ATP (> 8 and 3.6-fold increase in ED10 mmHg, respectively) but had no significant effect on intra-vesical pressure responses to substance P. Spinal stimulation evoked frequency-dependent increases in intra-vesical pressure (EF20 mmHg (frequency required to produce 20 mmHg increase in intra-vesical pressure) = 3.4 Hz). Blockade of muscarinic cholinoceptors and adrenoceptors with atropine (3 mg/kg, i.v.), propranolol (3 mg/kg, i.v.) and phentolamine (10 mg/kg, i.v.) produced marginal attenuation of the intra-vesical pressure responses to spinal stimulation indicating a major non-adrenergic non-cholinergic (NANC) component in the overall response. The NANC responses were significantly (p < 0.05) antagonized by alpha,beta-meATP (30 microg/kg per min, i.v.) and PPADS (10 mg/kg, i.v.) (> 2.6-fold increase in EF20 mmHg), consistent with involvement of a purinergic neurotransmitter, presumably ATP. Comparative studies in young (4-6 months) and old (21-23 months) Fischer rats revealed no age-dependent changes in the relative contribution of the cholinergic and purinergic systems, with the latter being the dominant one. These findings suggest that purinergic neurotransmission, presumably mediated by ATP acting via P2x purinoceptors, represents a major component of excitatory innervation to the urinary bladder in pithed rats.

Effect of exogenous adenosine and its monophosphate on the contractile response to acetylcholine in the human isolated detrusor muscle strips

1. Adenosine (0.1-1 mM) or its 5'-monophosphate (5'-AMP) induced a concentration-dependent relaxation of tension caused by acetylcholine (0.2 microM) in human urinary bladder detrusor strips. 2. This effect was antagonized concentration dependently by theophylline at an apparent pA2 value of about 5. 3. Maximum relaxation by adenosine or 5'-AMP never exceeded 50% and 80%, respectively, of acetylcholine-induced tension. Relaxation by some beta2-adrenoceptor agonists (0.1-0.2 mM) or norepinephrine was limited to about 50% of maximum. 4. The responses to adenosine and terbutaline were additive, causing full relaxation, and suggesting mobilization of distinct mechanisms underlying muscle relaxation.

Neuromuscular transmission and innervation in the urinary bladder of the insectivore Suncus murinus

In isolated preparations of the urinary bladder detrusor of the house musk shrew Suncus murinus (order: insectivora; family: Soricidae), electrical field stimulation (0.5-32 pulses/s) evoked neurogenic contractile responses that were markedly attenuated by atropine (1 microM). The non-cholinergic component was reduced but not abolished by the P2-purinoceptor antagonist, suramin (300 microM). Thus, neuromuscular transmission in the suncus urinary bladder is effected by cholinergic and purinergic nerves together with an as-yet unidentified component. Using immunohistochemical methods, the suncus urinary bladder was seen to be supplied by nerves containing neuropeptide Y, tyrosine hydroxylase, vasoactive intestinal polypeptide, galanin, substance P, calcitonin gene-related peptide and type I nitric oxide synthase. The pattern of responses to electrical field stimulation was more similar to that of humans and Old World primates, than to that of rodents or lagomorphs. The pattern of innervation of the bladder wall, in terms of the distribution of populations containing a given neuropeptide, was very similar to that in humans. Hence, Suncus murinus may provide a novel species for modelling the neuropharmacology of the human bladder, and also for studying the evolution of autonomic innervation.

Diadenosine polyphosphates inhibit adenosine kinase activity but decrease levels of endogenous adenosine in rat brain

Findings in peripheral tissues that diadenosine polyphosphates (Ap(n)As) activate 5'-nucleotidase activity and inhibit adenosine kinase activity in vitro led us to test the hypothesis that Ap(n)As and analogues thereof, through such actions on purine enzymes, increase brain levels of endogenous adenosine in vivo. Accordingly, we tested Ap(n)As for their effects on the in vitro activities of adenosine kinase, adenosine deaminase, AMP deaminase and 5'-nucleotidase and, following unilateral microinjections in rat striatum, on in vivo levels of endogenous adenosine. Adenosine kinase activity was not affected significantly by 5',5'''-P1,P2-diadenosine pyrophosphate (Ap2A) or by 5',5'''-P1,P3-diadenosine triphosphate (Ap3A), but was inhibited by 5',5'''-P1,P4-diadenosine tetraphosphate (Ap4A), 5',5'''-P1,P5-diadenosine pentaphosphate (Ap5A) and 5',5'''-P1,P6-diadenosine hexaphosphate (Ap6A); apparent IC50 values were 5.0, 3.3 and 500 microM, respectively. Inhibition of adenosine kinase activity by Ap4A and the four metabolically stable analogues of Ap4A tested was uncompetitive. Following unilateral intrastriatal injections, adenosine levels, relative to uninjected contralateral striatum, were decreased significantly (P < 0.05) by 48% with Ap4A and by 37% with AppCH2ppA, a metabolically stable analogue of Ap4A. Striatal levels of adenosine were not affected significantly by Ap5A or Ap6A. Cytosolic, but not particulate 5'-nucleotidase activity was inhibited and AMP deaminase activity was increased by some Ap(n)As. Although adenosine kinase inhibitors increase levels of endogenous adenosine and we showed here that Ap(n)As were potent inhibitors of this enzyme, these particular actions of Ap(n)As were not consistent with their effects on levels of endogenous adenosine.

Diinosine polyphosphates, a group of dinucleotides with antagonistic effects on diadenosine polyphosphate receptor

A new family of dinucleotide derivatives, diinosine polyphosphates, has been synthesized through the use of the enzyme 5' adenylic acid deaminase from Aspergillus sp., starting from the corresponding diadenosine polyphosphates. Functional studies were performed on rat brain synaptic terminals in which a dinucleotide receptor has been described that is specific for adenine dinucleotides. The results demonstrated that diinosine polyphosphates did not behave as agonists on the diadenosine polyphosphate receptor (also know as P4 purinoceptor), but they were very efficient as antagonists in abolishing the Ca2+ responses elicited by diadenosine pentaphosphate. The IC50 values for diinosine triphosphate, diinosine tetraphosphate, and diinosine pentaphosphate were 4.90 +/- 0.10 microM, 8.33 +/- 0.22 microM, and 4.23 +/- 0.12 nM, respectively. The diinosine polyphosphates also antagonized the ATP receptors present in synaptic terminals, showing IC50 values of 100.08 +/- 5.72 microM for diinosine triphosphate, 29.51 +/- 1.40 microM for diinosine tetraphosphate and 27.75 +/- 1.65 microM for diinosine pentaphosphate. The antagonistic ability of these diinosine nucleotides was studied in comparison with other P1 and P2 purinoceptor antagonists, such as suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, and 8-cyclopentyl-1,3-dipropylxanthine. These purinergic antagonists did not inhibit the response of the P4 purinoceptor; only the diinosine polyphosphates were able to act as antagonists on the dinucleotide receptor. Suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid attenuated the responses elicited by ATP, as did the diinosine polyphosphate compounds. The most antagonistic diinosine polyphosphate for the dinucleotide and ATP receptors was diinosine pentaphosphate, which was 6000 times more selective for the P4 purinoceptor than it was for the ATP receptor.

Vascular actions of diadenosine phosphates

1. Diadenosine phosphates were isolated from platelets, adrenal gland and autonomic nerves. The presence of diadenosine phosphates in storage pools releasable into the circulation suggests an important role in the control of blood pressure, and potentially to a modulation of the actions of catecholamines. 2. Besides a role of the diadenosine phosphates in platelet aggregation, these agents have potent vasoactive properties. Vasoactive actions of the diadenosine phosphates were demonstrated in numerous vascular models including most of the physiologically important elements of blood pressure regulation. Mostly, the vasoactive action depends on the number of phosphates in the diadenosine phosphates. Vasodilation can be observed in intact vessels after administration of Ap2A, Ap3A and Ap4A whereas contraction is affected by Ap5A and Ap5A and Ap6A. Vasocontraction induced by the diadenosine phosphates in vascular smooth muscle cells is mediated by an increase in intracellular free Ca2+. 3. In vivo, intravenous injection of Ap4A lowers blood pressure whereas injections of Ap5A and Ap6A caused a prolonged increase in blood pressure. In blood, in contrast to ATP, diadenosine phosphates are relatively long-lived molecules, suggesting that the action of the latter is of intermediate time span. In a similar manner to the vasoconstrictor angiotensin II, diadenosine phosphates also act as mitogens. It can be assumed that diadenosine phosphates may be involved in pathophysiological events of circulation including hypertension and atherosclerosis.

Negative chronotropic and inotropic effects exerted by diadenosine hexaphosphate (AP6A) via A1-adenosine receptors

1. Diadenosine hexaphosphate (AP6A) exerts vasoconstrictive effects. The purpose of this study was to investigate whether AP6A has any effect on cardiac function. 2. The effects of AP6A (0.1-100 microM) on cardiac contractility and frequency were studied in guinea-pig and human isolated cardiac preparations. Furthermore, the effects of AP6A on the amplitude of the L-type calcium current, on the adenosine 3':5'-cyclic monophosphate (cyclic AMP) content and on the phosphorylation of regulatory phosphoproteins, i.e. phospholamban and troponin inhibitor, were investigated in guinea-pig isolated ventricular myocytes. 3. In isolated spontaneously beating right atria of the guinea-pig AP6A exerted a negative chronotropic effect and reduced the rate of contraction maximally by 35% (IC20 = 35 microM). 4. In isolated electrically driven left atria of the guinea-pig AP6A exerted a negative inotropic effect and reduced force of contraction maximally by 23% (IC20 = 70 microM). 5. In isolated electrically driven papillary muscles of the guinea-pig AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction maximally by 23% (IC20 = 60 microM). Furthermore, AP6A attenuated the relaxant effect of isoprenaline. 6. In human isolated electrically driven ventricular preparations AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction by maximally 42% (IC20 = 18 microM). Moreover, AP6A attenuated the relaxant effect of isoprenaline. 7. All these effects of AP6A were abolished by the selective A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX, 0.3 microM), whereas the M-cholinoceptor antagonist atropine (10 microM) and the P2-purinoceptor antagonist suramin (300 microM) failed to abolish the effects of AP6A. 8. AP6A 100 microM had no effect on the amplitude of the L-type calcium current, but attenuated isoprenaline-stimulated L-type calcium current. The maximum of the current-voltage relationship (I-V curve) was shifted to the left by isoprenaline and additional application of AP6A shifted the I-V curve back to the right to the control value. The phosphorylation state of phospholamban and the troponin inhibitor was unchanged by AP6A alone, but was markedly attenuated by AP6A in the presence of isoprenaline. Cyclic AMP levels remained unchanged by AP6A, even after stimulation with isoprenaline. 9. In summary, AP6A exerts negative chronotropic and inotropic effects in guinea-pig and human cardiac preparations. These effects are mediated via A1-adenosine receptors as all effects were sensitive to the selective A1-adenosine receptor antagonist DPCPX. Furthermore, the effects of AP6A on cyclic AMP levels, protein phosphorylation and the L-type calcium current are in accordance with stimulation of A1-adenosine receptors.

The human P2x1 receptor: molecular cloning, tissue distribution, and localization to chromosome 17

A human urinary bladder cDNA library was screened with a rat P2x purinoceptor probe. A full-length cDNA was isolated, and like its rat homologue, the deduced protein consists of 399 amino acids (M(r) = 44980 Da), contains two hydrophobic, putative transmembrane, domains flanking a large presumed extracellular loop, and represents the P2x1 subtype of this multigene family (approximately 89% amino acid sequence identity to rat P2x1, but only 40-50% identity to rat P2x2-4). Expression of the P2x1 gene in human bladder was confirmed by Northern analysis, which demonstrated a major transcript of approximately 2.9 kb. Transcripts were also found in a variety of other tissues including adult peripheral leukocytes, pancreas, spleen, prostate, small intestine, colon, testis, and ovary, and in fetal liver. The gene encoding the human P2x1 receptor was localized to chromosome 17 by Southern analysis of DNAs isolated from a panel of somatic cell hybrids. The results support a role for P2x purinoceptors in the regulation of human bladder function.

Effects of PPADS and suramin on contractions and cytoplasmic Ca2+ changes evoked by AP4A, ATP and alpha, beta-methylene ATP in guinea-pig urinary bladder

1. The contraction and intracellular Ca2+ change evoked by diadenosine tetraphosphate (AP4A) were studied in the outer longitudinal muscle of the guinea-pig urinary bladder and compared with those evoked by ATP and alpha, beta-methylene ATP (a P2-purinoceptor agonist). 2. AP4A, ATP and alpha, beta-methylene ATP produced concentration-dependent transient contractions. These contractions were inhibited by PPADS (pyridoralphosphate-6-azophenyl- 2'-4'-disulphonic acid), 0.3- 30 microM, a P2x-purinoceptor antagonist, and suramin, 1-300 microM, a P2-purinoceptor antagonist in a concentration-dependent manner. From Schild plot analysis, the apparent pA2 values for PPADS for contractions evoked by AP4A, ATP and alpha, beta-methylene ATP were 6.86, 6.56, 6.74, and those for suramin were 6.01, 4.59 and 5.12, respectively; the Schild slopes for PPADS were 1.07, 1.14 and 1.06, and, those for suramin 0.75, 1.05 and 1.16, respectively. 3. AP4A (10 microM) and ATP (100 microM) failed to elicit any contraction of the tissue after a desensitization produced by repeated application of alpha, beta-methylene ATP (1 microM). 4. In fluorescence experiments with fura-2, the increases in [Ca2+]i and contraction evoked by AP4A were suppressed by suramin and nifedipine, an L-type Ca2+ channel blocker. 5. These findings suggest that P2x-purinoceptors, which are more sensitive to PPADS than suramin, exist on the outer longitudinal muscles of guinea-pig urinary bladder, and that the AP4A-evoked contraction results from Ca2+ influx.