Evidence that the intracellular effects of adenosine in the guinea-pig aorta are mediated by inosine

Adenosine and nitric oxide in exercise-induced human skeletal muscle vasodilatation

The vasoactive substances adenosine and nitric oxide (NO) are credible candidates in the local regulation of skeletal muscle blood flow. Adenosine and NO have both been shown to increase in skeletal muscle cells and interstitial fluid during exercise and the enzymes responsible for their formation, AMP 5'-nucleotidase and NO synthase (NOS), have been shown to be activated upon muscle contraction. In vitro as well as in vivo evidence suggest that the contraction-induced increase in interstitial adenosine concentration largely stems from extracellular formation via the membrane-bound ecto-form of AMP 5'-nucleotidase. It remains unclear whether the exercise-induced NO formation in muscle originates from endothelial NOS in the microvascular endothelium, or from neuronal NOS (nNOS) in nerve cells and muscle fibres. Functional evidence for the role of adenosine in muscle blood flow control stems from studies using adenosine receptor agonists and antagonists, adenosine deaminase or adenosine uptake inhibitors. The majority of these studies have been performed on laboratory animals and, although the results show some discrepancy, the majority of studies indicate that adenosine does participate in the regulation of muscle blood flow. In humans, evidence is lacking. The role of NO in the regulation of skeletal muscle blood flow has mainly been studied using NOS inhibitors. Despite a large number of studies in this area, the role of NO for the contraction-induced increase in skeletal muscle blood flow is uncertain. The majority, but not all, human and animal studies show that, whereas blockade of NOS reduces muscle blood flow at rest and in recovery from exercise, there is no effect on the exercise-induced increase in muscle perfusion. Conclusive evidence for the mechanisms underlying the precise regulation of the multiphased increase in skeletal muscle blood flow during exercise and the role and potency of various vasoactive substances, remain missing.

Adenosine and its receptor agonists regulate nitric oxide production and RAW 264.7 macrophages via both receptor binding and its downstream metabolites-inosine

Adenosine and its receptor agonists enhanced the production of nitric oxide (NO) in lipopolysaccharide (LPS)-treated RAW 264.7 cells. The enhancement of LPS-induced NO production by adenosine, as represented by the amount of its oxidation products, nitrite and nitrate, was inhibited by adenosine uptake inhibitors, such as dipyridamole, S(4-nitrobenzyl)-6-thioinosine (NBTI) and S(4-nitrobenzyl)-6-thioguanosine (NBTG). These indicate that the uptake of adenosine by macrophages is a prerequisite for the enhancement effects observed. A downstream metabolite of adenosine, inosine, also potentiated the LPS-induced NO production in a dose-dependent manner while its enhancement effect was also inhibited by dipyridamole. However, the degree of enhancement by inosine on NO production and nitric oxide synthase (NOS) activity in LPS-treated RAW 264.7 was weaker than the effect of adenosine. Furthermore, adenosine agonists also enhanced the NO production in a dose-dependent manner, but were not specific for A1, A2 nor A3 adenosine receptor. Adenosine uptake inhibitors had no effects on the enhancement activity of the adenosine receptor agonists. Thus, extracellular receptor/s may also play an important role in the observed enhancement responses. The results of this study indicate that the enhancement effects of adenosine on NO production in macrophages could be mediated by the extracellular adenosine receptors as well as the downstream metabolites of adenosine.

Axon outgrowth is regulated by an intracellular purine-sensitive mechanism in retinal ganglion cells

Although purinergic compounds are widely involved in the intra- and intercellular communication of the nervous system, little is known of their involvement in the growth and regeneration of neuronal connections. In dissociated cultures, the addition of adenosine or guanosine in the low micromolar range induced goldfish retinal ganglion cells to extend lengthy neurites and express the growth-associated protein GAP-43. These effects were highly specific and did not reflect conversion of the nucleosides to their nucleotide derivatives; pyrimidines, purine nucleotides, and membrane-permeable, nonhydrolyzable cyclic nucleotide analogs were all inactive. The activity of adenosine required its conversion to inosine, because inhibitors of adenosine deaminase rendered adenosine inactive. Exogenously applied inosine and guanosine act directly upon an intracellular target, which may coincide with a kinase described in PC12 cells. In support of this, the effects of the purine nucleosides were blocked with purine transport inhibitors and were inhibited competitively with the purine analog 6-thioguanine (6-TG). In PC12 cells, others have shown that 6-TG blocks nerve growth factor-induced neurite outgrowth and selectively inhibits the activity of protein kinase N, a partially characterized, nerve growth factor-inducible serine-threonine kinase. In both goldfish and rat retinal ganglion cells, 6-TG completely blocked outgrowth induced by other growth factors, and this inhibition was reversed with inosine. These results suggest that axon outgrowth in central nervous system neurons critically involves an intracellular purine-sensitive mechanism.

Compensation of endothelin-1-induced coronary vasoconstriction

The vasodilator capacity of the coronaries was determined by the reactive hyperemia (RH) test in open-chest anesthetized dogs. The myocardial release of adenine nucleosides (adenosine and inosine) was measured by the HPLC-UV method. In group I (n = 9) after the control RH test, a bolus injection of endothelin-1 (ET-1; 1.0 nmol i.c.) was administered and was followed by a second RH test. In group II (n = 9), glibenclamide (GLIB) was infused continuously (1.0 mumol/min i.c.) and RH tests were performed during the control period and then before and after bolus injection of ET-1. In contrast to the significant reduction of the RH response after ET-1 in group I and after GLIB in group II, the nucleoside release into the coronary sinus during the first minute of the RH test was significantly higher (adenosine release 0.05 +/- 0.02 vs. 0.10 +/- 0.04 mumol, and 0.02 +/- 0.00 vs. 0.08 +/- 0.02 mumol; p < 0.05). Injection of ET-1 did not result in further RH reduction in GLIB-pretreated dogs (group II) but significantly increased nucleoside release. High doses of ET-1 activated the metabolic compensatory mechanisms of the myocardium and thereby increased the release of adenine nucleosides into the venous blood of the heart. However, whether these metabolites can exert any significant compensatory vasodilator effects appears doubtful.

A biphasic response to adenosine in the coronary vasculature of the K(+)-arrested perfused rat heart

Biphasic vasodilatory responses to adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) were observed in the coronary vasculature of K(+)-arrested perfused rat hearts. Dose-response data for both agonists were best represented by two-site models. For adenosine, two sites with negative log ED50 (pED50) values of 8.1 +/- 0.1 (mean +/- S.E.M) and 5.2 +/- 0.1 were obtained, mediating 31 +/- 2% and 69 +/- 2% of the total response. In the presence of 8-phenyltheophylline, the vasodilatory response to adenosine remained best fitted to a two-site model with pED50 values of 7.0 +/- 0.2 and 5.4 +/- 0.2. The relative contribution of each site to the total response remained unchanged. For NECA, pED50 values of 9.6 +/- 0.1 and 6.8 +/- 0.2 were obtained, representing 48 +/- 3% and 52 +/- 3% of the sites, respectively. In contrast, ATP produced a monophasic response with a pED50 value of 8.8 +/- 0.1. These results provide evidence of adenosine receptor and response heterogeneity in the in situ coronary vasculature.

The role of the A(2A) adenosine receptor subtype in functional hyperaemia in the hindlimb of anaesthetized cats

1. The present study was designed to investigate the contribution of the A(2A) adenosine receptor subtype in the functional hyperaemia response during muscle contraction. 2. In cats anaesthetized with sodium pentobarbitone and breathing spontaneously following tracheotomy, the left sciatic and femoral nerves were electrically stimulated at 3 Hz for 20 min to induce muscle contraction, and hindlimb blood flow was measured with a flow probe. The contribution of the A(2A) adenosine receptor subtype was assessed using ZM 241385, a potent and selective A(2A) adenosine receptor antagonist. 3. In a control group, the muscle isometric tension measured in the extensor digitorum longus-tibialis anterior muscle group was 6.64 +/- 0.66 kg (100 g muscle mass)(-1) and hindlimb vascular conductance was 0.22 +/- 0.03 ml mmHg(-1)(kg body mass)(-1) at 20 min of contraction. Administration of vehicle did not affect these parameters upon a second contraction period: 6.31 +/- 0.61 kg (100 g muscle mass)(-1) and 0.23 +/- 0.03 ml mmHg(-1) (kg body mass)(-1), respectively. Total hindlimb conductance during contraction was unaffected (5.5 +/- 3.7% decrease). 4. ZM 241385 (1.0 mg kg(-1)) did not alter the amount of force produced by the muscle at 20 min of contraction. Hindlimb conductance response was reduced by 27.1 +/- 4.8% following the A(2A) selective adenosine receptor antagonist, similar to that observed with the non-selective antagonist 8-phenyltheophylline. 5. These results show that adenosine acting at the A(2A) subtype receptor can contribute up to 30% of the functional hyperaemia response in the hindlimb of anaesthetized cats.

The vasorelaxant effects of acetate: role of adenosine, glycolysis, lyotropism, and pHi and Cai2+

The mechanism of acetate vasorelaxation is unknown. In the rat caudal artery, acetate has a vasorelaxant effect and also increases cyclic AMP. Here we evaluate the role of adenosine, of possible glycolysis inhibition by acetate, of the lyotropic properties of acetate and other anions, and of intracellular calcium and pH. Adenosine per se did not relax the caudal artery in the range of 10(-8) to 10(-2) M. Preincubation with adenosine deaminase (ADA, 5.0 U/ml) or with 8-phenyltheophylline (8-PT, 10(-6) to 10(-4) M) increased, rather than blocked the vasorelaxant effect of acetate. Oxypurinol (10(-3) M) or the nucleoside transport inhibitor NBMPR (10(-4) M) had no effect on acetate relaxation. Whereas acetate increased tissue cyclic AMP content, 10(-3) M adenosine or 10(-6) M PIA had no effect. In strips studied under conditions of inhibited glycolysis (no glucose, with 11 mM 2-deoxyglucose, 1.0 mM pyruvate, and 0.5 mM 5-iodoacetate), acetate-induced relaxation, as well as acetate-induced cyclic AMP generation, tended to be reduced but not significantly so. Other anions relaxed vascular strips in relation to their lyotropic number, but only at higher doses, and they did not stimulate cyclic AMP formation. Acetate (10 mM) caused a transient fall in Ca2+i followed by a slight, sustained rise. A concomitant decrease in pHi was seen. DIDS, which blocks the relaxant and cyclic AMP effects of acetate, had no effect on the pHi decrease, but did decrease the rate of pHi recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of adenosine in dilator responses induced in arterioles and venules of rat skeletal muscle by systemic hypoxia

1. In experiments on anaesthetized rats, we have studied the role of adenosine in mediating responses induced in individual arterioles and venules of the spinotrapezius muscle by systemic hypoxia. 2. During systemic hypoxia induced by breathing 6% O2 for 3 min, some arterioles and venules dilated while others constricted. Topical application of the adenosine receptor antagonist, 8 phenyl-theophylline (8-PT), to the spinotrapezius had no effect on the constrictor responses but greatly reduced the dilator responses. The vessels nearest to the capillary bed-terminal arterioles and collecting venules--were most affected; their mean changes in diameter were reduced from 39 and 8% to 11 and -1.6% respectively. 3. In accord with these results, topical application of adenosine (2 x 10(-7)-2 x 10(-3) M) produced graded dilation of all sections of the arterial and venous trees; the terminal arterioles and collecting venules were most responsive, being dilated at maximum by 31 and 15% respectively. The dilator responses induced in those vessels that constricted during hypoxia were fully comparable with those that dilated during hypoxia. 4. Histochemical analysis of the spinotrapezius revealed that oxidative fibres that most readily release adenosine, glycolytic and mixed fibres were all evenly distributed throughout the muscle. There is no reason to suppose that some vessels are preferentially influenced by oxidative fibres. 5. These results indicate that adenosine plays a major role in dilating both arterioles and venules of muscle during systemic hypoxia. But, they are consistent with the idea that the adenosine that is important is not released from muscle fibres, but synthesized by 5'-nucleotidase localized to the blood vessels; its activity may decrease proximally along the vascular tree and may vary from one vessel to another depending on the local O2 tension.

The role of adenosine in exercise hyperaemia of the gracilis muscle in anaesthetized cats

1. A number of metabolites have been proposed to control the vascular tone of skeletal muscle during exercise. The present study was designed to investigate the role of adenosine in this response by determining the effect of the adenosine receptor antagonist 8-phenyltheophylline. 2. The gracilis muscle of anaesthetized cats was exposed and made to contract by stimulating the obturator nerve (at 1 Hz, 5 V, 0.1 ms) for 20 min. Gracilis muscle blood flow and tension were measured during exercise and for 20 min following exercise. Initially this was performed in each animal during the infusion of a vehicle solution (50% polyethylene glycol 400, 50% 0.1 M-NaOH, 0.1 ml min-1 I.V.). Exercise was then repeated during infusion of either further vehicle (group I), 8-phenyltheophylline (group II) or 3-propylxanthine (group III), both at 2.7 x 10(7) mol min-1 kg-1. 3. In group 1 (n = 4) gracilis muscle blood flow during the first exercise period increased by 47.5 +/- 11.3 ml min-1 (110 g)-1 and gracilis muscle tension by 8.6 +/- 1.3 kg (100 g muscle mass)-1 at 20 min of exercise. These responses were not significantly different when repeated. 4. In group II (n = 5), blood flow increased by 46.9 +/- 9.9 ml min-1 (100 g)-1 and tension by 6.5 +/- 0.7 kg (100 g muscle mass)-1 during vehicle infusion. Infusion of 8-phenyltheophylline at a rate which abolished the vasodilatation response to 2-chloroadenosine, significantly reduced the muscle blood flow increase to 19.8 +/- 2.7 ml min-1 (100 g muscle mass)-1 (P less than 0.05) but the tension response was unaffected (increased by 7.0 +/- 0.8 kg (100 g muscle mass)-1). 8-Phenyltheophylline did not affect gracilis muscle blood flow or tension at rest. 5. Administration of 3-propylxanthine, which did not modify the vasodilatation response to 2-chloroadenosine, failed to alter the vascular responses to muscle contraction. 6. These results suggest that activation of adenosine receptors can contribute to up to 40% of the vasodilatation observed during isometric twitch contraction of the gracilis muscle of cats.

Evidence for the involvement of cyclic GMP in adenosine-induced, age-dependent vasodilatation

1. Adenosine-induced dilatation of rat aorta was present in aorta taken from 4 week-old rats, attenuated with increase in age of rats to 8 weeks, and was virtually absent in the aorta from 12 week-old rats. 2. Removal of the endothelium by mechanical rubbing attenuated adenosine-induced dilatation. 3. Haemoglobin and methylene blue partly reversed the adenosine-induced endothelium-dependent dilatation. 4. The order of potency of adenosine derivatives was 5'-(N-ethylcarboxamido)adenosine (NECA) greater than 2-phenylaminoadenosine (CV-1808) greater than 2-chloroadenosine greater than N6-([R]-[-]-phenylisopropyl)adenosine (R-PIA) greater than adenosine greater than N6-cyclohexyladenosine (CHA) greater than N6-([S]-[+]-phenylisopropyl)adenosine (S-PIA), indicating that adenosine receptors mediating the dilatation are of the A2 subtype. 5. [3H]-NECA bound to preparations of membranes from rats of 4 weeks old; it was displaced more effectively by NECA and the A2 ligand CV-1808 than by the A1 ligands CHA and S-PIA. ligands CHA and S-PIA. 6. The number but not affinity of specific binding sites for NECA decreased considerably with increase in age of rats to 8 weeks, and binding sites for [3H]-NECA were hardly detected in membrane preparations from rats of 20 weeks old. 7. Adenosine caused a marked increase in cyclic GMP production, but did not induce an increase in the cyclic AMP level. 8. This increase in cyclic GMP production induced by adenosine was abolished by methylene blue or 8-phenyltheophylline, or by removal of the endothelium. 9. The age-associated decrease in adenosine-induced dilatation was found to be associated with a reduction in the formation of cyclic GMP, but not of cyclic AMP. 10. These results suggest that adenosine causes dilatation via A2 receptors by inducing production of an endothelium-derived relaxing factor (EDRF), which in turn stimulates soluble guanylate cyclase, and so increases production of cyclic GMP. It is also suggested that the main reason for the age-associated decrease in adenosine-induced dilatation is a decrease in the number of A2-receptors or the ability of the endothelium to produce EDRF, leading to decreased production of cyclic GMP.

Purine metabolite inosine is an adrenergic neurotrophic substance for cultured chicken sympathetic neurons

Purines are ubiquitous endogenous cellular metabolites that have been postulated as neurotransmitters or neuromodulators in the nervous system. Recently, we showed that a low-molecular-mass component present in liver-conditioned medium selectively enhances the adrenergic properties of dissociated chicken sympathetic neurons in culture. We report here that this substance is inosine, a purine metabolite. Indeed, analysis of the low-molecular-mass fraction of liver-conditioned medium by HPLC shows that the neurotrophic activity coelutes with and has the same absorption spectrum as inosine. Inosine increases incorporation of [3H]leucine into neuronal protein and stimulates catecholamine, but not acetylcholine, production by the sympathetic neurons in a dose-dependent fashion (half-maximal stimulation at 10(-6) M). This effect can be blocked by 5 x 10(-6) M dipyridamole, an inhibitor of nucleoside transport. Inosine therefore appears to be capable of modulating adrenergic phenotypic expression in cultured sympathetic neurons by acting via an as-yet-unknown intracellular pathway.

The response of isolated guinea-pig aorta to amrinone

1. In helically cut strips of aorta from reserpine-treated guinea-pigs, cumulative concentrations (30 microM, 0.3 mM and 3 mM) of amrinone progressively reduced the basal tone of the preparations and relaxed the smooth muscle contracted by 1 microM noradrenaline or by 20 mM K+. 2. The relaxing effect was completely suppressed by tissue pretreatment with adenosine deaminase (1 U ml-1). 3. Relaxation induced by amrinone was not affected by 50 microM indomethacin or by 0.1 mM 8-phenyltheophylline and was potentiated by 5 microM quinidine. 4. Like amrinone, exogenous adenosine reduced the basal tone of the guinea-pig aorta strips and relaxed the preparations contracted by 1 microM noradrenaline or by 20 mM k+ in a concentration-dependent manner. 5. The relaxing activity of exogenous adenosine was not affected by 50 microM indomethacin, was potentiated by 5 microM quinidine and was partially antagonized by 0.1 mM 8-phenyltheophylline. 6. These results indicate the involvement of endogenous adenosine in the relaxing effect of amrinone on guinea-pig aorta strips, but the specific mechanism of amrinone-adenosine interaction remains to be elucidated.