Physiological importance of ATP released from nerve terminals and its degradation to adenosine in humans

Role of purinergic receptors in cardiac sympathetic nerve injury in diabetes mellitus

Diabetic cardiac autonomic neuropathy is a common and serious chronic complication of diabetes, which can lead to sympathetic and parasympathetic nerve imbalance and a relative excitation of the sympathetic nerve. Purinergic receptors play a crucial role in this process. Diabetic cardiac sympathetic nerve injury affects the expression of purinergic receptors, and activated purinergic receptors affect the phosphorylation of different signaling pathways and the regulation of inflammatory processes. This paper introduces the abnormal changes of sympathetic nerve in diabetes mellitus and summarizes the recently published studies on the role of several purinergic receptor subtypes in diabetic cardiac sympathetic nerve injury. These studies suggest that purinergic receptors as novel drug targets are of great significance for the treatment of diabetic autonomic neuropathy. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

The infarct-sparing effect of IB-MECA against myocardial ischemia/reperfusion injury in mice is mediated by sequential activation of adenosine A3 and A 2A receptors

Conflicting results exist regarding the role of A3 adenosine receptors (A3ARs) in mediating cardioprotection during reperfusion following myocardial infarction. We hypothesized that the effects of the A3AR agonist IB-MECA to produce cardioprotection might involve activation of other adenosine receptor subtypes. C57Bl/6 (B6), A3AR KO, A2AAR KO, and A2AAR KO/WT bone marrow chimeric mice were assigned to 12 groups undergoing either hemodynamic studies or 45 min of LAD occlusion and 60 min of reperfusion. IB-MECA (100 μg/kg) or vehicle was administered by iv bolus 5 min before reperfusion. Radioligand binding assays showed that IB-MECA has high affinity for the mouse A3AR (K i = 0.17 ± 0.05 nM), but also can bind with lower affinity to the A1AR (9.0 ± 2.4 nM) or the A2AAR (56.5 ± 10.2 nM). IB-MECA caused bi-phasic hemodynamic changes, which were completely absent in A3AR KO mice and were modified by A2AAR blockade or deletion. IB-MECA stimulated histamine release, increased heart rate, and significantly reduced IF size in B6 mice from 61.5 ± 1.4 to 48.6 ± 2.4% of risk region (RR; 21% reduction, p < 0.05) but not in A3AR KO mice. Compared to B6, A3AR KO mice had significantly reduced IF size (p < 0.05). In B6/B6 bone marrow chimeras, IB-MECA caused a 47% reduction of IF size (from 47.3 ± 3.9 to 24.7 ± 4.5, p < 0.05). However, no significant cardioprotective effect of IB-MECA was observed in A2AARKO/B6 mice, which lacked A2AARs only on their bone marrow-derived cells. Activation of A3ARs induces a bi-phasic hemodynamic response, which is partially mediated by activation of A2AARs. The cardioprotective effect of IB-MECA is due to the initial activation of A3AR followed by activation of A2AARs in bone marrow-derived cells.

Extracellular ATP signaling in equine digital blood vessels

The functional distribution of ATP-activated P2 receptors is well characterized for many blood vessels, but not in the equine digital vasculature, which is a superficial vascular bed that displays thermoregulatory functions and has been implicated in ischemia-reperfusion injuries of the hoof. Isolated equine digital arteries (EDA) and veins (EDV) were submitted to isometric tension studies, whereby electric field stimulation (EFS) and concentration-response curves to exogenously applied agonists were constructed under low tone conditions. Additionally, immunofluorescent localization of P2X and P2Y receptor subtypes was performed. EFS-induced constriction was abolished by tetrodotoxin (1 μM, n=4). Endothelium denudation did not modify the EFS-induced constriction (n=3). The EFS-induced constriction in EDA was inhibited by phentolamine (67.7±1.8%, n=6; 10 μM), and by the non-selective P2 receptor antagonist suramin (46.2±1.3%, n=6; 10 μM). EFS-induced constriction in EDV was reduced by suramin (48.2±2.4%, n=6; 10 μM), the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (58.3±4.5%, n=6; 10 μM), and phentolamine (23.2±2.5%, n=6; 10 μM). Exogenous methoxamine and ATP mimicked EFS-induced constriction in EDA and EDV. Immunostaining for P2X1, P2X2 and P2X3, and, for P2X1 and P2X7 receptor subunits were observed in EDA and EDV smooth muscle and adventitia, respectively. ATP and noradrenaline are co-transmitters in sympathetic nerves supplying the equine digital vasculature, noradrenaline being the dominant agonist in EDA, and ATP in EDV. In conclusion, P2X receptors mediate vasoconstriction in EDA and EDV, although different P2X subunits are involved in these vessels. The physiological significance of this finding in relation to thermoregulatory functions and equine laminitis is discussed.

Sources of intravascular ATP during exercise in humans: critical role for skeletal muscle perfusion

Exercise hyperaemia is regulated by several factors, and one factor known to increase with exercise that evokes a powerful vasomotor action is extracellular ATP. The origin of ATP detected in plasma from exercising muscle of humans is, however, a matter of debate, and ATP has been suggested to arise from sympathetic nerves, blood sources (e.g. erythrocytes), endothelial cells and skeletal myocytes, among others. Therefore, we tested the hypothesis that acute augmentation of sympathetic nervous system activity (SNA) results in elevated plasma ATP draining skeletal muscle, and that SNA superimposition during exercise increases ATP more than exercise alone. We showed that increased SNA via -40 mmHg lower body negative pressure (LBNP) at rest did not increase plasma ATP (51±8 nmol l(-1) at rest versus 58±7 nmol l(-1) with LBNP), nor did it increase [ATP] above levels observed during rhythmic hand-grip exercise (79±11 nmol l(-1) with exercise alone versus 71±8 nmol l(-1) with LBNP). Next, we tested the hypothesis that active perfusion of skeletal muscle is essential to observe increased plasma ATP during exercise. We showed that complete obstruction of blood flow to contracting muscle abolished exercise-mediated increases in plasma ATP (from 90±19 to 49±12 nmol l(-1)), and that cessation of blood flow prior to exercise completely inhibited the typical rise in ATP (3 versus 61%, obstructed versus intact perfusion). The lack of change in ATP during occlusion occurred in the face of continued muscular work and elevated SNA, indicating that the rise of intravascular ATP did not result from these extravascular sources. Our collective observations indicated that the elevation in extracellular ATP observed in blood during exercise was unlikely to originate from sympathetic nerves or the contacting muscle itself, but rather was dependent on intact skeletal muscle perfusion. We conclude that an intravascular source for ATP is essential, which indicates an important role for blood sources (e.g. red blood cells) in augmenting and maintaining elevated plasma ATP during exercise.

P2X(7) Receptors in Neurological and Cardiovascular Disorders

P2X receptors are ATP-gated cation channels that mediate fast excitatory transmission in diverse regions of the brain and spinal cord. Several P2X receptor subtypes, including P2X(7), have the unusual property of changing their ion selectivity during prolonged exposure to ATP, which results in a channel pore permeable to molecules as large as 900 daltons. The P2X(7) receptor was originally described in cells of hematopoietic origin, and mediates the influx of Ca(2+) and Na(+) and Ca(2+) and Na(+) ions as well as the release of proinflammatory cytokines. P2X(7) receptors may affect neuronal cell death through their ability to regulate the processing and release of interleukin-1beta, a key mediator in neurodegeneration, chronic inflammation, and chronic pain. Activation of P2X(7), a key mediator in neurodegeneration, chronic inflammation, and chronic pain. Activation of P2X(7) receptors provides an inflammatory stimulus, and P2X(7) receptor-deficient mice have substantially attenuated inflammatory responses, including models of neuropathic and chronic inflammatory pain. Moreover, P2X(7) receptor activity, by regulating the release of proinflammatory cytokines, may be involved in the pathophysiology of depression. Apoptotic cell death occurs in a number of vascular diseases, including atherosclerosis, restenosis, and hypertension, and may be linked to the release of ATP from endothelial cells, P2X(7) receptor activation, proinflammatory cytokine production, and endothelial cell apoptosis. In this context, the P2X(7) receptor may be viewed as a gateway of communication between the nervous, immune, and cardiovascular systems.

Neurotransmitter localization in the neuroepithelial cells and unipolar neurons of the respiratory tract in the bichir, Polypterus bichir bichir G. ST-HIL

Immunohistochemical localisation of neurotransmitters was used to determine the distribution of unipolar neurons and neuroepithelial cells (NECs) in the respiratory tract of the bichir, Polypterus bichir bichir. NECs were commonly encountered in the mucociliated epithelium of the lung. Unipolar neurons were located in the submucosal and muscle layers of the glottis. The results suggest the presence of tyrosine hydroxylase (TH) and nNOS immunoreactivities in NECs. In addition, ACh-E/nNOS and TH/nNOS nerve fibers were also found associated with these cells. Unipolar neuronal cells showed a chemical code including the presence of 5-HT, ACh-E, peptides and P2x2 receptors. The present findings indicate that nitric oxide (NO) is a primitive transmitter of neuroepithelial oxygen-sensitive chemoreceptor cells together with acetylcholine. The coexistence of ACh-E with other substances in the unipolar neurons, but not with NO, may be a property of vagal postganglionic neurons since the emergence of the cranial autonomic pathways in the earliest vertebrates. It would be interesting to know about the provenance of the nerves in contact with NECs, which appear to have a complex innervation pattern.

Increased expression of adenosine A2A receptors in patients with spontaneous and head-up-tilt-induced syncope

BACKGROUND

Adenosine may play a role in the triggering of neurocardiogenic syncope, but no information on adenosine receptors is available at the present time.

OBJECTIVE

The purpose of this study was to investigate whether adenosine A2A receptors expression is altered in patients with neurocardiogenic syncope.

METHODS

Adenosine plasma levels (APLs), the expression of A2A receptors, were measured (mean +/- standard error of the mean) during tilt testing. Expression of receptors was assessed on mononuclear cells using a selective receptor ligand.

RESULTS

At baseline, the APLs of 16 patients with a positive test were higher than those of 17 patients with a negative test and of those of a control group (2.10 +/- 0.30 vs. 0.40 +/- 0.05 and 0.41 +/- 0.06 muM, respectively; P <.0001). The number of receptors was higher in patients tested positive than in patients tested negative or in the control group (122 +/- 10 vs. 38 +/- 4 and 44 +/- 4 fmol/g of proteins, respectively; P <.0001). No difference was found in the affinity or synthesis among the three groups.

CONCLUSION

This study showed an increased number and an up-regulation of adenosine A2A receptors in patients with spontaneous syncope and a positive head-up tilt, which in the context of high APLs may play a role in the recurrence of syncopal episodes.

Effects of dynamic exercise on mean blood velocity and muscle interstitial metabolite responses in humans

We examined the effects of dynamic one-legged knee extension exercise on mean blood velocity (MBV) and muscle interstitial metabolite concentrations in healthy young subjects (n = 7). Femoral MBV (Doppler), mean arterial pressure (MAP) and muscle interstitial metabolite (adenosine, lactate, phosphate, K(+), pH, and H(+); by microdialysis) concentrations were measured during 5 min of exercise at 30 and 60% of maximal work capacity (W(max)). MAP increased (P < 0.05) to a similar extent during the two exercise bouts, whereas the increase in MBV was greater (P < 0.05) during exercise at 60% (77.00 +/- 6.77 cm/s) compared with 30% W(max) (43.71 +/- 3.71 cm/s). The increase in interstitial adenosine from rest to exercise was greater (P < 0.05) during the 60% (0.80 +/- 0.10 microM) compared with the 30% W(max) bout (0.57 +/- 0.10 microM). During exercise at 60% W(max), interstitial K(+) rose at a greater rate than during exercise at 30% W(max) (P < 0.05). However, pH increased (H(+) decreased) at similar rates for the two exercise intensities. During exercise, interstitial lactate and phosphate increased (P < 0.05) with no difference observed between the two intensities. After 5 min of recovery, MBV decreased to baseline levels after exercise at 30% W(max) (4.12 +/- 1.10 cm/s), whereas MBV remained above baseline levels after exercise at 60% W(max) (Delta19.46 +/- 2.61 cm/s; P < 0.05). MAP and interstitial adenosine, K(+), pH, and H(+) returned toward baseline levels. However, interstitial lactate and phosphate continued to increase during the recovery period. Thus an increase in exercise intensity resulted in concomitant changes in MBV and muscle interstitial adenosine and K(+), whereas similar changes were not observed for MAP or muscle interstitial pH, lactate, or phosphate. These data suggest that K(+) and/or adenosine may play an active role in the regulation of skeletal muscle blood flow during exercise.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Adenosine 5'-triphosphate and neuropeptide Y are co-transmitters in conjunction with noradrenaline in the human saphenous vein

1. Human saphenous veins were used to assess the cooperative participation of adenosine 5-triphosphate (ATP), neuropeptide Y (NPY), and noradrenaline (NA) in the vasomotor responses elicited following electrical depolarization of the perivascular nerve terminals. Rings from recently dissected human biopsies were mounted to record isometric muscular contractions; the motor activity elicited in the circular muscle layer following electrical depolarization (2.5-20 Hz, 50 V, 0.5 msec) were recorded. 2. Incubation of the biopsies with either 100 nM tetrodotoxin (TTX) or 1 microM guanethidine abolished the vasomotor response elicited by electrical nerve depolarization. The independent application of either ATP or NA to vein rings induced concentration-dependent contractions. 3. Tissue incubation with 30 microM suramin or 10 nM prazosin produced 10 fold rightward displacements of the alpha,beta-methylene ATP and NA concentration-response curves respectively. NPY contracted a limited number of biopsies, the vasoconstriction elicited was completely blocked by 1 microM BIBP 3226. A 5 min incubation of the biopsies with 10-100 nM NPY synergized, in a concentration-dependent fashion, both the ATP and the ATP analogue-induced contractions. Likewise, tissue preincubation with 10 nM NPY potentiated the vasomotor responses evoked with 20-60 nM NA. 4. Neither suramin, BIBP 3226, nor prazosin was individually able to significantly modify the derived frequency-tension curves. In contrast, the co-application of 30 microM suramin and 10 nM prazosin or 30 microM suramin and 1 microM BIBP 3226, elicited a significant (P<0.01) downward displacement of the respective frequency-tension curves. 5. The simultaneous application of the three antagonists-30 microM suramin, 1 microM BIBP 3226 and 10 nM prazosin-caused a significantly greater displacement of the frequency-tension curve than that achieved in experiments using two of these antagonists. 6. Electrically-evoked vasomotor activity is blocked to a larger extent by tissue incubation with 2.5 microM chloroethylclonidine and 30 microM suramin rather than with 10 nM 5 methyl urapidil and 30 microM suramin. As a result, the alpha1-adrenoceptor involved in the vasomotor activity has tentatively been associated with the alpha1B adrenoceptor family subtype. 7. Results support the physiological role of ATP in sympathetic neurotransmission. The present results are consistent with the working hypothesis that human sympathetic vasomotor reflexes involve the coordinated motor action of ATP, NPY, and NA acting on vascular smooth muscle cells. The present results support the concept of sympathetic co-transmission in the human saphenous vein.

A possible dual physiological role of extracellular ATP in the modulation of platelet aggregation

ATP and ADP are simultaneously released from activated platelets in equimolar concentrations. Micromolar concentrations of ATP inhibit platelet aggregation by both competitive and non-competitive mechanisms. The current studies addressed the question of how platelets respond to agonists in the presence of nanomolar and micromolar concentrations of ATP and ADP alone or in combination. This is a significant issue since the concentration of ATP +/- ADP may vary widely within a microenvironment depending upon the source and cause for the release of the nucleotides. ATP (1-10 nM) was found to significantly enhance the thromboxane A2 analog, U44619-, collagen- and thrombin-induced platelet aggregations. Conversely, ATP at 1-100 microM inhibited these same reactions. ADP, in general, behaved exactly opposite to ATP. When equal amounts of ATP and ADP were added together the ADP response appeared to predominate. The observed ATP-induced response was not due to a hydrolytic product as evidenced by an unaltered response to ATP in the presence of adenosine deaminase or the ATP generating system, creatine phosphate plus creatine phosphokinase. Adenosine (1-10 nM), like ADP, inhibited agonist-induced platelet aggregation. The stimulation of agonist-induced platelet aggregation by 1-10 nM extracellular ATP appears to depend upon the phosphorylation of platelet membrane ecto proteins. The ATP analog, beta gamma-methylene ATP, that is incapable of serving as a phosphate donor for protein kinases, inhibited rather than stimulated agonist-induced platelet aggregation. The dual response of platelets to low and high concentrations of extracellular ATP +/- ADP may play a physiological role in hemostasis and thrombosis under normal and pathological conditions.

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.

Aggregation of human and canine platelets: modulation by purine nucleotides

This study compared the responses of canine and human platelets to various aggregating agonists in the presence or absence of extracellular ATP and ATP analogues. Canine and human platelets were approximately equally reactive with ADP or collagen while the canine platelets were about 10 fold more sensitive to thrombin. Canine platelets were insensitive to the thromboxane mimetic U46619 but were synergistically aggregated by a mixture of ADP and U46619. Human platelets were very sensitive to U46619. Aggregations of human platelets with all of the above agonists were inhibited by extracellular ATP; beta, gamma methylene ATP (beta gamma ATP) and benzoyl ATP (BzATP) with a rank order suggestive of an interaction with P2x-like purinoceptors which support our previous findings. The comparable aggregations of canine platelets were likewise inhibited by ATP and its analogues but with a rank order suggestive of an interaction with P2y-like purinoceptors. ATP inhibited U46619- and ADP-induced aggregation of human platelets and ADP-induced aggregation of canine platelets, presumably, in part, due to competition for the ADP P2T receptor. However, when U46619 was added to either ATP or ATP analogue-inhibited ADP-treated canine platelets, the inhibition was nullified. Furthermore, we demonstrated, for the first time, that the canine thromboxane receptor becomes reactive to U46619 alone after incubation at room temperature for 3.5-5 hrs while human platelets become inactive under similar conditions. The implication of these studies is that there are significant differences in the canine and human platelet thromboxane and purine receptors. The future characterization of these differences and the mechanism by which they function should further our understanding of the impact of extracellular ATP on hemostasis and thrombosis.

Occupancy of P2 purinoceptors with unique properties modulates the function of human platelets

This report demonstrates that platelets possess P2 purinoceptors with unique properties that distinguish them from the ADP (P2T) receptor. Extracellular ATP, and its poorly hydrolyzable analogues, inhibit collagen- and U46619 (a thromboxane mimetic)-induced platelet aggregations. Adenosine deaminase was without effect on ATP action while reversing the inhibitory effect of adenosine. A unique aspect of the P2 receptor is the sensitivity to UTP and CTP and insensitivity to GTP. The rank order of inhibition by beta gamma-methylene ATP, alpha beta-methylene ATP > ATP indicates that a P2x-like receptor is present on the platelet membrane. This conclusion is further supported by the nearly complete desensitization to ATP by pre-exposure of platelets to alpha beta-methylene-ATP. However, unlike previously described P2x purinoceptors, the inhibition of platelet aggregation by extracellular ATP appears to result, at least in part, from the ATP-induced increase of intracellular cyclic AMP levels apparently coupled through a Gs protein. The combined addition of iloprost (0.14 to 1.39 nM) and ATP (18 microM) or ATP (20-40 microM) and the phosphodiesterase inhibitor theophylline (0.5 to 1 mM) synergistically inhibited platelet aggregation implying a common interactive site with adenylate cyclase. This is further substantiated by the ability of the adenylate cyclase inhibitor, 2',5'-dideoxyadenosine, to abrogate the inhibitory effects of ATP. The protein kinase A (PKA) inhibitor H1004 blocks ATP inhibition of platelet aggregation while the protein kinase C inhibitor H7 did not. This implies that the generation of cyclic AMP, with the subsequent activation of PKA and phosphorylation of selected proteins is required, in part, for the action of ATP.

Activation of sympathetic tone during dipyridamole test

Cardiac imaging with dipyridamole infusion has been proposed as an exercise-independent tool for the diagnosis of coronary artery disease. Dipyridamole acts through the accumulation of adenosine, which reduces sympathetic tone in vasomotor nuclei of the brainstem and inhibits norepinephrine release in noradrenergic neurons but also activates arterial chemoreceptors. The aim of this study was to assess whether dipyridamole administration (up to 0.84 mg/kg over 10 minutes, a dosage commonly employed for diagnostic testing) may modulate sympathetic activity either directly or indirectly through blood pressure reduction or myocardial ischemia, which may be evoked by dipyridamole infusion and represent two recognized sympathetic stimuli. Twenty patients were studied with infusion combined with two-dimensional echocardiography and 12-lead ECG monitoring. Blood pressure was recorded each minute by a cuff sphygmomanometer. In all patients, we obtained venous blood samples for epinephrine (an index of adrenomedullary catecholamine release) and norepinephrine (an index of neuronal activity) both in resting conditions and at peak dipyridamole, ie, at the first minute after termination of dipyridamole infusion in negative cases or in the presence of obvious ischemia in positive cases (ie, as soon as a regional ventricular dyssynergy or an ST segment depression greater than 0.1 mV appeared). Epinephrine and norepinephrine determinations were made by a high performance liquid chromatography (HPLC) method. After dipyridamole, there was a significant rise in norepinephrine, while epinephrine did not change significantly. Dipyridamole-induced percentage variations of norepinephrine from baseline were not significantly correlated with mean blood pressure changes (r = .1, p = ns) and were of a similar extent in patients with (n = 10) and without (n = 10) dipyridamole-induced ischemia (+68 vs +73 percent, p = ns). Dipyridamole administration provokes an activation of sympathetic tone which can be detected even in the absence of myocardial ischemia and is not related to blood pressure changes. The increased catecholamine release appears to be of neuronal rather than adrenomedullary origin.