Release of vasoactive substances from endothelial cells by shear stress and purinergic mechanosensory transduction

Effect of flow and stretch on the [Ca2+]i response of principal and intercalated cells in cortical collecting duct

An acute increase in tubular fluid flow rate in the microperfused cortical collecting duct (CCD), associated with a approximately 20% increase in tubular diameter, leads to an increase in intracellular Ca2+ concentration ([Ca2+]i)in both principal and intercalated cells (Woda CB, Leite M Jr, Rohatgi R, and Satlin LM. Am J Physiol Renal Physiol 283: F437-F446, 2002). The apical cilium present in principal but not intercalated cells has been proposed to be a flow sensor. To determine whether flow across the cilium and/or epithelial stretch mediates the [Ca2+]i response, CCDs from New Zealand White rabbits were microperfused in vitro, split-open (to isolate the effect of flow across cilia), or occluded (to examine the effect of stretch and duration/magnitude of the flow impulse), and [Ca2+]i was measured using fura 2. In perfused and occluded CCDs, a rapid (<1 s) but not slow (>3 min) increase in luminal flow rate and/or circumferential stretch led to an approximately threefold increase in [Ca2+]i in both principal and intercalated cells within approximately 10 s. This response was mediated by external Ca2+ entry and inositol 1,4,5-trisphosphate-mediated release of cell Ca2+ stores. In split-open CCDs, an increase in superfusate flow led to an approximately twofold increase in [Ca2+]i in both cell types within approximately 30 s. These experimental findings are interpreted using mathematical models to predict the fluid stress on the apical membranes of the CCD and the forces and torques on and deformation of the cilia. We conclude that rapid increases in luminal flow rate and circumferential stretch, leading to shear or hydrodynamic impulses at the cilium or apical membrane, lead to increases in [Ca2+]i in both principal and intercalated cells.

Regulation of phospholipase D by P2X7 receptors in submandibular ductal cells

ATP (1 mM) increased the phospholipase D (PLD) activity of rat submandibular gland (RSMG) ductal cells in a concentration-dependent and calcium-sensitive manner. The response to ATP was reproduced by benzoylbenzoyl-ATP (Bz-ATP, 100 microM) and also partly by adenosine 5'-(gamma-thio)triphosphate (ATPgammaS, 1 mM). A similar stimulation was observed in control mice (P2X7R+/+ mice) but not in mice lacking the P2X7 receptors (P2X7R-/- mice). Oxidized ATP and Coomassie blue or the addition of magnesium or nickel to the incubation medium inhibited the response to ATP. The stimulation of PLD by purinergic agonist was inhibited by about 50% by calphostin C and chelerythrine, two protein kinase C (PKC) inhibitors. The stimulation of PLD by Bz-ATP and by o-tetradecanoylphorbol 13-acetate (TPA), a phorbol ester which activates PKC, were not additive. From these results we can conclude that the activation of P2X7 receptors in RSMG ductal cells is coupled to the activation of a PLD. This activation is partly mediated by protein kinase C.

Unusual absence of endothelium-dependent or -independent vasodilatation to purines or pyrimidines in the rat renal artery

BACKGROUND

Adenosine triphosphate (ATP) is a cotransmitter with noradrenaline (NA) in sympathetic perivascular nerves. It has a dual role in the maintenance of vascular tone as ATP, released from endothelial cells during shear stress or hypoxia, induces vasodilatation via endothelial P2Y receptors or by direct action on smooth muscle. The role and distribution of P2 receptors is well characterized for many blood vessels but not for the rat renal artery. This study aims to determine whether ATP is a vasoconstrictor cotransmitter with NA and whether ATP induces vasodilatation via the endothelium or smooth muscle.

METHODS

On isolated rat renal arteries, electrical field stimulation (EFS) in the absence and presence of antagonists to P2X receptors and alpha1-adrenoceptors was examined. Concentration-response curves were constructed to NA, ATP, alpha,beta-methylene ATP (alpha,beta-meATP), uridine triphosphate (UTP), and 2-methylthio ADP (2-MeSADP) on low tone. Curves to acetylcholine (ACh), 2-MeSADP, and UTP were constructed on raised tone. Immunofluorescent localization of P2X and P2Y receptor subtypes was performed.

RESULTS

Electrical field stimulation induced vasoconstriction, partially inhibited by the P2X receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, and predominantly by prazosin. Exogenous NA and ATP mimicked EFS; immunostaining for P2X1 and P2X2 receptors was expressed on vascular smooth muscle. Unusually, ATP, 2-MeSADP, and UTP failed to induce vasodilatation. Acetylcholine induced vasodilatation. alpha,beta-meATP, 2-MeSADP, and UTP induced vasoconstriction via P2X1, P2Y1, and P2Y2 receptors, respectively. Immunostaining for P2X1, P2Y1, and P2Y2 receptors was expressed on the vascular smooth muscle.

CONCLUSION

Adenosine triphosphate and NA are cotransmitters in sympathetic nerves supplying the rat renal artery, NA being the dominant partner. The novel feature of this vessel is that purines and pyrimidines do not produce either endothelium-dependent or -independent vasodilatation; P2X1, P2Y1, and P2Y2 receptors on the smooth muscle all mediate vasoconstriction.

Eicosanoids in cirrhosis and portal hypertension

In the last decade, the knowledge of the pathogenesis of portal hypertension and cirrhosis has increased dramatically. In portal hypertension, almost all the known vasoactive systems/substances are activated or increased and the most recent studies have stressed the importance of the endothelial factors, in particular, prostaglandins. Prostaglandins are formed following the oxygenation of arachidonic acid by the cyclooxygenase (Cox) pathway. An important consideration in portal hypertension and cirrhosis in the periphery is the altered hemodynamic profile and its contributory role in controlling endothelial release of these vasoactive substances. Prostaglandins are released from the endothelium in response to both humoral and mechanical stimuli and can profoundly affect both intrahepatic and peripheral vascular resistance. Within the liver, intrahepatic resistance is altered due to a diminution in sinusoidal responsiveness to vasodilators and an increase in prostanoid vasoconstrictor responsiveness. This review will examine the contributory role of both hormonal and/or hemodynamic force-induced changes in prostaglandin production and signaling in cirrhosis and portal hypertension and the consequence of these changes on the structural and functional response of both the vasculature and the liver.

Extracellular ATP signaling in the rabbit lung: erythrocytes as determinants of vascular resistance

Previously, it was reported that red blood cells (RBCs) are required to demonstrate participation of nitric oxide (NO) in the regulation of rabbit pulmonary vascular resistance (PVR). RBCs do not synthesize NO; hence, we postulated that ATP, present in millimolar amounts in RBCs, was the mediator, which evoked NO synthesis in the vascular endothelium. First, we found that deformation of RBCs, as occurs on passage across the pulmonary circulation with increasing flow rate, evoked increments in ATP release. Here, ATP (300 nM), administered to isolated, salt solution-perfused (PSS) rabbit lungs, decreased total and upstream (arterial) PVR, a response inhibited by NG-nitro-L-arginine methyl ester (L-NAME, 100 microM). In lungs perfused with PSS containing RBCs, L-NAME increased total and upstream PVR. In lungs perfused with PSS containing glibenclamide-treated RBCs, which inhibits ATP release, L-NAME was without effect. Apyrase grade VII (8 U/ml), which degrades ATP to AMP, was without effect on PVR in PSS-perfused lungs. These results are consistent with the hypothesis that ATP, released from RBCs as they traverse the pulmonary circulation, evokes endogenous NO synthesis.

Increased mitogenic and decreased contractile P2 receptors in smooth muscle cells by shear stress in human vessels with intact endothelium

OBJECTIVE

We investigated the role of shear stress in regulating P2 receptors in human umbilical vein.

METHODS AND RESULTS

Using a novel, computerized, biomechanical perfusion model, parallel vessel segments were randomized to simultaneous perfusion under high (25 dyn/cm2) or low (<4 dyn/cm2) shear stress at identical mean perfusion pressure (20 mm Hg) for 6 hours. In the endothelium, no significant P2 receptor mRNA differences were found. In smooth muscle cells (SMCs), high shear stress decreased P2X1 receptors, whereas P2Y2 and P2Y6 receptors were upregulated. These findings were consistent at the mRNA level (real-time reverse transcription-polymerase chain reaction), protein level (Western blot), and morphologically (immunohistochemistry). The changes were more pronounced in the subintimal layer of the media.

CONCLUSIONS

Our findings suggest that shear stress might regulate gene expression in SMCs more than in the endothelium in intact vessels. Decreased expression of the contractile P2X1 receptor could lead to reduced vascular tonus and increased blood flow. Because P2Y2 and P2Y6 receptors stimulate growth and migration of SMCs, increased expression of these receptors could promote vascular remodeling induced by shear stress. The pattern of upregulation of mitogenic P2Y receptors and downregulation of contractile P2X1 receptor is similar to changes seen in the phenotypic shift from contractile to synthetic SMCs.

Release and interconversion of P2 receptor agonists by human osteoblast-like cells

Nucleotides, acting as agonists at P2 receptors, are important extracellular signaling molecules in many tissues. In bone they affect both bone-forming osteoblast and bone-resorbing osteoclast cell activity. The presence of nucleotides in the extracellular microenvironment is largely determined by their release from cells and metabolism by ecto-enzymes, both of which have scarcely been studied in bone. We have investigated adenosine 5'-triphosphate (ATP) release from SaOS-2 osteoblastic cells and the activities of cell surface ecto-enzymes on ATP metabolism. ATP, but not LDH, was detected in SaOS-2 cell conditioned medium, suggesting these cells were actively releasing ATP. Introduction of ADP resulted in increased ATP concentrations in the medium, which was found not to be receptor mediated. Nucleotide inhibition and substrate specificity studies revealed an ecto-nucleoside diphosphokinase (ecto-NDPK) was responsible for the ADP-->ATP conversion; PCR and immunocytochemistry confirmed its presence. Analysis of ATP metabolism over time demonstrated overall ATP degradation was increased by inhibiting ecto-NDPK activity; confirming that the combined action of multiple osteoblast-expressed ecto-enzymes affected extracellular nucleotide concentration. The data establish the coexistence of ATP-consuming, and for the first time, ATP-generating activities on the osteoblast cell surface, the discovery of which has significant implications for studies involving P2 receptor subtypes in bone.

Pharmacology of vagal afferent nerve activity in guinea pig airways

The excitability and activity of vagal afferent nerves innervating the airways can be pharmacologically increased and decreased. Autacoids released as a result of airway inflammation can lead to substantial increases in afferent nerve activity, consequently altering pulmonary reflex physiology. In a manner analogous to hyperalgesia associated with inflammation in the somato-sensory system, increases in vagal afferent nerve activity in inflamed airways may lead to a heightened cough reflex, and increases in autonomic activity in the airways. These effects may contribute to many of the symptoms of inflammatory airway disease. Here we provide a brief overview of some of the mechanisms by which the afferent activity in airway nerves can be pharmacologically modified.

Recent developments in the management of detrusor overactivity

PURPOSE OF REVIEW

Detrusor overactivity is a relatively common yet embarrassing symptom complex with significant impact on quality of life. The mainstay of current pharmacological treatment involves use of muscarinic receptor antagonists, but their therapeutic efficacy is limited by their troublesome side effects resulting in the non-continuance of treatment in a significant number of patients. Therefore, the development of new drugs can proceed by targeting alternative pathways affecting detrusor overactivity. In this article, the pharmacological basis for the current therapeutic alternatives for managing detrusor overactivity and possible future developments are discussed.

RECENT FINDINGS

It is clear that far from being a passive container for urine, the urothelium is a crucial part of the bladder. Its functions are complex, dynamic and important, and only now becoming understood. The release of ATP from urothelium in response to distension and its action on P2X receptors resulting in activating both motor and sensory neurons is being increasingly recognised. In the normal bladder, muscarinic receptor stimulation produces the main part of detrusor contraction. However, in functionally abnormal bladders, a non-cholinergic activation via the purinergic receptors may occur. The central nervous mechanisms controlling the micturition reflex have also recently attracted attention.

SUMMARY

Recent research has suggested that several transmitters may modulate voiding. However, few drugs with clinical benefits have been developed so far. Present treatments for overactive bladders have significant non-compliance rates. Hopefully, future research will lead to drugs with greater therapeutic benefits and better tolerance.

Potent P2Y6 receptor mediated contractions in human cerebral arteries

BACKGROUND

Extracellular nucleotides play an important role in the regulation of vascular tone and may be involved in cerebral vasospasm after subarachnoidal haemorrhage. This study was designed to characterise the contractile P2 receptors in endothelium-denuded human cerebral and omental arteries. The isometric tension of isolated vessel segments was recorded in vitro. P2 receptor mRNA expression was examined by RT-PCR.

RESULTS

In human cerebral arteries, the selective P2Y6 receptor agonist, UDPbetaS was the most potent of all the agonists tested (pEC50 = 6.8 PlusMinus; 0.7). The agonist potency; UDPbetaS > alphabeta-MeATP > UTPgammaS > ATPgammaS > ADPbetaS = 0, indicated the presence of contractile P2X1 P2Y2, P2Y4 and P2Y6, but not P2Y1 receptors, in human cerebral arteries. In human omental arteries, UDPbetaS was inactive. The agonist potency; alphabeta-MeATP > ATPgammaS = UTPgammaS > ADPbetaS = UDPbetaS = 0, indicated the presence of contractile P2X1, and P2Y2 receptors, but not P2Y1 or P2Y6 receptors, in human omental arteries. RT-PCR analysis of endothelium-denuded human cerebral and omental arteries demonstrated P2X1, P2Y1, P2Y2 and P2Y6 receptor mRNA expression. There were no bands for the P2Y4 receptor mRNA in the omental arteries, while barely detectable in the cerebral arteries.

CONCLUSIONS

P2Y6 receptors play a prominent role in mediating contraction of human cerebral arteries. Conversely, no such effect can be observed in human omental arteries and previous results confirm the absence of P2Y6 receptors in human coronary arteries. The P2Y6 receptor might be a suitable target for the treatment of cerebral vasospasm.

Overactive bladder--pharmacological aspects

The micturition reflex can be initiated by contraction or distension of detrusor smooth muscle cells, or by signals from the urothelium. It has been shown that bladder distension causes release of ATP from the urothelium, and that ATP can activate P2X3 receptors on suburothelial afferent nerve terminals to evoke a neural discharge. However, most probably the activation of afferent fibres during bladder filling involves not only ATP, but a cascade of inhibitory and stimulatory transmitters/mediators. These mechanisms may be targets for future drugs. Both in the normal and functionally disturbed bladder, muscarinic receptor stimulation produces the main part of detrusor contraction, but evidence is accumulating that in disease states, such as neurogenic bladders, outflow obstruction, idiopathic detrusor instability, interstitial cystitis, and also in the ageing bladder, a non-cholinergic activation via purinergic receptors may occur. If this component of activation is responsible not only for part of the bladder contractions, but also for the symptoms of the overactive bladder, it should be considered an important target for therapeutic interventions. Drags blocking different P2X receptor subtypes, or counteracting bladder contraction via other mechanisms, e.g. beta3-adrenoceptor stimulation, may be developed for treatment of the overactive bladder.

Dual sensory innervation of pulmonary neuroepithelial bodies

The characteristics of the different populations of sensory nerve terminals that selectively contact pulmonary neuroepithelial bodies (NEBs) in rat lungs were investigated after chemical denervation with capsaicin and compared with control lungs. Vagal calbindin D28k and P2X(3) purinoceptor immunoreactive (IR) afferent nerve terminals contacting NEBs appeared to have their origin in the nodose ganglion. Thick CB/P2X(3)-IR nerve fibers were seen to be myelinated and to lose their myelin sheaths just before branching and protruding intraepithelially between the NEB cells. This vagal sensory component of the innervation of NEBs was not affected by capsaicin nor expressed capsaicin receptors (vanilloid receptor subtype 1). A second sensory nerve fiber population that selectively innervates pulmonary NEBs in the rat lung consists of thin unmyelinated nonvagal substance P/calcitonin gene-related peptide IR nerve fibers, contacting mainly the basal pole of pulmonary NEBs, and having their origin in dorsal root ganglia. In concordance with vanilloid receptor 1 expression on these nerve terminals, the spinal sensory substance P/calcitionin gene-related peptide-IR component of the innervation of NEBs was depleted by systemic capsaicin treatment. The complex sensory innervation pattern of pulmonary NEBs characterized in the present study strongly suggests that, physiologically, pulmonary NEBs represent a group of intraepithelial receptors that may be able to accommodate various local and central reflex actions, in relation to both chemo- and mechanosensory stimuli.

Control of epithelial transport via luminal P2 receptors

P2 membrane receptors are specifically activated by extracellular nucleotides like ATP, ADP, UTP, and UDP. P2 receptors are subdivided into metabotropic P2Y and ionotropic P2X receptors. They are expressed in all tissues and induce a variety of biological effects. In epithelia, they are found in both the basolateral and the luminal membranes. Their widespread luminal expression in nearly all transporting epithelia and their effect on transport are summarized. The P2Y(2) receptor is a prominent luminal receptor in many epithelia. Other luminal P2 receptors include the P2X(7), P2Y(4), and P2Y(6) receptors. Functionally, luminal P2Y(2) receptor activation elicits differential effects on ion transport. In nearly all secretory epithelia, intracellular Ca(2+) concentration-activated ion conductances are stimulated by luminal nucleotides to induce Cl(-), K(+), or HCO(3)(-) secretion. This encompasses respiratory and various gastrointestinal epithelia or tissues like the conjunctiva of the eye and the epithelium of sweat glands. In the distal nephron, all active transport processes appear to be inhibited by luminal nucleotides. P2Y(2) receptors inhibit Ca(2+) and Na(+) absorption and K(+) secretion. Commonly, in all steroid-sensitive epithelia (lung, distal nephron, and distal colon), luminal ATP/UTP inhibits epithelial Na(+) channel-meditated Na(+) absorption. ATP is readily released from epithelial cells onto their luminal aspect, where ecto-nucleotidases promote their metabolism. Adenosine generated by the action of 5'-nucleotidase may elicit further effects on ion transport, often opposite those of ATP. ATP release from epithelia continues to be poorly understood. Integrated functional concepts for luminal P2 receptors are suggested: 1) luminal P2 receptors are part of an epithelial "secretory" defense mechanism; 2) they may be involved in the regulation of cell volume when transcellular solute transport is out of balance; 3) ATP and adenosine may be important autocrine/paracrine regulators mediating cellular protection and regeneration after ischemic cell damage; and 4) ATP and adenosine have been suggested to mediate renal cyst growth and enlargement in polycystic kidney disease.

Functional morphology of pulmonary neuroepithelial bodies: extremely complex airway receptors

Innervated groups of neuroendocrine cells, called neuroepithelial bodies (NEBs), are diffusely spread in the epithelium of intrapulmonary airways in many species. Our present understanding of the morphology of NEBs in mammalian lungs is comprehensive, but none of the proposed functional hypotheses have been proven conclusively. In recent reviews on airway innervation, NEBs have been added to the list of presumed physiological lung receptors. Microscopic data on the innervation of NEBs, however, have given rise to conflicting interpretations. Using neuronal tracing, denervation, and immunostaining, we recently demonstrated that the innervation of NEBs is much more complex than the almost unique vagal nodose sensory innervation suggested by other authors. The aim of the present work is to summarize our present understanding about the origin and chemical coding of the profuse nerve terminals that selectively contact pulmonary NEBs. A thorough knowledge of the complex interactions between the neuroendocrine cells and at least five different nerve fiber populations is essential for defining the position(s) of NEBs among the many pulmonary receptors characterized by lung physiologists.

Nitric oxide, cholesterol oxides and endothelium-dependent vasodilation in plasma of patients with essential hypertension

The objective of the present study was to identify disturbances of nitric oxide radical (.NO) metabolism and the formation of cholesterol oxidation products in human essential hypertension. The concentrations of.NO derivatives (nitrite, nitrate, S-nitrosothiols and nitrotyrosine), water and lipid-soluble antioxidants and cholesterol oxides were measured in plasma of 11 patients with mild essential hypertension (H: 57.8 +/- 9.7 years; blood pressure, 148.3 +/- 24.8/90.8 +/- 10.2 mmHg) and in 11 healthy subjects (N: 48.4 +/- 7.0 years; blood pressure, 119.4 +/- 9.4/75.0 +/- 8.0 mmHg). Nitrite, nitrate and S-nitrosothiols were measured by chemiluminescence and nitrotyrosine was determined by ELISA. Antioxidants were determined by reverse-phase HPLC and cholesterol oxides by gas chromatography. Hypertensive patients had reduced endothelium-dependent vasodilation in response to reactive hyperemia (H: 9.3 and N: 15.1% increase of diameter 90 s after hyperemia), and lower levels of ascorbate (H: 29.2 +/- 26.0, N: 54.2 +/- 24.9 micro M), urate (H: 108.5 +/- 18.9, N: 156.4 +/- 26.3 micro M), beta-carotene (H: 1.1 +/- 0.8, N: 2.5 +/- 1.2 nmol/mg cholesterol), and lycopene (H: 0.4 +/- 0.2, N: 0.7 +/- 0.2 nmol/mg cholesterol), in plasma, compared to normotensive subjects. The content of 7-ketocholesterol, 5alpha-cholestane-3beta,5,6beta-triol and 5,6alpha-epoxy-5alpha-cholestan-3alpha-ol in LDL, and the concentration of endothelin-1 (H: 0.9 +/- 0.2, N: 0.7 +/- 0.1 ng/ml) in plasma were increased in hypertensive patients. No differences were found for.NO derivatives between groups. These data suggest that an increase in cholesterol oxidation is associated with endothelium dysfunction in essential hypertension and oxidative stress, although.NO metabolite levels in plasma are not modified in the presence of elevated cholesterol oxides.

Pharmacologic perspective on the physiology of the lower urinary tract

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

Effects of dimethyl sulphoxide and heparin on stretch-activated ATP release by bladder urothelial cells from patients with interstitial cystitis

OBJECTIVE

To determine whether dimethyl sulphoxide (DMSO) and heparin reduce the greater stretch-activated ATP release in interstitial cystitis (IC), as ATP serves as a nocio-neurotransmitter in the bladder, and thus explain their beneficial effects in patients with IC (a disease characterized by hypersensory bladder symptoms).

MATERIALS AND METHODS

Bladder epithelia in IC release more ATP in response to stretch than do control samples. Both DMSO and heparin are used intravesically to treat IC; such agents can modulate urothelial function because they directly contact bladder urothelium. Biopsies taken from patients with IC and from control subjects were grown in primary cultures using established cell-culture techniques. Cultured urothelial cells were stretched with the Flexcell device (Flexcell International Corp., McKeesport, PA, USA) and supernatant ATP was measured, using a luciferin-luciferase assay. DMSO (0.1%, 0.5% and 1%) or heparin (50, 200, 800 and 1600 U) was added to the cells at the start of the stretch experiments and the ATP released into the supernatant measured. Cell viability was also determined using Trypan Blue staining.

RESULTS

IC cells released significantly more ATP in response to stretch than did control cells. This increased release of ATP by stretched IC cells was significantly blocked by adding DMSO or heparin at all concentrations used. Heparin appeared to have a greater dose-dependent effect on ATP release than did DMSO.

CONCLUSIONS

These findings are consistent with the hypothesis that the urothelium provides sensory input via ATP release and that this process is increased in IC. Furthermore, stretch-activated ATP release was blocked by adding DMSO and heparin, both intravesical agents commonly used to treat the symptoms of IC. This study supports the notion that purinergic-targeted therapy is warranted in treating IC. Further studies are needed to determine the mechanisms of increased ATP release by IC urothelial cells.

Intravesical adenosine triphosphate stimulates the micturition reflex in awake, freely moving rats

PURPOSE

Adenosine triphosphate (ATP) (Sigma Chemical Co., St. Louis, Missouri) is known to contract animal as well as human detrusor muscle and recent investigations have shown an involvement of ligand gated purinergic-1 receptors in detrusor contraction. In addition, ligand gated purinergic-3 receptors have been demonstrated on suburothelial sensory nerves (C-fibers) and may be involved in distention induced initiation of the micturition reflex. We tested the hypothesis that ATP given intravesically can stimulate afferent nerves and initiate the micturition reflex.

MATERIALS AND METHODS

Continuous cystometry was performed in conscious, freely moving, normal female Sprague-Dawley rats. Cystometric parameters were evaluated before and after drug administration.

RESULTS

Instilled intravesically ATP (10 mM.) induced bladder overactivity in 6 animals with a mean increase in voiding pressure plus or minus standard error of 73 +/- 9 to 107 +/- 9 cm. water (p <0.01), mean baseline pressure increase of 5.32 +/- 0.58 to 12.71 +/- 1.01 cm. water (p <0.01) and mean bladder capacity decrease of 1.13 +/- 0.25 to 0.75 +/- 021 ml. (p <0.01). Lower concentrations had no significant effect. The effects of ATP were abolished by pretreatment with the ganglion blocker hexamethonium (40 mg./kg. ), nitric oxide synthase substrate L-arginine (Sigma Chemical Co.) (200 mg./kg. ) and neurokinin-2 receptor antagonist 123 (S)-N-methyl-N 123 4-(acetylamino-4-phenyl piperidone)-2-(3,4-dichlorophenyl) butyl 125 benzamide (Molecular Probes, Leiden, The Netherlands) (4 nmol.) given intravenously, the ligand gated purinergic-3 antagonist 2'-(or 3')-O-(trinitrophyl)adenosine 5'-triphosphate (50 microM./kg.) given intravenously and the k channel opener ZD6169 given intravesically.(ATP).

CONCLUSIONS

ATP given intravesically can induce bladder overactivity, probably by stimulating suburothelial C-fibers. The data suggest that several mediators and mechanisms are involved in mechano-afferent transduction in the bladder.

External GTP binding and induction of cell division in starved Tetrahymena thermophila

The extracellular nucleotide, guanosine 5'-triphosphate (GTP) is known to be a chemorepellent for ciliated protozoa such as Paramecium and Tetrahymena. Here, we studied the surface localization of GTP binding sites and also its effects on the cell division of Tetrahymena thermophila. When a ribose-modified and fluorescent analog of GTP, 2'-(or -3')-O-trinitrophenyl (TNP)-GTP was added to the cells starved in non-nutrient buffer, a remarkable fluorescence was observed at the compound cilia of the oral area, while it was weak at other cilia and the somatic membrane. Following transfer of the cells to the starvation medium, the intensity of TNP-GTP fluorescence at the oral area gradually increased and was saturated at 3-4 hours. Addition of GTP to the starved cell induced not only an avoiding reaction in swimming, but also induced a synchronous cell division that occurred 2 hours later. An attempt to search for other stimuli, which induced cell division, revealed that mechanical stimulation by a short period of centrifugation was almost as effective as the addition of GTP. The supernatant after centrifugation had the ability to induce cell division, and such activity was abolished after the supernatant was treated with the phosphatase, apyrase, suggesting the release of GTP by the mechanical stimulation. These results indicate that the released GTP binds mainly to the oral area and this then induces the cell division of starved T. thermophila.

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.

Regulation of an ATP-conductive large-conductance anion channel and swelling-induced ATP release by arachidonic acid

Mouse mammary C127 cells responded to hypotonic stimulation with activation of the volume-dependent ATP-conductive large conductance (VDACL) anion channel and massive release of ATP. Arachidonic acid downregulated both VDACL currents and swelling-induced ATP release in the physiological concentration range with K(d) of 4- 6 microM. The former effect observed in the whole-cell or excised patch mode was more prominent than the latter effect observed in intact cells. The arachidonate effects were direct and not mediated by downstream metabolic products, as evidenced by their insensitivity to inhibitors of arachidonate-metabolizing oxygenases, and by the observation that they were mimicked by cis-unsaturated fatty acids, which are not substrates for oxygenases. A membrane-impermeable analogue, arachidonyl coenzyme A was effective only from the cytosolic side of membrane patches suggesting that the binding site is localized intracellularly. Non-charged arachidonate analogues as well as trans-unsaturated and saturated fatty acids had no effect on VDACL currents and ATP release, indicating the importance of arachidonate's negative charge and specific hydrocarbon chain conformation in the inhibitory effect. VDACL anion channels were inhibited by arachidonic acid in two different ways: channel shutdown (K(d) of 4- 5 microM) and reduced unitary conductance (K(d) of 13-14 microM) without affecting voltage dependence of open probability. ATP(4-)-conducting inward currents measured in the presence of 100 mM ATP in the bath were reversibly inhibited by arachidonic acid. Thus, we conclude that swelling-induced ATP release and its putative pathway, the VDACL anion channel, are under a negative control by intracellular arachidonic acid signalling in mammary C127 cells.

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

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

ATP release in human kidney cortex and its mitogenic effects in visceral glomerular epithelial cells

BACKGROUND

In chronic renal failure the sympathetic nervous system is activated. Sympathetic cotransmitters released within the kidney may contribute to the progression of renal disease through receptor-mediated proliferative mechanisms.

METHODS

In human renal cortex electrical stimulation induced adenosine 5'-triphosphate (ATP; luciferin-luciferase-assay) and norepinephrine (HPLC) release was measured. ATP release also was induced by alpha1- and alpha2-adrenergic agonists. [3H]-thymidine uptake was tested in human visceral glomerular epithelial cells (vGEC) and mitogen-activated protein kinase (MAPK42/44) activation in vGEC and kidney cortex. The involved P2-receptors were characterized pharmacologically and by RT-PCR.

RESULTS

Sympathetic nerve stimulation and alpha-adrenergic agonists induced release of ATP from human kidney cortex. Seventy-five percent of the ATP released originated from non-neuronal sources, mainly through activation of alpha2-adrenergic receptors. ATP (1 to 100 micromol/L) and related nucleotides (1 to 100 micromol/L) increased [3H]-thymidine uptake. The adenine nucleotides ATP, ATPgammaS, ADP and ADPbetaS were about equally potent. UTP, UDP and alpha,beta-methylene ATP had no effect. ATP, ADPbetaS but not alpha,beta-methylene ATP activated MAPK42/44. ATP induced MAPK42/44 activation, and [3H]-thymidine uptake was abolished in the presence of the MAPK inhibitor PD 98059 (100 micromol/L). mRNA for P2X4,5,6,7 and P2Y1,2,4,6,11 were detected in human vGEC by RT-PCR.

CONCLUSIONS

In human renal cortex, adrenergic stimulation releases ATP from neuronal and non-neuronal sources. ATP has mitogenic effects in vGEC and therefore the potential to contribute to progression in chronic renal disease. The pattern of purinoceptor agonist effects on DNA synthesis together with the mRNA expression suggests a major contribution of a P2Y1-like receptor.

Reciprocal regulation of endothelin-1 and nitric oxide: relevance in the physiology and pathology of the cardiovascular system

The endothelium plays a crucial role in the regulation of cardiovascular structure and function by releasing several mediators in response to biochemical and physical stimuli. These mediators are grouped into two classes: (1) endothelium-derived constricting factors (EDCFs) and (2) endothelium-derived relaxing factors (EDRFs), the roles of which are considered to be detrimental and beneficial, respectively. Endothelin-1 (ET-1) and nitric oxide (NO) are the prototypes of EDCFs and EDRFs, respectively, and their effects on the cardiovascular system have been studied in depth. Numerous conditions characterized by an impaired availability of NO have been found to be associated with enhanced synthesis of ET-1, and vice versa, thereby suggesting that these two factors have a reciprocal regulation. Experimental studies have provided evidence that ET-1 may exert a bidirectional effect by either enhancing NO production via ETB receptors located in endothelial cells or blunting it via ETA receptors prevalently located in the vascular smooth muscle cells. Conversely, NO was found to inhibit ET-1 synthesis in different cell types. In vitro and in vivo studies have started to unravel the molecular mechanisms involved in this complex interaction. It has been clarified that several factors affect in opposite directions the transcription of preproET-1 and NO-synthase genes, nuclear factor-KB and peroxisome proliferator-activated receptors playing a key role in these regulatory mechanisms. ET-1 and NO interplay seems to have a great relevance in the physiological regulation of vascular tone and blood pressure, as well as in vascular remodeling. Moreover, an imbalance between ET-1 and NO systems may underly the mechanisms involved in the pathogenesis of systemic and pulmonary hypertension and atherosclerosis.

Purinergic signaling and vascular cell proliferation and death

Evidence for the role of purinergic signaling (via P1 and P2Y receptors) in the proliferation of vascular smooth muscle and endothelial cells is reviewed. The involvement of the mitogen-activated protein kinase second-messenger cascade in this action is clearly implicated, although details of the precise intracellular pathways involved still remain to be determined. Synergistic actions of purines and pyrimidines with growth factors occur in promoting cell proliferation. Interaction between purinergic signaling for vascular cell proliferation and cell death mediated by P2X7 receptors is discussed. There is evidence of the release of ATP from endothelial cells, platelets, and sympathetic nerves as well as from damaged cells in atherosclerosis, hypertension, restenosis, and ischemia; furthermore, there is evidence that vascular smooth muscle and endothelial cells proliferate in these pathological conditions. Thus, the involvement of ATP and its breakdown product, adenosine, is implicated; it is hoped that with the development of selective P1 (A2) and P2Y receptor agonists and antagonists, new therapeutic strategies will be explored.

ATP is released from guinea pig ureter epithelium on distension

Distension of the perfused guinea pig ureter at pressures from 20 to 700 cmH(2)O increased the amount of ATP released from the epithelium in a pressure-dependent manner. During basal perfusion (40 microl/min), the perfusate contained 10 pmol/ml ATP; this increased 10- to 50-fold at various distending pressures. ATP was released from epithelial cells during distension as mechanical removal of the urothelium blocked release. No lactate dehydrogenase was detected in the perfusate, and scanning electron microscopy confirmed an intact urothelium after distension. ATP was not released due to the activation of stretch-activated channels, as gadolinium (10 microM) failed to affect ATP release. Glibenclamide (10 microM), known to inhibit two members of the ATP-binding cassette (ABC) protein family, did not affect ATP release after distension; nor did verapamil (10 microM). In contrast, both monensin (100 microM) and brefeldin A (10 microM), which interfere with vesicular formation or trafficking, inhibited distension-evoked ATP release, which was Ca(2+)-dependent. This suggests that ATP release from the ureter epithelium might be mediated by vesicular exocytosis. The role of ATP released by distension of hollow visceral organs is discussed in relation to the concept of purinergic mechanosensory transductions, with special reference to nociception and the activation of P2X(3) receptors on the subepithelial sensory nerves.

P2 receptors: new potential players in atherosclerosis

Atherosclerosis is a focal inflammatory disease of the arterial wall. It starts with the formation of fatty streaks on the arterial wall that evolve to form a raised plaque made of smooth muscle cells (SMCs), and infiltrating leukocytes surrounding a necrotic core. The pathogenesis of the atherosclerotic lesion is incompletely understood, but it is clear that a dysfunction of the endothelium, recruitment and activation of inflammatory cells and SMC proliferation have a pivotal role. Over recent years receptors for extracellular nucleotides, the P2 receptors, have been recognized as fundamental modulators of leukocytes, platelets, SMCs and endothelial cells. P2 receptors mediate chemotaxis, cytokine secretion, NO generation, platelet aggregation and cell proliferation in response to accumulation of nucleotides into the extracellular milieu. Clinical trials have shown the benefit of antagonists of the ADP platelet receptor(s) in the prevention of vascular accidents in patients with atherosclerosis. Therefore, we anticipate that a deeper understanding of the involvement of P2 receptors in atheroma formation will open new avenues for drug design and therapeutic intervention.

Activity-dependent development of P2X7 current and Ca2+ entry in rabbit osteoclasts

Bone remodeling is regulated by local factors and modulated by mechanical stimuli. Mechanical stimulation can cause release of ATP, an agent that stimulates osteoclastic resorption at low concentrations and inhibits at high concentrations. We examined whether osteoclasts express P2X(7) receptors, which are activated by high concentrations of ATP and can behave as ion channels or cause the formation of membrane pores. Rabbit osteoclasts were studied using patch clamp techniques. Successive or prolonged applications of 2'- & 3'-O-(4-benzoylbenzoyl)-ATP (BzATP, a relatively potent P2X(7) agonist) or high concentrations of ATP caused the development of a slowly deactivating inward current. The underlying channel was permeable only to small cations, ruling out pore formation. Divalent cations reduced current magnitude, consistent with the presence of P2X(7) receptors, a finding confirmed in rat osteoclasts by immunocytochemistry. Successive applications of BzATP also elicited [Ca(2+)](i) elevations that required extracellular Ca(2+). The BzATP-induced current and the rise of [Ca(2+)](i) were temporally associated, and both were inhibited by PPADS, a P2X(7) antagonist. This study demonstrates that high concentrations of ATP activate P2X(7) receptors and provides the first functional evidence for an extracellular ligand-gated Ca(2+) influx pathway in osteoclasts. ATP released in response to mechanical stimuli may act through P2X(7) receptors to inhibit osteoclastic resorption.

Purinergic signalling: ATP release

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

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

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

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

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

Inhibition of mechanical activation of guinea-pig airway afferent neurons by amiloride analogues

1. The aim of this study was to investigate a role for Epithelial Sodium Channels (ENaCs) in the mechanical activation of low-threshold vagal afferent nerve terminals in the guinea-pig trachea/bronchus. 2. Using extracellular single-unit recording techniques, we found that the ENaC blocker amiloride, and its analogues dimethylamiloride and benzamil caused a reduction in the mechanical activation of guinea-pig airway afferent fibres. 3. Amiloride and it analogues also reduced the sensitivity of afferent fibres to electrical stimulation such that greater stimulation voltages were required to induce action potentials from their peripheral terminals within the trachea/bronchus. 4. The relative potencies of these compounds for inhibiting electrical excitability of afferent nerves were similar to that observed for inhibition of mechanical stimulation (dimethylamiloride approximately benzamil > amiloride). This rank order of potency is incompatible with the known rank order of potency for blockade of ENaCs (benzamil > amiloride >> dimethylamiloride). 5. As voltage-gated sodium channels play an important role in determining the electrical excitability of neurons, we used whole-cell patch recordings of nodose neuron cell bodies to investigate the possibility that amiloride analogues caused blockade of these channels. At the concentration required to inhibit mechanical activation of vagal nodose afferent fibres (100 microM), benzamil caused significant inhibition of voltage-gated sodium currents in neuronal cell bodies acutely isolated from guinea-pig nodose ganglia. 6. Combined, our findings suggest that amiloride and its analogues did not selectively block mechanotransduction in airway afferent neurons, but rather they reduced neuronal excitability, possibly by inhibiting voltage-gated sodium currents.

Nitric oxide modulates Ca(2+) channels in dorsal root ganglion neurons innervating rat urinary bladder

The effect of a nitric oxide (NO) donor on high-voltage-activated Ca(2+) channel currents (I(Ca)) was examined using the whole cell patch-clamp technique in L(6)-S(1) dorsal root ganglion (DRG) neurons innervating the urinary bladder. The neurons were labeled by axonal transport of a fluorescent dye, Fast Blue, injected into the bladder wall. Approximately 70% of bladder afferent neurons exhibited tetrodotoxin (TTX)-resistant action potentials (APs), and 93% of these neurons were sensitive to capsaicin, while the remaining neurons had TTX-sensitive spikes and were insensitive to capsaicin. The peak current density of nimodipine-sensitive L-type Ca(2+) channels activated by depolarizing pulses (0 mV) from a holding potential of -60 mV was greater in bladder afferent neurons with TTX-resistant APs (39.2 pA/pF) than in bladder afferent neurons with TTX-sensitive APs (28.9 pA/pF), while the current density of omega-conotoxin GVIA-sensitive N-type Ca(2+) channels was similar (43-45 pA/pF) in both types of neurons. In both types of neurons, the NO donor, S-nitroso-N-acetylpenicillamine (SNAP) (500 microM), reversibly reduced (23.4-26.6%) the amplitude of I(Ca) elicited by depolarizing pulses to 0 mV from a holding potential of -60 mV. SNAP-induced inhibition of I(Ca) was reduced by 90% in the presence of omega-conotoxin GVIA but was unaffected in the presence of nimodipine, indicating that NO-induced inhibition of I(Ca) is mainly confined to N-type Ca(2+) channels. Exposure of the neurons for 30 min to 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM), an inhibitor of NO-stimulated guanylyl cyclase, prevented the SNAP-induced reduction in I(Ca). Extracellular application of 8-bromo-cGMP (1 mM) mimicked the effects of NO donors by reducing the peak amplitude of I(Ca) (28.6% of reduction). Action potential configuration and firing frequency during depolarizing current pulses were not altered by the application of SNAP (500 microM) in bladder afferent neurons with TTX-resistant and -sensitive APs. These results indicate that NO acting via a cGMP signaling pathway can modulate N-type Ca(2+) channels in DRG neurons innervating the urinary bladder.

Identification of two distinct vasodilator pathways activated by ATP in the mesenteric bed of the rat

Adenosine 5'-triphosphate (ATP) has important roles in the cardiovascular system, modulating vascular tone by acting as both a vasoconstrictor and a vasodilator. The dilator function of ATP is traditionally thought to be monophasic and mediated primarily by nitric oxide (NO). Here we have identified the endothelium-dependent biphasic nature of ATP-induced vasodilatation of the rat isolated mesenteric bed and investigated the two distinct pathways involved. ATP, at doses of 1x10(-11) to 1x10(-8) moles, induced transient relaxations that were inhibited by the NO synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME: 1x10(-4) M), the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ: 3x10(-6) M) and KCl (6x10(-2) - 1.2x10(-1) M). At doses upwards of 1x10(-8) moles (1x10(-8) - 3x10(-7) moles), ATP also induced prolonged vasodilatations which were unaltered by L-NAME, L-NAME (1x10(-3) M) and indomethacin (1x10(-5) M), or by ODQ, but were abolished in the presence of KCl. In addition, the cannabinoid CB(1) receptor antagonist SR141716A (1x10(-5) M) was found to inhibit the second prolonged phase of vasodilatation. However, at the concentration used SR141716A is reported to be non-selective. A second CB(1) receptor antagonist, AM251 (1x10(-6) M), had a small but significant inhibitory effect on the second phase of ATP-induced vasodilatation. SR141716A, AM251 and KCl (6x10(-2) - 1.2x10(-1) M) all inhibited anandamide-induced relaxation of the isolated mesenteric bed. These observations demonstrate that ATP stimulates vasodilatation of the mesenteric bed by two distinct mechanisms involving the release of NO and an EDHF. In the absence of better pharmacological tools we can only speculate as to the involvement of an endogenous CB(1) receptor ligand in these responses.

Brefeldin A block of integrin-dependent mechanosensitive ATP release from Xenopus oocytes reveals a novel mechanism of mechanotransduction

Many animal cells release ATP into the extracellular medium, and often this release is mechanosensitive. However, the mechanisms underlying this release are not well understood. Using the luciferin-luciferase bioluminescent assay we demonstrate that a Xenopus oocyte releases ATP at a basal rate approximately 0.01 fmol/s, and gentle mechanical stimulation can increase this to 50 fmol/s. Brefeldin A, nocodazole, and progesterone-induced- maturation block basal and mechanosensitive ATP release. These treatments share the common feature of disrupting the Golgi complex and vesicle trafficking to the cell surface and thereby block protein secretion and membrane protein insertion. We propose that ATP release occurs when protein transport vesicles enriched in ATP fuse with the plasma membrane. Collagenase, integrin-binding peptides, and cytochalasin D also block ATP release, indicating that extracellular, membrane and cytoskeletal elements are involved in the release process. Elevation of intracellular Ca(2+) does not evoke ATP release but potentiates mechanosensitive ATP release. Our study indicates a novel mechanism of mechanotransduction that would allow cells to regulate membrane trafficking and protein transport/secretion in response to mechanical loading.

Activation of P2Y but not P2X(4) nucleotide receptors causes elevation of [Ca2+]i in mammalian osteoclasts

Extracellular nucleotides cause elevation of cytosolic free Ca2+ concentration ([Ca2+](i)) in osteoclasts, although the sources of Ca2+ are uncertain. Activation of P2Y receptors causes Ca2+ release from stores, whereas P2X receptors are ligand-gated channels that mediate Ca2+ influx in some cell types. To examine the sources of Ca2+, we studied osteoclasts from rat and rabbit using fura 2 fluorescence and patch clamp. Nucleotide-induced rise of ([Ca2+](i)) persisted on removal of extracellular Ca2+ (Ca), indicating involvement of stores. Inhibition of phospholipase C (PLC) with U-73122 or inhibition of endoplasmic reticulum Ca(2+)-ATPase with cyclopiazonic acid or thapsigargin abolished the rise of ([Ca2+](i)). After store depletion in the absence of Ca, addition of Ca led to a rise of ([Ca2+](i)) consistent with store-operated Ca2+ influx. Store-operated Ca2+ influx was greater at negative potentials and was blocked by La(3+). In patch-clamp studies where PLC was blocked, ATP induced inward current indicating activation of P2X(4) nucleotide receptors, but with no rise of ([Ca2+](i)). We conclude that nucleotide-induced elevation of [Ca(2+)](i) in osteoclasts arises primarily through activation of P2Y nucleotide receptors, leading to release of Ca2+ from intracellular stores.

Immunohistochemical identification of cells expressing ATP-gated cation channels (P2X receptors) in the adult rat thyroid

We carried out immunohistochemistry and western blotting of fresh frozen sections and crude extracts from adult rat thyroids. The histochemical and immunoblotting studies were performed with P2X receptor antibodies from 2 different sources. P2X-immunopositive cells were identified by fluorescence double labelling and confocal microscopy. Results of the western blotting experiments showed double bands of approximately 70 kDa and 140 kDa for all 7 P2X receptor subtypes with both sets of antibodies. Histochemical stains with antibodies from both sources also gave essentially identical results. P2X1, P2X, and P2X6 receptors were detected exclusively in vascular smooth muscle; P2X5 and P2X7 receptors were also present on vascular smooth muscle. Endothelial cells stained for P2X3, P2X4 and P2X7 receptors. Thyroid follicular cells displayed immunoreactivity for P2X3, P2X4 and P2X5 receptors. No immunostaining for P2X receptors was observed on C-cells. Possible roles for the broad expression of P2X receptor subtypes in the rat thyroid are discussed.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Purine-mediated signalling in pain and visceral perception

Receptor subtypes for purines have been identified in a variety of tissues, increasing interest in the roles of purine-mediated signalling in pathophysiological processes. Growing evidence supports the involvement of one of the purinoceptor subtypes, P2X3, in nociception. In this article, recent studies of purine-mediated nociception and visceral pain will be discussed. Furthermore, a novel hypothesis is proposed for purine-mediated mechanosensory transduction where ATP released during distension from epithelial cells lining tubes (such as ureter and gut) and sacs (such as the bladder) acts on P2X3 receptors on a subepithelial nerve plexus to initiate impulses that are relayed via the spinal cord to pain centres in the brain.