Purinergic regulation of vascular tone and remodelling

1 Purinergic signalling is involved both in short‐term control of vascular tone and in longer‐term control of cell proliferation, migration and death involved in vascular remodelling. 2 There is dual control of vascular tone by adenosine 5′‐triphosphate (ATP) released from perivascular nerves and by ATP released from endothelial cells in response to changes in blood flow (shear stress) and hypoxia. 3 Both ATP and its breakdown product, adenosine, regulate smooth muscle and endothelial cell proliferation. 4 These regulatory mechanisms are important in pathological conditions, including hypertension, atherosclerosis, restenosis, diabetes and vascular pain.     

Gender differences in the regulation of vascular tone

1. The greater incidence of hypertension and coronary artery disease in men and post-menopausal women compared with premenopausal women has suggested vascular protective effects of the female sex hormone oestrogen. However, vascular effects of the female sex hormone progesterone and the male sex hormone testosterone have also been suggested. 2. Oestrogen, progesterone and testosterone receptors have been identified in the plasmalemma, cytosol and nuclear compartments of vascular cells. The interaction of sex hormones with their specific receptors triggers not only long-term genomic vascular effects, but also acute non-genomic vascular responses. 3. Sex hormones may activate endothelium-dependent vascular relaxation pathways, including the nitric oxide-cGMP and prostacyclin-cAMP pathways and a hyperpolarizing factor pathway. 4. Sex hormones may also inhibit the mechanisms of vascular smooth muscle contraction, such as [Ca2+]i, protein kinase C and other protein kinases. 5. The sex hormone-induced stimulation of endothelium-dependent vascular relaxation and inhibition of vascular smooth muscle contraction may contribute to the gender differences in vascular tone and may represent potential beneficial vascular effects of hormone-replacement therapy during natural and surgically induced deficiencies of gonadal hormones.     

Mechanisms involved in the regulation of bovine pulmonary vascular tone by the 5-HT1B receptor

Background and purpose:

5-HT_1B receptors may have a role in pulmonary hypertension. Their relationship with the activity of BK_Ca, a T-type voltage-operated calcium channel (VOCC) and cyclic nucleotide-mediated relaxation was examined.

Experimental approach:

Ring segments of bovine pulmonary arteries were mounted in organ baths in modified Krebs–Henseleit buffer (37^oC) under a tension of 20 mN and gassed with 95% O₂/5% CO₂. Isometric recordings were made using Chart 5 software.

Key results:

Contractile responses to 5-HT (10 nM–300 µM) were inhibited similarly by the 5-HT_1B receptor antagonist SB216641 (100 nM) and the T-type VOCC blockers mibefradil (10 µM) and NNC550396 (10 µM) with no additive effect between SB216641 and mibefradil. Inhibition by SB216641 was prevented by the potassium channel blocker, charybdotoxin (100 nM). 5-HT_1B receptor activation and charybdotoxin produced a mibefradil-sensitive potentiation of responses to U46619. Bradykinin (0.1 nM–30 µM), sodium nitroprusside (0.01 nM–3 µM), zaprinast (1 nM–3 µM), isoprenaline (0.1 nM–10 µM) and rolipram (1 nM–3 µM) produced 50% relaxation of arteries constricted with 5-HT (1–3 µM) or U46619 (30–50 nM) in the presence of 5-HT_1B receptor activation, but full relaxation of arteries constricted with U46619, the 5-HT_2A receptor agonist 2,5 dimethoxy-4 iodoamphetamine (1 µM) or 5-HT in the presence of 5-HT_1B receptor antagonism. Enhanced relaxation of 5-HT-constricted arteries by cGMP-dependent pathways, seen in the presence of the 5-HT_1B receptor antagonist, was reversed by charybdotoxin whereas cAMP-dependent relaxation was only partly reversed by charybdotoxin.

Conclusions and implications:

5-HT_1B receptors couple to inhibition of BK_Ca, thus increasing tissue sensitivity to contractile agonists by activating a T-type VOCC and impairing cGMP-mediated relaxation. Impaired cAMP-mediated relaxation was only partly mediated by inhibition of BK_Ca.     

Effects of dinucleoside polyphosphates on regulation of coronary vascular tone

The aim of the present study was to investigate the effects of Xp(5)X and Xp(6)X (X = guanosine (G) or adenosine (A); n = 5 and 6), which have been identified in human platelets, on coronary vascular tone. The activation of purinoceptors in rat coronary vasculature by Xp(5)X and Xp(6)X was evaluated by measuring their effects on perfusion pressure in the Langendorff perfused rat. Ap(5)X and Ap(6)X induced dose-dependent vasodilation that was due to P2Y(1) receptor activation, as evidenced by use of the selective P2Y(1) receptor antagonist 2'-deoxy-N(6)-methyl-adenosine 3',5'-diphosphate diammonium (MRS2179). Vasodilation was induced by NO release, as evidenced by inhibition of nitric oxide synthases (NO synthases) by N(G)-nitro-L-arginine methyl ester (L-NAME). The dose-dependent decrease in coronary perfusion pressure induced by Ap(5)X and Ap(6)X was converted to a dose-dependent increase in perfusion pressure after inhibition of NO synthases by L-NAME. After endothelium removal, the vasodilation elicited by Ap(5)X and Ap(6)X was converted to a vasoconstriction which could be inhibited by P2X receptor blockade. Ap(5)A, Ap(5)G, Ap(6)A and Ap(6)G are vasodilating or vasoconstricting nucleotides that activate P2Y(1) or P2X receptors depending on the status of the coronary vascular endothelium.     

Evolving the concept of regulation of vascular tone in humans

Elucidation of mechanisms regulating microcirculatory vascular tone is a key issue in the knowledge of human pathophysiology. Anandamide is an endogenous lipidic cannabinoid (CB) characterized by potent vasodilator activity acting mainly through the activation of CB receptors, located on the vessel walls, and the vanilloid receptor 1, located on sensory peptidergic nerve endings within the external layers of vessel walls. In humans, cutaneous anandamide administration causes forearm skin vasodilation by activating vanilloid receptor 1 presumably on primary sensory nerves, while intrabrachial infusion of the same compound is devoid of effect on forearm muscle microcirculation. Taken together, these results indicate that, apart from a possible distrectual difference, the effect of anandamide is specific for the abluminal, but not for the endoluminal, part of the vessel wall. Thus, it is conceivable that, at least in the peripheral microcirculation, this compound could act as an autocrine/paracrine agent and not as a circulating hormone. In line with this possibility, it has been demonstrated that anandamide can be produced by macrophages and therefore its biological effect might increase in clinical conditions characterized by augmented activity of this cell line, including cardiogenic, hemorrhagic and endotoxic shock and even in atherosclerosis, inflammation and ischemia. Moreover, increased serum values of anandamide have been found in patients with endotoxic shock. However, decisive information concerning the role of anandamide in humans will be obtained when specific antagonists or inhibitors will be available. In that case, the anandamide system might represent a potential target for the treatment of important cardiovascular conditions, including severe shock.     

Inwardly rectifying potassium channels in the regulation of vascular tone

Potassium ion (K+) channel activity is one of the major determinants of vascular muscle cell membrane potential and thus vascular tone. Four types of K+ channels are functionally important in the vasculature-Ca(2+)-activated K+ (KCa) channels, voltage-dependent K+ (Kv) channels, ATP-sensitive K+ (KATP) channels, and inwardly rectifying K+ (KIR) channels, and the latter type will be the subject of this review. Recent advances in vascular KIR channel research indicate that this channel: 1) is present in vascular muscle; 2) modulates basal arterial tone; 3) mediates powerful hyperpolarization and vasodilator responses to small but physiological increases in extracellular K+; 4) may contribute to vasodilatation in response to flow-induced shear stress; 5) may be inhibited by protein kinase C activity; 6) may be involved in vasorelaxation mediated by endothelium-derived hyperpolarizing factor; and 7) may be functionally altered in cardiovascular diseases. Vascular effects of KIR channels have so far been most extensively studied in the cerebral circulation where KIR function may be important in coupling cerebral metabolism and blood flow.     

内源性硫化氢对高肺血流大鼠肺血管重构及血管活性物质的影响

目的探讨内源性硫化氢(hydrogen sulfide,H2S)对大鼠高肺血流性肺血管结构重建及血管活性肽的影响。方法♂SD大鼠32只随机分为分流组、分流+PPG(炔丙基甘氨酸,内源性H2S合成酶的抑制剂)组、对照组和对照+PPG组,每组8只。经腹主动脉-下腔静脉穿刺建立动物模型。分流4wk后,应用敏感硫电极法测定大鼠肺组织硫化氢(H2S)含量;应用放免试剂盒测定血浆中内皮素(ET-1)、心钠素(ANP)、降钙素基因相关肽(CGRP)及肾上腺髓质素(ADM)的含量;观察肺小血管的显微结构变化;结果分流4wk后,大鼠肺组织H2S水平升高,肺血管结构重建,血浆ET-1、ANP、CGRP及ADM升高。PPG干预后,肺组织H2S水平降低,肺血管结构重建加重,血浆ET-1、ANP、CGRP升高,而ADM降低。结论内源性H2S对高肺血流性肺血管结构重建具有重要的保护作用,并对多种血管活性肽具有调节作用。     

牛磺酸对大鼠高血压的影响

在大鼠腹主动脉狭窄和高盐摄入引起的高血压模型上,观察到口服牛磺酸治疗可显著降低动物血压,(20.8±4.0kPa VS非治疗组27.3±3.2 kPa.P<0.01)抑制高血压动物血浆CGRP的减少.改善动物血管条的舒张反应。实验结果提示,牛磺酸可能具有临床上抗高血压作用。     

Thapsigargin-induced endothelium-dependent triphasic regulation of vascular tone in the porcine renal artery.

1. To elucidate the role of thapsigargin-induced Ca2+ entry in endothelial cells in the regulation of vascular tone, changes in Ca2+ and force of smooth muscle were simultaneously monitored in fura-2-loaded strips of porcine renal artery. 2. During phenylephrine-induced sustained contraction, thapsigargin caused an endothelium-dependent triphasic response; an initial relaxation, a subsequent transient contraction, and a sustained relaxation. The initial relaxation and the contraction were associated with a decrease and an increase in [Ca2+]i, respectively. There was no apparent [Ca2+]i decrease during the sustained relaxation. Thapsigargin-induced responses were observed at 10-8 M and higher concentrations, with the maximum response observed at 10-6 M. 3. The transient contraction was inhibited by a cyclo-oxygenase inhibitor (10-5 M indomethacin), a thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor antagonist (10-5 M ONO-3708), and a TXA2 synthase inhibitor (10-5 M OKY-046). 4. During the phenylephrine-induced contraction in the presence of indomethacin, thapsigargin caused an initial, but not a sustained relaxation, in the presence of Nomega-nitro-L-arginine methylester (L-NAME). During the contraction induced by phenylephrine plus 40 mM K+-depolarization in the presence of indomethacin, thapsigargin induced both a transient and a sustained relaxation. However, these relaxations were completely abolished in the presence of L-NAME. 5. Thapsigargin caused a large Ca2+ elevation in cultured endothelial cells of the renal artery. The concentration-response relation was thus similar to that for force development in the arterial strips. 6. In conclusion, thapsigargin-induced Ca2+ entry in endothelial cells led to triphasic changes in the tone of the porcine renal artery. The endothelium-dependent contraction was mediated mainly by TXA2. Nitric oxide and hyperpolarizing factor are both involved in the initial relaxation. However, a sustained relaxation was observed which mainly depended on nitric oxide.     

染料木黄酮对慢性缺氧性肺血管结构改建的干预作用

目的　探讨染料木黄酮对大鼠缺氧性肺血管结构改建的干预作用及其作用机制。方法　将♂Wistar大鼠随机分为 3组 ,常氧对照组 (C)、慢性缺氧组 (H)和慢性缺氧 +染料木黄酮组 (H +G) ,对肺血管进行显微形态学观察。通过免疫组织化学方法检测肺动脉平滑肌细胞增殖细胞核抗原(PCNA)的表达。结果　与常氧对照组相比 ,慢性缺氧组大鼠肺血管管壁增厚 ,管腔狭窄 ,肌型动脉百分比显著增加 (P<0 .0 1) ;肺动脉平滑肌增殖细胞核抗原表达增加。与H组比较 ,H +G组大鼠肺动脉管壁增厚程度及血管腔狭窄程度显著减轻 ,肌型动脉百分比显著降低 (P <0 0 1) ;肺动脉平滑肌PCNA表达明显减少。结论　实验结果提示 ,染料木黄酮可以显著抑制缺氧性肺血管结构改建 ,其作用机制可能与其抑制缺氧诱导的肺动脉平滑肌细胞增殖有关。     

培哚普利对慢性缺氧高二氧化碳大鼠肺血管重建的影响

目的 探讨培哚普利对常压慢性缺氧高二氧化碳性肺动脉高压及肺血管重建的影响。方法 47只SD大鼠随机分为：对照组（A）、缺氧高二氧化碳处理4wk组（B1）、培哚普利治疗4wk组（B2）、缺氧高二氧化碳处理8wk组（C1）、培哚普利治疗8wk组（C2)。采用常压低氧高二氧化碳肺动脉高压大鼠模型,观察肺动脉平均压（mPAP),血浆血管紧张素Ⅱ（AngⅡ） 及肺小血管平滑肌外膜、内膜超微结构的变化,以及培哚普利对以上变化的影响。结果 缺氧高二氧化碳使mPAP,AngⅡ上升,超微结构发生变化,培哚普利治疗4wk组mPAP,AngⅡ下降,平滑肌超微结构病变减轻。培哚普利治疗8wk组,平滑肌超微结构病变与对照组相似。结论 提示缺氧高二氧化碳使肺血管重建,且与AngⅡ有关;培哚普利能在一定程度上能减轻肺血管重建。     

Thrombin causes endothelium-dependent biphasic regulation of vascular tone in the porcine renal interlobar artery

Using a method employing front-surface fura-2 fluorometry to measure the cytosolic Ca(2+) concentration, [Ca(2+)](i), the mechanism of endothelium-dependent regulation of vascular tone by thrombin was studied in porcine renal interlobar arterial strips. At concentrations lower than 3 u ml(-1), thrombin evoked only early transient relaxation, while at 3 u ml(-1) and higher concentrations, thrombin caused an early relaxation and a subsequent transient contraction. Both thrombin-induced relaxation and contraction were abolished by removing the endothelium. Similar biphasic responses were observed with a protease-activated receptor-1-activating peptide. Early relaxation was associated with a decrease in [Ca(2+)](i), while the transient contraction was not associated with a change in [Ca(2+)](i) of smooth muscle cells. A thromboxane A(2) (TXA(2))/prostaglandin H(2) (PGH(2)) receptor antagonist (10(-5) M ONO-3708) completely inhibited the thrombin-induced contraction, whereas a thromboxane A(2) synthase inhibitor (10(-5) M OKY-046) only partly inhibited it. When the thrombin-induced contraction was inhibited by ONO-3708, either pretreatment with N(omega)-nitro-L-arginine methylester (L-NAME) or an increase in the amount of external K(+) to 40 mM did not abolish thrombin-induced relaxation during phenylephrine-induced sustained contraction. However, the combination of pretreatment with L-NAME and an elevation of external K(+) to 40 mM completely abolished the relaxation. There was no significant difference in the concentration-dependent effects of thrombin on the initial early relaxation between conditions in which the contractile components either were or were not inhibited. Thrombin is thus considered to mainly activate protease-activated receptor-1 and cause a biphasic response, early relaxation and a transient contraction, in the porcine renal interlobar artery in an endothelium-dependent manner. The thrombin-induced endothelium-dependent relaxation was mediated by nitric oxide and hyperpolarizing factors, while the contraction was mediated by TXA(2) and PGH(2).     

Paracrine regulation of vascular tone, inflammation and insulin sensitivity by perivascular adipose tissue

A small amount of adipose tissue associated with small arteries and arterioles is encountered both in mice and man. This perivascular adipose tissue (PVAT) has a paracrine effect on the vascular tone regulation. PVAT is expanded in obesity and in diabetes. This expansion not only involves enlargement of fat cells, but also the accumulation of inflammatory cells and a shift in the production of adipokines and cytokines. This effect is illustrated in this review by the effect of PVAT-derived factors of insulin-mediated vasoregulation in mouse resistance arteries. Insulin sensitivity of endothelial cells is also involved in the insulin-mediated regulation of muscle glucose uptake. Insulin affects vasoregulation by acting on different signaling pathways regulating NO and endothelin-1 release. This process is influenced by various adipokines and inflammatory mediators released from PVAT, and is affected by the degree of expansion and content of inflammatory cells. It is modulated by adiponectin (via 5' adenosine monophosphate-activated protein kinase, AMPK), TNFα (via c-jun N-terminal kinase) and free fatty acids (via protein kinase C-θ). PVAT thus provides an important site of control of vascular (dys)function in obesity and type 2 diabetes. An altered profile of adipokine and cytokine production by PVAT of resistance arteries may also contribute to or modulate hypertension, but a causal role in hypertension has still to be established.     

Mechanisms controlling vascular tone in pulmonary arterial hypertension: implications for vasodilator therapy

Pulmonary vasoconstriction is believed to be an early component of pulmonary arterial hypertension. Intracellular calcium concentration ([Ca(2+)](i)) is a major trigger for pulmonary vasoconstriction; however, it is now well known that contractions and relaxations may also be elicited through Ca(2+)-independent mechanisms. A variety of intracellular protein kinases and cyclic nucleotides have been identified as key determinants in controlling pulmonary vascular tone. Herein, we provide an overview of the main signaling pathways, which include protein kinase C, Rho kinases and cyclic nucleotides (cAMP and cGMP). This review also focuses on the role of store-operated Ca(2+) channels and voltage-gated K(+) channels, which are currently considered especially attractive in the pulmonary circulation and may represent new targets in the treatment of pulmonary arterial hypertension.     

Vasodilator effect of gaseous sulfur dioxide and regulation of its level by Ach in rat vascular tissues

To explore the toxicological and physiological role of gaseous SO₂ on vascular contractility and its level in vascular tissues, a vasodilation study of isolated rat thoracic aortic rings by gaseous SO₂ was carried out. The level of SO₂ in vascular tissue was assayed using a modified high-performance liquid chromatographic method with fluorescence detection (HPLC-FD). The results show that gaseous SO₂ (from 1?μM to 2000?μM) relaxed rat thoracic aortic rings in a dose-dependent manner. The physiological concentrations of SO₂ in thoracic aortic tissues and plasma in rats were 127.76?±?31.34?μM and 16.77±8.24?μM, respectively; The vasorelaxant effect of gaseous SO₂ at physiological and low concentrations (<450?μM) was endothelium dependent, and at high concentrations (>500?μM) was endothelium independent. The results also show that SO₂ could be endogenously generated in vascular tissues, and mainly in vascular endothelial cells; acetylcholine (Ach) increased the SO₂ level in vascular tissue, and noradrenaline (NE) decreased the SO₂ level. These findings demonstrate that gaseous SO₂ is a vasorelaxant substance, and the vasorelaxant effect of gaseous SO₂ is much stronger than that of its derivatives sulfite and bisulfite, which result from the inactivation process of SO₂ gas transmitter by which SO₂ is hydrated to form sulfite, and the latter is enzymatically oxidized to form sulfate. These findings also demonstrate that endogenous SO₂ level in vascular tissue may be regulated by Ach and NE.     

Endothelial potassium channels, endothelium-dependent hyperpolarization and the regulation of vascular tone in health and disease

1. The elusive nature of endothelium-derived hyperpolarizing factor (EDHF) has hampered detailed study of the ionic mechanisms that underlie the EDHF hyperpolarization and relaxation. Most studies have relied on a pharmacological approach in which interpretations of results can be confounded by limited specificity of action of the drugs used. Nevertheless, small-, intermediate- and large-conductance Ca2+-activated K+ channels (SKCa, IKCa and BKCa, respectively) have been implicated, with inward rectifier K+ channels (KIR) and Na+/K+-ATPase also suggested by some studies. 2. Endothelium-dependent membrane currents recorded using single-electrode voltage-clamp from electrically short lengths of arterioles in which the smooth muscle and endothelial cells remained in their normal functional relationship have provided useful insights into the mechanisms mediating EDHF. Charybdotoxin (ChTx) or apamin reduced, whereas apamin plus ChTx abolished, the EDHF current. The ChTx- and apamin-sensitive currents both reversed near the expected K+ equilibrium potential, were weakly outwardly rectifying and displayed little, if any, time- or voltage-dependent gating, thus having the biophysical and pharmacological characteristics of IKCa and SKCa channels, respectively. 3. The IKCa and SKCa channels occur in abundance in endothelial cells and their activation results in EDHF-like hyperpolarization of these cells. There is little evidence for a significant number of these channels in healthy, contractile vascular smooth muscle cells. 4. In a number of blood vessels in which EDHF occurs, the endothelial and smooth muscle cells are coupled electrically via myoendothelial gap junctions. In contrast, in the adult rat femoral artery, in which the smooth muscle and endothelial layers are not coupled electrically, EDHF does not occur, even though acetylcholine evokes hyperpolarization in the endothelial cells. 5. In vivo studies indicate that EDHF contributes little to basal conductance of the vasculature, but it contributes appreciably to evoked increases in conductance. 6. Endothelium-derived hyperpolarizing factor responses are diminished in some diseases, including hypertension, pre-eclampsia and some models of diabetes. 7. The most economical explanation for EDHF in vitro and in vivo in small vessels is that it arises from the activation of IKCa and SKCa channels in endothelial cells. The resulting endothelial hyperpolarization spreads via myoendothelial gap junctions to result in the EDHF-attributed hyperpolarization and relaxation of the smooth muscle.     

Beta2-adrenergic activity modulates vascular tone regulation in lecithin:cholesterol acyltransferase knockout mice

Lecithin:cholesterol acyltransferase (LCAT) deficiency is associated with hypoalphalipoproteinemia, generally a predisposing factor for premature coronary heart disease. The evidence of accelerated atherosclerosis in LCAT-deficient subjects is however controversial. In this study, the effect of LCAT deficiency on vascular tone and endothelial function was investigated in LCAT knockout mice, which reproduce the human lipoprotein phenotype. Aortas from wild-type (Lcat^wt) and LCAT knockout (Lcat^KO) mice exposed to noradrenaline showed reduced contractility in Lcat^KO mice (P < 0.005), whereas acetylcholine exposure showed a lower NO-dependent relaxation in Lcat^KO mice (P < 0.05). Quantitative PCR and Western blotting analyses suggested an adequate eNOS expression in Lcat^KO mouse aortas. Real-time PCR analysis indicated increased expression of β2-adrenergic receptors vs wild-type mice. Aorta stimulation with noradrenaline in the presence of propranolol, to abolish the β-mediated relaxation, showed the same contractile response in the two mouse lines. Furthermore, propranolol pretreatment of mouse aortas exposed to L-NAME prevented the difference in responses between Lcat^wt and Lcat^KO mice. The results indicate that LCAT deficiency leads to increased β2-adrenergic relaxation and to a consequently decreased NO-mediated vasodilation that can be reversed to guarantee a correct vascular tone. The present study suggests that LCAT deficiency is not associated with an impaired vascular reactivity.     

Raloxifene, tamoxifen and vascular tone

1. Oestrogen deficiency causes progressive reduction in endothelial function. Despite the benefits of hormone-replacement therapy (HRT) evident in earlier epidemiological studies, recent randomized trials of HRT for the prevention of heart disease found no overall benefit. Instead, HRT users had higher incidences of stroke and heart attack. Most women discontinue HRT because of its many side-effects and/or the increased risk of breast and uterine cancer. This has contributed to the development of selective oestrogen receptor modulators (SERMs), such as tamoxifen and raloxifene, as alternative oestrogenic agents. 2. A SERM is a molecule that binds with high affinity to oestrogen receptors but has tissue-specific effects distinct from oestrogen, acting as an oestrogen agonist in some tissues and as an antagonist in others. Clinical and animal studies suggest multiple cardiovascular effects of SERMs. For example, raloxifene lowers serum levels of cholesterol and homocysteine, attenuates oxidation of low-density lipoprotein, inhibits endothelial-leucocyte interaction, improves endothelial function and reduces vascular smooth muscle tone. 3. Available evidence suggests that raloxifene and tamoxifen are capable of acting directly on both endothelial cells and the underlying vascular smooth muscle cells and cause a multitude of favourable modifications of the vascular wall, which jointly contribute to improved local blood flow. The outcome of the Raloxifene Use for the Heart (RUTH) trial will determine whether raloxifene, currently approved for the treatment of post-menopausal osteoporosis, could substitute for HRT in alleviating cardiovascular symptoms in post-menopausal women.     

Dual regulation of pancreatic vascular tone by P2X and P2Y purinoceptor subtypes

The effects of ATP on the pancreatic vascular bed of the rat were studied under resting tone. ATP exerted two different effects depending on the concentration used: a slight vasodilatation in the 1.65-49.5 microM range which was statistically significant only at 16.5 microM and a concentration-related vasoconstriction in the 495-4 950 microM range. Theophylline, a P1 purinoceptor antagonist, did not modify the vasodilator effect of ATP. The existence of two P2 purinoceptor subtypes (P2y and P2x) in our preparation may be responsible for the dual effect of ATP. The P2y antagonist 2,2'pyridylisatogen (PIT) used at 5 microM, revealed a vasoconstrictor effect of ATP 165 microM, a concentration without effect per se. Furthermore, the transient vasoconstrictor effect of ATP 495 microM was changed into a long-lasting one in the presence of PIT. On the other hand, the blockade of P2x purinoceptors by the desensitizing agent, alpha,beta-methylene ATP, increased the vasodilator effect of ATP 16.5 microM. In conclusion, two subtypes of P2 purinoceptor do exist on the pancreatic vascular bed: P2y inducing vasodilatation and P2x inducing vasoconstriction. At vascular resting tone, the effect observed with ATP therefore depends on the concentration used and on the balance between P2y/P2x purinoceptors.