A dual function for adenosine 5'-triphosphate in the regulation of vascular tone. Excitatory cotransmitter with noradrenaline from perivascular nerves and locally released inhibitory intravascular agent

P2 receptors in cardiovascular regulation and disease

The role of ATP as an extracellular signalling molecule is now well established and evidence is accumulating that ATP and other nucleotides (ADP, UTP and UDP) play important roles in cardiovascular physiology and pathophysiology, acting via P2X (ion channel) and P2Y (G protein-coupled) receptors. In this article we consider the dual role of ATP in regulation of vascular tone, released as a cotransmitter from sympathetic nerves or released in the vascular lumen in response to changes in blood flow and hypoxia. Further, purinergic long-term trophic and inflammatory signalling is described in cell proliferation, differentiation, migration and death in angiogenesis, vascular remodelling, restenosis and atherosclerosis. The effects on haemostasis and cardiac regulation is reviewed. The involvement of ATP in vascular diseases such as thrombosis, hypertension and diabetes will also be discussed, as well as various heart conditions. The purinergic system may be of similar importance as the sympathetic and renin-angiotensin-aldosterone systems in cardiovascular regulation and pathophysiology. The extracellular nucleotides and their cardiovascular P2 receptors are now entering the phase of clinical development.

Inhibitory effects of excess sympathetic activity on parasympathetic vasodilation in the rat masseter muscle

The present study was designed to examine the effect of sympathetic tonic activity on parasympathetic vasodilation evoked by the trigeminal-mediated reflex in the masseter muscle in urethane-anesthetized rats. Sectioning of the superior cervical sympathetic trunk (CST) ipsilaterally increased the basal level of blood flow in the masseter muscle (MBF). Electrical stimulation of the peripheral cut end of the CST for 2 min using 2-ms pulses ipsilaterally decreased in a dependent manner the intensity (0.5-10 V) and frequency (0.1-5 Hz) of the MBF. The CST stimulation for 2 min at <0.5 Hz with 5 V using 2-ms pulses seems to be comparable with the spontaneous activity in the CST fibers innervating the masseter vasculature, because this stimulation restored the basal level of the MBF to the presectioned values. Parasympathetic vasodilation evoked by electrical stimulation of the central cut end of the lingual nerve in the masseter muscle was markedly reduced by CST stimulation for 2 min with 5 V using 2-ms pulses in a frequency-dependent manner (0.5-5 Hz). Intravenous administration of phentolamine significantly reduced the vasoconstriction induced by CST stimulation in a dose-dependent manner (0.1-1 mg/kg), but pretreatment with either phentolamine or propranolol failed to affect the sympathetic inhibition of the parasympathetic vasodilation. Our results suggest that 1) excess sympathetic activity inhibits parasympathetic vasodilation in the masseter muscle, and 2) alpha- and beta-adrenoceptors do not contribute to sympathetic inhibition of parasympathetic vasodilation, and thus some other types of receptors must be involved in this response.

Modulation of ATP/ADP concentration at the endothelial surface by shear stress: effect of flow disturbance

The adenine nucleotides ATP and ADP modulate the release of endothelial-derived relaxing factors and hence play an important role in flow-mediated arterial vasoregulation. Adenine nucleotide concentration at the endothelial cell (EC) surface within an artery is determined by a balance of convective-diffusive delivery of blood-borne nucleotides to the EC surface, hydrolysis of these nucleotides at the cell surface, and flow-induced ATP release from ECs. Previous numerical simulations in a parallel plate flow chamber had demonstrated that flow-induced ATP release has a profound effect on nucleotide concentration under both steady and pulsatile flow conditions. In the present study, we have extended this analysis to probe the impact of disturbed flow downstream of a backward facing step on adenine nucleotide concentration at the EC surface. The results have demonstrated that over a wide range of applied wall shear stress, the ATP concentration at the EC surface drops abruptly within the disturbed flow zone due to increased nucleotide residence time within this region. The concentration is intricately sensitive to the kinetics of flow-induced ATP release, and this sensitivity is more pronounced at lower levels of wall shear stress.

Modulation of ATP/ADP concentration at the endothelial surface by shear stress: effect of flow recirculation

The extracellular presence of the adenine nucleotides ATP and ADP induces calcium mobilization in vascular endothelial cells (ECs). ATP/ADP concentration at the EC surface is determined by a balance of convective-diffusive transport to and from the EC surface, hydrolysis by ectonucleotidases at the cell surface, and flow-induced ATP release from ECs. Our previous numerical simulations in a parallel plate geometry had demonstrated that flow-induced ATP release has a profound effect on nucleotide concentration at the EC surface. In the present study, we have extended the modeling to probe the impact of flow separation and recirculation downstream of a backward facing step (BFS) on ATP/ADP concentration at the EC surface. The results show that for both steady and pulsatile flow over a wide range of wall shear stresses, the ATP+ADP concentration at the EC surface is considerably lower within the flow recirculation region than in areas of undisturbed flow outside the recirculation zone. Pulsatile flow also leads to sharp temporal gradients in nucleotide concentration. If confirmed experimentally, the present findings suggest that disturbed and undisturbed flow may affect EC calcium mobilization differently. Such differences might, in turn, contribute to the observed endothelial dysfunction in regions of disturbed flow.

Direct comparison of adenosine and adenosine 5'-triphosphate as pharmacologic stress agents in conjunction with Tl-201 SPECT: Hemodynamic response, myocardial tracer uptake, and size of perfusion defects in the same subjects

BACKGROUND

Adenosine 5'-triphosphate (ATP), a potent and inexpensive coronary vasodilator, was introduced as a pharmacologic stress agent for thallium 201 single photon emission computed tomography (SPECT). However, there has been no direct comparison of ATP and adenosine as myocardial stressors in the same subjects.

METHODS AND RESULTS

Thirty-six patients underwent consecutive Tl-201 SPECT imaging with adenosine and ATP in a randomly assigned order. There were no changes in clinical status and no invasive procedures were performed between the two tests. The hemodynamic response and side effects were monitored, and myocardial tracer uptake was assessed by use of a visual grading system and quantitative analysis via a CEqual map. The hemodynamic changes and adverse effects did not differ significantly between the two groups. There were no changes in the detection of any perfusion defect on a per-subject basis, except in one. The exact agreement rate for the visual grading of the myocardial tracer uptake was 84.8%. However, the average extent of the perfusion defect and the severity score were higher with adenosine.

CONCLUSION

The hemodynamic changes and the degree of myocardial uptake were similar between the adenosine and ATP infusion. However, quantitative analysis by use of a CEqual map revealed smaller perfusion defects and lower severity scores in subjects undergoing Tl-201 SPECT with ATP.

Metabotropic Ca(2+) channel-induced Ca(2+) release and ATP-dependent facilitation of arterial myocyte contraction

Voltage-gated Ca(2+) channels in arterial myocytes can mediate Ca(2+) release from the sarcoplasmic reticulum and, thus, induce contraction without the need of extracellular Ca(2+) influx. This metabotropic action of Ca(2+) channels (denoted as calcium-channel-induced calcium release or CCICR) involves activation of G proteins and the phospholipase C-inositol 1,4,5-trisphosphate pathway. Here, we show a form of vascular tone regulation by extracellular ATP that depends on the modulation of CCICR. In isolated arterial myocytes, ATP produced facilitation of Ca(2+)-channel activation and, subsequently, a strong potentiation of CCICR. The facilitation of L-type channel still occurred after full blockade of purinergic receptors and inhibition of G proteins with GDPbetaS, thus suggesting that ATP directly interacts with Ca(2+) channels. The effects of ATP appear to be highly selective, because they were not mimicked by other nucleotides (ADP or UTP) or vasoactive agents, such as norepinephrine, acetylcholine, or endothelin-1. We have also shown that CCICR can trigger arterial cerebral vasoconstriction in the absence of extracellular calcium and that this phenomenon is greatly facilitated by extracellular ATP. Although, at low concentrations, ATP does not induce arterial contraction per se, this agent markedly potentiates contractility of partially depolarized or primed arteries. Hence, the metabotropic action of L-type Ca(2+) channels could have a high impact on vascular pathophysiology, because, even in the absence of Ca(2+) channel opening, it might mediate elevations of cytosolic Ca(2+) and contraction in partially depolarized vascular smooth muscle cells exposed to small concentrations of agonists.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

Multiple P2Y receptors couple to calcium-dependent, chloride channels in smooth muscle cells of the rat pulmonary artery

BACKGROUND

Uridine 5'-triphosphate (UTP) and uridine 5'-diphosphate (UDP) act via P2Y receptors to evoke contraction of rat pulmonary arteries, whilst adenosine 5'-triphosphate (ATP) acts via P2X and P2Y receptors. Pharmacological characterisation of these receptors in intact arteries is complicated by release and extracellular metabolism of nucleotides, so the aim of this study was to characterise the P2Y receptors under conditions that minimise these problems.

METHODS

The perforated-patch clamp technique was used to record the Ca2+-dependent, Cl- current (I(Cl,Ca)) activated by P2Y receptor agonists in acutely dissociated smooth muscle cells of rat small (SPA) and large (LPA) intrapulmonary arteries, held at -50 mV. Contractions to ATP were measured in isolated muscle rings. Data were compared by Student's t test or one way ANOVA.

RESULTS

ATP, UTP and UDP (10(-4) M) evoked oscillating, inward currents (peak = 13-727 pA) in 71-93% of cells. The first current was usually the largest and in the SPA the response to ATP was significantly greater than those to UTP or UDP (P < 0.05). Subsequent currents tended to decrease in amplitude, with a variable time-course, to a level that was significantly smaller for ATP (P < 0.05), UTP (P < 0.001) and UDP (P < 0.05) in the SPA. The frequency of oscillations was similar for each agonist (mean approximately to 6-11.min(-1)) and changed little during agonist application. The non-selective P2 receptor antagonist suramin (10(-4) M) abolished currents evoked by ATP in SPA (n = 4) and LPA (n = 4), but pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (10(-4) M), also a non-selective P2 antagonist, had no effect (n = 4, 5 respectively). Currents elicited by UTP (n = 37) or UDP (n = 14) were unaffected by either antagonist. Contractions of SPA evoked by ATP were partially inhibited by PPADS (n = 4) and abolished by suramin (n = 5). Both antagonists abolished the contractions in LPA.

CONCLUSION

At least two P2Y subtypes couple to ICl,Ca in smooth muscle cells of rat SPA and LPA, with no apparent regional variation in their distribution. The suramin-sensitive, PPADS-resistant site activated by ATP most resembles the P2Y11 receptor. However, the suramin- and PPADS-insensitive receptor activated by UTP and UDP does not correspond to any of the known P2Y subtypes. These receptors likely play a significant role in nucleotide-induced vasoconstriction.

Attenuation of extracellular ATP response in cardiomyocytes isolated from hearts subjected to ischemia-reperfusion

Extracellular ATP is known to augment cardiac contractility by increasing intracellular Ca2+ concentration ([Ca2+]i) in cardiomyocytes; however, the status of ATP-mediated Ca2+ mobilization in hearts undergoing ischemia-reperfusion (I/R) has not been examined previously. In this study, therefore, isolated rat hearts were subjected to 10-30 min of global ischemia and 30 min of reperfusion, and the effect of extracellular ATP on [Ca2+]i was measured in purified cardiomyocytes by fura-2 microfluorometry. Reperfusion for 30 min of 20-min ischemic hearts, unlike 10-min ischemic hearts, revealed a partial depression in cardiac function and ATP-induced increase in [Ca2+]i; no changes in basal [Ca2+]i were evident in 10- or 20-min I/R preparations. On the other hand, reperfusion of 30-min ischemic hearts for 5, 15, or 30 min showed a marked depression in both cardiac function and ATP-induced increase in [Ca2+]i and a dramatic increase in basal [Ca2+]i. The positive inotropic effect of extracellular ATP was attenuated, and the maximal binding characteristics of 35S-labeled adenosine 5'-[gamma-thio]triphosphate with crude membranes from hearts undergoing I/R was decreased. ATP-induced increase in [Ca2+]i in cardiomyocytes was depressed by verapamil and Cibacron Blue in both control and I/R hearts; however, this response in I/R hearts, unlike control hearts, was not affected by ryanodine. I/R-induced alterations in cardiac function and ATP-induced increase in [Ca2+]i were attenuated by treatment with an antioxidant mixture and by ischemic preconditioning. The observed changes due to I/R were simulated in hearts perfused with H2O2. The results suggest an impairment of extracellular ATP-induced Ca2+ mobilization in I/R hearts, and this defect appears to be mediated through oxidative stress.

Endothelin (ET)-1-induced inhibition of ATP release from PC-12 cells is mediated by the ETB receptor: differential response to ET-1 on ATP, neuropeptide Y, and dopamine levels

During sympathetic neurotransmitter release, there is evidence for differential modulation of cotransmitter release by endothelin (ET)-1. Using nerve growth factor (NGF)-differentiated PC12 cells, the effects of ET-1 on K(+)-stimulated release of ATP, dopamine (DA), and neuropeptide Y (NPY) were quantified using high-pressure liquid chromatography or radioimmunoassay. ET-1, in a concentration-dependent manner, inhibited the release of ATP, but not DA and NPY. Preincubation with the ET(A/B) antagonist, PD 142893 (N-acetyl-beta-phenyl-D-Phe-Leu-Asp-Ile-Ile-Trp), reversed the inhibitory effect of ET-1 on ATP release, which remained unaffected in the presence of the ET(A)-specific antagonist BQ123 [cyclo(D-Asp-Pro-D-Val-Leu-D-Trp)]. The ET(B) agonists, sarafotoxin 6c (Cys-Thr-Cys-Asn-Asp-Met-Thr-Asp-Glu-Glu-Cys-Leu-Asn-Phe-Cys-His-Gln-Asp-Val-Ile-Trp), BQ 3020 (N-acetyl-[Ala(11,15)]-endothelin 1 fragment 6-21Ac-Leu-Met-Asp-Lys-Glu-Ala-Val-Tyr-Phe-Ala-His-Leu-Asp-IIe-IIe-Trp), and IRL 1620 (N-succinyl-[Glu(9), Ala(11,15)]-endothelin 1 fragment 8-21Suc-Asp-Glu-Glu-Ala-Val-Tyr-Phe-Ala-His-Leu-Asp-Ile-Ile-Trp), decreased K(+)-stimulated release of ATP in a dose-dependent manner, and this effect was reversed by the ET(B) antagonists RES 701-1 [cyclic (Gly1-Asp9) (Gly-Asn-Trp-His-Gly-Thr-Ala-Pro-Asp-Trp-Phe-Phe-Asn-Tyr-Tyr-Trp)] and BQ 788 (N-[N-[N-[(2,6-dimethyl-1-piperidinyl)carbonyl]-4-methyl-l-leucyl]-1-(methoxycarbonyl)-D-tryptophyl]-D-norleucine sodium salt). Preincubation of PC12 cells with pertussis toxin reversed the ET-1-induced inhibition of the K(+)-evoked ATP release. Real-time intracellular calcium level recordings were performed on PC-12 cell suspensions, and ET-1 induced a dose-dependent decrease in the K(+)-evoked calcium levels. Nifedipine, the L-type voltage-dependent Ca(2+) channel antagonist, caused inhibition of the K(+)-stimulated ATP release, but the N-type Ca(2+) channel antagonist, omega-conotoxin GVIA, did not reverse the effect on ATP release. These data suggest that ET-1 modulates the release of ATP via the ET(B) receptor and its associated G(i/o) G-protein through attenuation of the influx of extracellular Ca(2+) through L-type channels.

Circulating ATP-induced vasodilatation overrides sympathetic vasoconstrictor activity in human skeletal muscle

Despite increases in muscle sympathetic vasoconstrictor activity, skeletal muscle blood flow and O2 delivery increase during exercise in humans in proportion to the local metabolic demand, a phenomenon coupled to local reductions in the oxygenation state of haemoglobin and concomitant increases in circulating ATP. We tested the hypothesis that circulating ATP contributes to local blood flow and O2 delivery regulation by both inducing vasodilatation and blunting the augmented sympathetic vasoconstrictor activity. In eight healthy subjects, we first measured leg blood flow (LBF) and mean arterial pressure (MAP) during three hyperaemic conditions: (1) intrafemoral artery adenosine infusion (vasodilator control), (2) intrafemoral artery ATP infusion (vasodilator), and (3) mild knee-extensor exercise (approximately 20 W), and then compared the responses with the combined infusion of the vasoconstrictor drug tyramine, which evokes endogenous release of noradrenaline from sympathetic nerve endings. In all three hyperaemic conditions, LBF equally increased from approximately 0.5 +/- 0.1 l min(-1) at rest to approximately 3.6 +/- 0.3 l min(-1), with no change in MAP. Tyramine caused significant leg vasoconstriction during adenosine infusion (53 +/- 5 and 56 +/- 5% lower LBF and leg vascular conductance, respectively, P < 0.05), which was completely abolished by both ATP infusion and exercise. In six additional subjects resting in the sitting position, intrafemoral artery infusion of ATP increased LBF and leg vascular conductance 27 +/- 3-fold, despite concomitant increases in venous noradrenaline and muscle sympathetic nerve activity of 2.5 +/- 0.2- and 2.4 +/- 0.1-fold, respectively. Maximal ATP-induced vasodilatation at rest accounted for 78% of the peak LBF during maximal bicycling exercise. Our findings in humans demonstrate that circulating ATP is capable of regulating local skeletal muscle blood flow and O2 delivery by causing substantial vasodilatation and negating the effects of increased sympathetic vasoconstrictor activity.

Intracoronary Angiotensin II causes inotropic and vascular effects via different paracrine mechanisms

We hypothesized that Angiotensin II (Ang II), like other circulating hormones, acts exclusively intravascularly. To activate or block solely intravascular Ang II receptors, Ang II and its peptide receptor blocker saralasin (Sar) were covalently coupled to a inert polymer (POL, MW >4000 kD) forming Ang II-POL and Sar-POL. These two nonpermeable polymers, Ang II and Sar, were intracoronarily administered into the isolated, saline-perfused rat hearts. Ang II-POL and Ang II caused a dose-dependent ventricular positive inotropic (+I) and vasoconstrictor effects (+V) which were blocked by Sar. Sar-POL blocked their +I but not their +V. Thus, Ang II and Ang II-POL act on endothelial luminal receptors through paracrine mechanisms. +I were blocked solely by purinoceptor antagonists and paralleled by augmented venous release of ATP degradation products (adenosine, inosine and hypoxanthine). In contrast, +V were blocked solely by aspirin, indomethacin or a thromboxane A2 receptor antagonist. Intracoronary administration of ATP-gamma-S and U46169, a purinergic, and TXA2 agonists, respectively, mimicked +I and +V. The results indicate that ATP is the paracrine inotropic mediator while thromboxane A2 is the vasoconstrictor mediator. Thus, the +I and +V distinct effects by intracoronary Ang II indicate that its diverse mechanism of action along the coronary vascular tree may be due to a functionally heterogeneous endothelium.

Do P2X purinergic receptors regulate skeletal muscle blood flow during exercise?

Although there is evidence that sympathetic nerves release ATP as a neurotransmitter to produce vasoconstriction via P2X purinergic receptors, the role of these receptors in the regulation of blood flow to exercising skeletal muscle has yet to be determined. We hypothesized that there is tonic P2X receptor-mediated vasoconstriction in exercising skeletal muscle. To test this hypothesis, the effect of P2X receptor blockade on skeletal muscle blood flow was examined in six exercising mongrel dogs. P2X receptor antagonism was accomplished with pyridoxal-phosphate-6-azophenyl-2'4'-disulfonic acid (PPADs). Animals were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and a catheter in one femoral artery. PPADs (40 mg) was infused as a bolus into the femoral artery catheter during steady-state exercise at 6 miles/h. Intra-arterial infusion of PPADs increased iliac blood flow from 542 +/- 55 to 677 +/- 69 ml/min (P < 0.05) and iliac vascular conductance from 5.17 +/- 0.62 to 6.53 +/- 0.80 ml.min(-1).mmHg(-1). The PPADs infusion did not affect blood flow in the contralateral iliac artery. These data support the hypothesis that P2X purinergic receptors produce vasoconstriction in exercising skeletal muscle.

Vasoconstriction in active skeletal muscles: a potential role for P2X purinergic receptors?

There is evidence that ATP acts as a neurotransmitter in vascular smooth muscle and is coreleased with norepinephrine from sympathetic nerves. We hypothesized that P2X-receptor stimulation with the selective P2X-receptor agonist alpha,beta-methylene ATP would produce vasoconstriction in resting and exercising skeletal muscle. Six mongrel dogs were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and a catheter in one femoral artery. The selective P2X agonist alpha,beta-methylene ATP was infused as a bolus into the femoral artery catheter at rest and during mild, moderate, and heavy exercise. Intra-arterial infusions of alpha,beta-methylene ATP elicited reductions in vascular conductance of 54 +/- 5, 49 +/- 8, 39 +/- 8, and 30 +/- 6% at rest, 3 miles/h, 6 miles/h, and 6 miles/h at a 10% grade, respectively. The agonist infusions did not affect blood flow in the contralateral iliac artery. To examine whether nitric oxide is responsible for the attenuated vasoconstrictor response to P2X stimulation, the infusions were repeated in the presence of NG-nitro-l-arginine methyl ester. After nitric oxide synthase blockade, intra-arterial infusions of alpha,beta-methylene ATP elicited reductions in vascular conductance of 56 +/- 7, 61 +/- 8, 52 +/- 9, and 40 +/- 7% at rest, 3 miles/h, 6 miles/h, and 6 miles/h at a 10% grade, respectively. P2X-receptor responsiveness was attenuated during exercise compared with rest. Blockade of nitric oxide production did not affect the attenuation of P2X-receptor responsiveness during exercise. These data support the hypothesis that P2X purinergic receptors can produce vasoconstriction in exercising skeletal muscle.

Conducted dilations initiated by purines in arterioles are endothelium dependent and require endothelial Ca2+

The signaling pathways underlying the regulation of vascular resistance by purines in intact microvessels and particularly in communication of remote vasomotor responses are unclear. One process by which remote regions of arterioles communicate is via transmission of signals axially along the vessel wall. In this study, we identified a pathway for local and conducted dilations initiated by purines. Adenosine (Ado) or ATP (bind P1 and P2 purinergic receptors, respectively) was micropipette applied to arterioles (maximum diameter approximately 40 microm) in the cheek pouch of anesthetized hamsters. Observations were made at the site of stimulation (local) or approximately 1200 microm upstream along the same vessel. P2 antagonists (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium and suramin) inhibited local constriction to ATP, whereas local and upstream dilations were unaffected. In contrast, during inhibition of P1 receptors (with xanthine amine congener) the local constriction was unchanged, whereas both local and upstream dilations to ATP were inhibited. Hydrolysis of ATP to Ado is implicated in the dilator response as blocking 5'-ectonucleotidase (with alpha,beta-methyleneadenosine 5'-diphosphate) attenuated ATP-induced dilations. After endothelium denudation, constriction to ATP was unchanged, but dilations to both ATP and Ado were inhibited, identifying endothelial cells (ECs) as the primary target for P1-mediated dilation. Purines increased EC Ca2+ locally and upstream. Chelation of EC Ca2+ (with BAPTA) abolished the local and upstream dilations to P1 receptor stimulation. Collectively, these data demonstrate that stimulation of P1 receptors on ECs produces a vasodilation that spreads to remote regions. There is an associated increase in EC Ca2+, which is a required signaling intermediate in the manifestation of both the local and axially communicated arteriolar dilations.

Erythrocyte and the regulation of human skeletal muscle blood flow and oxygen delivery: role of circulating ATP

Blood flow to contracting skeletal muscle is tightly coupled to the oxygenation state of hemoglobin. To investigate if ATP could be a signal by which the erythrocyte contributes to the regulation of skeletal muscle blood flow and oxygen (O2) delivery, we measured circulating ATP in 8 young subjects during incremental one-legged knee-extensor exercise under conditions of normoxia, hypoxia, hyperoxia, and CO+normoxia, which produced reciprocal alterations in arterial O2 content and thigh blood flow (TBF), but equal thigh O2 delivery and thigh O2 uptake. With increasing exercise intensity, TBF, thigh vascular conductance (TVC), and femoral venous plasma [ATP] augmented significantly (P<0.05) in all conditions. However, with hypoxia, TBF, TVC, and femoral venous plasma [ATP] were (P<0.05) or tended (P=0.14) to be elevated compared with normoxia, whereas with hyperoxia they tended to be reduced. In CO+normoxia, where femoral venous O2Hb and (O2+CO)Hb were augmented compared with hypoxia despite equal arterial deoxygenation, TBF and TVC were elevated, whereas venous [ATP] was markedly reduced. At peak exercise, venous [ATP] in exercising and nonexercising limbs was tightly correlated to alterations in venous (O2+CO)Hb (r2=0.93 to 0.96; P<0.01). Intrafemoral artery infusion of ATP at rest in normoxia (n=5) evoked similar increases in TBF and TVC than those observed during exercise. Our results in humans support the hypothesis that the erythrocyte functions as an O2 sensor, contributing to the regulation of skeletal muscle blood flow and O2 delivery, by releasing ATP depending on the number of unoccupied O2 binding sites in the hemoglobin molecule.

Regional variation in P2 receptor expression in the rat pulmonary arterial circulation

The P2 receptors that mediate contraction of the rat isolated small (SPA, 200-500 micro m i.d.) and large (LPA, 1-1.5 mM i.d.) intrapulmonary arteries were characterized. 2 In endothelium-denuded vessels the contractile order of potency was alpha,beta-methyleneATP (alpha,beta-meATP)>>UDP=UTP=ATP=2-methylthioATP>ADP in the SPA and alpha,beta-meATP=UTP>or=UDP>2-methylthioATP, ATP>>ADP in the LPA. alpha,beta-meATP, 2-methylthioATP and ATP had significantly greater effects in the SPA than the LPA (P<0.001), but there was no difference in the potency of UTP or UDP between the vessels. 3 In the SPA, P2X1 receptor desensitisation by alpha,beta-meATP (100 microM) inhibited contractions to alpha,beta-meATP (10 nM-300 microM), but not those to UTP or UDP (100 nM-300 microM). In the LPA, prolonged exposure to alpha,beta-meATP (100 microM) did not desensitize P2X receptors. 4 Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), suramin and reactive blue 2 (RB2) (30-300 microM) inhibited contractions evoked by alpha,beta-meATP. UTP and UDP were potentiated by PPADS, unaffected by RB2 and inhibited, but not abolished by suramin. 1 and 3 mM suramin produced no further inhibition, indicating suramin-resistant components in the responses to UTP and UDP. 5 Thus, both P2X and P2Y receptors mediate contraction of rat large and small intrapulmonary arteries. P2Y agonist potency and sensitivity to antagonists were similar in small and large vessels, but P2X agonists were more potent in small arteries. This indicates differential expression of P2X, but not P2Y receptors along the pulmonary arterial tree.

Calcium Influx Induced by Stimulation of ATP Receptors on Neurons Cultured from Rat Dorsal Root Ganglia

A combination of microspectrofluorimetry and single cell voltage-clamp was used to examine the response to ATP of cultured neurons from rat dorsal root ganglia. ATP activated an inward current and a rise in internal calcium concentration that was dependent on the external calcium concentration and on the magnitude of the ATP-induced current response. The response was not affected by prerelease of internal calcium stores with caffeine. The rise in internal calcium was increased at hyperpolarized membrane potentials as the calcium driving force was increased. These results demonstrate that the ATP-gated channels in these cells can admit a significant amount of calcium in a physiological calcium gradient. This alternative calcium entry pathway could provide an internal calcium signal that is spatially distinct to that generated by voltage-gated calcium entry.

In vitro responses of equine colonic arterial and venous rings to adenosine triphosphate

OBJECTIVE

To evaluate the in vitro effects of adenosine tryphosphate (ATP) on vasomotor tone of equine colonic vasculature.

SAMPLE POPULATION

Arteries and veins from the left ventral colon of 14 mixed-breed horses euthanatized for reasons unrelated to cardiovascular or gastrointestinal tract disease.

PROCEDURES

Endothelium-intact and -denuded arterial and venous rings were precontracted with 10(-7) and 1.8 x 10(-8) M endothelin-1, respectively. In 1 trial, endothelium-intact rings were also incubated with 10(-4) M N omega-nitro-L-arginine methyl ester (L-NAME) to inhibit nitric oxide (NO) production. Adenosine triphosphate (10(-8) to 10(-3) M) was added in a noncumulative manner, and relaxation percentage versus time curves were generated. Areas under the curves (ie, percentage of relaxation time) were calculated.

RESULTS

Relaxation response of arterial and venous rings to ATP was dose-dependent. Percentage of relaxation time in response to 10(-4) and 10(-3) MATP was significantly greater, compared with that for rings not treated with ATP Removal of endothelium attenuated but did not eliminate the relaxation response. Addition of L-NAME did not attenuate the relaxation response in arteries. At higher concentrations, the vascular response to ATP was biphasic.

CONCLUSIONS AND CLINICAL RELEVANCE

ATP applied to equine colonic arterial and venous rings with and without intact endothelium induced a biphasic response characterized by transient contraction followed by slow, substantial, and sustained relaxation. This ATP-induced response is possibly mediated by a mechanism other than NO. Adenosine triphosphate may be a useful treatment to modulate colonic vasomotor tone in horses with strangulating volvulus of the ascending colon.

Alterations in systemic and local colonic hemodynamic variables associated with intravenous infusion of ATP-MgCl2 in healthy anesthetized horses

OBJECTIVE

To characterize alterations in systemic and local colonic hemodynamic variables associated with IV infusion of ATP-MgCl2 in healthy anesthetized horses.

ANIMALS

12 adult horses.

PROCEDURE

Six horses were given ATP-MgCl2, IV, beginning at a rate of 0.1 mg of ATP/kg of body weight/min with incremental increases until a rate of 1.0 mg/kg/min was achieved. The remaining 6 horses were given an equivalent volume of saline (0.9% NaCl) solution over the same time period. Colonic and systemic hemodynamic variables and colonic plasma nitric oxide (NO) concentrations were determined before, during, and after infusion.

RESULTS

Infusion of ATP-MgCl2 caused a rate-dependent decrease in systemic and colonic vascular resistance, principally via its vasodilatory effects. A rate of 0.3 mg of ATP/kg/min caused a significant decrease in systemic and colonic arterial pressure and colonic vascular resistance without a significant corresponding decrease in colonic arterial blood flow. Consistent alterations in NO concentrations of plasma obtained from colonic vasculature were not detected, despite profound vasodilatation of the colonic arterial vasculature.

CONCLUSIONS AND CLINICAL RELEVANCE

Results revealed that IV infusion of ATP-MgCl2 may be beneficial in maintaining colonic perfusion in horses with ischemia of the gastrointestinal tract, provided a sufficient pressure gradient exists to maintain blood flow.

Sp1-mediated downregulation of P2X4 receptor gene transcription in endothelial cells exposed to shear stress

Endothelial purinoceptors play an important role in vascular responses to extracellular adenine nucleotides and hemodynamic forces. Here we report that P2X4 purinoceptor expression in human umbilical vein endothelial cells is transcriptionally downregulated by fluid shear stress. When human umbilical vein endothelial cells were subjected to a laminar shear stress of 15 dyn/cm(2), P2X4 mRNA levels began to decrease within 1 h and further decreased with time, reaching 60% at 24 h. Functional analysis of the 1.9-kb P2X4 5'-promoter indicated that a 131-bp segment (-112 to +19 bp relative to the transcription start site) containing a consensus binding site for the Sp1 transcription factor was critical for the shear stress responsiveness. Mutations of the Sp1 site decreased the basal level of transcription and abolished the response of the P2X4 promoter to shear stress. Electrophoretic mobility shift assays showed a marked decrease in binding of Sp1 to the Sp1 consensus element in shear-stressed cells, suggesting that Sp1 mediates the shear stress-induced downregulation of P2X4 gene transcription.

Mechanism of prolonged vasorelaxation to ATP in the rat isolated mesenteric arterial bed

1. This study investigated the mechanism of prolonged relaxation to ATP in the rat isolated perfused mesenteric arterial bed. 2. In methoxamine pre-constricted preparations, ATP elicited dose-dependent, endothelium-dependent, rapid relaxation at 5 pmol - 0.05 micromol (R(max) 76+/-5.6%, pD(2) 9.2+/-0.2), and contraction, followed by prolonged endothelium-independent vasorelaxation at 0.05, 0.5 and 5 micromol (56+/-3.0, 87+/-2.9 and 85+/-4.6%). Suramin (100 microM), attenuated rapid (pD(2) 7.8+/-0.1) and prolonged relaxation to ATP. The selective P2 receptor antagonist PPADS (10 microM) reduced prolonged, but not rapid relaxation. Neither phase of relaxation was affected by 8-sulphophenyltheophylline (1 microM) or indomethacin (10 microM). 3. alpha,beta-methylene ATP (alpha,beta-meATP; 10 microM) attenuated prolonged relaxation to ATP (relaxations at 0.05 and 0.5 micromol were 25+/-8.3 and 48+/-9.0%, respectively). alpha,beta-meATP blocked contractions and revealed rapid relaxation to ATP at 0.05 - 5 micromol. 4. Capsaicin pre-treatment did not affect either phase of vasorelaxation to ATP. alpha,beta-meATP (10 microM) had no effect on vasorelaxation mediated by electrical stimulation of capsaicin-sensitive sensory nerves. 5. High K(+) (25 mM) attenuated prolonged relaxation to ATP (21+/-2.6 and 64+/-5.8%, at 0.05 and 0.5 micromol, respectively), but had no effect on rapid relaxation. Ouabain (1 mM), an inhibitor of Na(+)/K(+)-ATPase, and glibenclamide (10 microM), an inhibitor of K(ATP) channels, also attenuated prolonged relaxation to ATP. Charybdotoxin (100 nM), a selective inhibitor of K(Ca) channels, and tetraethylammonium (10 mM) had no effect on rapid or prolonged relaxations. 6. These results show that the prolonged phase of vasorelaxation to ATP in the rat isolated mesenteric arterial bed, which may be mediated by P2Y receptors, is endothelium-independent, involves activation of Na(+)/K(+)-ATPase and K(ATP) channels, and is inhibited by alpha,beta-meATP. Neither prolonged nor rapid vasorelaxation to ATP involves capsaicin-sensitive sensory nerves, adenosine P1 receptors, prostanoids or K(Ca) channels.

Effects of arginine vasopressin in the heart are mediated by specific intravascular endothelial receptors

Arginine vasopressin induces vascular, inotropic and arrhythmogenic effects in the heart. Existing evidence, obtained indirectly, suggests that these effects occur through paracrine endothelial mechanisms. To demonstrate this, vasopressin was confined to the intravascular space by covalent coupling to high molecular weight (2x10(6) Da, vasopresin-dextran) dextran. Isolated guinea pig hearts were infused with equivalent concentrations of vasopressin and vasopressin-dextran. The negative inotropic and coronary vasopressor effects of vasopressin-dextran were similar to those evoked by vasopressin; in both cases effects were reversible. Free dextran had no effect on vascular resistance nor in ventricular developed pressure. The inotropic and vascular effects of both vasopressin and vasopressin-dextran were blocked by the vasopressin receptor antagonist [Adamantaneacetyl(1), o-Et-D-Tyr(2), Val(4), Aminobutyryl(6), Arg(8,9)]vasopressin (Adam-vasopressin), indicating that the effects of the two agonists were vasopressin receptor-mediated. To elucidate possible endothelial intermediaries of these effects, isolated guinea pig hearts were infused simultaneously with vasopressin or vasopressin-dextran and several inhibitors either of synthesis or blockers of receptors of possible endothelial mediators. Only reactive blue 2, a P(2y) purinoceptor antagonist, and suramin, a P(2y) and a P(2x) purinoceptor antagonist, caused a total reversal of vascular and inotropic effects of vasopressin and vasopressin-dextran. Pyridoxalphosphate-6-Azophenyl-2'-4'disulphonic acid, a P(2x) purinoceptor antagonist, was without effect. Our results provide direct evidence that the short-term cardiac effects of vasopressin are due to selective activation of intravascular purinoceptors and suggest that an intermediary of these effects is ATP.

Effect of diadenosine tetraphosphate (AP4A) on coronary arterial microvessels in the beating canine heart

Diadenosine tetraphosphate (AP4A) can be released from activated platelets and the present study examined its effect on coronary arterial microvessels. The role of purinoceptors in the coronary microcirculation in vivo was also investigated. In open chest dogs, coronary arterioles were observed using a microscope with a floating objective. In Protocol 1, AP4A (1, 10, 100 and 1,000 micromol/L) was superfused onto the heart surface before and during the superfusion of 10 micromol/L of 8-phenyltheophylline (8-PT), a P1 purinoceptor blocker. In Protocol 2, AP4A (0.1, 1, 10, and 100 nmol x kg(-1) x min(-1)) was infused into the left anterior descending coronary artery before and during the superfusion of 10 micromol/L of 8-PT. In addition to 8-PT, 30 micromol/L of pyridoxalphosphate-6-azophenyl 2',4'-disulphonic acid (PPADS), a P2X purinoceptor blocker in Protocol 3, or 300 micromol/L of N(omega)-nitro-L-arginine (LNNA) in Protocol 4, was continuously superfused, and 4 doses of AP4A were cumulatively superfused as in Protocol 1. In Protocol 5, 10 micromol/L of alpha,beta-methylene ATP, an agonist of P2X purinoceptors, was superfused for 60 min. Superfused AP4A dilated arterioles in a dose-dependent manner. The magnitude of dilatation was greater in smaller arterioles (small vessel < or = 150 microm: 24.5+/-2.2% vs large vessel > 150 microm: 10.6+/-1.5% at a dose of 1,000 micromol/L, p<0.001). On the other hand, intraluminally applied AP4A also dilated arterioles, but no size dependency was shown. In the presence of 8-PT, vasodilatory responses to superfused and intraluminally applied AP4A were attenuated and the lower doses of AP4A constricted arterioles. This vasoconstrictor effect was not affected by PPADS. The vasodilatory effect of the higher doses of AP4A was almost abolished in the presence of LNNA. Alpha,beta-methylene ATP had no effect on coronary microvascular diameters. AP4A has bidirectional effects on coronary arterial microvessels: vasodilatory effects mediated by P1 purinoceptors and NO, which might be mediated by P2Y purinoceptors, and a vasoconstrictor effect, which is not mediated by P2X purinoceptors.

Downregulation of adenosine and P2X receptor-mediated cardiovascular responses in heart failure rats

Neurohormonal changes in congestive heart failure (CHF) include an enhanced peripheral sympathetic nerve activity which results in increased release of noradrenaline, neuropeptide Y and ATP. To examine if such changes in CHF would modulate peripheral pre- and postsynaptic receptors of ATP and its degradation product adenosine, experiments were performed in a rat model of ischaemic CHF. In this model, ischaemia was induced in rats by ligation of the left coronary artery. Our results demonstrate that there is a selective downregulation of P2X receptor-mediated pressor effects, while the hypotensive effects mediated by the endothelial P2Y receptors are unaffected in CHF. Moreover, the adenosine-mediated inhibitory effects on heart rate and blood pressure were also attenuated in the CHF rats. The most important changes in adenosine and P2-receptor function induced by ischaemic CHF were the reduced pressor effect mediated by the P2X receptor and the increased heart rate due to an attenuated inhibitory effect of adenosine.

Dissociation of muscle sympathetic nerve activity and leg vascular resistance in humans

We examined the hypothesis that the increase in inactive leg vascular resistance during forearm metaboreflex activation is dissociated from muscle sympathetic nerve activity (MSNA). MSNA (microneurography), femoral artery mean blood velocity (FAMBV, Doppler), mean arterial pressure (MAP), and heart rate (HR) were assessed during fatiguing static handgrip exercise (SHG, 2 min) followed by posthandgrip ischemia (PHI, 2 min). Whereas both MAP and MSNA increase during SHG, the transition from SHG to PHI is characterized by a transient reduction in MAP but sustained elevation in MSNA, facilitating separation of these factors in vivo. Femoral artery vascular resistance (FAVR) was calculated (MAP/MBV). MSNA increased by 59 +/- 20% above baseline during SHG (P < 0.05) and was 58 +/- 18 and 78 +/- 18% above baseline at 10 and 20 s of PHI, respectively (P < 0.05 vs. baseline). Compared with baseline, FAVR increased 51 +/- 22% during SHG (P < 0.0001) but returned to baseline levels during the first 30 s of PHI, reflecting the changes in MAP (P < 0.005) and not MSNA. It was concluded that control of leg muscle vascular resistance is sensitive to changes in arterial pressure and can be dissociated from sympathetic factors.

Role of A(2A)-adenosine receptor activation for ATP-mediated coronary vasodilation in guinea-pig isolated heart

1. Adenosine-5'-triphosphate (ATP) and adenosine are potent coronary vasodilators. ATP is rapidly converted to adenosine by ectonucleotidases. We examined whether coronary vasodilation caused by exogenous ATP is mediated by P(2) receptor activation or by A(2A)-adenosine receptor activation. 2. Effects of interventions on coronary conductance were determined by measuring coronary perfusion pressure in guinea-pig isolated hearts perfused at a constant flow of 10 ml min(-1). 3. ATP and adenosine both caused sustained, concentration-dependent increases of coronary conductance. Maximal responses to both agonists were equivalent. The values of pD(2) (+/-s.e.mean) for ATP and adenosine were 6.68+/-0.04 and 7.06+/-0.05, respectively. Adenosine was significantly more potent than ATP (P<0. 0001, n=10). 4. The values of pIC(50) for the selective A(2A)-adenosine receptor antagonist SCH58261 to antagonize equivalent responses to ATP and adenosine were 8.28+/-0.08 and 8.28+/-0.06 (P=0.99, n=6), respectively. 5. The non-selective adenosine receptor antagonists xanthine amine congener (XAC) and CGS15943 antagonized similarly the equivalent vasodilations caused by ATP (pIC(50) values 7.48+/-0.04 and 7.45+/-0.06, respectively) and adenosine (pIC(50) values 7. 37+/-0.13 and 7.56+/-0.11). 6. In contrast to ATP and adenosine, the two P(2) agonists 2-methylthio-ATP and uridine-5'-triphosphate failed to cause stable increases of coronary conductance, caused desensitization of vasodilator responses, and were not antagonized by SCH 58261, 8-parasulphophenyltheophylline, or XAC. 7. Glibenclamide attenuated coronary vasodilations caused by ATP and adenosine by 88 and 89%, respectively, but failed to attenuate those caused by 2-methylthio-ATP. 8. These results strongly suggest that sustained, submaximal coronary vasodilation caused by exogenous ATP is entirely mediated by adenosine acting upon A(2A)-adenosine receptors.

Red blood cell regulation of microvascular tone through adenosine triphosphate

The matching of blood flow with metabolic need requires a mechanism for sensing the needs of the tissue and communicating that need to the arterioles, the ultimate controllers of tissue perfusion. Despite significant strides in our understanding of blood flow regulation, the identity of the O(2) sensor has remained elusive. Recently, the red blood cell, the Hb-containing O(2) carrier, has been implicated as a potential O(2) sensor and contributor to this vascular control by virtue of its concomitant carriage of millimolar amounts of ATP, which it is able to release when exposed to a low-O(2) environment. To evaluate this possibility, we exposed perfused cerebral arterioles to low extraluminal O(2) in the absence and presence of red blood cells or 6% dextran and determined both vessel diameter and ATP in the vessel effluent. Only when the vessels were perfused with red blood cells did the vessels dilate in response to low extraluminal O(2). In addition, this response was accompanied by a significant increase in vessel effluent ATP. These findings support the hypothesis that the red blood cell itself serves a role in determining O(2) supply to tissue.

Systemic adenosine increase during cold pressor test is dependent on sympathetic activation

1. Following local vasoconstriction-inducing stimuli, such as the cold pressor test (CPT), significant changes occur in haemodynamics, with a rise in arterial blood pressure and heart rate (HR) due to the activation of the sympathetic nervous system. Among the compensatory mechanisms to local ischaemia, the endogenous nucleoside adenosine (ADO) has been suggested to play a relevant role by contributing to sympathetic stimulation. The possibility was investigated that CPT-induced increases in plasma ADO levels were not only an expression of the increased production of ADO in the ischaemic area, but also a consequence of systemic sympathoexcitatory mechanisms, thus showing a bidirectional involvement of the mechanisms of ADO formation. 2. The CPT was performed in 15 volunteers and mean arterial blood pressure (MABP) and HR were evaluated, together with plasma levels of noradrenaline (NA) and ADO in the tested and contralateral arm. The 15 subjects were then divided into three groups of five that were treated with either 5 mg transdermal clonidine weekly, 100 mg atenolol daily or 600 mg aminophylline twice daily. After 1 week treatment, the same test was repeated in the respective groups. 3. The CPT induced a rise in MABP and HR and an increase in plasma levels of NA and ADO. Increases in ADO were more pronounced in the tested arm. Clonidine blunted the haemodynamic response and NA release, while increases in ADO increase were reduced to a greater extent in the contralateral arm rather than the tested arm. Atenolol only affected MABP and HR without any effect on NA and ADO levels. Theophylline did not show any effect on CPT-induced changes. 4. In conclusion, local vasoconstriction and ischaemia induced in one arm following CPT are associated with haemodynamic changes dependent on the activation of the sympathetic system. The observed increase in plasma levels of ADO seems to be, in part, a direct expression of local responses to ischaemia (pre-dominant in the tested arm), but also appears as the consequence of systemic sympathoexcitatory mechanisms. Such increases in ADO are not dependent on a beta 1-adrenoceptor-mediated mechanism. Finally, theophylline, at a therapeutic dose, has no effect on the response to CPT.

Interaction between purinoceptor subtypes on hippocampal serotonergic transmission using in vivo microdialysis

The effects of purinoceptor subtypes on hippocampal extracellular serotonin levels were determined by using in vivo microdialysis. Perfusion with adenosine-5'-triphosphate (ATP) for 20 min produced concentration-dependent changes in hippocampal extracellular serotonin levels, which consisted of an initial rise phase, with levels increasing to 309% of control with 100 microM ATP, followed by a later rebound reduction phase, with levels decreasing to 6% of control. The P2X1-7 active P2 purinoceptor agonist, 2-methylthioATP (2-MeSATP: 100 microM) increased the extracellular serotonin level drastically (638%), while the P2X1,3 active P2 purinoceptor agonist, alpha, beta-methylene-L-ATP (alpha, beta-meATP: 100 microM) produced a small increase (132%) in the serotonin level. The P2X1,2,3,5,7 active P2 purinoceptor antagonist, suramin (100 microM), reduced the basal serotonin level (86%) and the ATP-evoked initial rise phase (from 309 to 254%) without affecting the late reduction phase. The adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT: 50 microM) potentiated the rising phase (167%) and abolished the subsequent ATP-evoked reduction phase. Perfusion with CPT and an adenosine A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX: 10 microM), reduced the ATP-evoked initial rise (to 181%) and abolished the late reduction phases of serotonin release. These results indicate that ATP-evoked hippocampal serotonin release is composed of an initial rise phase and a later reduction phase. The ATP-evoked initial rise phase might be produced by an activation of P2X purinoceptor function, whereas the late reduction phase was modulated by the activation of adenosine A1 receptor function by adenosine, metabolized from ATP in the synaptic cleft.

ATP receptors for the protection of hippocampal functions

The inhibitory effects of extracellular adenosine 5'-triphosphate (ATP) are reviewed in the present paper. ATP inhibits the release of the excitatory transmitter glutamate and stimulates the release of the inhibitory transmitter GABA from hippocampal neurons. Also, ATP activates potassium conductance directly through G protein, resulting in hyperpolarization of membrane potential. ATP activates microglia to secrete plasminogen that promotes the development of mesencephalic dopaminergic neurons and enhances neurite outgrowth from explants of neocortical tissue. Moreover, ATP may protect hippocampal neurons from excitotoxic cell death by preserving mitochondrial function. Thus, ATP may have a role in the protection of the function of hippocampus from over-stimulation by glutamate.

Regional differences in sympathetic neurotransmission to cutaneous arteries in the guinea-pig isolated ear

The effects of sympathetic nerve stimulation on different cutaneous arteries were examined in arteries isolated from guinea-pig ears, by measuring membrane potential changes in smooth muscle cells in response to electrical field stimulation. Resting membrane potential (RMP) was similar in proximal (main ear artery) and distal (3rd or 4th branch order) cutaneous arteries (mean -71 mV). Single stimuli evoked excitatory junction potentials (EJPs) in all arteries. The EJPs in proximal arteries were twice the amplitude, and the time constant of EJP decay was almost half the value, compared with distal cutaneous arteries. EJP amplitude was reduced by > 90% by suramin (30 microM) or alpha,beta,methylene-ATP (alpha,beta,m-ATP)(1 microM) in all proximal, and most distal arteries. Residual responses in distal arteries were resistant to tetrodotoxin. The N-type calcium channel blocker, omega-conotoxin GVIA (30 nM), reduced EJP amplitude by 70-100% in both proximal and distal arteries. Successive EJPs evoked by trains of stimuli at 1 to 5 Hz were depressed in amplitude in proximal arteries, but showed facilitation in distal arteries. EJP depression in proximal arteries was reversed to facilitation by the alpha2-adrenoceptor antagonist, yohimbine (30 nM). Trains of stimuli delivered at 10-20 Hz produced summation of EJPs and active membrane responses in 30% of proximal arteries. Active responses were never detected in distal arteries. Slow depolarizations following the EJPs were detected in most arteries after trains of stimuli, and were abolished by prazosin (0.3 microM) or omega-conotoxin GVIA (30 nM). The density of the perivascular plexus of axons innervating proximal arteries, demonstrated with catecholamine fluorescence histochemistry, was twice that in distal cutaneous arteries. These regional differences in sympathetic neurotransmission suggest that cutaneous vasoconstriction in response to thermoregulatory stimuli, which occurs predominantly in distal cutaneous segments, is likely to be qualitatively different from cutaneous vasoconstriction of proximal arteries in response to other physiological stimuli.

Effects of chylomicrons and chylomicron remnants on endothelium-dependent relaxation of rat aorta

The effects of chylomicrons and chylomicron remnants on endothelium-dependent relaxation of rat aorta were studied in vitro. Chylomicrons and chylomicron remnants were prepared in vivo. Aortic rings were incubated with the lipoproteins for 45 min before the vessels were constricted with phenylephrine and concentration relaxation response curves constructed to carbachol, ATP, A23187 and S-nitroso-N-acetylpenicillamine. Maximum % relaxations to carbachol were significantly reduced by both chylomicrons and chylomicron remnants but responses to ATP and S-nitroso-N-acetylpenicillamine were unaffected. In addition, chylomicrons significantly inhibited A23187-induced relaxation, causing an increase in the EC50 value. Chylomicron remnants cause selective inhibition of carbachol-induced relaxation suggesting an action at the receptor or G protein-coupled component of the receptor-mediated activation of the L-arginine-nitric oxide pathway. Chylomicrons appear to be less selective in their inhibition of the endothelium-dependent relaxation. This study demonstrates that lipoprotein particles of dietary origin may cause endothelial cell dysfunction.

Alterations in the expression of P2X1 receptors in failing and nondiseased human atria

This is the first report of the analysis of the ATP-specific P2X1 receptor subunit in human hearts. We have examined homogenate samples of human left atria for the presence of P2X1 receptors using Western blots. Anti-P2X1 immunoreactivity was detected in populations of nondiseased atria as well as in atria from explanted hearts from patients with terminally failing heart conditions such as dilated cardiomyopathy. At least three groups of P2X1 immunoreactive proteins were detected in the Western blots with approximate molecular mass values of 50, 70, and 160 kDa. We report changes in expression of their 50 and 70 kDa components. These changes may be related to the type of deficit in these hearts since the changes have been observed in hearts with decreased ejection fractions characteristic of dilated cardiomyopathy.

Evidence that ATP acts at two sites to evoke contraction in the rat isolated tail artery

1. The site(s) at which P2-receptor agonists act to evoke contractions of the rat isolated tail artery was studied by use of P2-receptor antagonists and the extracellular ATPase inhibitor 6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP (ARL 67156). 2. Suramin (1 microM(-1) mM) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (0.3-300 microM) inhibited contractions evoked by equi-effective concentrations of alpha,beta-methyleneATP (alpha,beta-meATP) (5 microM), 2-methylthioATP (2-meSATP) (100 microM) and adenosine 5'-triphosphate (ATP) (1 mM) in a concentration-dependent manner. Responses to alpha,beta-meATP and 2-meSATP were abolished, but approximately one third of the peak response to ATP was resistant to suramin and PPADS. 3. Contractions evoked by uridine 5'-triphosphate (UTP) (1 mM) were slightly inhibited by suramin (100 and 300 microM) and potentiated by PPADS (300 microM). 4. Desensitization of the P2X1-receptor by alpha,beta-meATP abolished contractions evoked by 2-meSATP (100 microM) and reduced those to ATP (1 mM) and UTP (1 mM) to 15+/-3% and 68+/-4% of control. 5. Responses to alpha,beta-meATP (5 microM) and 2-meSATP (100 microM) were abolished when tissues were bathed in nominally calcium-free solution, while the peak contractions to ATP (1 mM) and UTP (1 mM) were reduced to 24+/-6% and 61+/-13%, respectively, of their control response. 6. ARL 67156 (3-100 microM) potentiated contractions elicited by UTP (1 mM), but inhibited responses to alpha,beta-meATP (5 microM), 2-meSATP (100 microM) and ATP (1 mM) in a concentration-dependent manner. 7. These results suggest that two populations of P2-receptors are present in the rat tail artery; ligand-gated P2X1-receptors and G-protein-coupled P2Y-receptors.

Safety and optimal dose of intracoronary adenosine 5'-triphosphate for the measurement of coronary flow reserve

BACKGROUND

Adenosine 5'-triphosphate (ATP) has been demonstrated to have similar vasodilator potency and fewer hemodynamic or electrocardiographic derangements compared with papaverine in the measurement of coronary flow reserve. However, there is little data about its optimal dose and the effect on myocardial lactate metabolism.

METHODS

Under continuous monitoring of the left anterior descending coronary flow velocity with a Doppler guide wire, we investigated the changes of hemodynamics, electrocardiogram, and myocardial lactate metabolism before and after the administration of 50 microg ATP and 10 mg papaverine into the left coronary artery in 18 patients with normal coronary arteries. To determine the optimal dose of ATP for the coronary flow reserve in the left coronary artery, we measured coronary flow velocity with five incremental doses of intracoronary ATP (0.5, 5, 15, 30, and 50 microg) and 10 mg of papaverine in another seven patients.

RESULTS

In contrast to papaverine, ATP did not produce any significant changes in hemodynamics or the electrocardiogram. The increase in the coronary flow velocity of the two agents was similar. Although all patients showed lactate production after the administration of papaverine, only three patients showed lactate production after ATP (p < 0.001). The coronary flow reserve derived from > or = 215 microg of ATP was similar to that derived from papaverine. There was a significant correlation between the coronary flow reserve obtained with > or = 5 microg of ATP and that obtained with papaverine.

CONCLUSIONS

These results suggest that maximal coronary vasodilation in the left coronary artery can be safely obtained with doses > or = 15 microg of intracoronary ATP in patients with normal coronary arteries.

Stretch activates Jun N-terminal kinase/stress-activated protein kinase in vascular smooth muscle cells through mechanisms involving autocrine ATP stimulation of purinoceptors

Mechanical strain has been implicated in phenotypic changes, including alteration of gene expression in vascular smooth muscle cells; however, the molecular basis for mechanotransduction leading to nuclear gene expression is largely unknown. We demonstrate in the present study that cyclic stretching of vascular smooth muscle cells dramatically activates Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) through an autocrine mechanism. Stretch causes time- and strength-dependent rise of the ATP concentration in media. The stretch-induced activation JNK/SAPK is attenuated by the addition of hexokinase or apyrase that scavenge ATP in media. Both the P2 receptor antagonist and the A1 subtype-selective P1 receptor antagonist partially inhibit stretch-induced activation of JNK/SAPK. The conditioned medium from stretched cells contains an activity to stimulate JNK/SAPK. The JNK-stimulating activity in the conditioned medium from stretched cells is attenuated by the addition of apyrase or P1 and P2 receptor antagonists. The addition of exogenous ATP or adenosine induces dose-dependent activation of JNK/SAPK. These results indicate that stretch activates JNK/SAPK in vascular smooth muscle cells through mechanisms involving autocrine stimulation of purinoceptors by ATP and its hydrolyzed product adenosine.

ADP-induced platelet activation

Platelet activation is central to the pathogenesis of hemostasis and arterial thrombosis. Platelet aggregation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is the first known and an important agonist for platelet aggregation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation induced by ADP and other agonists. ADP also induces platelet shape change, secretion from storage granules, influx and intracellular mobilization of Ca2+, and inhibition of stimulated adenylyl cyclase activity. The ADP-receptor protein mediating ADP-induced platelet responses has neither been purified nor cloned. Therefore, signal transduction mechanisms underlying ADP-induced platelet responses either remain uncertain or less well understood. Recent contributions from chemists, biochemists, cell biologists, pharmacologists, molecular biologists, and clinical investigators have added considerably to and enhanced our knowledge of ADP-induced platelet responses. Although considerable efforts have been directed toward identifying and cloning the ADP-receptor, these have not been completely successful or without controversy. Considerable progress has been made toward understanding the mechanisms of ADP-induced platelet responses but disagreements persist. New drugs that do not mimic ADP have been found to inhibit fairly selectively ADP-induced platelet activation ex vivo. Drugs that mimic ADP and selectively act at the platelet ADP-receptor have been designed, synthesized, and evaluated for their therapeutic efficacy to block selectively ADP-induced platelet responses. This review examines in detail the developments that have taken place to identify the ADP-receptor protein and to better understand mechanisms underlying ADP-induced platelet responses to develop strategies for designing innovative drugs that block ADP-induced platelet responses by acting selectively at the ADP-receptor and/or by selectively interfering with components of ADP-induced platelet activation mechanisms.

Small-conductance Ca(2+)-dependent K+ channels activated by ATP in murine colonic smooth muscle

The patch-clamp technique was used to determine the ionic conductances activated by ATP in murine colonic smooth muscle cells. Extracellular ATP, UTP, and 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP) increased outward currents in cells with amphotericin B-perforated patches. ATP (0.5-1 mM) did not affect whole cell currents of cells dialyzed with solutions containing ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Apamin (3 x 10(-7) M) reduced the outward current activated by ATP by 32 +/- 5%. Single channel recordings from cell-attached patches showed that ATP, UTP, and 2-MeS-ATP increased the open probability of small-conductance, Ca(2+)-dependent K+ channels with a slope conductance of 5.3 +/- 0.02 pS. Caffeine (500 microM) enhanced the open probability of the small-conductance K+ channels, and ATP had no effect after caffeine. Pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS, 10(-4) M), a nonselective P2 receptor antagonist, prevented the increase in open probability caused by ATP and 2-MeS-ATP. PPADS had no effect on the response to caffeine. ATP-induced hyperpolarization in the murine colon may be mediated by P2y-induced release of Ca2+ from intracellular stores and activation of the 5.3-pS Ca(2+)-activated K+ channels.

ATP-induced cochlear blood flow changes involve the nitric oxide pathway

Although control mechanisms of cochlear blood flow (CBF) have been intensively studied since laser Doppler flowmetry was introduced for CBF measurement in animals and humans, the role of adenosine 5'-triphosphate (ATP) in CBF regulation is not known. Since ATP is a potent vasoactive agent in other organs, the aim of this study is to examine ATP-induced changes in CBF and to test whether the nitric oxide pathway is involved in ATP-induced CBF changes. The anterior inferior cerebellar artery (AICA) of anesthetized pigmented guinea pigs was exposed, and ATP was perfused into the AICA. For CBF measurement, the bulla was opened and the 0.7 mm laser probe of a Perimed PF2B flowmeter was positioned on the basal turn of the cochlea. AICA perfusion of an ATP solution caused dose-dependent transient CBF increases. The maximum CBF increase induced was 220% of the baseline. In some animals, CBF showed a dual effect; a transient decrease followed by a longer-lasting increase. The perfusions of sodium nitroprusside (SNP) also resulted in dose-dependent CBF changes. The intravenous application of N(omega)-nitro-L-arginine methyl ester (L-NAME) significantly attenuated ATP-induced CBF increases, and enhanced ATP-induced decreases, but did not affect SNP-induced CBF changes. The ATP-induced CBF responses indicate that ATP plays a role in CBF regulation. The biphasic characteristic of the ATP-induced CBF change suggests the involvement of both P2x- and P2y-subtype purinoceptors. That L-NAME caused attenuation of the ATP-induced CBF increase implies that the ATP-induced CBF increase is mediated by the release of endothelium-derived relaxing factor, nitric oxide, following activation of endothelial P2y-purinoceptors in the cochlear vascular bed and/or cochlear supplying vessels.

Noradrenergic and peptidergic sympathetic regulation of cutaneous microcirculation in the rat

Cutaneous microcirculatory changes were measured by laser-Doppler flowmetry in response to electrical stimulation of sympathetic efferent fibres of the rat's saphenous nerve. After perineural capsaicin (2%) pretreatment, electrical stimulation of the peripheral stump of the cut saphenous nerve evoked a reduction in blood flow (vasoconstriction) followed by a minimal enhancement. This late vasodilatation was further reduced by resiniferatoxin (1 microg/kg i.v.), and vasoconstriction was abolished by guanethidine (8 mg/kg i.v.), indicating the involvement of sensory and sympathetic fibres in the respective responses. The vasoconstrictor response was analysed after blockade of antidromic vasodilatation by combined capsaicin-resiniferatoxin pretreatment. alpha-Adrenoceptor antagonists (1 mg/kg phentolamine, 0.5 mg/kg prazosin and 1 mg/kg GYKI-12743 (RS-2-(3)N-(2-benzo;1,4i-dioxanyl)-methylamino(propyl)-3(2H) -piridazinone hydrochloride) inhibited, but did not eliminate the blood flow reduction evoked by 3 Hz stimulation. At 10 Hz stimulation significant inhibition was obtained only with GYKI-12743. No inhibition was observed with propranolol (10 microg/kg) on any occasion. A functional neuropeptide Y antagonist, alpha-trinositol (D-myo-inositol-1,2,6-trisphosphate, PP56; 50 mg/kg i.v.), markedly diminished the vasocontrictor response remaining after treatments with the alpha-adrenoceptor blocking agents. Inhibition was more pronounced at 10 Hz. Since 3 Hz corresponds to an average, and 10 Hz approaches the maximal firing rate of the sympathetic efferents, these results emphasise the significant role of neuropeptide Y in regulation of the cutaneous microcirculation by sympathetic fibres under physiological circumstances, particularly during high activity.

Nonadrenergic contractile response of guinea pig portal vein to electrical field stimulation mimics response to UTP but not to ATP

Transmural electrical field stimulation (EFS, 4-32 Hz) produced a biphasic contractile response consisting of a rapid and transient contraction (first phase) followed by a slow contraction (second phase) in ring preparations of guinea pig portal veins. Both contractions were enhanced by the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 30 microM). In the presence of L-NAME, tetrodotoxin (1 microM) and guanethidine (3 microM) inhibited both contractions and phentolamine (10 microM), and reserpine treatment abolished the first-phase contraction without affecting the second-phase contraction. These results suggest that the first-phase contraction is caused by norepinephrine released from the perivascular nerves. In the presence of phentolamine and L-NAME, the second-phase contraction was inhibited by the nonselective P2-purinoceptor antagonist suramin (30-300 microM) and the P(2Y)-purinoceptor antagonist reactive blue 2 (RB2; 10-100 microM). alpha,beta-Methylene-adenosine triphosphate (alpha,beta-mATP; 3-30 microM), which desensitizes P(2X)-purinoceptors, and the P(2X)-purinoceptor antagonist 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS; 1-10 microM) had a little effect. Exogenous ATP (0.1-3 mM) and UTP (0.1-3 mM) in the presence of L-NAME produced contractions in a concentration-dependent manner. The ATP-induced contraction was enhanced by suramin, RB2, and DIDS but unaltered by alpha,beta-mATP. The UTP-induced contraction was inhibited by suramin and RB2 but unaltered by alpha,beta-mATP and DIDS. These results indicate that in the guinea pig portal vein, the classic P(2X)-purinoceptors do not contribute to the nonadrenergic component of sympathetic neurotransmission. Furthermore, the pharmacology of the nonadrenergic component of neurotransmission resembles that of vasoconstrictor responses to exogenous UTP rather than to ATP.

Involvement of phosphatidylcholine hydrolysis by phospholipase D in extracellular ATP-induced arachidonic acid release in aortic smooth muscle cells

We investigated the effect of extracellular ATP on phosphatidylcholine-hydrolyzing phospholipase D activity and the role of phospholipase D activation in extracellular ATP-induced arachidonic acid release in cultured rat aortic smooth muscle cells. ATP significantly stimulated the formation of choline in a dose-dependent manner in the range between 0.01 and 0.5 mmol/L. However, ATP had no effect on the formation of phosphocholine. Staurosporine, an inhibitor of protein kinases, did not affect the ATP-induced formation of choline. ATP significantly stimulated arachidonic acid release in a dose-dependent manner in the range between 0.01 and 0.5 mmol/L. DL-Propranolol hydrochloride (propranolol), an inhibitor of phosphatidic acid phosphohydrolase, significantly inhibited the ATP-induced release of arachidonic acid. 1,6-Bis(cyclohexyloximinocarbonylamino)-hexane (RHC-80267), a potent and selective inhibitor of diacylglycerol lipase, reduced ATP-induced arachidonic acid release. Quinacrine, a phospholipase A2 inhibitor, suppressed ATP-induced arachidonic acid release. Both propranolol and RHC-80267 markedly inhibited the ATP-induced synthesis of 6-ketoprostaglandin F1 alpha, a stable metabolite of prostacyclin. These results strongly suggest that extracellular ATP activates phosphatidylcholine-hydrolyzing phospholipase D independently of protein kinase C in aortic smooth muscle cells and that the arachidonic acid release induced by extracellular ATP is mediated, at least in part, through phosphatidylcholine hydrolysis by phospholipase D activation.