Origin of ATP release in the rat vas deferens: concomitant measurement of [3H]noradrenaline and [14C]ATP

The Pharmacological Effects of Phenylephrine are Indirect, Mediated by Noradrenaline Release from the Cytoplasm

Phenylephrine (PE) is a canonical α₁-adrenoceptor-selective agonist. However, unexpected effects of PE have been observed in preclinical and clinical studies, that cannot be easily explained by its actions on α₁-adrenoceptors. The probability of the involvement of α₂- and β-adrenoceptors in the effect of PE has been raised. In addition, our earlier study observed that PE released noradrenaline (NA) in a [Ca²⁺]_o-independent manner. To elucidate this issue, we have investigated the effects of PE on [³H]NA release and α₁-mediated smooth muscle contractions in the mouse vas deferens (MVD) as ex vivo preparation. The release experiments were designed to assess the effects of PE at the presynaptic terminal, whereas smooth muscle isometric contractions in response to electrical field stimulation were used to measure PE effect postsynaptically. Our results show that PE at concentrations between 0.3 and 30 µM significantly enhanced the resting release of [³H]NA in a [Ca²⁺]_o-independent manner. In addition, prazosin did not affect the release of NA evoked by PE. On the contrary, PE-evoked smooth muscle contractions were inhibited by prazosin administration indicating the α₁-adrenoceptor-mediated effect. When the function of the NA transporter (NAT) was attenuated with nisoxetine, PE failed to release NA and the contractions were reduced by approximately 88%. The remaining part proved to be prazosin-sensitive. The present work supports the substantial indirect effect of PE which relays on the cytoplasmic release of NA, which might explain the reported side effects for PE.

ATP released from astrocytes modulates action potential threshold and spontaneous excitatory postsynaptic currents in the neonatal rat prefrontal cortex

Maternal immune activation during pregnancy is a risk factor for neurodevelopmental disorders, such as schizophrenia; however, a full mechanistic understanding has yet to be established. The activity of a transient cell population, the subplate neurons, is critical for the development of cortical inhibition and functional thalamocortical connections. Sensitivity of these cells to factors released during inflammation, therefore, may offer a link between maternal immune activation and the aberrant cortical development underlying some neuropsychiatric disorders. An elevated extracellular ATP concentration is associated with inflammation and has been shown to have an effect on neuronal activity. Here, we investigated the effect of ATP on the electrophysiological properties of subplate neurons. Exogenous ATP increased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) at micromolar concentrations. Further, ATP released by astrocytes activated by the PAR-1 agonist, TFLLR-NH₂, also increased the amplitude and frequency of sEPSCs in subplate neurons. The electrophysiological properties of subplate neurons recorded from prefrontal cortical (PFC) slices from neonatal rats were also disrupted in a maternal immune activation rat model of schizophrenia, with a suramin-sensitive increase in frequency and amplitude of sEPSCs. An alternative neurodevelopmental rat model of schizophrenia, MAM-E17, which did not rely on maternal immune activation, however, showed no change in subplate neuron activity. Both models were validated with behavioral assays, showing schizophrenia-like endophenotypes in young adulthood. The purinergic modulation of subplate neuron activity offers a potential explanatory link between maternal immune activation and disruptions in cortical development that lead to the emergence of neuropsychiatric disorders.

P2X receptors up-regulate the cell-surface expression of the neuronal glycine transporter GlyT2

Glycinergic inhibitory neurons of the spinal dorsal horn exert critical control over the conduction of nociceptive signals to higher brain areas. The neuronal glycine transporter 2 (GlyT2) is involved in the recycling of synaptic glycine from the inhibitory synaptic cleft and its activity modulates intra and extracellular glycine concentrations. In this report we show that the stimulation of P2X purinergic receptors with βγ-methylene adenosine 5'-triphosphate induces the up-regulation of GlyT2 transport activity by increasing total and plasma membrane expression and reducing transporter ubiquitination. We identified the receptor subtypes involved by combining pharmacological approaches, siRNA-mediated protein knockdown, and dorsal root ganglion cell enrichment in brainstem and spinal cord primary cultures. Up-regulation of GlyT2 required the combined stimulation of homomeric P2X₃ and P2X₂ receptors or heteromeric P2X_2/3 receptors. We measured the spontaneous glycinergic currents, glycine release and GlyT2 uptake concurrently in response to P2X receptor agonists, and showed that the impact of P2X3 receptor activation on glycinergic neurotransmission involves the modulation of GlyT2 expression or activity. The recognized pro-nociceptive action of P2X₃ receptors suggests that the fine-tuning of GlyT2 activity may have consequences in nociceptive signal conduction.

Exaggerated increases in blood pressure during isometric muscle contraction in hypertension: role for purinergic receptors

Physical activity is a cornerstone therapy for the primary prevention and treatment of hypertension, which is becoming increasingly prevalent in modern societies. During exercise, heart rate and blood pressure (BP) increase in order to acutely meet the metabolic demands of the working skeletal muscle. In hypertensive adults, isometric exercise-induced increases in BP are excessive, potentially increasing the risk of an acute cardiovascular event during or after physical activity. Recently, the skeletal muscle metaboreflex has emerged as a significant contributor to the development of aberrant cardiovascular control during isometric exercise in this clinical population. Our laboratory has conducted a series of studies characterizing the skeletal muscle metaboreflex in hypertensive humans. We and others have demonstrated that hypertension is characterized by greater increases in muscle sympathetic nerve activity and BP during selective activation of the metaboreflex during post-exercise muscle ischemia compared to the increases noted in healthy age-matched normotensive adults, suggesting that the skeletal muscle metaboreflex is exaggerated in human hypertension. The focus of this review is the skeletal muscle metaboreflex (i.e., the metabolic component of the exercise pressor reflex) in hypertension, with particular emphasis on the potential role of purinergic receptors in mediating the exaggerated responses to muscle metaboreflex activation.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Neuronal and non-neuronal modulation of sympathetic neurovascular transmission

Noradrenaline, neuropeptide Y and adenosine triphosphate are co-stored in, and co-released from, sympathetic nerves. Each transmitter modulates its own release as well as the release of one another; thus, anything affecting the release of one of these transmitters has consequences for all. Neurotransmission at the sympathetic neurovascular junction is also modulated by non-sympathetic mediators such as angiotensin II, serotonin, histamine, endothelin and prostaglandins through the activation of specific pre-junctional receptors. In addition, nitric oxide (NO) has been identified as a modulator of sympathetic neuronal activity, both as a physiological antagonist against the vasoconstrictor actions of the sympathetic neurotransmitters, and also by directly affecting transmitter release. Here, we review the modulation of sympathetic neurovascular transmission by neuronal and non-neuronal mediators with an emphasis on the actions of NO. The consequences for co-transmission are also discussed, particularly in light of hypertensive states where NO availability is diminished.

ATP overflow in skeletal muscle 1A arterioles

The purpose of this study was to investigate the sources of ATP in the 1A arteriole, and to investigate age-related changes in ATP overflow. Arterioles (1A) from the red portion of the gastrocnemius muscle were isolated, cannulated and pressurized in a microvessel chamber with field stimulation electrodes. ATP overflow was determined using probes specific for ATP and null probes that were constructed similar to the ATP probes, but did not contain the enzyme coating. ATP concentrations were determined using a normal curve (0.78 to 25 micromol l(-1) ATP). ATP overflow occurred in two phases. Phase one began in the first 20 s following stimulation and phase two started 35 s after field stimulation. Tetrodotoxin, a potent neurotoxin that blocks action potential generation in nerves, abolished both phases of ATP overflow. alpha1-Receptor blockade resulted in a small decrease in ATP overflow in phase two, but endothelial removal resulted in an increase in ATP overflow. ATP overflow was lowest in 6-month-old rats and highest in 12- and 2-month-old rats (P<0.05). ATP overflow measured via biosensors was of neural origin with a small contribution from the vascular smooth muscle. The endothelium seems to play an important role in attenuating ATP overflow in 1A arterioles.

Therapeutic potential of extracellular ATP and P2 receptors in nervous system diseases

Extracellular adenosine 5 inch-triphosphate (ATP) is a key signaling molecule present in the central nervous system (CNS), and now is receiving greater attention due to its role as a messenger in the CNS during different physiological and pathological events. ATP is released into the extracellular space through vesicular exocytosis or from damaged and dying cells. Once in the extracellular environment, ATP binds to the specific receptors termed P2, which mediate ATP effects and are present broadly in both neurons and glial cells. There are P2X, the ligand-gated ionotropic receptors, possessing low affinity for ATP and responsible for fast excitatory neurotransmission, and P2Y, the metabotropic G-protein-coupled receptors, possessing high affinity for ATP. Since massive extracellular release of ATP often occurs after stress, brain ischemia and trauma, the extracellular ATP is considered relating to or involving in the pathological processes of many nervous system diseases. Conversely, the trophic functions have also been extensively described for the extracellular ATP. Therefore, extracellular ATP plays a very complex role in the CNS and its binding to P2 receptors can be related to toxic and/or beneficial effects. In this review, we described the extracellular ATP acting via P2 receptors as a potent therapeutic target for treatment of nervous system diseases.

New insights into purinergic receptor signaling in neuronal differentiation, neuroprotection, and brain disorders

Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system and participate in the synaptic process particularly associated with acetylcholine, GABA, and glutamate neurotransmission. As a result of activation, the P2 receptors promote the elevation of free intracellular calcium concentration as the main signaling pathway. Purinergic signaling is present in early stages of embryogenesis and is involved in processes of cell proliferation, migration, and differentiation. The use of new techniques such as knockout animals, in vitro models of neuronal differentiation, antisense oligonucleotides to induce downregulation of purinergic receptor gene expression, and the development of selective inhibitors for purinergic receptor subtypes contribute to the comprehension of the role of purinergic signaling during neurogenesis. In this review, we shall discuss the participation of purinergic receptors in developmental processes and in brain physiology, including neuron-glia interactions and pathophysiology.

Varicosity-Schwann cell interactions mediated by ATP in the mouse vas deferens

Schwann cells, from a variety of sources, are known to possess P2Y purinergic metabotropic receptors. However, it is not known if Schwann cells associated with autonomic nerve terminals possess such receptors and if so whether these receptors are activated by the endogenous release of ATP from the nerve terminals. We show that such Schwann cells in the vas deferens give evoked calcium transients on nerve stimulation. These transients are mediated, at least in part, by the endogenous release of ATP, which acts on Schwann cell P2Y receptors to release calcium from within the cells. This work suggests the possibility that Schwann cells are active participants in the process of junctional transmission in the autonomic nervous system.

Modulation of neurotransmitter release by NO is altered in mesenteric arterial bed of spontaneously hypertensive rats

Nitric oxide (NO) reacts with catecholamines resulting in their deactivation. In the present study with the use of the perfused mesenteric arterial bed as a model of the sympathetic neuroeffector junction, the NO synthase (NOS) inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) resulted in the enhancement of the periarterial nerve stimulation-induced increase in perfusion pressure and norepinephrine overflow while decreasing neuropeptide Y (NPY) overflow. These changes were prevented by l-arginine, demonstrating that the effects of l-NAME were specific to the inhibition of NOS. From the fact that norepinephrine acts on prejunctional α2-adrenoceptors to inhibit the evoked release of sympathetic cotransmitters, we carried out experiments in the presence of the α2-adrenergic receptor antagonist yohimbine to investigate the possibility that the decrease in NPY observed in the presence of l-NAME was due to the increase in bioactive norepinephrine acting on its autoreceptor. Periarterial nerve stimulation in the presence of both l-NAME and yohimbine prevented the previously observed decrease in NPY, indicating that the cause of this decrease was, as predicted, due to α2-adrenoceptor activation. The periarterial nerve stimulation-induced increase of norepinephrine overflow was greater in the spontaneously hypertensive rat compared with normotensive rats. In contrast to what was observed in the isolated perfused mesenteric arterial bed obtained from normotensive animals, inhibition of NOS did not result in a further increase in the overflow of norepinephine or in a subsequent decrease in NPY. These results demonstrate that, in addition to being a direct vasodilator, NO, by deactivating norepinephrine, can modulate sympathetic neurotransmission and that this modulation is altered in the spontaneously hypertensive rat.

Nitric oxide decreases the biological activity of norepinephrine resulting in altered vascular tone in the rat mesenteric arterial bed

Nitric oxide (NO) reacts with catecholamines resulting in their deactivation. In this study, we demonstrated that coincubation of NO donors with sympathetic neurotransmitters decreased the amount of norepinephrine detected but not ATP or neuropeptide Y (NPY). Furthermore, we found that the ability of norepinephrine to increase perfusion pressure in the isolated perfused mesenteric arterial bed of the rat was attenuated by the incubation of norepinephrine with the NO donor diethylamine NONOate. Conversely, the vasoconstrictive ability of NPY and ATP was unaffected by incubation with NONOate. Periarterial nerve stimulation in the presence of the NO synthase (NOS) inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) resulted in an increase in both perfusion pressure response and norepinephrine levels. This was prevented by l-arginine, demonstrating that the effects of l-NAME were indeed specific to the inhibition of NOS. To confirm that NO was not altering the release of norepinephrine from the sympathetic nerve via presynaptic activation of guanylate cyclase, we repeated the experiments in the presence of the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxaloine-one (ODQ). Unlike l-NAME, ODQ infusion did not increase norepinephrine overflow, demonstrating that modulation of norepinephrine by NO at the vascular neuroeffector junction of the rat mesenteric vascular bed is not the result of presynaptic guanylate cyclase activation. These results demonstrate that, in addition to being a direct vasodilatator, NO can also alter vascular reactivity at the sympathetic neuroeffector junction in the rat mesenteric bed by deactivating the vasoconstrictor norepinephrine.

Interaction between ATP and catecholamines in stimulation of platelet aggregation

Platelets, on activation by endothelial damage, release ADP, ATP, serotonin, epinephrine, and norepinephrine. Although ATP is known to augment the action of norepinephrine in cardiovascular and endocrine systems, the possible interaction between ATP and catecholamines in regulation of platelet reactivity has not been reported. The addition of ATP (1-5 microM) to human platelet-rich plasma did not induce platelet aggregation; however, it selectively augmented the aggregatory response to norepinephrine and epinephrine, but not to serotonin. This potentiating action of ATP was dose dependent and was not due to contamination by, or hydrolysis to, ADP. The action of ATP was blocked by 10 microM of adenosine 3'-phosphate 5'-phosphosulfate, a selective P(2)Y(1) receptor antagonist. ATP alone did not cause release of intracellular Ca(2+), but produced a significant Ca(2+) response in the presence of norepinephrine. In contrast, the P(2)X(1) receptor agonists P(1),P(6)-diadenosine-5' hexophosphate and alpha,beta-methylene-ATP had no effect on norepinephrine-induced platelet aggregation even when added at 100 microM. This synergistic interaction between ATP and norepinephrine in stimulating platelet aggregation may have significant clinical implications and suggests a prothrombotic role for ATP in stress.

ATP stimulates chemically sensitive and sensitizes mechanically sensitive afferents

We examined whether ATP stimulation of P2X purinoceptors would raise blood pressure in decerebrate cats. Femoral arterial injection of the P2X receptor agonist alpha,beta-methylene ATP into the blood supply of the triceps surae muscle induced a dose-dependent increase in arterial blood pressure. The maximal increase in mean arterial pressure (MAP) evoked by 0.1, 0.2, and 0.5 mM alpha,beta-methylene ATP (0.5 ml/min injection rate) was 6.2 +/- 2.5, 22.5 +/- 4.4, and 35.2 +/- 3.9 mmHg, respectively. The P2X receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (2 mM ia) attenuated the increase in MAP elicited by intra-arterial alpha,beta-methylene ATP (0.5 mM), whereas the P2Y receptor antagonist reactive blue 2 (2 mM ia) did not affect the MAP response to alpha,beta-methylene ATP. In a second group of experiments, we tested the hypothesis that ATP acting through P2X receptors would sensitize muscle afferents and, thereby, augment the blood pressure response to muscle stretch. Two kilograms of muscle stretch evoked a 26.5 +/- 4.3 mmHg increase in MAP. This MAP response was enhanced when 2 mM ATP or 0.1 mM alpha,beta-methylene ATP (0.5 ml/min) was arterially infused 10 min before muscle stretch. Furthermore, this effect of ATP on the pressor response to stretch was attenuated by 2 mM pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (P < 0.05) but not by the P1 purinoceptor antagonist 8-(p-sulfophenyl)-theophylline (2 mM). These data indicate that activation of ATP-sensitive P2X receptors evokes a skeletal muscle afferent-mediated pressor response and that ATP at relatively low doses enhances the muscle pressor response to stretch via engagement of P2X receptors.

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.

Distinct mechanisms underlying alpha1-adrenoceptor and P2x purinoceptor operated ATP release and contraction in the guinea-pig vas deferens

The temperature-dependence of ATP release and contraction response evoked by different agonists were investigated in superfused guinea-pig vas deferens. Alpha-adrenoceptor agonists, i.e. noradrenaline (300 microM), and alpha-methyl-noradrenaline (300 microM), increased the basal ATP outflow, measured by the luciferin-luciferase assay, and induced biphasic contractile response. Cooling the bath temperature to 12 degrees C almost completely inhibited ATP release and twitch contraction evoked by alpha-adrenoceptor agonists, whereas the phasic contraction remained unaffected. In contrast, twitch contraction and subsequent ATP release induced by beta,gamma-methylene-ATP, a selective P2 receptor agonist (100 microM), was not reduced by low temperature. The ectoATPase activity, measured by HPLC technique was not significantly different at 37 degrees C and 12 degrees C. Nifedipine (1 microM), the voltage sensitive Ca2+ channel blocker eliminated beta,gamma-methylene-ATP evoked twitch contraction but not ATP release. In conclusion, alpha-adrenoceptor and P2 receptor agonists utilize distinct mechanisms to elicit ATP release and contraction: alpha-adrenoceptor-mediated ATP release and contraction is temperature-dependent, indicating the involvement of a carrier-mediated process in it, whereas P2x purinoceptor evoked ATP release and twitch is mediated by a different mechanism.

Nucleotide-evoked relaxation of rat vas deferens--a possible role for endogenous ATP released upon alpha(1)-adrenoceptor stimulation

The possibility was tested that endogenous ATP released upon alpha(1)-adrenoceptor activation causes relaxation of the rat vas deferens smooth muscle. ATP, 2-methylthio ATP and adenosine relaxed the vas deferens precontracted with 80 mM K(+). The metabolically stable P2 receptor agonists alpha,beta-methylene ATP (alpha,beta-MeATP) and adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS) had little or no effect. The adenosine P1 receptor antagonist 8-(para-sulfophenyl)theophylline did not significantly affect the response to ATP. The P2 receptor antagonist reactive blue 2 markedly reduced the relaxation (by up to 73%); suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and acid blue 129 caused no change. ATP, but not alpha,beta-MeATP, also attenuated contractions elicited by noradrenaline at resting tension; reactive blue 2 blocked the inhibitory effect of ATP. Reactive blue 2, by itself, enhanced the response to noradrenaline (by up to 36%); suramin, PPADS and acid blue 129 caused no change. In the presence of the ATP-degrading enzymes apyrase and nucleotide pyrophosphatase, the facilitatory effect of reactive blue 2 was lost. Apyrase, by itself, enhanced the response to noradrenaline (by 13%). The results indicate that endogenous ATP, released from rat vas deferens smooth muscle upon alpha(1)-adrenoceptor stimulation, causes relaxation. The site of action of ATP is not a typical smooth muscle P2Y receptor.

Co-transmitter function of ATP in central catecholaminergic neurons of the rat

Intracellular recordings were made in a mid-pontine slice preparation of the rat brain containing the nucleus locus coeruleus. Focal electrical stimulation evoked biphasic synaptic potentials consisting of early depolarizing (d.p.s.p.) and late hyperpolarizing (i.p.s.p.) components. The alpha(2)-adrenoceptor antagonist idazoxan inhibited the i.p.s.p. without altering the d.p.s.p. All of the following experiments were carried out in the presence of kynurenic acid and picrotoxin to block the glutamatergic and GABAergic fractions of the d.p.s.p., respectively. Guanethidine, which is known to inhibit noradrenaline and ATP release from nerve terminals of postganglionic sympathetic nerves, depressed both the d.p.s.p. and the i.p.s.p. in a concentration-dependent manner. Damage of catecholaminergic nerve terminals by 6-hydroxydopamine also decreased both the d.p.s.p. and the i.p.s.p. The P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) depressed the d.p.s.p., whereas the i.p.s.p. remained unaffected. The further application of PPADS did not increase the depression of the d.p.s.p. by guanethidine. Superfusion with the mixed alpha-adrenoceptor agonist noradrenaline or the selective P2 receptor agonist adenosine 5'-O-(2-thiodiphosphate) inhibited both the d.p.s.p. and the i.p.s.p. The inhibitory effects of these agonists were prevented by the respective antagonists idazoxan or suramin. In the presence of suramin noradrenaline failed to inhibit the residual d.p.s.p. Superfused noradrenaline potentiated rather than inhibited responses to pressure-applied alpha,beta-methylene-ATP; superfused adenosine 5'-O-(2-thiodiphosphate) did not interact with pressure-applied noradrenaline. In conclusion, we present electrophysiological evidence for the co-release of ATP and catecholamines in the CNS. At the cell somata of neurons in the locus coeruleus, noradrenaline and ATP activate inhibitory alpha(2)-adrenoceptors and excitatory P2 receptors, respectively. In addition, inhibitory presynaptic autoreceptors of the alpha(2) and P2 types appear to regulate release of the two co-transmitters.

P2X2 receptor expression by interstitial cells of Cajal in vas deferens implicated in semen emission

Male reproduction is dependent upon seminal emission mediated by vas deferens contraction. This drives spermatic fluid to the prostatic urethra during ejaculation. We localize interstitial cells of Cajal (ICC), which express P2X2 receptor, subunits of ATP-gated ion channels, to rat, mouse and guinea-pig vas deferens submucosa. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of rat vas deferens resolved two functional splice variant transcripts of the P2X2 receptor subunit. The P2X2 receptor mRNA was localized principally within the lamina propria (submucosal) region of the rat vas deferens using in situ hybridization (ISH) and in situ RT-PCR-ISH. Immunohistochemistry using rat, mouse and guinea-pig vas deferens tissues confirmed expression of P2X2 receptor protein within the lamina propria, particularly within a dense column of small spindle-shaped cells adjacent to the columnar epithelial cells which line the lumen. This immunoreactivity was co-localized with neurone-specific enolase (NSE) and c-Kit protein expression, gene markers for ICC. Mucosal mast cells were distinguished from ICC by toluidine blue staining. Choline acetyltransferase immunoreactivity, a marker for post-ganglionic parasympathetic innervation, occurred on the lateral margin of the lamina propria and extended into the inner longitudinal muscle layer. P2X1 receptor immunolabelling was associated with sympathetic innervation of the smooth muscle in the outer longitudinal and circular muscle layers, but not the inner longitudinal layer. The physiological significance of the vas deferens ICC which express P2X2 receptors remains to be established. Possible roles include regulation of smooth muscle activity or mucosal secretion utilizing local ATP signaling, both of which would affect semen transport.

Stimulation-dependent release, breakdown, and action of endogenous ATP in mouse hemidiaphragm preparation: the possible role of ATP in neuromuscular transmission

In this study the in vitro mouse phrenic nerve- hemidiaphragm preparation was utilized to study the release and extracellular catabolism of endogenous ATP and its action on the postsynaptic site, i.e. on the contraction force evoked by nerve stimulation. ATP, measured by the luciferin-luciferase assay, was released stimulation-dependently from the mouse hemidiaphragm in response to electrical field stimulation at 10 Hz. Blockade of the Na(+) channel activity by tetrodotoxin inhibited the majority of the release of ATP in response to stimulation, showing that it is related to neuronal activity. The nicotinic receptor antagonists d-tubocurarine, and alpha-bungarotoxin and cooling the bath temperature to 7 degrees C also reduced stimulation-induced ATP outflow, suggesting that nicotinic receptors are responsible for the part of the release of ATP that is released from postsynaptic sites in a carrier-mediated manner. Exogenous ATP (20-500 microM) added to the bath was degraded to ADP and AMP by the action of ectoATPase and ectoATPdiphosphohydrolase; the K(m) and v(max) values of these enzymes were 185.8 microM and 55.16 nmol/min.g respectively. However, the total amount of nucleotides ([ATP+ADP+AMP]) was increased after the addition of ATP, indicating that ATP itself promoted further adenine nucleotide release. Twitch contractions of the rat hemidiaphragm preparation evoked by low frequency electrical stimulation was blocked concentration-dependently by the non-depolarizing muscle relaxants d-tubocurarine and pancuronium. Suramin (100 microM-1 mM) reversed neuromuscular blockade by d-tubocurarine and pancuronium; i.e., it shifted their concentration-response curves to the right Taken together our data, that endogenous ATP is released by stimulation and subsequently catabolized in the hemidiaphragm preparation and that suramin inhibits ecto-ATPase activity could be interpreted as meaning that suramin prolongs the action of endogenous ATP to elicit twitch contraction, which points to a new, undefined role of ATP in neuromuscular transmission. The source of ATP is partly postsynaptic, released from the muscle in response to activation of nicotinic ACh receptors expressed on the muscle.

Autoinhibition, sympathetic cotransmission and biphasic contractile responses to trains of nerve stimulation in the rodent vas deferens

1. The present review critically discusses the evidence for and against the various hypotheses that have been proposed to explain the biphasic contractile response of the rodent vas deferens to trains of electrical field stimulation (EFS). 2. It is widely accepted that the initial component of the biphasic response of the rodent isolated vas deferens to trains of EFS is mediated by ATP and the second slower tonic contractions is mediated by noradrenaline (NA). This theory is based on the ability of antagonists of the post-junctional receptors for these neurotransmitters to inhibit the respective components of the biphasic response and on the ability of exogenous application of either ATP or NA to mimic the responses of each phase. 3. Prejunctional autoinhibition has also been proposed as the cause of the biphasic response. This is based primarily on the ability of alpha 2-adrenoceptor antagonists to transform responses from biphasic to monophasic and on the ability of neuronal NA uptake inhibitors to accentuate the separation of the two phases. 4. Atypical or extrajunctional NA receptors have also been proposed to be the mediators of the component of the response to nerve stimulation that is resistant to the traditional alpha-adrenoceptor antagonists. 5. Different contractile mechanisms and/or sources of calcium have also been postulated to cause the biphasic response. Blockers of intracellular Ca2+ mobilization are able to block the initial component, while blockers of extracellular Ca2+ entry inhibit the second tonic phase. 6. It is concluded that because alpha 1-adrenoceptor antagonists and blockers of P2 purinoceptors have also been shown to block both phases of the response to trains of EFS, prejunctional auto-inhibitory mechanisms perhaps provide the most sound explanation for the phenomenon of the biphasic contractile response to trains of EFS.

Existence of postsynaptic dopamine D2 receptor as an enhancer of contractile response in vas deferens

Effects of dopamine and (+/-)-2-(N-phenylethyl-N-propyl)amino-5-hydroxy-tetralin hydrochloride (N-0434), a dopamine D2 receptor agonist, in the presence of prazosin on the ATP- and acetylcholine-induced contraction were investigated in the guinea-pig vas deferens in order to test for the existence of postsynaptic dopamine receptors. The contraction induced by ATP was potentiated by dopamine and N-0434. This potentiation was antagonized by spiperone, a dopamine D2 receptor antagonist, but not by a dopamine D1 receptor antagonist and an alpha2-adrenoceptor antagonist. Similar results were also observed by acetylcholine as well as ATP. The contraction induced by transmural nerve stimulation in the presence of alpha-adrenoceptor antagonists was also potentiated by N-0434, and this potentiation was antagonized by spiperone. The results suggest that dopamine D2 receptors are located on the postsynaptic site of guinea-pig vas deferens and that the contractile responses to ATP and acetylcholine are potentiated via activation of dopamine D2 receptor.

Co-release of endogenous ATP and [3H]noradrenaline from rat hypothalamic slices: origin and modulation by alpha2-adrenoceptors

The release of endogenous ATP, measured by the luciferin-luciferase assay, and of [3H]noradrenaline from the in vitro superfused rat hypothalamic slices were studied. ATP and [3H]noradrenaline were released simultaneously during resting conditions and in response to low and high frequency field electrical stimulation; the release of both substances were frequency dependent between 2 Hz and 16 Hz. The stimulation-induced release of ATP and [3H]noradrenaline was diminished by more than 80% under Ca2+-free conditions. Tetrodotoxin inhibited the majority of the evoked release of both ATP and [3H]noradrenaline, however, it was less effective in reducing the release of [3H]noradrenaline, than that of ATP. Bilateral stereotaxic injection of 6-hydroxydopamine (4 microg/side) to the ventral part of the ventral noradrenergic bundle, originating from the A1 cell group in the brainstem, resulted in a 55% reduction of endogenous noradrenaline content of the hypothalamic slices, and the tritium uptake and the stimulation-evoked release of [3H]noradrenaline was also markedly reduced. While the basal release of ATP was not affected, the evoked release was diminished by 72% by this treatment. Perfusion of the slices with noradrenaline (100 microM) initiated rapid and continuous tritium release; on the other hand, it did not release any ATP. In contrast, 6 min perfusion of (-)nicotine and 1,1-dimethyl-4-phenyl-piperazinium iodide evoked parallel release of ATP and [3H]noradrenaline which was inhibited by the nicotinic receptor antagonist mecamylamine; 6-hydroxydopamine lesion of the ventral part of the ventral noradrenergic bundle did not affect the nicotine-evoked ATP and [3H]noradrenaline release. While CH 38083, a non subtype-selective alpha2-antagonist and BRL44408, the subtype-selective alpha2AD antagonist augmented the evoked release of [3H]noradrenaline, ARC239, a selective alpha2BC antagonist was without effect. In contrast, neither of the alpha2-antagonists significantly affected the evoked-release of ATP. In summary, we report here that endogenous ATP and [3H]noradrenaline are co-released stimulation-dependently from superfused rat hypothalamic slices. A significant part of the release of both compounds is derived from the nerve terminals, originating from the A1 catecholaminergic cell group of brainstem nuclei. Unlike that from the peripheral sympathetic transmission, noradrenaline and alpha1-adrenoceptor agonists were unable to promote the release of ATP. Conversely, parallel ATP and noradrenaline release could be induced by nicotine receptor activation, but this release does not originate from the same nerve endings. The evoked-release of [3H]noradrenaline is inhibited by endogenous noradrenaline via alpha2AD subtype of adrenoreceptors, while the release of ATP is not subject to this autoinhibitory modulation. In conclusion, our results support the view that ATP is involved in the neurotransmission in the hypothalamus, but the sources of the released ATP and noradrenaline seem to be not identical under different stimulatory and modulatory conditions.

Effects of hypophysectomy on purinergic and noradrenergic contractility of the rat vas deferens

1. The effects of removal of the pituitary on vas deferens contractile function were examined in young adult male rats having undergone hypophysectomy 7 days previously. Sham-operated age-matched rats served as controls. Responses to electrical field stimulation (EFS), and exogenous noradrenaline (NA) and beta, gamma-methylene ATP (beta, gamma-meATP) were tested. 2. Hypophysectomized rats lost weight over the 7 days. Body weights were 267.2 +/- 3.9 g (n = 7) in controls and 195.3 +/- 1.56 g (n = 5) with hypophysectomy. The wet weight of the vas deferens from rats with hypophysectomy, 22.8 +/- 1.8 mg (n = 5) was approximately half that of the controls, 41.3 +/- 1.2 mg (n = 7). The contractile response to KCl was smaller in hypophysectomized preparations (0.97 +/- 0.9 g, n = 5) than in controls (2.91 +/- 0.2 g, n = 7). 3. The alpha 1-adrenoceptor antagonist prazosin (1 microM) was more effective as an inhibitor of the tonic than of the twitch component of the contractile response, indicating a dominant adrenergic component. However, alpha, beta-methylene ATP was also effective at inhibiting the tonic component, indicating that ATP is also released secondarily to NA. 4. Absolute contractions to EFS of vas deferens from hypophysectomized rats were smaller than those of the controls; however, when corrected for the difference in smooth muscle function (expressed as a percentage of the contraction to 120 mM KCl), the twitch response was significantly greater than in the controls at frequencies of up to 16 Hz, maximal responses being unaffected. In contrast, the tonic response was not significantly different between the two groups. 5. The sensitivity of contractions evoked by NA (1-300 microM) was less in vas deferens from hypophysectomized rats than in the controls. 6. Contractions of the vas deferens to beta, gamma-meATP (1-300 microM) were greater in hypophysectomized rats than in the controls. 7. In conclusion, the present results indicate that hypophysectomy of rats causes an increase in the twitch, but not the tonic component of the vas deferens contractile response to EFS. This appears to be due to an increase in the number or sensitivity of postjunctional P2X-purinoceptors, there being a reduction in sensitivity to NA.

Biochemistry, localization and functional roles of ecto-nucleotidases in the nervous system

Nucleotides such as ATP, ADP, UTP or the diadenosine polyphosphates and possibly even NAD+ are extracellular signaling substances in the brain and in other tissues. Enzymes located on the cell surface catalyze the hydrolysis of these compounds and thus limit their spatio-temporal activity. As a final hydrolysis product they generate the nucleoside and phosphate. The paper discusses the biochemical properties, cellular localization and functional properties of surface-located enzymes that hydrolyse nucleotides released from nervous tissue. This is preceded by a brief discussion of nucleotide receptors, cellular storage and mechanisms of nucleotide release. In nervous tissue nucleoside 5'-triphosphates are hydrolysed by ecto-ATP-diphosphohydrolase and possibly in addition also by ecto-nucleoside triphosphatase and ecto-nucleoside diphosphatase. The molecular identity of the ATP-diphosphohydrolase has now been revealed. The hydrolysis of nucleoside 5'-monophosphates is catalysed by 5'-nucleotidase whose biochemical properties and molecular structure have been studied in detail. Little is known about the molecular properties of the diadenosine polyphosphatases. Surface located enzymes for the extracellular hydrolysis of NAD+ and also ecto-protein kinases are discussed briefly. The cellular localization of the ecto-nucleotidases is only partly defined. Whereas in adult mammalian brain activity for hydrolysis of ATP and ADP may be associated with nerve cells or glial cells 5'-nucleotidase appears to have a preferential glial allocation in the adult mammal. The extracellular hydrolysis of the nucleotides is of functional importance not only during synaptic transmission where it functions in signal elimination. It plays a crucial role also for the survival and differentiation of neural cells in vitro and presumably during neuronal development in vivo.

Selective enhancement by an adenosine A1 receptor agonist of agents inducing contraction of the rat vas deferens

The adenosine analogue N6-cyclopentyladenosine (CPA), acting via postjunctional A1 receptors, has been shown to enhance contractions of the rat vas deferens induced by adenosine 5'-triphosphate (ATP), the sympathetic cotransmitter in this tissue. The aim of the present study was to examine the ability of CPA to enhance contractions induced by other contractile agents. CPA (0.01-0.3 microM) enhanced contractions induced by exogenous ATP (10 microM), 5-hydroxytryptamine (5-HT) (3 microM), tyramine (10 microM), 2-methyl-5-hydroxytryptamine (2-Me-5-HT) (10 microM) and KCl (35 mM) and this enhancement was blocked by an A1-selective concentration (3 nM) of 1, 3-dipropyl-8-cyclopentylxanthine (DPCPX). CPA failed to enhance contractions induced by exogenous noradrenaline (NA) (1 microM or 10 microM), bradykinin (0.1 microM), phenylephrine (3 microM) or carbachol (10 microM). The contractions induced by ATP (10 microM), 5-HT (3 microM), 2-Me-5-HT (10 microM) and KCl (35 mM) were unaffected by tetrodotoxin (1 microM) as well as by desensitisation of the P2x-purinoceptors with the ATP analogue adenosine 5'-(alpha, beta-methylene) triphosphonate. The contractions induced by tyramine (10 microM) and 2-Me-5-HT (10 microM) were blocked by prazosin (100 nM) or by imipramine (1 microM). Ketanserin (10 nM) antagonised the response to 5-HT giving a dose-ratio of 12.9 corresponding to an apparent pA2 of 9.1. In conclusion, the A1-mediated effect was clearly selective for certain contractile agents and not due to a non-specific increase in contractility of the tissue. CPA enhanced contractions induced by both ATP and indirect sympathomimetics which release endogenous NA, and this enhancement of the two sympathetic cotransmitters may have a functional significance, and demonstrates the complexity of the neuromodulatory effects of adenosine in the rat vas deferens.

A1-receptor-mediated effect of adenosine on the release of acetylcholine from the myenteric plexus: role and localization of ecto-ATPase and 5'-nucleotidase

No attempt has been made so far to classify the subtypes of presynaptic inhibitory adenosine receptors located in the myenteric plexus and to localize ecto-ATPase and 5'-nucleotidase in the intestine. The release of [3H]acetylcholine and smooth muscle responses to acetylcholine were measured and the effect of selective adenosine receptor ligands was studied using field-stimulated isolated longitudinal muscle strips of guinea-pig ileum. Release of ATP and its hydrolysis rate were also measured using the luciferin-luciferase technique. A histochemical method combined with electron microscopy was used for localization of ecto-ATPase and 5'-nucleotidase, enzymes responsible for destruction of extracellular ATP, ADP and AMP. Subtype-selective A1-receptor agonists and antagonists inhibited and enhanced, respectively, the release of acetylcholine associated with neuronal activity. A significant amount of ATP was released in response to electrical stimulation and administration of carbamylcholine. The release of ATP was inhibited by atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide, an M3-receptor antagonist. Hydrolysis of ATP was rapid and resulted in an accumulation of extracellular adenosine involved in presynaptic A1-receptor-mediated inhibition of acetylcholine release. While the inhibitory effect of adenosine and ATP was significantly potentiated by dipyridamol, an adenosine uptake blocker, that of 2-ms ATP was not. The effect of ATP was not competitively antagonized by 8-cyclopentyl-1,3-dipropylxanthine, a selective A1-receptor antagonist. In conclusion, axon terminals of cholinergic interneurons are equipped with inhibitory A1- and P2 gamma-receptors. Therefore, both adenosine and ATP control the release of acetylcholine through these receptors. ATP is mainly released from the smooth muscle in response to stimulation of M3-muscarinic receptors by endogenous acetylcholine (cascade transmission [Vizi E. S. et al. (1992) Neuroscience 50, 455-465]) and is rapidly hydrolysed by ecto-ATPase localized on the surface of the smooth muscle and axon terminals producing ADP and AMP, and by 5'-nucleotidase present only on the surface of smooth muscle cells producing adenosine.

Alpha-adrenoceptor modulation of norepinephrine and ATP release in isolated kidneys of spontaneously hypertensive rats

The present study investigates sympathetic cotransmission and its alpha-adrenoceptor-mediated modulation in kidneys of spontaneously hypertensive rats (SHR, 12 to 14 weeks) and age-matched normotensive Wistar-Kyoto rats (WKY). In the presence of cocaine and corticosterone, renal nerve stimulation at 1 Hz (30 seconds) induced a greater outflow of norepinephrine in SHR (4.2 +/- 0.2 pmol/g kidney) than in WKY (3.0 +/- 0.2 pmol/g kidney). The alpha 2-adrenoceptor antagonist rauwolscine (0.01 to 1 mumol/L) increased the stimulation-induced norepinephrine outflow to a greater extent in SHR than in WKY. In contrast, the alpha 1-adrenoceptor antagonist prazosin (0.03 to 3 mumol/L) increased the stimulation-induced norepinephrine outflow to a greater extent in WKY than in SHR. This difference was not observed in the presence of the P1-purinoceptor antagonist 8-(p-sulfophenyl)theophylline (100 mumol/L). Stimulation at 4 Hz (30 seconds) induced an outflow of ATP (SHR, 12.7 +/- 3.3 pmol/g kidney; WKY, 16.7 +/- 2.1 pmol/g kidney; perfusion solution without cocaine and corticosterone). Prazosin (0.03 mumol/L) markedly reduced pressor responses to stimulation and inhibited the induced ATP outflow by 60% to 70%. When prazosin (0.03 mumol/L) was present, rauwolscine (0.1 mumol/L) increased the induced outflow of norepinephrine and ATP and markedly enhanced prazosin-resistant pressor responses. These pressor responses were abolished by the P2-purinoceptor antagonist suramin (300 mumol/L). The results demonstrate an increased alpha 2-adrenoceptor-mediated automodulation of norepinephrine release in SHR kidneys caused by increased intrasynaptic norepinephrine levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrically-evoked release of taurine in the rat vas deferens: evidence for a purinoceptor-mediated effect

Release of taurine evoked by electrical stimulation (2700 pulses; 5 Hz; 10 mA unless stated otherwise) and its dependence on noradrenaline and ATP was studied in isolated, perifused rat vas deferens. Outflow of noradrenaline was also measured in some experiments. The basal outflow of taurine averaged 3.90 +/- 0.32 nmol/g tissue per min. Electrical stimulation increased the outflow to about 4 times basal values. The electrically-evoked overflow averaged 128.0 +/- 11.7 nmol/g. An increase in current strength to 40 mA increased the evoked overflow by about 50%. At either current strength, the evoked overflow of taurine (and noradrenaline) was abolished by tetrodotoxin. Ca(2+)-deprivation blocked the overflow of taurine elicited by 10 mA and increased the overflow elicited by 40 mA pulses (but abolished noradrenaline overflow under either condition). Neither prazosin nor pretreatment of the rats with reserpine reduced electrically-evoked overflow of taurine (although reserpine pretreatment abolished evoked noradrenaline overflow). Tyramine (100 mumols/l; 9 min) caused an overflow of taurine 36% of that caused by electrical stimulation (but an overflow of noradrenaline 3 times higher than that evoked by electrical stimulation). Exogenous noradrenaline (9 min) caused a concentration-dependent overflow of taurine with a maximal effect at 162 mumol/l, amounting to 33% of the electrically-evoked overflow. alpha,beta-Methylene ATP (19 mumols/l) elicited an overflow of taurine that faded despite continued exposure to the drug and amounted to 62% of the response to electrical stimulation. Thirty minutes after the start of application of alpha,beta-methylene ATP, electrically-evoked overflow of taurine was greatly reduced. Suramin (100 mumols/l) also reduced taurine overflow in response to electrical stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of neural noradrenaline and ATP release by angiotensin II and prostaglandin E2 in guinea-pig vas deferens

Effects of angiotensin II and prostaglandin E2 on contractions, release of noradrenaline and release of ATP elicited by electrical stimulation (210 pulses, 7 Hz) were studied in the isolated vas deferens of the guinea pig. Release of noradrenaline was assessed as overflow of tritium after preincubation with [3H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique. In some experiments postsynaptic alpha 1-adrenoceptors and P2X-purinoceptors were blocked by prazosin and suramin, respectively, to isolate the neural fraction of the overflow of ATP. Electrical stimulation elicited an overflow of tritium and ATP and, in the absence of prazosin and suramin, contraction. In the absence of prazosin and suramin, angiotensin II (1-100 nM) enhanced contractions as well as the evoked overflow of tritium and ATP. All parameters were increased by about the same percentage for a given concentration of angiotensin II. The effect of prostaglandin E2 (1-100 nM) was complex. Contractions were mainly enhanced, the evoked overflow of tritium was reduced, whereas the evoked overflow of ATP was predominantly increased. No or almost no contraction remained in the presence of prazosin and suramin, and the evoked overflow of ATP was decreased to about 16%. Angiotensin II (1-100 nM) again enhanced the evoked overflow of tritium and ATP. Both were increased by about the same percentage for a given concentration of angiotensin II and also were increased by about the same percentage as obtained in the absence of prazosin and suramin.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of endogenous ATP from the vasa deferentia of the rat and guinea-pig by the indirect sympathomimetic tyramine

1. Adenosine 5'triphosphate (ATP) as well as [3H]-noradrenaline ([3H]-NA) is released by perfusion of the vas deferens with the indirect sympathomimetic tyramine (100 microM); this result is consistent with the concept of sympathetic cotransmission. 2. While tyramine produced a strong contraction in the vas deferens of the rat, it had little mechanical action in the guinea-pig vas deferens. This appears to be largely because tyramine induces considerably lower levels of release of both ATP and NA from the guinea-pig vas deferens compared to that of the rat. Furthermore, NA released by tyramine appears to release ATP from a secondary pool in the rat vans deferens, but not that of the guinea-pig, since prazosin reduced the tyramine-induced release of ATP in the rat vas deferens. 3. alpha,beta-Methylene ATP (alpha,beta-meATP) increased both the spontaneous release of ATP and the tyramine-evoked efflux of ATP and [3H]-NA. The basal and tyramine-induced efflux of [3H]-NA was also enhanced by the alpha 1-adrenoceptor antagonist, prazosin, suggesting that prejunctional alpha 1-adrenoceptors may modulate neurotransmitter release.

P1-purinoceptor-mediated modulation of neural noradrenaline and ATP release in guinea-pig vas deferens

The effect of P1-purinoceptor activation on contractions, release of noradrenaline and release of ATP elicited by electrical field stimulation (210 pulses, 7 Hz) was studied in the superfused vas deferens of the guinea pig. Release of noradrenaline was assessed as overflow of total tritium after preincubation with [3H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique. Electrical stimulation elicited reproducible contraction, tritium overflow and ATP overflow. In the absence of other drugs, adenosine (10-100 microM) did not change evoked contractions but reduced the evoked overflow of tritium and ATP. In subsequent experiments alpha 1-adrenoceptors were blocked by prazosin, P2-purinoceptors by suramin and alpha 2-adrenoceptors by rauwolscine. No or almost no contraction remained under these conditions. The evoked overflow of tritium was 505% and the evoked overflow of ATP 34% of that observed in the absence of prazosin, suramin and rauwolscine. Adenosine (1-100 microM) again reduced the evoked overflow of tritium and ATP, and so did the A1-selective agonist 2-chloro-N6-cyclopentyladenosine (CCPA; 0.032-0.32 microM). Adenosine and CCPA decreased the evoked overflow of ATP to a greater extent than the evoked overflow of tritium. It is concluded that neural release of both postganglionic sympathetic cotransmitters, noradrenaline and ATP, is decreased upon activation of prejunctional P1- (A1-) purinoceptors in guinea-pig vas deferens. The A1-receptor-mediated inhibition of the release of ATP is more marked than the inhibition of the release of noradrenaline, a pattern opposite to the inhibition produced by activation of prejunctional alpha 2-autoreceptors.

Co-release of noradrenaline and ATP from cultured sympathetic neurons

The vesicles of postganglionic sympathetic axons store both noradrenaline and ATP. The theory of noradrenaline-ATP co-transmission implies that both compounds are released by nerve action potentials and elicit postjunctional effects. Many properties of postjunctional responses support the theory. However, neural release of ATP has been difficult to detect biochemically: by far the major part of the overflow of ATP from intact tissues upon sympathetic nerve stimulation comes from non-neural elements, especially smooth muscle and endothelial cells. Here we describe a parallel electrically evoked overflow of [3H]noradrenaline and endogenous ATP from cultured chick sympathetic neurons. The overflow was abolished by tetrodotoxin, omega-conotoxin and withdrawal of Ca2+, was increased by tetraethylammonium and 4-aminopyridine, and was not changed by prazosin or suramin. The results demonstrate directly the action potential-evoked, Ca(2+)-dependent and presumably vesicular and exocytotic release of ATP from postganglionic sympathetic neurons. They support the co-transmitter theory and suggest that cultured sympathetic neurons are a preparation in which noradrenaline-ATP co-release can be examined free from postjunctional components.

Evidence for participation of nitric oxide in excitatory neurotransmission in rat vas deferens

A depression of the fast, non-adrenergic, and also of the slow, adrenergic, components of muscle contraction in response to intramural nerve stimulation was induced by the blocker of nitric oxide synthase NG-nitro-L-arginine methyl ester (L-NAME), in rat vas deferens. Effects of exogenous noradrenaline or ATP were not reduced by L-NAME. However, L-arginine also caused an inhibition of electrically induced effects in most of the preparations, contrary to the expectations for a precursor of nitric oxide synthesis. In spite of these difficulties L-arginine antagonized the action of L-NAME. These results indicate that nitric oxide is involved in excitatory nerve-muscle transmission in vas deferens.

A study of ATP as a sympathetic cotransmitter in human saphenous vein

1. Strips of human saphenous veins were superfused with Krebs-Henseleit solution at either 25 degrees C or 37 degrees C. Constrictor responses to electrical stimulation (10 Hz, 40 s) but not to exogenous noradrenaline (0.1, 1 microM) were abolished by guanethidine (10 microM) and tetrodotoxin (1 microM). Hence, responses to electrical stimulation are due to action potential-induced release of sympathetic neurotransmitters. 2. Constrictor responses to electrical stimulation and noradrenaline were reduced by the alpha 1-adrenoceptor antagonist, prazosin (0.3 microM) as well as by the alpha 2-adrenoceptor antagonist, rauwolscine (1 microM). The combination of prazosin and rauwolscine abolished constrictor responses to noradrenaline at 25 degrees C and 37 degrees C. However, constrictor responses to electrical stimulation were partly resistant to alpha-adrenoceptor blockade by prazosin and rauwolscine (at 25 degrees C about 30%). Residual constrictor responses to electrical stimulation were also observed in the presence of the combination of prazosin (3 microM) and rauwolscine (10 microM) as well as in the presence of phenoxybenzamine (10 microM). 3. Veins, incubated with [3H]-noradrenaline, released tritium upon electrical stimulation (10 Hz, 40 s). Moreover, electrical stimulation also induced an overflow of ATP amounting to 4.8 +/- 1.5 pmol g-1 at 25 degrees C and 2.0 +/- 0.5 pmol g-1 at 37 degrees C. Both tritium and ATP overflow were abolished by tetrodotoxin (0.5 microM). The combination of prazosin (0.3 microM) and rauwolscine (1 microM) increased tritium overflow at either 25 degrees C or 37 degrees C by about 120%, but reduced ATP overflow by about 70%. Hence, a significant percentage of the electrically evoked ATP overflow seems to be released from non-neuronal cells upon activation of alpha-adrenoceptors by endogenous noradrenaline. The remaining ATP overflow, which was resistant to alpha-adrenoceptor blockade, may reflect neuronally released ATP.4. ATP (300 MicroM) and alpha,Beta-methylene-ATP (1, 10 MicroM), both induced constrictor responses. The P2-purinoceptor antagonist, suramin (300 MicroM) markedly inhibited constrictor responses to ATP and alpha, beta-methylene-ATP, but not those to electrical stimulation and to noradrenaline. Moreover, suramin(300 MicroM) failed to diminish the alpha-adrenoceptor blockade-resistant constrictor response to 10 Hz.5. In conclusion, constrictor responses to sympathetic nerve stimulation in human saphenous veins are mainly but not exclusively mediated by neuronally released noradrenaline. There is a concomitant release of ATP and noradrenaline. P2-purinoceptors which mediate vasoconstriction are present; however,a role of neuronally released ATP in constrictor responses to electrical stimulation could not be established. Therefore, the nature of the sympathetic transmitter responsible for alpha-adrenoceptor blockade-resistant constrictor responses remains unknown.

Methoxamine enhances the release of endogenous noradrenaline from rabbit ear artery: possible involvement of ATP

The effect of methoxamine, an alpha 1-adrenoceptor agonist, on the electrically-evoked release of endogenous noradrenaline was examined in the isolated rabbit ear artery. Noradrenaline was quantified by high performance liquid chromatography-electrochemical detection. The release of adenine nucleotides and nucleosides by methoxamine was examined using high performance liquid chromatography-fluorescence detection. The release of noradrenaline evoked by electrical field stimulation (EFS) at 4 Hz was reduced by tetrodotoxin 0.3 mumol/l and clonidine 1 mumol/l by approximately 80% and 50%, respectively. On the other hand, methoxamine at 10 but not 1 mumol/l enhanced the release of noradrenaline to approximately twice the control, and the enhancement was prevented by prazosin 1 mumol/l. The facilitatory action of methoxamine was also abolished after desensitization of P2-purinoceptors by alpha,beta-methylene ATP 30 mumol/l as well as by the presumed P2-purinoceptor antagonist suramin given at 10 mumol/l. Exogenous ATP 10 mumol/l significantly enhanced the EFS-evoked release of noradrenaline, and the enhancement was abolished by alpha,beta-methylene ATP and suramin. None of the drugs changed the spontaneous outflow of noradrenaline. These results indicate that endogenous ATP, acting at prejunctional purinoceptors, may participate in the facilitatory effect of methoxamine. Indeed, methoxamine 10 mumol/l significantly enhanced the spontaneous outflow of ATP and, less so, ADP. The methoxamine evoked release of ATP and ADP was antagonized by prazosin 1 mumol/l. It is concluded that methoxamine releases endogenous ATP from postjunctional sites which then, via prejunctional purinoceptors, facilitates action potential-evoked release of noradrenaline in rabbit ear artery.

Evidence that the inhibition of ATP release from sympathetic nerves by adenosine is a physiological mechanism

1. Perfusion with the P1-purinoceptor agonist adenosine (1-500 microM) greatly reduced the stimulation-induced release of ATP and the initial contractile phase of the response of the guinea pig vas deferens to field stimulation. 2. The inhibitory effects of adenosine (100 microM) were readily antagonised by the P1-purinoceptor antagonist, 8-phenyltheophylline (10 microM). 3. Dipyridamole (10 microM), inhibited the stimulation-evoked release of ATP from the guinea pig vas deferens and reduced the initial component of contraction. 4. These results support the view that adenosine, resulting from ectoenzymatic breakdown of ATP released as a cotransmitter from sympathetic nerve terminals, acts as a physiological prejunctional regulator of transmitter release.

Is the neuronal ATP release from guinea-pig vas deferens subject to α2-adrenoceptor-mediated modulation?

The effects of a variety of alpha 2-adrenoceptor agonists and antagonists were studied on stimulation-evoked release of endogenous ATP, measured by the luciferin-luciferase assay, and on the release of [3H]noradrenaline from the guinea-pig vas deferens. The biphasic mechanical contraction of the guinea-pig smooth muscle was recorded concomitantly. The alpha 2-adrenoceptor agonist, xylazine (1 microM) inhibited the field stimulation-evoked (8 Hz, 0.1 ms, 480 shocks) release of ATP and [3H]noradrenaline, and both phases of the contraction. The inhibitory effect of xylazine on the release of ATP, noradrenaline and muscle contraction was prevented by the selective alpha 2-adrenoceptor antagonist, CH 38083 [7,8-(methylenedioxi)-14 alpha-alloberbanol, 1 microM]. In the presence of prazosin (0.1-1 microM) or WB 4101 [2-(2,6-dimethoxyphenoxyethyl)aminomethyl- 1,4-benzodioxane hydrochloride, 0.1-1 microM], i.e. under the condition when the effect of noradrenaline on postjunctional alpha 1-adrenoceptors was excluded, the stimulation-evoked release of [3H]noradrenaline was significantly enhanced, however, the release of endogenous ATP and also both phases of contraction were reduced. In the presence of prazosin, xylazine was able to inhibit the stimulation-evoked release of ATP. In vas deferens dissected from reserpine pretreated (2 x 5 mg/kg, i.p.) guinea-pigs, the content of noradrenaline was 0.5% of control and there was no detectable evoked release of noradrenaline. Under this condition, the release of ATP evoked by electrical stimulation was still detectable, but the amount of ATP was much smaller than that measured from control animals. Xylazine did not reduce the release of ATP. Oxymetazoline, a relatively selective alpha 2-adrenoceptor agonist failed to inhibit the release of [3H]noradrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Different sites of action for alpha 2-adrenoceptor antagonists in the modulation of noradrenaline release and contraction response in the vas deferens of the rat

Rat vas deferens was prepared, loaded with [3H]noradrenaline, and superfused to measure the release of tritium in resting conditions and in response to electrical field stimulation. The alpha 2-adrenoceptor antagonists yohimbine, CH-38083 (7,8-(methylenedioxi)-14 alpha-hydroxyalloberbane HCl), and idazoxan increased the electrically induced release of tritium in a concentration-dependent manner, whereas noradrenaline and the alpha 2-adrenoceptor agonist xylazine exerted opposite effects. The inhibitory effect of noradrenaline on electrically induced tritium release was antagonized by yohimbine, CH-38083, and idazoxan. Of the alpha 2-adrenoceptor antagonists tested, yohimbine and CH-38083 reversed the xylazine-induced inhibition of tritium release, and idazoxan was found to be completely ineffective against xylazine. Idazoxan, yohimbine and CH-38083 antagonized the inhibitory effect of xylazine on electrical stimulation-induced contractions of the vas deferens, as was evidenced by the apparent pA2 values. We conclude from the present experiments that noradrenaline and xylazine inhibit noradrenaline release by acting on distinct prejunctional alpha 2-adrenoceptors and that the receptor subtype that responds to xylazine is insensitive to idazoxan. In addition, inhibition by xylazine of contractility but not of noradrenaline release was antagonized by idazoxan, suggesting that besides noradrenergic neurotransmission, other motor transmitter systems (purinergic) may also be involved in the inhibition by alpha 2-adrenoceptor antagonists of mechanical responses in the rat vas deferens.

Evidence that ATP released from the postsynaptic site by noradrenaline, is involved in mechanical responses of guinea-pig vas deferens: cascade transmission

The release of endogenous ATP and [3H]noradrenaline, and the mechanical response of the guinea-pig vas deferens to field stimulation of its motor nerves were examined using a perfusion system. The release of ATP at rest was 0.83 +/- 0.13 pmol/g per min, and ATP released by field stimulation (8 Hz, 480 shocks) was 5.47 +/- 1.23 pmol/g. The evoked release was completely inhibited when Ca2+ was removed and 1 mM EGTA was added, or by 1 microM tetrodotoxin. The release of ATP and [3H]noradrenaline in response to field stimulation was constant with an S2/S1 ratio of 1.10 +/- 0.11 for ATP and 0.92 +/- 0.03 for [3H]noradrenaline, respectively (where S1 and S2 are stimulation periods). Prazosin (1 microM), a potent alpha 1-adrenoceptor antagonist, significantly reduced the stimulation-evoked release of ATP by 75% and significantly reduced both mechanical twitch and tonic responses, but enhanced the release of [3H]noradrenaline. This finding indicates that there is an alpha 1-adrenoceptor-mediated release of endogenous ATP. However, the prazosin-insensitive portion of ATP release (25%) is considered to be of presynaptic origin. The stimulation of alpha 1-adrenoceptors by 1-noradrenaline or methoxamine in concentrations ranging from 10 to 100 microM resulted in a concentration-dependent release of ATP and a biphasic contraction of the vas deferens: a twitch response was followed by a tonic contraction. Prazosin (1 microM) completely prevented the effect of 1-noradrenaline or methoxamine on both ATP release and mechanical response. When Ca2+ was omitted and EGTA (1 mM) was added, 1-noradrenaline was still able to release ATP but failed to produce contraction. Nifedipine, a Ca-channel and ATP receptor antagonist, reduced the twitch contraction and enhanced the release of ATP from muscle in response to noradrenaline administration. This finding indicates that the release of ATP from the muscle is not linked to mechanical contraction. When the vas deferens was made deficient in noradrenaline by 6-hydroxydopamine pretreatment (100 + 250 mg/kg, i.p.), electrical field stimulation failed to release [3H]noradrenaline and ATP. Under these conditions, exogenous 1-noradrenaline was much more effective in releasing ATP from the smooth muscle, and producing twitch responses, followed by a tonic contraction. After reserpine pretreatment (2 x 5 mg/kg, i.p.), the field stimulation-evoked release of ATP and both phases of contraction were markedly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationship between force and Ca2+ in anococcygeal and vas deferens smooth muscle cells of the mouse

We compared the changes of the cytoplasmic Ca2+ concentration ([Ca2+]i), as measured with the fluorescent Ca2+ indicator fura-2, and the force development in intact smooth muscle of the tonic anococcygeus (AC) and the phasic vas deferens (VD) of the mouse, during activation by K+ depolarization and by agonists. Resting [Ca2+]i was observed to be 33% lower in AC (80 nM) than in VD (115 nM), while the Ca2+ threshold for contraction was found to be about 120 nM in AC and 160 nM in VD. For a similar [Ca2+]i increase, the agonist stimulation induced a higher force development than the K+ depolarization in both muscle types. During prolonged depolarization, the force/calcium ratio increased in AC but strongly declined in VD. This decline of the force/calcium ratio in VD during depolarization was partially reversed by lowering [Ca2+]o. Our results indicate that the Ca2+ threshold for force development was about 150% of the resting [Ca2+]i in both cell types. The resting [Ca2+]i was lower in the tonic AC than in the phasic VD. Agonist-induced sensitization to Ca2+ occurred in both muscle types. The tonic and phasic smooth muscles essentially differed in the respective modulation of their Ca2+ sensitivity during contraction. The desensitization to Ca2+ was specific for phasic muscle, in which it occurred as an early, time- and Ca(2+)-dependent process that was partially reversible.

ATP overflow from the mouse isolated vas deferens

1. A modified form of the highly specific luciferin-luciferase assay was used to measure the overflow of adenosine 5'-triphosphate (ATP) from the field-stimulated mouse vas deferens in vitro. Precise timing of the stimulation, superfusate collection and assay minimized ATP degradation before assay, offering the opportunity for quantitative studies. 2. Stimulation with between 50 and 200 supramaximal pulses at 5 Hz increased ATP overflow by between 7 and 28 times over basal. 3. ATP overflow increased steadily with increasing numbers of stimuli from 50 to 200 pulses. Increasing the frequency of stimulation initially had no effect, but above 10 Hz there was increased overflow, suggesting an interaction between facilitated release and the rate of degradation by extracellular ATPases. 4. Contractions of the vas induced by exogenous (-)-phenylephrine produced only a small increase in ATP overflow, suggesting that the stimulation induced increase in ATP overflow is mostly pre-junctional in origin, though tetrodotoxin eliminated stimulation-induced overflow only in a proportion of preparations.

Efficient Ca2+ mobilization induced by neurokinin A in rat vas deferens

Although both neurokinin A (NKA) and norepinephrine (NE) induced similar maximal contractions in the epididymal and the prostatic site of vas deferens, NKA affected sensitivity more potently than did NE in both sites. The NKA-induced contractions were more strongly inhibited by nicardipine, a dihydropyridine Ca2+ entry blocker, or by elimination of extracellular Ca2+ (Cao2+) in both sites. However, ryanodine, which interferes with the release of intracellular Ca2+ (Cai2+), abolished the contractions caused by NKA in the prostatic site whereas it had no effect in the epididymal site. These results suggest that NKA-induced contraction utilizes both Cai2+ and Cao2+ in the prostatic site but mobilizes only Cao2+ in the epididymal site. Cai2+ concentration [( Ca2+]i) was measured directly with a Ca(2+)-sensitive fluorescent dye, fura-2. In the epididymal site NKA induced contractions with smaller increase in [Ca2+]i compared to that necessary for NE-induced contractions. These results suggest that NKA utilizes Ca2+ more efficiently than does NE and plays a role as a neuromodulator in rat vas deferens.

Effect of presynaptic P2 receptor stimulation on transmitter release

Because ATP is degraded to adenosine, its effect could be mediated by both P1 and P2 receptors. Hence, the actions of an ATP analogue, resistant to enzymatic breakdown (alpha, beta-methylene ATP), were studied on the resting and electrically evoked release of radioactivity from longitudinal muscle strips of guinea pig ileum, preloaded either with [3H]choline or with [3H]noradrenaline. Their effects were compared with the actions of adenosine and ATP. Although adenosine and ATP markedly decreased the [3H]acetylcholine release evoked by field stimulation, alpha,beta-methylene-ATP, a potent and selective agonist of P2x receptors, enhanced this release. However, 2-methyl-2-thio-ATP, an agonist of the P2y receptors, neither enhanced nor inhibited the [3H]-acetylcholine release. 8-Phenyltheophylline, an antagonist of P1 receptors, increased the stimulation-evoked release of acetylcholine, indicating that the release of acetylcholine is tonically controlled by endogenous adenosine via P1 receptors. When alpha,beta-methylene-ATP and 8-phenyltheophylline were added together, their potentiating effect on the acetylcholine release proved to be additive. Because alpha,beta-methylene-ATP failed to antagonize the presynaptic effect of adenosine on P1 purinoceptors, it seems very likely that its effect to enhance transmitter release is mediated via separate receptors, i.e., via P2x receptors, located on the axon terminals. Similarly, the stimulation-evoked release of [3H]noradrenaline was enhanced slightly by alpha,beta-methylene-ATP. Our results suggest that both cholinergic and noradrenergic axon terminals are equipped with P2 receptors through which the stimulation-evoked release of transmitter can be modulated by ATP in a positive manner.(ABSTRACT TRUNCATED AT 250 WORDS)