Calcium signalling through nucleotide receptor P2X1 in rat portal vein myocytes

Coupling switch of P2Y-IP3 receptors mediates differential Ca(2+) signaling in human embryonic stem cells and derived cardiovascular progenitor cells

Purinergic signaling mediated by P2 receptors (P2Rs) plays important roles in embryonic and stem cell development. However, how it mediates Ca(2+) signals in human embryonic stem cells (hESCs) and derived cardiovascular progenitor cells (CVPCs) remains unclear. Here, we aimed to determine the role of P2Rs in mediating Ca(2+) mobilizations of these cells. hESCs were induced to differentiate into CVPCs by our recently established methods. Gene expression of P2Rs and inositol 1,4,5-trisphosphate receptors (IP3Rs) was analyzed by quantitative/RT-PCR. IP3R3 knockdown (KD) or IP3R2 knockout (KO) hESCs were established by shRNA- or TALEN-mediated gene manipulations, respectively. Confocal imaging revealed that Ca(2+) responses in CVPCs to ATP and UTP were more sensitive and stronger than those in hESCs. Consistently, the gene expression levels of most P2YRs except P2Y1 were increased in CVPCs. Suramin or PPADS blocked ATP-induced Ca(2+) transients in hESCs but only partially inhibited those in CVPCs. Moreover, the P2Y1 receptor-specific antagonist MRS2279 abolished most ATP-induced Ca(2+) signals in hESCs but not in CVPCs. P2Y1 receptor-specific agonist MRS2365 induced Ca(2+) transients only in hESCs but not in CVPCs. Furthermore, IP3R2KO but not IP3R3KD decreased the proportion of hESCs responding to MRS2365. In contrast, both IP3R2 and IP3R3 contributed to UTP-induced Ca(2+) responses while ATP-induced Ca(2+) responses were more dependent on IP3R2 in the CVPCs. In conclusion, a predominant role of P2Y1 receptors in hESCs and a transition of P2Y-IP3R coupling in derived CVPCs are responsible for the differential Ca(2+) mobilization between these cells.

Blood brain barrier precludes the cerebral arteries to intravenously-injected antisense oligonucleotide

Alternative splicing of the ryanodine receptor subtype 3 (RyR3) produces a short isoform (RyR3S) able to negatively regulate the ryanodine receptor subtype 2 (RyR2), as shown in cultured smooth muscle cells from mice. The RyR2 subtype has a crucial role in the control of vascular reactivity via the fine tuning of Ca(2+) signaling to regulate cerebral vascular tone. In this study, we have shown that the inhibition of RyR3S expression by a specific antisense oligonucleotide (asRyR3S) was able to increase the Ca(2+) signals implicating RyR2 in cerebral arteries ex vivo. Moreover, we tried to inhibit the expression of RyR3S in vivo. The asRyR3S was complexed with JetPEI and injected intravenously coupled with several methods known to induce a blood brain barrier disruption. We tested solutions to induce osmotic choc (mannitol), inflammation (bacteria lipopolysaccharide and pertussis toxin), vasoconstriction or dilatation (sumatriptan, phenylephrine, histamine), CD73 activation (NECA) and lipid instability (Tween80). All tested technics failed to target asRyR3 in the cerebral arteries wall, whereas the molecule was included in hepatocytes or cardiomyocytes. Our results showed that the RyR3 alternative splicing could have a function in cerebral arteries ex vivo; however, the disruption of the blood brain barrier could not induce the internalization of antisense oligonucleotides in the cerebral arteries, in order to prove the function of RYR3 short isoform in vivo.

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Purinergic control of vascular tone in the retina

Purinergic control of vascular tone in the CNS has been largely unexplored. This study examines the contribution of endogenous extracellular ATP, acting on vascular smooth muscle cells, in controlling vascular tone in the in vivo rat retina. Retinal vessels were labelled by i.v. injection of a fluorescent dye and imaged with scanning laser confocal microscopy. The diameters of primary arterioles were monitored under control conditions and following intravitreal injection of pharmacological agents. Apyrase (500 units ml(-1)), an ATP hydrolysing enzyme, dilated retinal arterioles by 40.4 ± 2.8%, while AOPCP (12.5 mm), an ecto-5'-nucleotidase inhibitor that increases extracellular ATP levels, constricted arterioles by 58.0 ± 3.8% (P < 0.001 for both), demonstrating the importance of ATP in the control of basal vascular tone. Suramin (500 μm), a broad-spectrum P2 receptor antagonist, dilated retinal arterioles by 50.9 ± 3.7% (P < 0.001). IsoPPADS (300 μm) and TNP-ATP (50 μm), more selective P2X antagonists, dilated arterioles by 41.0 ± 5.3% and 55.2 ± 6.1% respectively (P < 0.001 for both). NF023 (50 μm), a potent antagonist of P2X1 receptors, dilated retinal arterioles by 32.1 ± 2.6% (P < 0.001). A438079 (500 μm) and AZ10606120 (50 μm), P2X7 antagonists, had no effect on basal vascular tone (P = 0.99 and P = 1.00 respectively). In the ex vivo retina, the P2X1 receptor agonist α,β-methylene ATP (300 nm) evoked sustained vasoconstrictions of 18.7 ± 3.2% (P < 0.05). In vivo vitreal injection of the gliotoxin fluorocitrate (150 μm) dilated retinal vessels by 52.3 ± 1.1% (P < 0.001) and inhibited the vasodilatory response to NF023 (50 μm, 7.9 ± 2.0%; P < 0.01). These findings suggest that vascular tone in rat retinal arterioles is maintained by tonic release of ATP from the retina. ATP acts on P2X1 receptors, although contributions from other P2X and P2Y receptors cannot be ruled out. Retinal glial cells are a possible source of the vasoconstricting ATP.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Role of purinergic P2X receptors in the control of liver homeostasis

It is now accepted that extracellular ATP and other nucleotides are potent signaling molecules, akin to neurotransmitters, hormones and lipid mediators. In the liver, several clues support a significant role for extracellular ATP-induced signaling pathways in the control of tissue homeostasis. First, ATP and other nucleotides are physiologically detected in extracellular fluids within the liver, including sinusoidal blood and intraductular bile, in various mammalian species including human and rodents. Moreover, finely tuned mechanisms of ATP release by different liver cell types have been described, under physiological cellular changes. In addition, most hepatic cells constitutively express, at the membrane level, several ATP-metabolizing ectoenzymes and ATP-sensitive receptors that modulate and transduce these mediator signals respectively. Finally, hepatic cells also express numerous membrane transporters that actively contribute to purinergic salvage pathways. Once released in the extracellular medium, unmetabolised ATP molecules can bind to purinergic P2X and P2Y receptors, and subsequently trigger various intracellular signal transduction pathways collectively referred to as purinergic signaling. In the liver, purinergic signaling has been shown to regulate key basic cellular functions, such as glucose/lipid metabolism, protein synthesis and ionic secretion, and homeostatic processes, such as cell cycle, inflammatory response and immunity. Whilst the functional relevance of P2Y receptors in liver physiology has been well documented, limited information is available regarding the potential role of hepatic P2X receptors in the modulation of liver homeostasis.

Dual signaling pathways of arterial constriction by extracellular uridine 5'-triphosphate in the rat

We investigated actions of uridine 5'-triphosphate (UTP) in rat aorta, cerebral and mesenteric arteries, and their single myocytes. UTP (≥10 µM) elicited an inward-rectifying current strongly reminiscent of activation of P2X(1) receptor, and a similar current was also induced by α,β-methylene adenosine 5'-triphosphate (ATP) (≥100 nM). UTP desensitized α,β-methylene ATP-evoked current, and vice versa. The UTP-activated current was insensitive to G-protein modulators, TRPC3 inhibitors, or TRPC3 antibody, but was sensitive to P2-receptor inhibitors or P2X(1)-receptor antibody. Both UTP (1 mM) and α,β-methylene ATP (10 µM) elicited similar conductance single channel activities. UTP (≥10 µM) provoked a dose-dependent contraction of de-endothelialized aortic ring preparation consisting of phasic and tonic components. Removal of extracellular Ca(2+) or bath-applied 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) (30 µM) or nifedipine (10 µM) completely inhibited the phasic contraction while only partially reducing the tonic one. The tonic contraction was almost completely abolished by additional application of thapsigargin (2 µM). Similar biphasic rises in [Ca(2+)](i) were also evoked by UTP in rat aortic myocytes. In contrast to the low expression of TRPC3, significant expression of P2X(1) receptor was detected in all arteries by RT-PCR and immunoblotting, and its localization was limited to plasma membrane of myocytes as indicated by immunohistochemistry. These results suggest that UTP dually activates P2X(1)-like and P2Y receptors, but not TRPC3.

The decrease of expression of ryanodine receptor sub-type 2 is reversed by gentamycin sulphate in vascular myocytes from mdx mice

The mdx mouse, a model of the human Duchenne muscular dystrophy, displays impaired contractile function in skeletal, cardiac and smooth muscles. We explored the possibility that ryanodine receptor (RYR) expression could be altered in vascular muscle. The three RYR sub-types were expressed in portal vein myocytes. As observed through mRNA and protein levels, RYR2 expression was strongly decreased in mdx myocytes, whereas RYR3 and RYR1 expression were unaltered. The use of antisense oligonucleotide directed against RYR sub-types indicated that caffeine-induced Ca²⁺ response and Ca²⁺ spark frequency depended on RYR2 and RYR1. In mdx mice, caffeine-induced Ca²⁺ responses were decreased in both amplitude and maximal rate of rise, and the frequency of Ca²⁺ sparks was also strongly decreased. The gentamycin treatment was able to increase both the expression of RYR2 and the caffeine-induced Ca²⁺ response to the same level as that observed in wild-type mice. Taken together, these results confirm that both RYR1 and RYR2 are required for vascular Ca²⁺ signalling and indicate that inhibition of RYR2 expression may account for the decreased Ca²⁺ release from the SR in mdx vascular myocytes. Finally, we suggest that gentamycin can restore the Ca²⁺ signalling in smooth muscle from mdx mice by increasing RYR2 and dystrophin expression. These results may help explain the reduced efficacy of contraction in vascular myocytes of mdx mice and Duchenne muscular dystrophy–afflicted patients. Gentamycin treatment could be a good therapeutic tool to restore the vascular function.

Sympathetic neurogenic Ca2+ signalling in rat arteries: ATP, noradrenaline and neuropeptide Y

The sympathetic nervous system (SNS) plays an essential role in the control of total peripheral vascular resistance by controlling the contraction of small arteries. The SNS also exerts long-term trophic influences in health and disease; SNS hyperactivity accompanies most forms of human essential hypertension, obesity and heart failure. At their junctions with smooth muscle cells, the peri-arterial sympathetic nerves release ATP, noradrenaline (NA) and neuropeptide Y (NPY) onto smooth muscle cells. Confocal Ca(2+) imaging studies reveal that ATP and NA each produce unique types of postjunctional Ca(2+) signals and consequent smooth muscle cell contractions. Neurally released ATP activates postjunctional P2X(1) receptors to produce local, non-propagating Ca(2+) transients, termed 'junctional Ca(2+) transients', or 'jCaTs'. Neurally released NA binds to alpha(1)-adrenoceptors and can activate Ca(2+) waves or more uniform global changes in [Ca(2+)]. Neurally released NPY does not appear to produce Ca(2+) transients directly, but significantly modulates NA-induced Ca(2+) signalling. The neural release of ATP and NA, as judged by postjunctional Ca(2+) signals, electrical recording of excitatory junction potentials and carbon fibre amperometry to measure NA, varies markedly with the pattern of nerve activity. This probably reflects both pre- and postjunctional mechanisms, which are not yet fully understood. These phenomena, together with different temporal patterns of sympathetic nerve activity in different regional circulations, are probably an important mechanistic basis of the important selective regulation of regional vascular resistance and blood flow by the sympathetic nervous system.

The role of purinergic signaling in the liver and in transplantation: effects of extracellular nucleotides on hepatic graft vascular injury, rejection and metabolism

Extracellular nucleotides (e.g. ATP, UTP, ADP) are released by activated endothelium, leukocytes and platelets within the injured vasculature and bind specific cell-surface type-2 purinergic (P2) receptors. This process drives vascular inflammation and thrombosis within grafted organs. Importantly, there are also vascular ectonucleotidases i.e. ectoenzymes that hydrolyze extracellular nucleotides in the blood to generate nucleosides (viz. adenosine). Endothelial cell NTPDase1/CD39 has been shown to critically modulate levels of circulating nucleotides. This process tends to limit the activation of platelet and leukocyte expressed P2 receptors and also generates adenosine to reverse inflammatory events. This vascular protective CD39 activity is rapidly inhibited by oxidative reactions, such as is observed with liver ischemia reperfusion injury. In this review, we chiefly address the impact of these signaling cascades following liver transplantation. Interestingly, the hepatic vasculature, hepatocytes and all non-parenchymal cell types express several components co-ordinating the purinergic signaling response. With hepatic and vascular dysfunction, we note heightened P2- expression and alterations in ectonucleotidase expression and function that may predispose to progression of disease. In addition to documented impacts upon the vasculature during engraftment, extracellular nucleotides also have direct influences upon liver function and bile flow (both under physiological and pathological states). We have recently shown that alterations in purinergic signaling mediated by altered CD39 expression have major impacts upon hepatic metabolism, repair mechanisms, regeneration and associated immune responses. Future clinical applications in transplantation might involve new therapeutic modalities using soluble recombinant forms of CD39, altering expression of this ectonucleotidase by drugs and/or using small molecules to inhibit deleterious P2-mediated signaling while augmenting beneficial adenosine-mediated effects within the transplanted liver.

Sympathetically evoked Ca2+ signaling in arterial smooth muscle

The sympathetic nervous system plays an essential role in the control of total peripheral vascular resistance and blood flow, by controlling the contraction of small arteries. Perivascular sympathetic nerves release ATP, norepinephrine (NE) and neuropeptide Y. This review summarizes our knowledge of the intracellular Ca2+ signals that are activated by ATP and NE, acting respectively on P2X1 and alpha1-adrenoceptors in arterial smooth muscle. Each neurotransmitter produces a unique type of post-synaptic Ca2+ signal and associated contraction. The neural release of ATP and NE is thought to vary markedly with the pattern of nerve activity, probably reflecting both pre- and post-synaptic mechanisms. Finally, we show that Ca2+ signaling during neurogenic contractions activated by trains of sympathetic nerve fiber action potentials are in fact significantly different from that elicited by simple bath application of exogenous neurotransmitters to isolated arteries (a common experimental technique), and end by identifying important questions remaining in our understanding of sympathetic neurotransmission and the physiological regulation of contraction of small arteries.

Localisation, function and composition of primary Ca(2+) spark discharge region in isolated smooth muscle cells from guinea-pig mesenteric arteries

Smooth muscle cells (SMCs) contain numerous calcium release domains, grouped into regions discharging as a single unit. Laser scanning confocal microscopy, voltage clamp and immunocytochemistry of single SMCs from small mesenteric arteries of guinea-pig were used to study the localisation, function and macromolecular composition of such calcium discharge regions (CDRs). Use of the Ca(2+)-sensitive fluorescent dye fluo-3 or fluo-4 with BODIPY TR-X ryanodine (BTR), a fluorescent derivative of ryanodine, showed spontaneous Ca(2+) sparks originating from regions stained by BTR, located immediately under the plasma membrane, in the arch formed by the sarcoplasmic reticulum surrounding the nucleus. Membrane depolarisation or application of noradrenaline or alpha,beta-methylene ATP, a P2X purinoceptor agonist, elicited Ca(2+) sparks from the same, spontaneous Ca(2+) spark-discharging region. The most active (primary) CDR accounted for nearly 60% of spontaneous transient outward currents at -40 mV and these were of significantly higher amplitude than the ones discharged by secondary CDRs. Immunocytochemical staining for type 1 IP(3) receptors, BK(Ca) channels, P2X(1) purinoceptors or alpha(1) adrenoceptors revealed their juxtaposition with BTR staining at the location typical of the primary CDR. These data suggest the existence of a primary calcium discharge region in SMCs; its position can be predicted from the cell's structure, it acts as a key region for the regulation of membrane potential via Ca(2+) sparks and is a potential link between the external, neurohumoral and the cell's internal, calcium signalling system.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

P2 receptor-mediated Ca2+ transients in rat cerebral artery smooth muscle cells

Significant Ca(2+) release was previously noted with the activation of L-type Ca(2+) current in rat superior cerebral artery smooth muscle cells. Here we examined whether the P(2X) current that is partly carried by Ca(2+) also triggers Ca(2+) release in this preparation. Application of P(2X) agonists evoked membrane currents and concomitant Ca(2+) transients in whole cell voltage-clamped single cells. The expected increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) was calculated from the time-integrated P(2X) current by assuming Ca(2+) is the only charge carrier. The measured increase in [Ca(2+)](i) was plotted as a function of the expected increase in [Ca(2+)](i), and Ca(2+)-buffering power was obtained as a reciprocal of the linear fit to this relationship. Both ryanodine, a Ca(2+)-induced Ca(2+)-release inhibitor, and cADP ribose, a putative activator of Ca(2+)-induced Ca(2+) release, had no significant effects on Ca(2+)-buffering power. These results suggest that Ca(2+) influx through P(2X) receptors does not trigger significant Ca(2+) release. We then examined whether P(2X) responses influence the subsequent P(2Y) response. P(2Y) responses were characterized by measuring the rate of [Ca(2+)](i) increase obtained as the slope of the linear regression to the rising phase of the Ca(2+) transient. During simultaneous application of the P(2X) and P(2Y) agonist, the rate of [Ca(2+)](i) increase was facilitated or suppressed depending on the size of the P(2X) receptor-mediated [Ca(2+)](i) increase. Membrane depolarization close to the Ca(2+) equilibrium potential significantly promoted the rate of [Ca(2+)](i) increase. Our results suggest that the [Ca(2+)](i) increase and membrane depolarization caused by the P(2X) current may regulate the subsequent P(2Y) response.

The sources and sequestration of Ca(2+) contributing to neuroeffector Ca(2+) transients in the mouse vas deferens

The detection of focal Ca(2+) transients (called neuroeffector Ca(2+) transients, or NCTs) in smooth muscle of the mouse isolated vas deferens has been used to detect the packeted release of ATP from nerve terminal varicosities acting at postjunctional P2X receptors. The present study investigates the sources and sequestration of Ca(2+) in NCTs. Smooth muscle cells in whole mouse deferens were loaded with the Ca(2+) indicator Oregon Green 488 BAPTA-1 AM and viewed with a confocal microscope. Ryanodine (10 microM) decreased the amplitude of NCTs by 45 +/- 6 %. Cyclopiazonic acid slowed the recovery of NCTs (from a time course of 200 +/- 10 ms to 800 +/- 100 ms). Caffeine (3 mM) induced spontaneous focal smooth muscle Ca(2+) transients (sparks). Neither of the T-type Ca(2+) channel blockers NiCl2 (50 microM) or mibefradil dihydrochloride (10 microM) affected the amplitude of excitatory junction potentials (2 +/- 5 % and -3 +/- 10 %) or NCTs (-20 +/- 36 % and 3 +/- 13 %). In about 20 % of cells, NCTs were associated with a local, subcellular twitch that remained in the presence of the alpha1-adrenoceptor antagonist prazosin (100 nM), showing that NCTs can initiate local contractions. Slow (5.8 +/- 0.4 microm s(-1)), spontaneous smooth muscle Ca(2+) waves were occasionally observed. Thus, Ca(2+) stores initially amplify and then sequester the Ca(2+) that enters through P2X receptors and there is no amplification by local voltage-gated Ca(2+) channels.

Purinergic and adrenergic Ca2+ transients during neurogenic contractions of rat mesenteric small arteries

Contraction of small arteries is regulated by the sympathetic nervous system, but the Ca2+ transients during neurally stimulated contraction of intact small arteries have not yet been recorded. We loaded rat mesenteric small arteries with the fluorescent Ca2+ indicator fluo-4 and mounted them in a myograph that permitted simultaneous (i) high-speed confocal imaging of fluorescence from individual smooth muscle cells, (ii) electrical stimulation of perivascular nerves, and (iii) recording of isometric tension. Sympathetic neuromuscular transmission was achieved by electrical field stimulation (EFS) (frequency, 10 Hz; pulse voltage, 40 V; pulse duration, 0.2 ms) in the presence of capsaicin and scopolamine (to inhibit 'sensory' and cholinergic nerves, respectively). During the first 20 s of EFS, force rose to a small peak and then declined. During this time, junctional Ca2+ transients (jCaTs) were present at relatively high frequency. We have previously attributed jCaTs to influx of Ca2+ through post-junctional P2X receptors activated by ATP. Propagating asynchronous Ca2+ waves, previously associated with bath-applied alpha1-adrenoceptor agonists, were not initially present. During the next 2.5 min of EFS, force rose slowly, and asynchronous propagating Ca2+ waves appeared. The selective alpha1-adrenoceptor antagonist prazosin abolished both the slowly developing contraction and the Ca2+ waves, but reduced the initial transient contraction by only ~25 %. During 3 min of EFS in prazosin, the frequency of jCaTs declined markedly; at sites at which at least one jCaT occurred, the average probability of a jCaT was 0.008 +/- 0.002 pulse-1 in the first 20 s and 0.0007 +/- 0.0002 pulse-1 in the last 20 s. We suggest that (i) ATP released from sympathetic varicosities activates the initial, transient, contraction and the activator Ca2+ is derived largely from jCaTs, and (ii) sympathetically released noradrenaline (NA) activates the later, major contraction through mechanisms involving alpha1-adrenoceptors and which are associated with propagating Ca2+ waves.

Evoked and spontaneous purinergic junctional Ca2+ transients (jCaTs) in rat small arteries

Confocal microscopy of fluo-4 fluorescence in pressurized rat mesenteric small arteries subjected to low-frequency electrical field stimulation revealed Ca2+ transients in perivascular nerves and novel, spatially localized Ca2+ transients in adjacent smooth muscle cells. These muscle Ca2+ transients occur with a very brief latency to the stimulus pulse (most <3 ms). They are wider (approximately 5 micro m) and last longer (t(1/2), 145 ms) than Ca2+ sparks. They are abolished by the purinergic receptor (P2X) antagonist suramin, but they are totally unaffected by the alpha1 adrenoceptor antagonist prazosin or by capsaicin (which inhibits the function of perivascular sensory nerves). We conclude that these novel Ca2+ transients represent Ca2+ entering smooth muscle cells through P2X receptors activated by ATP released from sympathetic nerves, and we therefore call them "junctional Ca2+ transients" or jCaTs. As expected from spontaneous neurotransmitter release, jCaTs also occur spontaneously, with characteristics identical to evoked jCaTs. Visualization of sympathetic neurotransmission shows that purinergic components dominate at low frequencies of sympathetic nerve fiber activation.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Intermittent ATP release from nerve terminals elicits focal smooth muscle Ca2+ transients in mouse vas deferens

A confocal Ca2+ imaging technique has been used to detect ATP release from individual sympathetic varicosities on the same nerve terminal branch. Varicose nerve terminals and smooth muscle cells in mouse vas deferens were loaded with the Ca2+ indicator Oregon Green 488 BAPTA-1. Field (nerve) stimulation evoked discrete, focal increases in [Ca2+] in smooth muscle cells adjacent to identified varicosities. These focal increases in [Ca2+] have been termed 'neuroeffector Ca2+ transients' (NCTs). NCTs were abolished by alpha,beta-methylene ATP (1 microM), but not by nifedipine (1 microM) or prazosin (100 nM), suggesting that NCTs are generated by Ca2+ influx through P2X receptors without a detectable contribution from L-type Ca2+ channels or alpha(1)-adrenoceptor-mediated pathways. Action potential-evoked ATP release was highly intermittent (mean probability 0.019 +/- 0.002; range 0.001-0.10) at 1 Hz stimulation, even though there was no failure of action potential propagation in the nerve terminals. Twenty-eight per cent of varicosities failed to release transmitter following more than 500 stimuli. Spontaneous ATP release was very infrequent (0.0014 Hz). No Ca2+ transient attributable to noradrenaline release was detected even in response to 5 Hz stimulation. There was evidence of local noradrenaline release as the alpha(2)-adrenoceptor antagonist yohimbine increased the probability of occurrence of NCTs by 55 +/- 21 % during trains of stimuli at 1 Hz. Frequency-dependent facilitation preferentially occurred at low probability release sites. The monitoring of NCTs now allows transmitter release to be detected simultaneously from each functional varicosity on an identified nerve terminal branch on an impulse-to-impulse basis.

Ca(2+) oscillations, gradients, and homeostasis in vascular smooth muscle

Vascular smooth muscle shows both plasticity and heterogeneity with respect to Ca(2+) signaling. Physiological perturbations in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) may take the form of a uniform maintained rise, a transient uniform [Ca(2+)](i) elevation, a transient localized rise in [Ca(2+)](i) (also known as spark and puff), a transient propagated wave of localized [Ca(2+)](i) elevation (Ca(2+) wave), recurring asynchronous Ca(2+) waves, or recurring synchronized Ca(2+) waves dependent on the type of blood vessel and the nature of stimulation. In this overview, evidence is presented which demonstrates that interactions of ion transporters located in the membranes of the cell, sarcoplasmic reticulum, and mitochondria form the basis of this plasticity of Ca(2+) signaling. We focus in particular on how the junctional complexes of plasmalemma and superficial sarcoplasmic reticulum, through the generation of local cytoplasmic Ca(2+) gradients, maintain [Ca(2+)](i) oscillations, couple these to either contraction or relaxation, and promote Ca(2+) cycling during homeostasis.