Long-term administration of 1,3-dipropyl-8-sulfophenylxanthine causes arterial hypertension

Increased Gi protein signaling potentiates the negative chronotropic effect of adenosine in the SHR right atrium

Hypertension is a risk factor for cardiovascular diseases, which have been associated with dysfunction of sympathetic and purinergic neurotransmission. Therefore, herein, we evaluated whether modifications of adenosine receptor signaling may contribute to the cardiac dysfunction observed in hypertension. Isolated right atria from spontaneously hypertensive (SHR) or normotensive Wistar rats (NWR) were used to investigate the influence of adenosine receptor signaling cascade in the cardiac chronotropism. Our results showed that adenosine, the endogenous agonist of adenosine receptors, and CPA, a selective agonist of A₁ receptor, decreased the atrial chronotropism of NWR and SHR in a concentration- and time-dependent manner, culminating in cardiac arrest (0 bpm). Interestingly, a 3-fold lower concentration of adenosine was required to induce the negative chronotropic effect in SHR atria. Pre-incubation of tissues from both strains with DPCPX, a selective A₁ receptor antagonist, inhibited the negative chronotropic effect of CPA, while simultaneous inhibition of A₂ and A₃ receptors, with ZM241385 and MRS1523, did not change the adenosine chronotropic effects. Moreover, 1 μg/ml pertussis toxin, which inactivates the Gαi protein subunit, reduced by 80% the negative chronotropic effects of adenosine in the NWR atrium, with minor effects in SHR tissue. These data indicate that the negative chronotropic effect of adenosine in right atrium depends exclusively on the activation of A₁ receptors. Moreover, the distinct responsiveness of NWR and SHR atria to pertussis toxin reveals that the enhanced negative chronotropic response of SHR right atrium is probably due to an increased activity of Gαi protein-mediated.

Alpha2-adrenoceptor and adenosine A1 receptor within the nucleus tractus solitarii in hypertension development

Alpha2-adrenoceptor and A1 adenosine receptor systems within the nucleus tractus solitarii (NTS) play an important role in cardiovascular control. Deregulation of these systems may result in an elevated sympathetic tone, one of the root causes of neurogenic hypertension. The dorsomedial/dorsolateral and subpostremal NTS subnuclei of spontaneously hypertensive rats (SHR) show density changes in both receptors, even at 15 days of age, prior to the onset of hypertension. In addition, adenosine A1 receptors have been specifically reported to modulate alpha2-adrenoceptors in several brain regions, including the NTS, via a PLC-dependent pathway involving cross regulation between sympathetic neurons and astrocytes. The physiological cross talk between these receptor systems is also deregulated in SHR suggesting that alpha2-adrenoceptor and A1 adenosine receptor might be germane to the development of hypertension. In this review, we will focus on these systems within the NTS during development, pointing out some interesting modulations in processes, and chemical changes within specific subnuclei of NTS circuitry, that might have implications for neurogenic hypertension.

Differential inhibition of noradrenaline release mediated by inhibitory A₁-adenosine receptors in the mesenteric vein and artery from normotensive and hypertensive rats

Mesenteric arteries and veins are densely innervated by sympathetic nerves and are crucial in the regulation of peripheral resistance and capacitance, respectively, thus, in the control of blood pressure. Presynaptic adenosine receptors are involved in vascular tonus regulation, by modulating noradrenaline release from vascular postganglionic sympathetic nerve endings. Some studies also suggest that adenosine receptors (AR) may have a role in hypertension. We aim at investigating the role of presynaptic adenosine receptors in mesenteric vessels and establish a relationship between their effects (in mesenteric vessels) and hypertension, using the spontaneously hypertensive rats (SHR) as a model of hypertension. Adenosine receptor-mediated modulation of noradrenaline release was investigated through the effects of selective agonists and antagonists on electrically-evoked [(3)H]-noradrenaline overflow. CPA (A1AR selective agonist: 1-100 nM) inhibited tritium overflow, but the inhibition was lower in SHR mesenteric vessels. IB-MECA (A3AR selective agonist: 1-100 nM) also inhibited tritium overflow but only in WKY mesenteric veins. CGS 21680 (A2AAR selective agonist: up to 100 nM) failed to facilitate noradrenaline release in mesenteric veins, from both strains, but induced a similar facilitation in the mesenteric arteries. NECA (non-selective AR agonist: 1, 3 and 10μM), in the presence of A1 (DPCPX, 20 nM) and A3 (MRS 1523, 1 μM) AR selective antagonists, failed to change tritium overflow. In summary, the modulatory effects mediated by presynaptic adenosine receptors were characterized, for the first time, in mesenteric vessels: a major inhibition exerted by the A1 subtype in both vessels; a slight inhibition mediated by A3 receptors in mesenteric vein; a facilitation mediated by A2A receptors only in mesenteric artery (from both strains). The less efficient prejunctional adenosine receptor mediated inhibitory effects can contribute to an increase of noradrenaline in the synaptic cleft (both in arteries and veins), which might conduce to increased vascular reactivity.

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.

Scavenging of nitric oxide by an antagonist of adenosine receptors

Chronic treatment of rats with 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), an antagonist of adenosine receptors, causes hypertension, cardiovascular hypertrophy and hyperplasia and impaired endothelium-dependent vasodilatation. An accelerated degradation of nitric oxide (NO) by scavenging molecules could account for endothelial dysfunction and trophic changes in this hypertension. Our aim was to determine whether DPSPX is a scavenger of NO and if this putative effect is shared by caffeine (1,3,7-trimethylxanthine) and DPCPX (1,3-dipropyl-8-ciclopentylxanthine), which are also adenosine receptor antagonists but do not induce hypertension in rats. This effect was evaluated by electrochemical and spectrofluorometric assays. Urinary NOx (nitrate + nitrite) excretion was also evaluated in controls and DPSPX-treated rats as a marker for NO bioavailability. DPSPX behaved as a scavenger of NO in a concentration-dependent manner in the electrochemical and spectrofluorometric assays. Caffeine and DPCPX had no scavenging effect. DPSPX-treated rats had decreased excretion of urinary nitrites. We can conclude that: DPSPX has NO scavenging properties that may be involved in the alterations described for DPSPX-hypertensive rats; this NO-scavenging effect is not shared by caffeine and DPCPX, which are also xanthine derivatives and adenosine antagonists.

Age-dependent changes in adenosine A1 receptor distribution and density within the nucleus tractus solitarii of normotensive and hypertensive rats

This study shows the distribution and density of adenosine A1 receptor (A1R) within the nucleus tractus solitarii (NTS) of Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) from birth to adulthood (1, 15, 30 and 90 days old). The NTS shows heterogeneous distribution of A1R in dorsomedial/dorsolateral, subpostremal and medial/intermediate subnuclei. A1R decrease from rostral to caudal within dorsomedial/dorsolateral subnucleus in 15-, 30- and 90-day-old WKY and SHR. A1R increase from rostral to caudal subpostremal subnucleus in 30- and 90-day-old WKY, and in 15-, 30- and 90-day-old SHR. Furthermore, A1Rs are increased in SHR as compared with WKY within dorsomedial/dorsolateral in 30- and 90-day-old and within subpostremal of 15-, 30- and 90-day-old rats. Finally, A1Rs increase from 1- to 30-day-old rats. Medial/intermediate did not show any changes in A1R from rostral to caudal levels, age or strain. In summary, our result highlights the importance of A1 adenosine system regarding the neural control of blood pressure and the development of hypertension.

Role of superoxide and hydrogen peroxide in hypertension induced by an antagonist of adenosine receptors

Treatment of Wistar rats for 7 days with 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), an antagonist of adenosine receptors, induces long-lasting hypertension associated with marked changes in vascular structure and reactivity and renin-angiotensin system activation. This study aimed at evaluating the role of oxidative stress in the development of DPSPX-induced hypertension and also at identifying the relative contribution of superoxide radical (O2.-) vs hydrogen peroxide (H2O2). Vascular and systemic prooxidant/antioxidant status was evaluated in sham (saline, i.p., 7 days) and DPSPX (90 microg/kg/h, i.p., 7 days)-treated rats. Systolic blood pressure was determined by invasive and non-invasive methods. The activity of vascular NADPH oxidase, superoxide dismutase (SOD), catalase and glutathione peroxidase was assayed by fluorometric/spectrophotometric methods. H2O2 levels were measured using an Amplex Red Hydrogen Peroxide kit. Plasma thiobarbituric acid reactive substances and plasma antioxidant capacity were also measured. In addition we tested the effects of antioxidants or inhibitors of reactive oxygen species generation on blood pressure, vascular hyperplasia and oxidative stress parameters. DPSPX-hypertensive rats showed increased activity of vascular NADPH oxidase, SOD, catalase and glutathione peroxidase, as well as increased H2O2 generation. DPSPX-hypertensive rats also had increased plasma lipid peroxidation and decreased plasma antioxidant capacity. Treatment with apocynin (1.5 mmol/l, per os, 14 days), or with polyethylene glycol (PEG)-catalase (10,000 U/kg/day, i.p., 8 days), prevented the DPSPX-induced effects on blood pressure, vascular structure and H2O2 levels. Tempol (3 mmol/l, per os, 14 days) failed to inhibit these changes, unless PEG-catalase was co-administered. It is concluded that O2.- generation with subsequent formation of H2O2 plays a major role in the development of DPSPX-induced hypertension.

Lesion of the caudal ventrolateral medulla prevents the induction of hypertension by adenosine receptor blockade in rats

The continuous infusion for 7 days of the adenosine receptor antagonist 1,3-dipropyl-8-sulfophenylxanthine (DPSPX) causes a sustained hypertension in rats, with an enhancement of sympathetic neurotransmission and activation of the renin-angiotensin system. We studied the involvement of the caudal ventrolateral medulla in the establishment of this hypertensive model by evaluating the effect of local lesioning in blood pressure (BP). Male adult Wistar rats received stereotaxic injections of 0.3 mul of saline or quinolinic acid (QA; 180 mM) in the caudal ventrolateral medulla followed by abdominal implant of minipump for infusion of saline or DPSPX (90 microg(-1) kg(-1) h(-1)). BP was measured in conscious animals every 2 days for 12 days. The sustained increase of BP (22.1 mm Hg; P < 0.001) detected in rats infused with DPSPX was reverted (6.7 mm Hg; P > 0.05) from day six onwards in animals with lesion of the lateralmost part of caudal ventrolateral medulla (VLMlat). The present results suggest that the development of hypertension induced by adenosine receptor antagonist involves the participation of the VLMlat. They further add new data as to the functional complexity of this medullary area involved in a variety of functions such as cardiovascular, respiratory, motor and pain control.

Losartan and atenolol on hypertension induced by adenosine receptor blockade

1. The prolonged infusion of 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), a non-selective antagonist of adenosine receptors, induces hypertension, an increase in plasma renin activity and morphological cardiovascular changes. 2. The aim of this work was to evaluate the effects of losartan, a selective AT1 receptor antagonist, and atenolol, a beta-adrenoceptor antagonist, on DPSPX-induced hypertension. 3. Male Wistar rats (250-300 g, n = 4-6) were treated for 1 or 4 weeks with: saline i.p.; DPSPX (90 microg kg(-1) h(-1)) i.p.; losartan (15 mg kg(-1) day(-1)) p.o.; atenolol (25 mg kg(-1) day(-1)) p.o.; DPSPX (90 microg kg(-1) h(-1)) i.p. + losartan (15 mg kg(-1) day(-1)) p.o.; DPSPX (90 microg kg(-1) h(-1)) i.p. + atenolol (25 mg kg(-1) day(-1)) p.o. Blood pressure was measured by the 'tail-cuff' method in conscious animals. Fragments of the mesenteric and tail arteries were processed for morphological study and the mean diameter of the vascular smooth muscle cells was determined. 4. DPSPX increased blood pressure. Losartan and atenolol prevented this rise but had no effect on blood pressure of control rats. DPSPX-treated groups showed hypertrophy of the vascular smooth muscle cells and proliferation of subintimal cells. Losartan but not atenolol prevented these changes. Losartan had no effect on the vascular morphology of control rats, while treatment with atenolol for 4 weeks induced hypertrophy of the vascular smooth muscle cells. 5. Both losartan and atenolol counteract the development of DPSPX-induced hypertension but only losartan prevents the alterations in vascular morphology.

Angiotensin converting enzyme inhibition prevents trophic and hypertensive effects of an antagonist of adenosine receptors

The continuous infusion of 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), a non-selective antagonist of adenosine receptors, causes hypertension and marked cardiovascular structural changes in Wistar rats. Adenosine inhibits noradrenaline and renin release. We investigated the effects of sympathetic denervation, evaluated renin activity and the influence of angiotensin converting enzyme inhibition in DPSPX-treated rats. Captopril was given (30 or 100 mg kg(-l) day(-l); p.o.) from day -l to day 28. On day 0, constant infusions of DPSPX (90 microg kg(-l) h(-l); i.p.) or vehicle were started. On day 28, fragments of the left ventricle, mesenteric and tail arteries were processed for morphological studies. Plasma renin activity was increased in DPSPX-treated animals. Sympathetic denervation delayed and partially prevented blood pressure rise. Angiotensin converting enzyme inhibition prevented DPSPX-induced hypertension and morphological changes. Our results, although pointing to the involvement of the sympathetic system, suggest that other mechanisms are involved. We could not differentiate between the trophic and anti-hypertensive effects of angiotensin converting enzyme inhibition.

Somatostatin inhibits the release of noradrenaline induced by electrical stimulation of the rat mesenteric artery

Somatostatin, a peptide with antisecretory and antiproliferative effects, coexists with noradrenaline in sympathetic neurons. Octreotide, a stable somatostatin analogue, prevents hypertension and cardiovascular structural changes induced by prolonged infusion of DPSPX (1,3-dipropyl-8-sulfophenylxanthine, a non-selective adenosine receptor antagonist) in rats. In the present work we investigated the effect of somatostatin and its analogue octreotide on the release of [(3)H]noradrenaline from sympathetic nerves in the rat mesenteric artery. Rat mesenteric arteries were incubated for 60 min with [(3)H]noradrenaline (0.2 microm), mounted in perifusion chambers, washed out for 90 min and electrically stimulated (2 Hz, 5 min, 50 mA). Radioactivity was measured in the tissue and in the perifusion fluid before, during and after stimulation. Both somatostatin and octreotide inhibited tritium release evoked by electrical stimulation of in vitro preparations of rat mesenteric arteries preloaded with [(3)H]noradrenaline. The maximal effects produced by octreotide and somatostatin were a 56 and 70% inhibition of noradrenaline release, respectively. For somatostatin an EC(50)=0. 18 n m (0.01 n m-2.2 n m;n =16) was calculated. When used alone, the somatostatin receptor antagonist, cyclo(7-aminoheptanoyl-Phe- d -Trp-Lys-Thr[BZL]) (CYCAM; 1 microm), had no effect on noradrenaline release induced by electrical stimulation. However, it was able to significantly antagonize the inhibitory effects of octreotide and somatostatin. These results are compatible with a negative modulatory role of somatostatin on sympathetic neurotransmission.

Reduced responsiveness of [Ca2+]i to adenosine A1- and A2-receptor stimulation in the isoproterenol-stimulated ventricular myocytes of spontaneously hypertensive rats

To determine the modulatory action of adenosine-receptor stimulation on [Ca2+]i responses to beta-adrenoceptor stimulation in the heart of the spontaneously hypertensive rat (SHR), the electrically induced [Ca2+]i transient in response to isoproterenol (ISO) in single ventricular myocytes pretreated with adenosine agonists in SHRs and its normotensive control Wistar-Kyoto (WKY) rats was measured with a spectrofluorometric method by using fura-2/AM as the calcium indicator. In both types of rat, ISO at 0.001-1 microM augmented the electrically induced [Ca2+]i transient, and the effect was blocked by a beta-adrenoceptor blocker, propranolol. In SHRs that did not exhibit cardiac hypertrophy, the resting level of [Ca2+]i and the amplitude of the electrically induced [Ca2+]i transient were the same as those in WKY rats, whereas the augmentation of the electrically induced [Ca2+]i transient in response to ISO was significantly lower than that in WKY rats. In WKY rats, the effects of ISO on the electrically induced [Ca2+]i transient were inhibited by the adenosine A1-receptor agonist, R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) at 0.01-10 microM. In contrast, the effects of ISO were further enhanced by the adenosine A2-receptor agonist, N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl)]adenosine (DPMA) at 1-10 microM. In SHRs, the inhibitory effect of R-PIA was significantly reduced, whereas the excitatory effect of DPMA was absent. The effects of both adenosine-receptor agonists in both types of rat were abolished by the respective adenosine-receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 3,7-dimethyl-1-propargylxanthine (DMPX). The results indicate that the modulatory actions of adenosine-receptor stimulation on [Ca2+]i response to beta-adrenoceptor stimulation in the hearts of SHRs are reduced, which is independent of cardiac hypertrophy.

Endothelium-dependent vascular responses in 1,3-dipropyl-8-sulphophenylxanthine (DPSPX) hypertensive rats

The study was undertaken to test the endothelium-mediated vascular responses in rats rendered hypertensive by chronic administration of 1,3-dipropyl-8-sulphophenylxanthine (DPSPX). The relaxant effect of carbachol (an endothelium-dependent relaxing drug) and of sodium nitroprusside (an endothelium-independent relaxing drug) as well as the potentiation of the contractile effect of noradrenaline by NG-nitro-L-arginine methyl ester (L-NAME) were compared in aortic rings from normotensive and DPSPX-hypertensive rats. Carbachol and sodium nitroprusside caused concentration-dependent relaxations in aortic rings precontracted by 1 microM noradrenaline. The relaxant effect being of carbachol was significantly reduced in tissues of DPSPX-hypertensive rats: the maximal relaxant effect being 86 +/- 3% and 64 +/- 4% (of the pre-existing tone) in normal and hypertensive rats, respectively, while there were no significant differences in the relaxant effect of sodium nitroprusside. L-NAME (100 microM) significantly reduced the EC50 values of noradrenaline (3.71 +/- 0.28 times, n = 8 and 2.96 +/- 0.27 times, n = 7, in normal and hypertensive rats, respectively) and significantly enhanced the maximal contractile effect of noradrenaline (46 +/- 8%, n = 8 and 35 +/- 6%, n = 7, in normal and hypertensive rats respectively): the factors of reduction of EC50 values and the percentages of enhancement of the maximal contractile effect in the aorta of normal and hypertensive rats were not significantly different. The results obtained provide evidence of functional impairment of the endothelium in DPSPX-hypertensive rats.

Hypertension due to chronic blockade of P1-purinoceptors

1. Long-term treatment of rats with 1,3-dipropyl-8-sulphophenylxanthine (DPSPX), a non-selective antagonist of adenosine receptors, causes a hypertensive state. 2. In DPSPX-hypertensive rats, prejunctional alpha 2-adrenoceptors become supersensitive to the selective alpha 2-adrenoceptor agonist UK-14,304, while postjunctional adrenoceptor-mediated responses are not changed; furthermore, prejunctional beta-adrenoceptor-mediated facilitation of noradrenaline release is also enhanced. 3. In DPSPX-hypertensive rats, there are important morphological alterations of the small arteries, their lumina appearing strongly reduced and occasionally occluded by proliferation of the intimal cells. 4. In DPSPX-hypertensive rats, there is an increase in plasma renin, and captopril prevents not only the development of the hypertension but also the morphological changes in the arteries. 5. Other important changes occur in DPSPX-hypertensive rats: an alteration of the adrenergic regulation of the cardiac functions and an enhancement of perivascular neurotransmission. 6. These results suggest that adenosine may play an important role in the development of some kinds of human hypertension.

Augmented sensory-motor vasodilatation of the rat mesenteric arterial bed after chronic infusion of the P1-purinoceptor antagonist, DPSPX

1. The effect of long-term antagonism of P1-purinoceptors on vascular function was examined in the perfused mesenteric arterial bed isolated from rats which had received constant infusion of either the non-selective P1-purinoceptor antagonist, 1-3-dipropyl-8-sulphophenylxanthine (DPSPX, 30 micrograms kg-1 h-1, i.p.) or saline for seven days. Sympathetic and sensory-motor neurotransmission, smooth muscle and endothelial function were assessed. 2. Basal tone was similar in mesenteric arterial preparations from control and DPSPX-treated rats. Continuous perfusion with methoxamine (7-70 microM) induced similar increases in tone in control and DPSPX-treated preparations. In the presence of guanethidine (5 microM), electrical field stimulation (EFS; 1-12 Hz, 60V, 0.1 ms, 30 s) elicited frequency-dependent vasodilatation due to activation of sensory-motor nerves. In tissues from DPSPX-treated rats the nerve-mediated vasodilator responses were markedly augmented at all frequencies. Maximal relaxation at 8 Hz was 38.34 +/- 4.76% (n = 5) in controls and 65.92 +/- 3.68% (n = 5) after DPSPX-treatment (P < 0.01). Adenosine (3 microM) inhibited the frequency-dependent sensory-motor neurotransmission similar in preparations from controls and DPSPX-treated rats. 3. In raised-tone preparations calcitonin gene-related peptide (CGRP; 5,15 and 50 pmol), the principal vasodilator transmitter of sensory-motor nerves in rat mesenteric arteries, produced similar relaxations in control and DPSPX-treated preparations. Vasodilator responses to the sensory neurotoxin capsaicin (50 and 500 pmol) were also similar between the groups. 4. Assay of tissue CGRP levels of the superior mesenteric artery by enzyme-linked immunosorbent assay showed no significant difference in tissue levels of CGRP in controls, 120.25 +/- 26.34 pmol g-1 tissue (n = 6) and with DPSPX-treatment, 82.12 +/- 24.42 pmol g-1 tissue (n = 6). 5. In raised-tone preparations dose-dependent endothelium-dependent vasodilatation to acetylcholine and ATP, and endothelium-independent vasodilatation to sodium nitroprusside were similar in control and DPSPX-treated preparations. 6. EFS (4-32 Hz, 90V, 1 ms, 30 s) elicited frequency-dependent vasoconstriction due to activation of sympathetic nerves which was similar in controls and in DPSPX-treated preparations. Adenosine (10 and 30 microM) inhibited sympathetic neurotransmission similarly in control and DPSPX-treated preparations. Dose-dependent vasoconstriction to noradrenaline (NA) and ATP, and to KCI (0.15 mmol) was similar between the groups. 7. High performance liquid chromatographic analysis of tissue NA showed no significant difference in NA content of the superior mesenteric artery from DPSPX-treated (1.38 +/- 0.09 ng mg-1, n = 6) and control rats (1.46 +/- 0.17 ng mg-1, n = 6). 8. In conclusion, in rats with hypertension due to 7 days treatment with the P1-purinoceptor antagonist, DPSPX, there is an increase in sensory-motor vasodilatation of the mesenteric arterial bed. There is no change in sympathetic nerve, endothelial or smooth muscle function. Augmented sensory-motor neurotransmission, which does not involve a change in postjunctional responsiveness to CGRP or in the CGRP content of sensory-motor nerves, could be a compensatory change in response to the DPSPX- induced hypertension.

Adenosine inhibitory effect on enhanced growth of aortic smooth muscle cells from streptozotocin-induced diabetic rats

1. There is evidence to suggest that adenosine may regulate arterial smooth muscle cell (SMC) growth and proliferation, which is a key event in atherogenesis. This regulation may be mediated via adenylate cyclase. As diabetes is a known risk factor for atherosclerosis, we investigated the growth of aortic SMC from diabetic rats in primary culture and their sensitivity to adenosine and to adenylate cyclase activity. 2. Diabetes was induced with streptozotocin (STZ, 66 mg kg-1, i.p.) Aortic SMC primary cultures were prepared from STZ-diabetic and age-matched rats 5 weeks after the STZ injection. 3. SMC from STZ-diabetic rats grew faster and reached greater densities at confluence than those from non-diabetic animals. 4. Adenosine inhibited growth in both control and diabetic SMC. However, cells from STZ-diabetic rats were apparently more sensitive to adenosine. 5. Direct activation of adenylate cyclase by forskolin induced a dose-dependent growth inhibition, similar in both groups of cells. 6. Cholera toxin, an activator of stimulatory GTP-binding protein (Gs), induced a similar growth inhibitory response in non-diabetic and diabetic SMC. Pertussis toxin (PTX), an inactivator of inhibitory GTP-binding protein (Gi), did not itself affect SMC growth. However, PTX increased dose-dependently the growth inhibition induced by adenosine in SMC from non-diabetic rats but not in SMC from diabetic rats. 7. These findings suggest a functional abnormality in Gi activity in SMC from diabetic rats, that would explain the increased sensitivity to the nucleoside. This impaired inhibitory pathway may reflect changes in the growth regulation of SMC in experimental diabetic states.

Differential alteration of neuronal and cardiovascular responses to adenosine microinjected into the nucleus tractus solitarius of spontaneously hypertensive rats

We previously reported that adenosine elicited site-dependent neuronal and cardiovascular responses in two subareas of the nucleus tractus solitarius (NTS) of normotensive rats. Pressor and tachycardic responses were obtained from the rostral NTS (adenosine pressor system), and depressor and bradycardic responses were obtained from the caudal NTS (adenosine depressor system). In both areas, adenosine inhibited the firing rate of barosensitive neurons. The present study investigated whether spontaneously hypertensive rats (SHR) exhibit abnormal neuronal and cardiovascular responses mediated by the adenosine pressor and depressor systems within the NTS. Male SHR and Wistar-Kyoto rats (WKY) were anesthesized with urethane and prepared for blood pressure and heart rate recording, stereotaxic microinjection of adenosine into the NTS, and extracellular recording of single-unit neuronal activity of NTS neurons. Chemical identification of the targeted neuronal pool was made by L-glutamate (5 nmol) and confirmed by histology. SHR exhibited significantly higher mean arterial pressure and firing rate of caudal NTS neurons (45.0 +/- 4.5 versus 27.3 +/- 4.7 spikes per 2.5 seconds, P <.05) but similar heart rate and neuronal firing rate of rostral NTS neurons compared with WKY. Adenosine (0.1, 1, and 10 nmol) elicited dose-related neuronal and cardiovascular responses in both strains. However, SHR exhibited differential alterations in both adenosine systems. Compared with WKY, SHR exhibited attenuated pressor, tachycardic, and neuronal responses mediated by the adenosine pressor system and exaggerated depressor, bradycardic, and neuronal responses mediated by the adenosine depressor system. In both strains, the responses elicited by adenosine were virtually abolished by theophylline (10 mg/kg IV), suggesting that these responses were mediated by adenosine receptors in the NTS. Furthermore, the theophylline-evoked increase in blood pressure was twofold higher in SHR (15.0 +/- 1.7 versus 6.9 +/- 1.5 mm Hg, P <.05); larger but nonsignificant increases in heart rate and neuronal firing rate also were evident in SHR compared with WKY. These findings suggest differential alterations in adenosine pressor and depressor systems in the NTS of SHR, which may be implicated in the pathophysiology of this model of hypertension.

Functional characterization of adenosine receptors in the nucleus tractus solitarius mediating hypotensive responses in the rat

1. The aim of this study was to characterize adenosine receptors located in the nucleus tractus solitarius (NTS) that mediate decreases in blood pressure in the anaesthetized rat. To determine the adenosine receptor subtype involved, a range of selective agonists and antagonists were studied and their relative potencies evaluated. 2. The rank order of agonist potency in inducing decreases in diastolic blood pressure was N6-cyclopentyladenosine (CPA) > N6-cyclohexyladenosine (CHA) > N-ethyl-carboxamidoadenosine (NECA) > or = 2-phenylaminoadenosine (CV1808) > 2-p-(carboxyethyl)phenethylamino-5' N-ethylcarboxamidoadenosine (CGS 21680) > N6-(2-(4-aminophenyl)ethyl)-adenosine (APNEA). 3. The hypotensive action of CPA following microinjection into the NTS was antagonized by i.v. infusions (50 micrograms kg-1 min-1) of adenosine receptor antagonists, 8-cyclopentyl-1,3 dipropylxanthine (DPCPX), 8-phenyltheophylline (8-PT), 8-(p-sulphophenyl)theophylline (8-SPT), and 1,3-dipropyl-8-N-(2-diethylamino)ethyl)-N methyl-4-(2,3,6,7-tetrahydro-2,6-dioxo) benzenesulphonamidexanthine (PD 115199). The antagonist potency order was DPCPX > PD115199 > or = 8-PT. Intravenous infusion of 8-SPT had no effect on blood pressure responses to microinjection of CPA into the NTS. 4. The results suggest that adenosine A1 receptors in the NTS mediate hypotensive responses in the anaesthetized rat preparation.

Enhanced sympathetic neurotransmission in the tail artery of 1,3-dipropyl-8-sulphophenylxanthine (DPSPX)-treated rats

1. Sympathetic neurotransmission and noradrenaline content of the tail artery of Wistar rats treated for 7 days with the adenosine antagonist, 1,3-dipropyl-8-sulphophenylxanthine (DPSPX), were examined. 2. Systolic blood pressure of the DPSPX-treated rats (164.0 +/- 2.9 mmHg; n = 6) was significantly greater than saline-treated controls (140.0 +/- 2.8 mmHg; n = 5) after 7 days treatment. 3. The pressor responses of the arterial rings to transmural nerve stimulation (65 V, 0.1 ms, 4-64 Hz, for 1 s) were markedly enhanced in the DPSPX-treated compared with the saline-treated animals. Both noradrenergic and purinergic components of perivascular sympathetic neurotransmission were enhanced during DPSPX-induced hypertension. 4. Vasoconstrictor responses to exogenous noradrenaline (0.1-300 microM) and adenosine 5'-triphosphate (0.01-3 mM) were unaffected after DPSPX treatment, indicating prejunctional alteration of sympathetic cotransmission during DPSPX-induced hypertension. 5. Acute exposure to DPSPX (10 microM) did not modify vasoconstrictor responses to transmural nerve stimulation, thus supporting the claim that the enhancement of sympathetic neurotransmission only results from long-term DPSPX treatment. 6. The noradrenaline content of the tail arteries of DPSPX-treated (4.498 +/- 0.26 ng cm-1; n = 4) was significantly greater than saline-treated (3.440 +/- 0.30 ng cm-1; n = 5) animals. 7. These findings show that chronic inhibition of the actions of endogenous adenosine by DPSPX results in an elevation of systolic blood pressure accompanied by enhanced sympathetic cotransmission and enhanced noradrenaline content of the rat tail artery.

Hypertension and enhanced beta-adrenoceptor-mediated facilitation of noradrenaline release produced by chronic blockade of adenosine receptors

1. The study was undertaken to compare the beta-adrenoceptor-mediated facilitation of noradrenaline release in the tail artery of vehicle-treated rats and of rats rendered hypertensive by chronic administration of 1,3-dipropyl-8-sulphophenylxanthine (DPSPX). Artery rings were loaded with [3H]-noradrenaline, and five periods of electrical stimulation (1 Hz for 2 min) were applied. To eliminate the influence of prejunctional alpha 2-adrenoceptors, the tissues were pre-exposed to 1 microM phenoxybenzamine. 2. Isoprenaline caused a concentration-dependent increase of tritium overflow elicited by electrical stimulation. It was more effective in arteries from DPSPX-treated than in those from vehicle-treated rats; isoprenaline (27.8 nM) increased by 30% tritium overflow in vessels from vehicle-treated rats whereas isoprenaline (7.0 nM) produced a 30% increase in vessels from DPSPX-treated animals. Furthermore, the maximal effect of isoprenaline was a 32.6% increase in control rats but a 48.6% increase in DPSPX-treated rats. 3. These results show that the sympathetic nerve endings of the rat tail artery are endowed with prejunctional beta-adrenoceptors which mediate facilitation of noradrenaline release elicited by electrical stimulation. They also suggest that adenosine receptors and beta-adrenoceptors interact at the prejunctional level and that impairment of this 'talk' may lead to the development of a hypertensive state.

Long-term administration of 1,3-dipropyl-8-sulphophenylxanthine (DPSPX) alters alpha 2-adrenoceptor-mediated effects at the pre- but not at the postjunctional level

The present investigation was undertaken to see whether a long-term inhibition of adenosine receptors--leading to hypertension--interferes with alpha 2-adrenoceptor-mediated modulation of noradrenaline release. Rat tail arteries were removed from normal and from hypertensive animals obtained by chronic treatment with intraperitoneally infused DPSPX (1,3-dipropyl-8-sulphophenylxanthine) or orally administered L-NAME (NG-Nitro-L-arginine methyl ester). To study prejunctional effects, the influence of UK-14,304 (5-bromo-6(imidazoline-2-ylamino)-quinoxaline) and yohimbine on the overflow of tritium evoked by electrical stimulation (100 V; 1 Hz; 2 ms; 5 min) from tissues preloaded with 3H-noradrenaline was analysed. To study postjunctional effects, concentration-response curves to UK-14,304 were determined. In DPSPX-treated rats there was an enhancement of the prejunctional effects of UK-14,304: its Ec30% was reduced from 381 (250; 579) to 85 (73; 99) nmol.l-1 (n = 5; P < 0.05) and its maximal effect--expressed as percent reduction of tritium overflow-increased from 45 +/- 5% to 61 +/- 5% (n = 6; P < 0.05). In L-NAME-treated rats there was no change in either of these two parameters. At the postjunctional level, there was no change in the sensitivity to UK-14,304 in tissues from either DPSPX- or L-NAME-treated rats. Yohimbine (10-1000 nmol.l-1) caused a concentration-dependent increase of tritium overflow evoked by electrical stimulation in both control and hypertensive animals (either DPSPX- or L-NAME-treated).(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity of central adenosine A1 receptors is decreased in spontaneously hypertensive rats

Functional defects in purinergic neurotransmission have been related to the development of arterial hypertension in spontaneously hypertensive rats. In order to elucidate the molecular basis of this perturbation, we have directly characterized adenosine A1 receptors using radioligand binding to rat brain membranes of Wistar Kyoto (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP). Saturation studies with [3H]1,3-dipropylcyclopentylxanthine ([3H]DPCPX) showed a lower affinity in both 5- and 48-week-old SHRSP in comparison with age-matched WKY. Similarly, competition experiments with [3H]DPCPX showed lower affinity of R-N6-phenylisopropyladenosine for the low-affinity binding site in 5- and 48-week-old SHRSP in comparison with WKY. In both studies, the difference in KD values was abolished by guanosine-5'-triphosphate in 5-week-old rats and mitigated in 48-week-old animals. No differences in Bmax values were observed in 5-week-old rats, whereas in 48-week-old SHRSP the number of receptors was significantly higher in comparison with age-matched WKY. Saturation experiments with the A1-selective agonist [3H]2-chloro-N6-cyclopentyladenosine ([3H]CCPA) demonstrated a higher affinity in 5-week-old SHRSP, whereas in 48-week-old hypertensive animals it was lower than in control WKY rats. No difference in receptor number was detected in comparison with age-matched WKY. In conclusion, our data demonstrated a diminished affinity of central adenosine A1 receptors for antagonists and for the low affinity state of the agonist binding site in genetically hypertensive rats. This might be due to structural changes of the receptor protein, to an altered G protein or defective receptor-G protein coupling in arterial hypertension.

Mediation by adenosine of the trophic effects exerted by the sympathetic innervation of blood vessels

Chemical or surgical sympathetic denervation of blood vessels causes marked changes of the effector cells. Since postganglionic sympathetic cotransmission by noradrenaline and adenosine 5'-triphosphate is well established, the role of these transmitters as putative trophic factors was investigated. Whereas noradrenaline was ineffective in preventing morphological changes due to denervation, both adenosine and N-ethylcarboxamido-adenosine totally prevented them. In conscious rats, the adenosine receptor antagonist dipropylsulphophenylxantine (DPSPX) caused alterations of the blood vessel wall similar to those described for denervation. These results strongly suggest that adenosine is the trophic factor of sympathetic innervation.