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SYMPOSIUM REPORT |
1 Department of Physiology, The Medical School, Birmingham B15 2TT, UK
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Abstract |
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 Top Abstract IntroductionReferences |
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(Received 15 May 2007;
accepted after revision 3 July 2007;
first published online 5 July 2007)
Corresponding author J. M Marshall: Department of Physiology, The Medical School, Birmingham B15 2TT, UK. Email: j.m.marshall{at}bham.ac.uk
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Introduction |
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TopAbstractIntroductionReferences |
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Within this review, consideration is given to the evidence for and against adenosine or ATP contributing to exercise hyperaemia, the mechanisms by which they may be released or generated, and the mechanisms by which they may act.
Historical perspective
Early studies of Berne's group showed accumulation of adenosine, ADP and AMP in biopsies taken from dog hind limb muscle during contraction under ischaemic conditions (Berne et al. 1971; Dobson et al. 1971) and constant blood flow (Bockman et al. 1976). Adenosine concentration in the venous efflux also increased during contraction of dog muscle perfused at low, or resting blood flow (Bockman et al. 1975; Belloni et al. 1979).
However, release of adenosine into venous efflux was soon shown for dog muscle contracting with constant high flow (Ballard et al. 1987; see below) and during free flow (Fuchs et al. 1986; Karim et al. 1988). The positive outcomes of these studies may be explained by improvements in the sensitivity of the assay methods. Moreover, greater understanding of the rapidity with which adenosine is cleared from plasma led to improvement in the collection of samples for assay and in experimental design (see Ballard et al. 1987; Poucher et al. 1990 and below).
It also emerged that cat soleus, a slow oxidative (red) muscle produces more adenosine during contraction than the gracilis, a fast-twitch, glycolytic (white) muscle. Indeed, the cat soleus has a two-fold higher activity than the gracilis of 5'nucleotidase (5'N), which dephosphorylates AMP to adenosine, whereas the gracilis has a three-fold higher activity than the soleus, of AMP deaminase, which degrades AMP to inosine monophosphate (Rubio et al. 1973; Bockman & McKenzie, 1983). Significantly, the 5'N was shown to be widely distributed across soleus muscle, whereas in gracilis it was concentrated in the borders of muscle fibres adjacent to blood vessels and in vascular endothelium, the density increasing towards the capillaries (Rubio et al. 1973).
Results obtained with the aid of pharmacological antagonists intended to test the role of adenosine in exercise hyperaemia were far more equivocal. Thus, adenosine deaminase (ADA) reduced exercise hyperaemia by 30–40% in the highly oxidative, gracilis muscle of dogs (Kille & Klabunde, 1984) and in the soleus, but not gracilis of cats (Schwartz & McKenzie, 1990). Addition of ADA to the superfusate also reduced the arteriolar dilatation evoked in hamster cremaster by graded muscle contractions (Proctor, 1984). However, ADA reduced exercise hyperaemia in dog gracilis muscle by only 
11% (Karim & Goonewardene, 1989), and did not affect that induced in the rat by low-speed treadmill exercise (Klabunde et al. 1988).
Further, dipyridamole, which blocks the equilibrative transporter for adenosine, enhanced the hyperaemia evoked in various muscles of pigs performing maximum treadmill exercise (Laughlin et al. 1989), and that evoked by maximal contraction in soleus muscle of dogs (Kille & Klabunde, 1984), but not that evoked by submaximal contractions in dog gracilis (Klabunde, 1986).
Finally, studies involving the most selective adenosine receptor antagonist available at the time, and still the only one available for use in humans, theophylline (aminophylline), achieved mixed success. For example, it reduced exercise hyperaemia in dog gracilis (Tabaie et al. 1977), hamster cremaster (Proctor, 1984) and in dogs during treadmill exercise (Metting et al. 1986). By contrast, theophylline did not affect exercise hyperaemia in dog gracilis muscle (Honig & Frierson, 1980), or in dogs during treadmill exercise (Koch et al. 1990).
Even pharmacological findings that apparently support a role for adenosine in exercise hyperaemia, must be treated with caution. The breakdown product of ADA, inosine, is a vasodilator in its own right. Further, there is doubt over the extent to which ADA and dipyridamole can pass from the bloodstream into the interstitial space (see Poucher et al. 1990; Schwartz & McKenzie, 1990; Martin et al. 2007). This is important if most of the adenosine generated during muscle contraction is released into interstitium (see below). Moreover, dipyridamole stimulates prostacyclin production and inhibits phosphodiesterase activity (see Martin et al. 2007), and although theophylline is a relatively potent adenosine receptor antagonist (pA2 5), it inhibits phosphodiesterase activity (see Poucher et al. 1990). Such non-selective effects probably explain why both antagonists tend to cause resting vasodilatation, making their effects on exercise hyperaemia difficult to interpret.
Against this background, the study that is a landmark in the field is that of Ballard et al. (1987). In dog gracilis muscle perfused at a constant high flow rate equivalent to that recorded during contraction, arterial and venous samples were collected for assay of adenosine before, during and after contraction at 4 Hz for 20 min. The high blood flow avoided fatigue, and meant that the released adenosine was not diluted by an increase in blood flow, and was exposed to uptake and degradation at a constant rate. Subsequently, adenosine was infused into the muscle at a rate that achieved the venous concentration reached during contraction, and the magnitude of dilatation was measured. These results clearly showed release of adenosine into the venous efflux, reaching a peak around the 10th minute of contraction and returning to baseline 5–10 min after contraction (Fig. 1). Comparison of the contraction-induced vasodilatation and that evoked by infused adenosine indicated that adenosine contributed 15% of the total vasodilatation in the first minute of contraction, 40% between the 5th and 20th minute and 80% for 5 min after contraction (Fig. 1).
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40% from the third minute of contraction (see Poucher et al. 1990). More recently, theophylline reduced by 20%, hyperaemia induced by submaximal knee extensor exercise (Radegren & Calbet, 2001).
Taken together, these results indicate that adenosine makes a substantial contribution to the maintained phase of exercise hyperaemia even at submaximal workloads and in muscle that is mainly glycolytic. Thus, even though total 5'N activity is low in cat gracilis, its localization (see above) apparently enables vasoactive concentrations of adenosine to be produced close to resistance arterioles. Confirmatory evidence that adenosine is an essential contributor to exercise hyperaemia was provided by Murrant & Sarelius (2002). A characteristic of exercise hyperaemia is that vasodilatation initiated in terminal arterioles propagates retrogradely towards proximal arterioles. By applying xanthine amine congener, another specific adenosine receptor antagonist, to hamster cremaster muscle, they showed that adenosine contributed 30–40% of the local and remote vasodilatation evoked by muscle contraction.
Cellular origins of adenosine and adenine nucleotides
The assumption in many of the earlier studies was that adenosine that appears in venous efflux during contraction is generated within the skeletal fibres and released into the interstitial fluid. Certainly, Hellsten et al. (1998), who made the first measurements of adenosine in the interstitium by using microdialysis, showed that the interstitial adenosine concentration in vastus lateralis muscle increased progressively during graded knee extensor exercise. Further, the increase in interstitial adenosine was highly correlated with the increase in muscle blood flow, as would be expected if adenosine acts on receptors on the extraluminal surface of vascular smooth muscle to cause vasodilatation (Fig. 2). Similarly, interstitial adenosine concentration increased during contraction in dog gracilis and in the oxidative soleus and more glycolytic, extensor digitorum longus muscles of rats (Lo et al. 2001; Mo & Ballard, 2001).
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During spontaneous contractions of rat primary skeletal muscle cells in culture, the concentrations of adenine nucleotides and adenosine gradually increased in the medium (Hellsten & Frandsen, 1997). When the muscle cells were made to contract at different frequencies, there was a graded increase in adenosine in the medium, but no further accumulation of nucleotides. Nevertheless, maximal stimulation led to an 18% reduction in the intracellular concentrations of ATP and a trend for intracellular concentrations of ADP and AMP to increase (Hellsten & Frandsen, 1997).
The muscle cells were shown to have substantial extracellular ATPase, ADPase and AMP 5'N activities, although the ATPase and 5'N activities were only about one-quarter and one-sixtieth, respectively, of those measured in cultured microvascular endothelial cells. Nevertheless, the ecto 5'N activity of muscle cells was increased by 60% during contraction, and they generated far more adenosine when AMP rather than ATP was provided as substrate (Hellsten & Frandsen, 1997). Further, addition of medium from stimulated muscle cells to non-stimulated muscle cells caused a 20% increase in ecto 5'N activity. This suggested that contracting cells release a substance that enhances ecto 5'N activity. Addition of K+, lactate, or nitric oxide (NO), substances that might be released by muscle contraction, were not able to reproduce this effect. However, evidence was obtained that the increase in ecto 5'N activity might be partly explained by translocation of cytosolic vesicles containing 5'N to the sarcolemma (see Hellsten, 1999).
Another mechanism that may increase adenosine production via ecto 5'N was uncovered by Cheng et al. (2000). When the pH of buffer perfusing rat gracilis muscle at rest was reduced, both adenosine and AMP were released into the perfusate. Further, the affinity of ecto 5'N for AMP was increased when pH was decreased. Indeed, the combined effect of increase in substrate and increase in affinity of 5'N was estimated as a 10–20% increase in the ability of 5'N to generate adenosine for every 0.5 unit decrease in pH (Cheng et al. 2000). These findings agree with the linear relationship between venous adenosine concentration and decrease in venous pH during muscle contraction (Achike & Ballard, 1993).
That adenosine is mainly generated extracellularly during muscle contraction was confirmed by Lynge et al. (2001). When rat skeletal muscle cells in culture contracted in the presence of AOPCP (
methylene ADP), which inhibits ecto 5'N, adenosine accumulation in the medium was virtually attenuated (Fig. 3). By contrast, NBMPR (nitrobenzylthioinosine), which inhibits the equilibrative adenosine transporter, accentuated adenosine accumulation (Fig. 3). Thus the transporter is largely involved in the uptake of adenosine, rather than release of preformed adenosine. Importantly, in the presence of AOPCP, there was greater release of AMP (Fig. 3, Lynge et al. 2001).
Thus, these results demonstrated that during contraction, AMP is released from the skeletal muscle fibres into the interstitium and metabolized by muscle ecto 5'N to adenosine. Release of AMP and the affinity of ecto 5'N for AMP increases when acidosis accompanies muscle contraction, while the concentration of ecto 5'N may be increased by translocation from intracellular stores. The adenosine so generated may be taken up again by the equilibrative transporters on the muscle cells. The enzymes that metabolize adenosine, ADA and adenosine kinase, are mainly intracellular (see Cheng et al. 2000).
Release from motor and sympathetic nerves. The very fact ATP was not released from cultured muscle cells when they were made to contract (Hellsten & Frandsen, 1997), but did appear in the interstitium of human muscles during exercise and in dog muscle made to contract by stimulation of its mixed nerve supply (Hellsten et al. 1998; Mo & Ballard, 2001), is consistent with ATP being released from motor and/or sympathetic nerves. It is unlikely that ATP diffuses from the blood into the interstitial fluid during contraction, for ATP infused intra-arterially into dog gracilis muscle did not appear in the interstitium until the arterial concentration was >50 times higher than in the interstitium (Mo & Ballard, 2001). Similar observations were made for ADP, AMP and adenosine (Mo & Ballard, 2001). These findings are consistent with evidence that endothelium has an avid transport mechanism for adenosine and high activities of enzymes that metabolize adenine nucleotides and adenosine (see Hellsten & Frandsen, 1997).
Release from red blood cells and endothelium. Even if ATP and adenosine generated within the blood stream do not reach the interstitial fluid during contraction, it is still reasonable to question whether either contribute to exercise hyperaemia.
ATP is released from RBCs by mechanical deformation (Sprague et al. 2001) and during haemoglobin deoxygenation (Jagger et al. 2001). ATP is also released from endothelium in response to shear and hypoxia (Deussen et al. 1986; Burnstock, 2006). Clearly, mechanical compression and shear rate are likely to increase during muscle contraction. In resting muscle, O2 diffuses across arterial walls, such that two-thirds of the O2 is lost before the capillaries. Thus, when muscle O2 consumption increases during exercise and O2 is downloaded from haemoglobin, this longitudinal diffusion gradient increases (Pittman, 2000). Consequently, blood PO2 and haemoglobin O2 saturation must fall, providing potential stimuli for ATP release from RBCs and endothelium, at all sites from proximal arterioles to venules.
Certainly, consistent with the findings of Forrester & Lind (1969) on the forearm, ATP was released in a graded manner into venous efflux during graded knee-extensor exercise. Further, the ATP release was accentuated during exercise performed under hypoxic conditions, but reduced when exercise was performed during hyperoxia, or when 1% CO was added to the inspirate (González-Alonso et al. 2002). These results accord with evidence that ATP is released from RBCs when the haemoglobin molecule undergoes the conformational change from its oxygenated, to its deoxygenated state: the high-affinity binding of CO prevents this change (Jagger et al. 2001). Importantly, González-Alonso et al. 2002) confirmed that ATP infused intra-arterially under resting conditions in normoxia, can reproduce the muscle vasodilatation of exercise.
Actions of intraluminal ATP and adenosine
Application of ATP extraluminally to arterioles in hamster cheek pouch evoked vasoconstriction via P2X receptors (McCullough et al. 1997), as does ATP released from sympathetic nerve fibres in muscle (Johnson et al. 2001). However, intraluminal ATP evoked vasodilatation (McCullough et al. 1997) that was attributable to stimulation of endothelial P2Y receptors (Burnstock, 2006). Moreover, dilatation evoked by intraluminal application of ATP to arterioles was propagated retrogradely to proximal arterioles (McCullough et al. 1997; Fig. 5), while that evoked by intraluminal application to venules was conducted across the capillaries to arterioles (Collins et al. 1998). Thus, in these respects, ATP released intraluminally fits the criteria required of a candidate for exercise hyperaemia.
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Turning to adenosine, at least 50% of vasodilatation evoked in rat hind limb muscle by systemic hypoxia is mediated by adenosine and attenuated by 8-PT or 8-SPT (see Marshall, 2000). A similar finding was made in human forearm muscle by using aminophylline (Leuenberger et al. 1999). It seems likely that in systemic hypoxia, adenosine itself is released, rather than being generated extracellularly, for hypoxia-induced muscle vasodilatation was not affected by AOPCP (Skinner & Marshall, 1996). Further, the available evidence indicates that during hypoxia, adenosine is largely released by endothelial, rather than extravascular cells. (see Marshall, 2000). Notably, adenosine was released from coronary endothelium during hypoxic perfusion (Deussen et al. 1986). Moreover, systemic hypoxia released adenosine into the venous efflux of dog skeletal muscle, but had no effect on venous concentrations of ATP, ADP or AMP, or on interstitial concentrations of adenine nucleotides or adenosine (Mo & Ballard, 2001).
In human skeletal muscle, adenosine A1, A2A and A2B receptors are present on endothelium and vascular smooth muscle (Lynge & Hellsten, 2000). Accordingly, adenosine infused intra-arterially into rat hindlimb muscle produces vasodilatation by stimulating A1 and A2A receptors (Bryan & Marshall, 1999). However, muscle vasodilatation induced by systemic hypoxia was attenuated only by the selective A1 receptor antagonist DPCPX, and not by the selective A2A receptor antagonist, ZM241385 (Bryan & Marshall, 1999). Further, muscle vasodilatation evoked by hypoxia and by infused adenosine was largely attenuated by L-NAME. Moreover, hypoxia-induced dilatation was restored when the tonic level of NO was restored by infusion of NO donor, and this dilatation was also mediated by A1 receptors (Edmunds et al. 2003). Thus, the adenosine component of hypoxia-induced vasodilatation is mediated by A1 receptors and is NO dependent. When the tonic level of NO is restored after NOS inhibition, then adenosine released by hypoxia presumably acts on A1 receptors on vascular smooth muscle (Edmunds et al. 2003).
There is direct evidence that adenosine releases NO from the endothelium of rat aorta and iliac arteries, by stimulating A1 or A2A receptors (Ray et al. 2002). Moreover, systemic hypoxia releases NO into the venous efflux of rat hindlimb muscle by stimulating A1 receptors (Ray & Marshall, 2005). Thus, it can be concluded that systemic, and local hypoxia causes endothelial cells to release adenosine, which acts back on endothelial A1 receptors to induce muscle vasodilatation. Smits et al. (1995), who similarly restored vasodilator tone after NOS inhibition with NO donor, showed that vasodilatation evoked in human forearm by intra-arterial adenosine is partly mediated by NO.
Exercise hyperaemia versus intraluminally acting adenosine and ATP
Putting the results discussed in the section above together, a key question arises: does vasodilatation induced by adenosine generated from ATP, or released into the bloodstream by the relatively hypoxic conditions of exercise, or by intra-arterially infused adenosine, show similar characteristics to the contribution adenosine makes to exercise hyperaemia? This is considered below.
(i) The availability of the highly selective A2A receptor antagonist ZM24138, allowed Poucher (1996) to show it had an exactly comparable effect to 8-PT (Fig. 6), reducing exercise hyperaemia in cat gracilis muscle by 40%. Thus, in the cat, exercise hyperaemia is mediated by A2A and not by A1 or A2B receptors. In our recent experiments, ZM241385 reduced by 30%, vasodilatation evoked in rat hind limb by contractions at 4 Hz for 5 min. Moreover, subsequent administration of 8-SPT, which is non-selective between adenosine receptor subtypes, had no further effect (Ray & Marshall, 2007, unpublished findings). Thus, in the rat also, the adenosine component of exercise hyperaemia is attributable to A2A receptors, even though A1 receptors are responsible for vasodilatation evoked by systemic hypoxia (Bryan & Marshall, 1999).
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30%, by ZM24138 (Ray & Marshall, 2007 unpublished findings). This is fully consistent with exercise hyperaemia being evoked by adenosine acting on A2A receptors on vascular smooth muscle and not on endothelial A1 receptors that are stimulated during hypoxia and act via NO (see above). (iii) Martin et al. (2006a,b, 2007) recently reported that healthy male and female subjects can be designated ‘responders’ and ‘non-responders’ on the basis of their responsiveness to intra-arterially infused adenosine, although the two groups showed fully comparable exercise hyperaemia responses to three different workloads. Inhibition of NOS reduced muscle vasodilatation evoked by intra-arterial adenosine in ‘responders’, but not in ‘non-responders’, nor that evoked by exercise in either group (Martin et al. 2006a). Further, dipyridamole reduced vasodilatation evoked by intra-arterial adenosine in ‘responders’, so that it equalled that evoked in ‘non-responders’, but had no effect on exercise hyperaemia in either group (Martin et al. 2007). Moreover, the relatively weak adenosine receptor antagonist aminophylline, significantly reduced vasodilatation evoked by intra-arterial adenosine in ‘responders’ and had no effect in ‘non-responders’, but achieved a 15% attenuation of exercise hyperaemia in both groups (Martin et al. 2006b; Fig. 7). These results could be explained if the disparities between the groups mainly reflect greater endothelial uptake of intra-arterial adenosine in ‘non-responders’, so decreasing the concentration available to stimulate endothelial adenosine receptors that act via NO, and if intraluminal adenosine makes little contribution to exercise hyperaemia.
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Thus, exercise hyperaemia differs in several important respects from vasodilatation induced by intraluminal adenosine and indeed, by intraluminal ATP, given ATP's effects are NO-dependent and partly mediated by adenosine.
Conclusion
Adenosine is responsible for 20–40% of the maintained phase of exercise hyperaemia at submaximal and maximal workloads. Adenosine is largely formed extracellularly from AMP released by skeletal muscle fibres, by the action of ecto 5'N associated with the sarcolemma. This adenosine causes dilatation by acting on A2A receptors on the extraluminal surface of the arterial smooth muscle. By contrast, adenosine released from endothelial cells, or generated from ATP released from endothelial cells or RBCs, produces vasodilatation via endothelial A1 receptors, and makes little contribution to exercise hyperaemia. ATP itself may make a small contribution to exercise hyperaemia by acting on P2Y receptors. However, this possibility is unattractive since the dilator effect of ATP would be expected to show a strong dependency on NO, which is not a characteristic of exercise hyperaemia.
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Footnotes |
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References |
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P < 0.05 before versus after Aph for responders or non-responders. Reproduced from 