Autoregulation of superior mesenteric artery is blocked by adenosine antagonism

Pressure-flow autoregulation of the intact superior mesenteric artery (SMA) was demonstrated in the fasted, pentobarbital-anesthetized cat by use of a micrometer-controlled screw clamp to produce progressive decreases in vascular pressure. Administration (ia) of bolus doses of 8-phenyltheophylline (8-PT) were followed by infusion of adenosine to verify adenosine antagonism. 8-PT doses were progressively doubled until adenosine responses were blocked. If higher doses of 8-PT were used, SMA flow declined to very low levels and autoregulatory curves could not be obtained. Comparison of vasodilator responses to isoproterenol and adenosine before and after adenosine receptor blockade verified that, whereas adenosine responses were blocked, isoproterenol effects were not altered. The autoregulation was quantitated using three methods (the autoregulatory index, the percent decrease in vascular resistance, and the slope index) as blood pressure was reduced from a standardized control pressure of 110 mmHg (1 mmHg = 133.3 Pa). Maximal vasodilation appeared at a blood pressure of 56 +/- 5 mmHg (range 34-70). 8-PT resulted in dose-related antagonism of the dilator response to exogenous adenosine and autoregulation. All indices of autoregulation were significantly reduced by 8-PT. The data are compatible with the hypothesis that pressure-flow autoregulation in the SMA is not myogenic (responding to altered transmural pressure) but is dependent upon local concentrations of adenosine.     

Effect of raised portal venous pressure and postocclusive hyperemia on superior mesenteric arterial resistance in control and adenosine receptor blocked state in cats

Superior mesenteric arterial (SMA) blood flow was measured in pentobarbital-anesthetized cats using a noncannulating electromagnetic flowprobe. The selective adenosine antagonist 8-phenyltheophylline (8-PT) antagonized the dilator effect of infused adenosine but not isoproterenol. The vasodilation in response to reduced arterial perfusion pressure (autoregulation) was blocked by the adenosine receptor blockade, which also reduced the degree of postocclusive (1 min) hyperemia by one-half to two-thirds. The remainder of the hyperemia may have been due partially to adenosine, since exogenous adenosine still produced a small vasodilation (26%), so effects of endogenous adenosine could also still be expected to exert a small effect. Myogenic effects appear unlikely to be the mechanism of the small remaining hyperemia, since venous pressure increments within physiologically relevant ranges did not cause altered SMA conductance, and arterial dilation in response to large decreases in arterial pressure could be blocked by adenosine antagonism. Portal pressure was increased using hepatic nerve stimulation (8 Hz) to raise pressure from 7.0 to 12.4 mmHg (1 mmHg = 133.3 Pa). The small vasoconstriction seen in the SMA was due to the rise in systemic blood pressure, since prevention of a rise in SMA pressure prevented the response and 8-PT blocked the response (previously shown to block arterial pressure-flow autoregulation). An equal rise in PVP imposed by partial occlusion of the portal vein did not lead to changes in SMA vascular conductance. Thus, we conclude that within physiologically relevant ranges of arterial and portal venous pressure, the SMA does not show myogenic responses of the resistance vessels.     

Coronary blood flow regulation in exercising swine involves parallel rather than redundant vasodilator pathways

In dogs, only combined blockade of vasodilator pathways [via adenosine receptors, nitric oxide synthase (NOS) and ATP-sensitive K+ (KATP) channels] results in impairment of metabolic vasodilation, which suggests a redundancy design of coronary flow regulation. Conversely, in swine and humans, blocking KATP channels, adenosine receptors, or NOS each impairs coronary blood flow (CBF) at rest and during exercise. Consequently, we hypothesized that these vasodilators act in parallel rather than in redundancy to regulate CBF in swine. Swine exercised on a treadmill (0-5 km/h), during control and after blockade of KATP channels (with glibenclamide), adenosine receptors [with 8-phenyltheophylline (8-PT)], and/or NOS [with Nomega-nitro-l-arginine (l-NNA)]. l-NNA, 8-PT, and glibenclamide each reduced myocardial O2 delivery and coronary venous O2 tension. These effects of l-NNA, 8-PT, and glibenclamide were not modified by simultaneous blockade of the other vasodilators. Combined blockade of KATP channels and adenosine receptors with or without NOS inhibition was associated with increased H+ production and impaired myocardial function. However, despite an increase in O2 extraction to >90% during administration of l-NNA + 8-PT + glibenclamide, vasodilator reserve could still be recruited during exercise. Thus in awake swine, loss of KATP channels, adenosine, or NO is not compensated for by increased participation of the other two vasodilator mechanisms. These findings suggest a parallel rather than a redundancy design of CBF regulation in the porcine circulation.     

Adenosine receptor-mediated changes in cyclic AMP production and DNA synthesis in cultured arterial smooth muscle cells

The effects of adenosine and two analogs, L-phenylisopropyladenosine (L-PIA) and 5'-N-ethylcarboxamidoadenosine (NECA), on cAMP production and on platelet-derived growth factor (PDGF)-stimulated initiation of DNA synthesis in growth-arrested cultures of rat arterial smooth muscle cells (SMC) were studied. The intracellular cAMP concentration was dose-dependently enhanced by micromolar concentrations of adenosine and its analogs, with the potency order NECA greater than adenosine greater than L-PIA. The effect was antagonized, in a competitive manner, by the adenosine receptor antagonist 8-phenyltheophylline (8-PT). The stimulatory effect of adenosine was enhanced by 3 microM dipyridamole an adenosine-uptake blocker. DNA synthesis was inhibited in a parallel manner, showing the same potency order. The inhibition was antagonized by 8-PT. Forskolin, a diterpene with the ability to stimulate the catalytic unit of adenylate cyclase and thereby cAMP formation, potentiated the effects of micromolar concentrations of NECA and L-PIA. Forskolin, by itself, stimulated cAMP production and inhibited DNA synthesis. The forskolin-stimulated increase in cAMP was inhibited by L-PIA at nanomolar concentrations. L-PIA in the nanomolar concentration range also stimulated DNA synthesis when initiation was stimulated with suboptimal concentrations of PDGF. These findings suggest the presence of adenosine receptors of both the A1- and A2-subtype on SM-mediating bidirectional changes of cAMP and DNA synthesis.     

Adenosine is not responsible for local metabolic control of coronary blood flow in dogs during exercise

The purpose of this investigation was to quantitatively evaluate the role of adenosine in coronary exercise hyperemia. Dogs (n = 10) were chronically instrumented with catheters in the aorta and coronary sinus, and a flow probe on the circumflex coronary artery. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous plasma concentrations using a previously tested mathematical model. Coronary blood flow, myocardial oxygen consumption, heart rate, and aortic pressure were measured at rest and during graded treadmill exercise with and without adenosine receptor blockade with either 8-phenyltheophylline (8-PT) or 8-p-sulfophenyltheophylline (8-PST). In control vehicle dogs, exercise increased myocardial oxygen consumption 4.2-fold, coronary blood flow 3.8-fold, and heart rate 2.5-fold, whereas mean aortic pressure was unchanged. Coronary venous plasma adenosine concentration was little changed with exercise, and the estimated interstitial adenosine concentration remained well below the threshold for coronary vasodilation. Adenosine receptor blockade did not significantly alter myocardial oxygen consumption or coronary blood flow at rest or during exercise. Coronary venous and estimated interstitial adenosine concentration did not increase to overcome the receptor blockade with either 8-PT or 8-PST as would be predicted if adenosine were part of a high-gain, negative-feedback, local metabolic control mechanism. These results demonstrate that adenosine is not responsible for local metabolic control of coronary blood flow in dogs during exercise.     

Influence of adenosine on cerebral blood flow during hypoxic hypoxia in the newborn piglet

This study investigated the role of adenosine in the regulation of neonatal cerebral blood flow (CBF) during moderate (arterial PO2 = 47 +/- 9 Torr) and severe (arterial PO2 = 25 +/- 4 Torr) hypoxia. Twenty-eight anesthetized and ventilated newborn piglets were assigned to four groups: 8 were injected intravenously with the vehicle (controls, group 1); 13 received an intravenous injection of 8-phenyltheophylline (8-PT), a potent adenosine receptor blocker, either 4 mg/kg (group 2, n = 6, mean cerebrospinal fluid (CSF) levels less than 1 mg/l) or 8 mg/kg (group 3, n = 7, mean CSF levels less than 3.5 mg/l); and 7 received an intracerebroventricular injection of 10 micrograms 8-PT (group 4). During normoxia, CBF was not altered by vehicle or 8-PT injections. In group 1, 10 min of moderate and severe hypoxia increased total CBF by 112 +/- 36 and 176 +/- 28% (SE), respectively. Compared with controls, the cerebral hyperemia during moderate hypoxia was not altered in group 2, attenuated in group 3 (to 53 +/- 13%, P = NS), and completely blocked in group 4 (P less than 0.01). CBF increase secondary to severe hypoxia was attenuated only in group 4 (74 +/- 29%, P less than 0.05). CSF concentrations of adenosine and adenosine metabolites measured by high-performance liquid chromatography increased during hypoxia. Arterial O2 content was inversely correlated (P less than 0.005) to maximal CSF levels of adenosine (r = 0.73), inosine (r = 0.87), and hypoxanthine (r = 0.80).(ABSTRACT TRUNCATED AT 250 WORDS)     

肾缺血增强大鼠延髓腹外侧头端区神经元电话动和Fos蛋白表达

在67只切断两侧缓冲神经的麻醉Sprague-Dawley大鼠,应用细胞外记录的电生理方法和免疫组织化学技术,分别观察肾缺血对延髓腹外侧头端区巨细胞旁外侧核神经元自发放电活动和Fos蛋白表达的影响.所得结果如下(1)左肾动脉阻断后,28个单位的放电频率由11.40±1.08增至21.1±1.74spikes/s(P<0.001),血压和心率无明显变化(P>0.05);(2)在17个放电单位中,应用腺苷受体拮抗剂8-苯茶碱(8-phenyltheophylline,10mg/kg)可明显抑制肾缺血的兴奋效应(P<0.05);(3)肾缺血后,延髓腹外侧头端区的Fos蛋白样免疫反应神经元显著增加(P<0.01);(4)预先应用8-苯茶碱可明显减弱肾缺血所激活的Fos蛋白表达反应(P<0.05).以上结果提示肾缺血增强延髓腹外侧头端区神经元的放电活动和Fos蛋白表达,而此作用可能与肾脏缺血所产生的腺苷激活肾内感受器有关.     

The effects of adenyl compounds on the heart of the axolotl (Ambystoma mexicanum)

In the spontaneously beating axolotl atrium, adenosine 5'-triphosphate (ATP) produced initial excitation followed by inhibition and then by a secondary excitation. This third phase of the ATP response was only seen in electrically driven preparations in the presence of 8-phenyltheophylline (8-PT), an adenosine receptor antagonist. alpha,beta-Methylene ATP (APCPP), a stable analogue of ATP, produced only excitatory effects, while adenosine and beta,gamma-methylene ATP (APPCP), a slowly degradable ATP analogue, produced inhibition or inhibition followed by excitation. 2-Chloroadenosine produced inhibition. The excitatory effects were not antagonized by 8-PT, indomethacin or propranolol and phentolamine. The negative inotropic responses of these compounds were antagonized by 8-PT and, with the exception of 2-chloroadenosine, potentiated by dipyridamole, an adenosine uptake blocker. In the ventricle, ATP, APCPP and APPCP produced positive inotropic effects, which were not affected by 8-PT, indomethacin or propranolol and phentolamine. Adenosine produced a negative inotropic effect which was not antagonized by 8-PT nor atropine nor potentiated by dipyridamole. The effects of adenyl compounds on the axolotl (urodele) heart suggest that, like the frog (anuran) heart, both P1- and P2-purinoceptors are present in the axolotl atrium and that only P2-purinoceptors are present in the axolotl ventricle, although adenosine does produce an inhibitory effect on the ventricle which is probably mediated via the release of a neurotransmitter other than acetylcholine.     

Mechanisms underlying the supra-maximal thermogenesis elicited by aminophylline in rats

Previous studies showed that acute treatment with aminophylline (AMPY) significantly elevated maximum thermogenesis and improved cold tolerance in rats and man in severe cold. However, the exact mechanism by which AMPY enhances thermogenesis was unknown. Rats receiving enprofylline (ENPRO) (1.5 and 15 mg/kg, i.p.), a selective phosphodiesterase inhibitor, failed to show enhanced thermogenesis. In contrast, treatment with a selective adenosine receptor antagonist, 8-phenyltheophylline(8-PT; 2.5 to 10 mg/kg, i.p.), significantly increased (p less than 0.05) thermogenesis and cold tolerance. However, the maximal thermogenic effect by optimal dose of 8-PT (5 mg/kg) was significantly lower than that with optimal dose of AMPY (18.7 mg/kg, i.p.); the deficit could be eradicated by combining optimal 8-PT dose with a low dose of AMPY (1.25 mg/kg), but not with ENPRO. These results indicate that the thermogenic effect of AMPY is not by inhibition of phosphodiesterase but at least partially by antagonism of adenosine receptors. It is also apparent that older mechanisms in addition to adenosine antagonism are also involved in AMPY's thermogenic action.     

Blockade of nitric oxide synthase potentiates the suppression of vasodilators by norepinephrine in the hepatic artery.

We have previously shown that nitric oxide (NO) and adenosine suppress vasoconstriction induced by norepinephrine infusion and sympathetic nerve stimulation in the hepatic artery and superior mesenteric artery. NO is involved in the control of basal vascular tone in the superior mesenteric artery but not the hepatic artery. The vasodilation induced by adenosine is inhibited by NO in the superior mesenteric artery but not in the hepatic artery. Based on these known interactions of catecholamines, adenosine, and NO, the objective of this study was to test the hypothesis that NO modulates the interaction between vasoconstrictors and vasodilators in the hepatic artery. We examined the ability of norepinephrine to suppress adenosine-mediated vasodilation and the role of NO in this interaction. Hepatic arterial blood flow and pressure were monitored in pentobarbital-anesthetized cats. The maximum hepatic arterial vasoconstrictor response to norepinephrine infusion was potentiated by blockade of NO production using Nomega-nitro-L-arginine methyl ester (L-NAME), and the potentiation was reversed by L-arginine. The maximum dilator response to adenosine was only slightly suppressed (14.0+/-5.8%, P < 0.05) by norepinephrine infusion; however, after the NO blockade, the suppression by norepinephrine of the vasodilation induced by adenosine was substantially potentiated (45.2+/-9.1%, P < 0.05). Similar results were obtained for isoproterenol-induced vasodilation. We conclude that the interaction between these vasodilators and norepinephrine was modulated by NO which inhibited the vasoconstriction and the suppression of vasodilators caused by norepinephrine and that in the absence of NO production, norepinephrine-induced constriction and the ability to antagonize dilation is substantially potentiated.     

Effect of temperature on atrial and ventricular adenosine A1 receptors of the guinea pig

Cardiac adenosine receptors are coupled to adenylate cyclase inhibition. In the guinea pig heart, the relative agonist potencies observed for adenylate cyclase inhibition were R-N6-phenylisopropyladenosine (R-PIA) = N6-cyclohexyladenosine greater than 5'-N-ethylcarboxamidoadenosine much greater than S-PIA. In both atrial and ventricular membranes, the antagonists 8-phenyltheophylline (8-PT) and isobutylmethylxanthine (IBMX) also showed similar affinities for atrial and ventricular adenosine receptors. The same pattern of relative agonist potencies was observed in experiments performed at either 25 or 37 degrees C. However, the maximal inhibition produced by R-PIA in atrial membranes decreased from 30.8 +/- 3.2% (n = 7) at 25 degrees C to 18.8 +/- 1.6% (n = 4) at 37 degrees C. No such difference in maximal inhibition was observed with ventricular membranes at these two temperatures (34.5 +/- 1.6%, n = 6 at 25 degrees C and 35.3 +/- 0.9%, n = 11 at 37 degrees C). While there was no change in agonist potencies, the affinities of the antagonists 8-PT and IBMX at cardiac adenosine A1 receptors were affected by temperature. At 25 degrees C, the pKD values for 8-PT and IBMX in ventricular membranes were 4.65 +/- 0.21 (n = 3) and 4.55 +/- 0.20 (n = 3), respectively. Their affinities were 7- to 19-fold higher at 37 degrees C, the pKD values being 5.93 +/- 0.12 (n = 7) (p less than 0.02) and 5.38 +/- 0.18 (n = 3) (p less than 0.05), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Systemic circulatory effects of pentagastrin

Effects of pentagastrin on systemic circulation were studied in anesthetized cats. Systemic arterial, central venous and portal pressure were monitored with electromanometers and blood flow through the superior mesenteric artery, common carotid artery, femoral artery and ascending aorta were measured with an electromagnetic blood flow meter. Pentagastrin injected intravenously at a doses of 2.0, 4.0 and 8.0 micrograms/kg induced a dose-dependent fall in arterial pressure, heart rate and cardiac output, increased mesenteric blood flow, decreased common carotid artery blood flow, did not change femoral artery blood flow and slightly rose central venous pressure. Atropine blocked observed effects. After repeated injections of the peptide, tachyphylaxis quickly developed. The obtained results indicate that pentagastrin influences general hemodynamics probably via interaction with cholinergic receptors.     

Sites of action of adenosine in interorgan preconditioning of the heart

The mechanism underlying interorgan preconditioning of the heart remains elusive, although a role for adenosine and activation of a neurogenic pathway has been postulated. We tested in rats the hypothesis that adenosine released by the remote ischemic organ stimulates local afferent nerves, which leads to activation of myocardial adenosine receptors. Preconditioning with a 15-min mesenteric artery occlusion (MAO15) reduced infarct size produced by a 60-min coronary artery occlusion (60-min CAO) from 68 +/- 2% to 48 +/- 4% (P < 0.05). Pretreatment with the ganglion blocker hexamethonium or 8-(p-sulfophenyl)theophylline (8-SPT) abolished the protection by MAO15. Intramesenteric artery (but not intraportal vein) infusion of adenosine (10 microg/min) was as cardioprotective as MAO15, which was also abolished by hexamethonium. Whereas administration of hexamethonium at 5 min of reperfusion following MAO15 had no effect, 8-SPT at 5 min of reperfusion abolished the protection. Permanent reocclusion of the mesenteric artery before the 60-min CAO enhanced the cardioprotection by MAO15 (30 +/- 5%), but all protection was abolished when 8-SPT was administered after reocclusion of the mesenteric artery. Together, these findings demonstrate the involvement of myocardial adenosine receptors. We therefore conclude that locally released adenosine during small intestinal ischemia stimulates afferent nerves in the mesenteric bed during early reperfusion, initiating a neurogenic pathway that leads to activation of myocardial adenosine receptors.     

Voltage-dependent K+ channels regulate the duration of reactive hyperemia in the canine coronary circulation

We previously demonstrated a role for voltage-dependent K(+) (K(V)) channels in coronary vasodilation elicited by myocardial metabolism and exogenous H(2)O(2), as responses were attenuated by the K(V) channel blocker 4-aminopyridine (4-AP). Here we tested the hypothesis that K(V) channels participate in coronary reactive hyperemia and examined the role of K(V) channels in responses to nitric oxide (NO) and adenosine, two putative mediators. Reactive hyperemia (30-s occlusion) was measured in open-chest dogs before and during 4-AP treatment [intracoronary (ic), plasma concentration 0.3 mM]. 4-AP reduced baseline flow 34 +/- 5% and inhibited hyperemic volume 32 +/- 5%. Administration of 8-phenyltheophylline (8-PT; 0.3 mM ic or 5 mg/kg iv) or N(G)-nitro-L-arginine methyl ester (L-NAME; 1 mg/min ic) inhibited early and late portions of hyperemic flow, supporting roles for adenosine and NO. 4-AP further inhibited hyperemia in the presence of 8-PT or L-NAME. Adenosine-induced blood flow responses were attenuated by 4-AP (52 +/- 6% block at 9 microg/min). Dilation of arterioles to adenosine was attenuated by 0.3 mM 4-AP and 1 microM correolide, a selective K(V)1 antagonist (76 +/- 7% and 47 +/- 2% block, respectively, at 1 microM). Dilation in response to sodium nitroprusside, an NO donor, was attenuated by 4-AP in vivo (41 +/- 6% block at 10 microg/min) and by correolide in vitro (29 +/- 4% block at 1 microM). K(V) current in smooth muscle cells was inhibited by 4-AP (IC(50) 1.1 +/- 0.1 mM) and virtually eliminated by correolide. Expression of mRNA for K(V)1 family members was detected in coronary arteries. Our data indicate that K(V) channels play an important role in regulating resting coronary blood flow, determining duration of reactive hyperemia, and mediating adenosine- and NO-induced vasodilation.     

Stimulation of adenylate cyclase by adenosine and other agonists in mesenteric artery smooth muscle cells in culture

An adenosine-sensitive adenylate cyclase has been characterized in cultured mesenteric artery smooth muscle cells. N-Ethylcarboxamide-adenosine (NECA), N-Methylcarboxamide-adenosine (MECA), L-N6-phenylisopropyladenosine (PIA) and 2-chloroadenosine (2-cl-Ado) all stimulated adenylate cyclase in a concentration dependent manner. NECA was the most potent analog (EC50, 1 microM), whereas PIA (EC50, 15 microM), 2-Cl-Ado (EC50, 15 microM) and MECA (EC50, 24 microM), were less potent and had efficacies relative to NECA of 0.61, 0.61 and 0.65, respectively. Adenosine showed a biphasic effect: stimulation at lower concentrations and inhibition at higher concentrations, whereas 2' deoxyadenosine only inhibited adenylate cyclase activity. The stimulatory effect of NECA on adenylate cyclase was dependent on metal ion concentration and was blocked by 3-isobutyl-l-methylxanthine (IBMX) and 8-phenyltheophylline (8-PT). Adenylate cyclase from these cultured cells was also stimulated by other agonists such as epinephrine, norepinephrine, prostaglandins, dopamine, NaF and forskolin. The stimulation of adenylate cyclase by isoproterenol, epinephrine and norepinephrine was blocked by propranolol but not by phentolamine. On the other hand, phentolamine, propranolol and flupentixol all inhibited dopamine-stimulated adenylate cyclase activity. In addition, the stimulation by an optimal concentration of PIA was additive or almost additive with maximal stimulation caused by catecholamines and prostaglandins. These data indicate the presence of adenosine (Stimulatory "Ra"), catecholamine and prostaglandin receptors in mesenteric artery smooth muscle cells and suggest that these agents may exert their physiological actions through their interaction with their respective receptors coupled to adenylate cyclase.     

On the Adenosine Receptor Involved in the Excitatory Action of Adenosine on Respiration: Antagonist Profile

Abstract

The use of xanthines to characterize the adenosine receptor involved in excitation of respiration revealed that XCC, PD 115, 199 and XAC have much higher affinities than DPCPX followed by PACPX, DPX and 8-PT, which is compatible with an A2 type of adenosine receptor.     

Maintenance of hepatic arterial blood flow during hemorrhage is mediated by adenosine

The effect of hemorrhage (1.91 mL/min, 10 mL/kg) on splanchnic blood flow was determined in cats anesthetized with pentobarbital. The hepatic artery (HA) is relatively protected during hemorrhage and does not constrict, whereas the superior mesenteric artery (SMA) undergoes significant vasoconstriction. Adenosine receptor antagonism with 8-phenyltheophylline blocks the dilator response to infused adenosine selectively (does not block responses to isoproterenol). The dilator response to reduced portal blood flow (the HA buffer response) is also antagonized and adenosine receptor blockade converts the HA response to hemorrhage to one similar to that of the SMA. Thus, the protective dilation of the HA during hemorrhage is mediated by adenosine. In contrast, the vasodilation of the HA seen with reinfusion of the shed blood is not altered by adenosine receptor antagonism.     

Effect of synthetic physalaemin on splanchnic circulation in dogs

Physalaemin has been reported as one of the most potent vasodilator and hypotensive peptides (1-4). In spite of these studies, however, the effect of the peptide on splanchnic circulation is not known precisely. In the present study, the effect of synthetic physalaemin on superior mesenteric arterial blood flow, portal venous blood flow and pancreatic capillary blood flow was investigated in dogs. Dose dependent increases of superior mesenteric arterial blood flow and portal venous blood flow were induced in response to physalaemin (0.1-10.0 ng/kg). Superior mesenteric arterial blood flow and portal venous blood flow attained maximal increases of 77 +/- 8.9% and 70 +/- 8.6%, respectively, at a dose of 5 ng/kg. Physalaemin caused a dose-related decrease in systemic arterial blood pressure. Pancreatic capillary blood flow did not show significant change with the administration of physalaemin. These data suggest that physalaemin may play some physiological roles in the regulation of splanchnic circulation.     

Adenosine modulation of vasoconstrictor responses to stimulation of sympathetic nerves and norepinephrine infusion in the superior mesenteric artery of the cat

Vasoconstriction induced by sympathetic nerve stimulation and by norepinephrine infusion in the superior mesenteric artery of cats anesthetized with pentobarbital was inhibited by adenosine infusions in a dose-related way. The responses to nerve stimulation were not inhibited to a greater extent than the responses to norepinephrine, thus suggesting no presynaptic modulation of sympathetic nerves supplying the resistance vessels of the feline intestinal vascular bed. Blockade of adenosine receptors using 8-phenyltheophylline did not alter the degree of constriction induced by nerve stimulation or norepinephrine infusion, indicating that in the fasted cat, endogenous adenosine co-released or released subsequent to constriction does not affect the peak vasoconstriction reached. Isoproterenol caused similar degrees of vasodilation as adenosine but did not show significant antagonism of the pooled responses to nerve stimulation or norepinephrine infusion; there was no tendency for the degree of dilation induced by isoproterenol to correlate with the inhibition of constrictor responses. Thus, the effect of adenosine on nerve- and norepinephrine-induced constriction is not secondary to nonspecific vasodilation.     

Inhibition of glucose uptake in murine cardiomyocyte cell line HL-1 by cardioprotective drugs dilazep and dipyridamole

Inhibition of adenosine reuptake by nucleoside transport inhibitors, such as dipyridamole and dilazep, is proposed to increase extracellular levels of adenosine and thereby potentiate adenosine receptor-dependent pathways that promote cardiovascular health. Thus adenosine can act as a paracrine and/or autocrine hormone, which has been shown to regulate glucose uptake in some cell types. However, the role of adenosine in modulating glucose transport in cardiomyocytes is not clear. Therefore, we investigated whether exogenously applied adenosine or inhibition of adenosine transport by S-(4-nitrobenzyl)-6-thioinosine (NBTI), dipyridamole, or dilazep modulated basal and insulin-stimulated glucose uptake in the murine cardiomyocyte cell line HL-1. HL-1 cell lysates were subjected to SDS-PAGE and immunoblotting to determine which GLUT isoforms are present. Glucose uptake was measured in the presence of dipyridamole (3-300 microM), dilazep (1-100 microM), NBTI (10-500 nM), and adenosine (50-250 microM) or the nonmetabolizable adenosine analog 2-chloro-adenosine (250 microM). Our results demonstrated that HL-1 cells possess GLUT1 and GLUT4, the isoforms typically present in cardiomyocytes. We found no evidence for adenosine-dependent regulation of basal or insulin-stimulated glucose transport in HL-1 cardiomyocytes. However, we did observe a dose-dependent inhibition of glucose transport by dipyridamole (basal, IC(50) = 12.2 microM, insulin stimulated, IC(50) = 13.09 microM) and dilazep (basal, IC(50) = 5.7 microM, insulin stimulated, IC(50) = 19 microM) but not NBTI. Thus our data suggest that dipyridamole and dilazep, which are widely used to specifically inhibit nucleoside transport, have a broader spectrum of transport inhibition than previously described. Moreover, these data may explain previous observations, in which dipyridamole was noted to be proischemic at high doses.