The role of adenosine in mediating vasodilatation in mesenteric circulation of the rat in acute and chronic hypoxia

Impaired function of prejunctional adenosine A1 receptors expressed by perivascular sympathetic nerves in DOCA-salt hypertensive rats

Increased sympathetic nervous system activity contributes to deoxycorticosterone acetate (DOCA)-salt hypertension in rats. ATP and norepinephrine (NE) are coreleased from perivascular sympathetic nerves. NE acts at prejunctional α2-adrenergic receptors (α2ARs) to inhibit NE release, and α2AR function is impaired in DOCA-salt rats. Adenosine, an enzymatic ATP degradation product, acts at prejunctional A1 adenosine receptors (A1Rs) to inhibit NE release. We tested the hypothesis that prejunctional A1R function is impaired in sympathetic nerves supplying mesenteric arteries (MAs) and veins (MVs) of DOCA-salt rats. Electrically evoked NE release and constrictions of blood vessels were studied in vitro with use of amperometry to measure NE oxidation currents and video microscopy, respectively. Immunohistochemical methods were used to localize tyrosine hydroxylase (TH) and A1Rs in perivascular sympathetic nerves. TH and A1Rs colocalized to perivascular sympathetic nerves. Adenosine and N(6)-cyclopentyl-adenosine (CPA, A1R agonist) constricted MVs but not MAs. Adenosine and CPA (0.001-10 µM) inhibited neurogenic constrictions and NE release in MAs and MVs. DOCA-salt arteries were resistant to adenosine and CPA-mediated inhibition of NE release and constriction. The A2A adenosine receptor agonist CGS21680 (C23H29N7O6.HCl.xH2O) (0.001-0.1 μM) did not alter NE oxidation currents. We conclude that there are prejunctional A1Rs in arteries and both pre- and postjunctional A1Rs in veins; thus, adenosine selectively constricts the veins. Prejunctional A1R function is impaired in arteries, but not veins, from DOCA-salt rats. Sympathetic autoreceptor dysfunction is not specific to α2ARs, but there is a more general disruption of prejunctional mechanisms controlling sympathetic neurotransmitter release in DOCA-salt hypertension.

Hemodynamic and ventilatory response to different levels of hypoxia and hypercapnia in carotid body-denervated rats

OBJECTIVE

Chemoreceptors play an important role in the autonomic modulation of circulatory and ventilatory responses to changes in arterial O(2) and/or CO(2). However, studies evaluating hemodynamic responses to hypoxia and hypercapnia in rats have shown inconsistent results. Our aim was to evaluate hemodynamic and respiratory responses to different levels of hypoxia and hypercapnia in conscious intact or carotid body-denervated rats.

METHODS

Male Wistar rats were submitted to bilateral ligature of carotid body arteries (or sham-operation) and received catheters into the left femoral artery and vein. After two days, each animal was placed into a plethysmographic chamber and, after baseline measurements of respiratory parameters and arterial pressure, each animal was subjected to three levels of hypoxia (15, 10 and 6% O(2)) and hypercapnia (10% CO(2)).

RESULTS

The results indicated that 15% O(2) decreased the mean arterial pressure and increased the heart rate (HR) in both intact (n = 8) and carotid body-denervated (n = 7) rats. In contrast, 10% O(2) did not change the mean arterial pressure but still increased the HR in intact rats, and it decreased the mean arterial pressure and increased the heart rate in carotid body-denervated rats. Furthermore, 6% O(2) increased the mean arterial pressure and decreased the HR in intact rats, but it decreased the mean arterial pressure and did not change the HR in carotid body-denervated rats. The 3 levels of hypoxia increased pulmonary ventilation in both groups, with attenuated responses in carotid body-denervated rats. Hypercapnia with 10% CO(2) increased the mean arterial pressure and decreased HR similarly in both groups. Hypercapnia also increased pulmonary ventilation in both groups to the same extent.

CONCLUSION

This study demonstrates that the hemodynamic and ventilatory responses varied according to the level of hypoxia. Nevertheless, the hemodynamic and ventilatory responses to hypercapnia did not depend on the activation of the peripheral carotid chemoreceptors.

Influence of acute hypoxia combined with nitrous oxide on cardiovascular variability in conscious hypertensive rats

Anesthetics have been reported to depress autonomic nervous system (ANS) responses to hypoxia. The mechanisms by which cardiovascular variability responds to acute progressive hypoxia (APH) under nitrous oxide (N(2)O) inhalation, however, remain unclear. Additionally, the effect of hypertension on ANS responses in such cases has not been fully clarified. The present study examined the influence of APH (10% O(2)) under 60% N(2)O inhalation on cardiovascular variability in conscious, spontaneously hypertensive rats (SHR). Twenty-seven male SHR were randomly assigned to 3 treatment groups receiving N(2)O inhalation alone, APH stress alone or APH stress under N(2)O inhalation, using Wistar Kyoto rats (WKY) or non-N(2)O inhalation rats as controls. Systolic blood pressure (SBP) and heart rate (HR) variability were evaluated time-dependently using the wavelet method. While inhalation of N(2)O alone induced more powerful sympathomimetic actions in SHR than in WKY, circulatory and parasympathetic reactions were weaker. APH stress alone evoked significant inhibition of cardiac parasympathetic activity from immediately after exposure to hypoxic stress in SHR in contrast to WKY, facilitating tachycardia. This inhibition of parasympathetic activity in SHR continued without coupled changes in sympathetic activity. In SHR, APH under N(2)O inhalation decreased SBP and sympathetic activity more prominently and earlier than APH alone, and earlier than APH under N(2)O inhalation in WKY. Additionally, APH under N(2)O inhalation inhibited cardiac parasympathetic activity in SHR as compared to APH stress alone. In conclusion, APH under N(2)O inhalation in SHR potentially results in exacerbation of circulatory suppression from the earlier hypoxic phase, compared with non-N(2)O inhalation.

Influence of acute progressive hypoxia on cardiovascular variability in conscious spontaneously hypertensive rats

The purpose of this study is to examine the influence of acute progressive hypoxia on cardiovascular variability and striatal dopamine (DA) levels in conscious, spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). After preparation for measurement, the inspired oxygen concentration of rats was decreased to 10% within 5 min (descent stage), maintained at 10% for 10 min (fixed stage), and then elevated back to 20% over 5 min (recovery stage). The systolic blood pressure (SBP) and heart rate (HR) variability at each stage was calculated to evaluate the autonomic nervous system response using the wavelet method. Striatal DA during each stage was measured using in vivo microdialysis. We found that SHR showed a more profound hemodynamic response to progressive hypoxia as compared to WKY. Cardiac parasympathetic activity in SHR was significantly inhibited by acute progressive hypoxia during all stages, as shown by the decrease in the high frequency band of HR variability (HR-HF), along with transient increase in sympathetic activity during the early hypoxic phase. This decrease in the HR-HF continued even when SBP was elevated. Striatal DA levels showed the transient similar elevation in both groups. These findings suggest that acute progressive hypoxic stress in SHR inhibits cardiac parasympathetic activity through reduction of baroreceptor reflex sensitivity, with potentially severe deleterious effects on circulation, in particular on HR and circulatory control. Furthermore, it is thought that the influence of acute progressive hypoxia on striatal DA levels is similar in SHR and WKY.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

Microvascular blood flow and oxygenation during hemorrhagic hypotension

Understanding microvascular oxygen transport requires the knowledge of microvessel topology and geometry, blood flow and oxygen levels. Microvascular hemodynamic responses to hemorrhagic hypotension (HH) such as size-dependent vasoconstriction and blood flow reduction could lead to increased longitudinal oxygen partial pressure (PO(2)) gradients. However, the mesenteric microvascular PO(2) has never been evaluated during HH. Therefore, we studied hemodynamic variables and PO(2) distribution in 165 mesenteric microvessels from 39 anesthetized rats to investigate whether HH-induced vasoconstriction and blood flow reduction were associated with changes in longitudinal PO(2) gradients. Vessels were analyzed according to their position in the network, as well as a few interstitial PO(2) areas. We found that during baseline a small PO(2) gradient exists, but HH is accompanied by more pronounced microvascular longitudinal PO(2) gradients. Decreased blood flow did not seem to completely explain these findings, since blood flow was uniformly diminished in arterioles and venules, independent of diameter and position in the network. During HH, some microvessels presented higher PO(2) than during baseline despite blood flow reduction, possibly due to a combination of systemic hyperoxia and low oxygen consumption of mesentery. The data suggest that blood flow measurements may be a poor indicator of the oxygenation status in some regions of the mesentery. The enhanced mesenteric longitudinal PO(2) gradient may lead to regions with different levels of other physiologically active compounds.

Role of vascular heme oxygenase in reduced myogenic reactivity following chronic hypoxia

OBJECTIVE

Exposure to chronic hypoxia (CH) results in a persistent endothelium-dependent vascular smooth muscle hyperpolarization that diminishes vasoconstrictor reactivity. Experiments were performed to test the hypothesis that products of both cytochrome P450 epoxygenase (CYP) and heme oxygenase (HO) are required for the persistent diminished myogenic reactivity following CH.

METHODS

The authors examined myogenic responses of mesenteric arteries isolated from control and CH (48 h; P(B) = 380 mmHg) rats in the presence of a HO inhibitor (zinc protoporphyrin IX; ZnPPIX) or combined HO and CYP epoxygenase inhibition (sulfaphenazole). Arteries were isolated and cannulated and the vascular smooth muscle was loaded with the Ca2+ indicator Fura-2.

RESULTS

Control vessels maintained their internal diameter in response to step increases in intraluminal pressure, whereas arteries from CH animals passively distended. ZnPPIX augmented myogenic reactivity and [Ca2+] in arteries from CH animals. Combined administration of sulfaphenazole and ZnPPIX did not have an additional effect compared to ZnPPIX alone. Myogenic reactivity in control vessels was not altered by ZnPPIX or ZnPPIX + sulfaphenazole.

CONCLUSIONS

HO appears to play a role in regulating myogenic reactivity following CH. Furthermore, these data suggest that products of HO and CYP are both required for the observed attenuation in vasoreactivity following CH.

Oxygen delivery and oxygen consumption in rat hindlimb during systemic hypoxia: role of adenosine

1. In anaesthetised rats, the increase in femoral vascular conductance (FVC) evoked by moderate systemic hypoxia is mediated by adenosine acting on A(1) receptors. It is also nitric oxide (NO) dependent: it is attenuated by NO synthase (NOS) inhibition, but restored when baseline FVC is restored by sodium nitroprusside (SNP), a NO donor. However, under these conditions there was in increase in the critical O(2) delivery (D(O2,crit)) at which hindlimb O(2) consumption (V(O2)) becomes directly dependent upon O(2) delivery (D(O2)), indicating that V(O2) is regulated by newly synthesised NO. 2. In the present study, after NOS inhibition, when baseline FVC was restored with SNP infusion, the increases in FVC evoked by breathing 12 and 8 % O(2) were reduced by the A(1) receptor antagonist DPCPX, by 60 and 40 %, respectively (n = 8). The A(2A) receptor antagonist ZM241385 reduced the FVC increase evoked by 12 % O(2) (by 45 %, n = 8), but did not alter that evoked by 8 % O(2). 3. DPCPX also reduced the increases in FVC evoked by graded systemic hypoxia, breathing 14-6 % O(2) and increased D(O2,crit), from 0.64 +/- 0.06 to 0.95 +/- 0.07 ml O(2) min(-1) kg(-1) (control vs. DPCPX). However, ZM241385 (n = 8) had no effect on the FVC increases or on D(O2,crit) (0.70 +/- 0.02 ml O(2) min(-1) kg(-1), n = 8). 4. Thus, the increases in FVC evoked by mild to severe systemic hypoxia are mediated by A(1) receptors. These responses, which are attributable to proximal arteriolar dilatation, help maintain D(O2). Even after NOS inhibition, adenosine still increases FVC via A(2A) (moderate hypoxia only) and A(1) receptors, providing baseline levels of NO are present. Furthermore, adenosine, acting via A(1) receptors, is important in determining D(O2,crit) and therefore in maintaining V(O2). We propose that this is achieved by A(1)-evoked dilatation of terminal arterioles and is mediated by increased synthesis of NO.

Vasodilatation, oxygen delivery and oxygen consumption in rat hindlimb during systemic hypoxia: roles of nitric oxide

We have investigated the relationship between O2 delivery (DO2) and O2 consumption (VO2) in hindlimb muscle of anaesthetised rats during progressive systemic hypoxia. Since muscle vasodilatation that occurs during hypoxia is nitric oxide (NO) dependent, we examined the effects of the NO synthase (NOS) inhibitor nitro-L-arginine methyl ester (L-NAME). In control rats (n = 8), femoral vascular conductance (FVC) increased at each level of hypoxia. Hindlimb DO2 decreased with the severity of hypoxia, but muscle VO2 was maintained until the critical DO2 value (DO2,crit) was reached at 0.64 +/- 0.06 ml O2 min-1 kg-1; below this VO2 declined linearly with DO2. This is a novel finding for the rat but is comparable to the biphasic relationship seen in the dog. In another group of rats (n = 6), L-NAME caused hindlimb vasoconstriction and attenuated the hypoxia-evoked increases in FVC DO2 was so low after L-NAME administration that VO2 was dependent on DO2 at all levels of hypoxia. In a further group (n = 8), femoral blood flow and DO2 were restored after L-NAME by infusion of the NO donor sodium nitroprusside (20 g x kg(-1) x min(-1). Thereafter, hypoxia-evoked increases in FVC were fully restored. Nevertheless, DO2,crit was increased relative to control (0.96 +/- 0.07 ml O2 min(-1) x kg(-1), P < 0.01). As NOS inhibition limited the ability of muscle to maintain VO2 during hypoxia, we propose that hypoxia-induced dilatation of terminal arterioles, which improves tissue O2 distribution, is mediated by NO. However, since the hypoxia-evoked increase in FVC was blocked by L-NAME but restored by the NO donor, we propose that the dilatation of proximal arterioles is dependent on tonic levels of NO, rather than mediated by NO.

Conducted vasoconstriction in rat mesenteric arterioles: role for dihydropyridine-insensitive Ca(2+) channels

The aim of this study was to evaluate the role of voltage-operated Ca(2+) channels in the initiation and conduction of vasoconstrictor responses to local micropipette electrical stimulation of rat mesenteric arterioles (28 +/- 1 microm, n = 79) in vivo. Local and conducted (600 microm upstream from the pipette) vasoconstriction was not blocked by TTX (1 micromol/l, n = 5), nifedipine, or nimodipine (10 micromol/l, n = 9). Increasing the K(+) concentration of the superfusate to 75 mmol/l did not evoke vasoconstriction, but this depolarizing stimulus reversibly abolished vasoconstrictor responses to current stimulation (n = 7). Addition of the T-type Ca(2+) antagonist mibefradil (10 micromol/l, n = 6) to the superfusate reversibly blocked local and conducted vasoconstriction to current stimulation. With the use of RT-PCR techniques, it was demonstrated that rat mesenteric arterioles <40 microm do not express mRNA for L-type Ca(2+) channels (alpha(1C)-subunit), whereas mRNA coding for T-type subunits was found (alpha(1G)- and alpha(1H)-subunits). The data indicate that L-type Ca(2+) channels are absent from rat mesenteric arterioles (<40 microm). Rather, the vasoconstrictor responses appear to rely on other types of voltage-gated, dihydropyridine-insensitive Ca(2+) channels, possibly of the T-type.

Adenosine and muscle vasodilatation in acute systemic hypoxia

Adenosine is released by skeletal and cardiac muscles when their metabolism increases: it serves to couple O2 supply with O2 demand by causing vasodilatation. This review argues that adenosine plays a similar role in skeletal muscle in systemic hypoxia. It accounts for approximately 50% of the increase in muscle vascular conductance and, within muscle, it causes dilatation of individual arterioles, thus maximizing the distribution of O2 and allowing O2 consumption to remain constant when O2 delivery is reduced. In vivo and in vitro studies have indicated that adenosine can induce dilatation in several different ways. This review argues that during systemic hypoxia, adenosine is predominantly released from the endothelium and acts on endothelial A1 receptors to produce dilatation in a nitric oxide (NO)-dependent manner. A1 receptor stimulation increases the synthesis of NO by a process initiated by opening of ATP-sensitive K+ (KATP) channels. Moreover, recent findings suggest that prostaglandins also make a major contribution to the hypoxia-induced dilatation, but that the dilator pathways for adenosine, NO and prostaglandins are interdependent. In addition, adenosine released from the skeletal muscle fibres contributes indirectly to the dilatation by stimulating A1 and A2 receptors on the muscle fibres, opening KATP channels and allowing efflux of K+, which is a vasodilator. Finally, by acting on endothelial A1 receptors, adenosine attenuates the vasoconstrictor effects of constant or bursting patterns of sympathetic activity. This limits the extent to which the sympathetic nervous system can reduce O2 delivery to muscle when it is already compromised by systemic hypoxia.

Hypoxia attenuates hepatic arterial vasodilatation and enhances portal venous vasoconstriction to ATP in the perfused rabbit liver

Dose-related responses to acetylcholine, adenosine 5'-triphosphate (ATP), adenosine and sodium nitroprusside were studied in an in vitro perfused rabbit liver gassed with (95% N(2)/5% CO(2), Group 1) and without carbon dioxide (100% N(2), Group 2). At raised tone, achieved by addition of methoxamine to the perfusate, significantly attenuated hepatic arterial vasodilatation to sodium nitroprusside, acetylcholine, ATP and adenosine was measured in Group 1 and responses to all but sodium nitroprusside were abolished in Group 2. Portal venous responses to acetylcholine, adenosine and sodium nitroprusside were not significantly altered in either Group 1 or Group 2. However, portal venous vasoconstriction to ATP was significantly enhanced in Group 1 and less so in Group 2. It is concluded that carbon dioxide-free hypoxia attenuated hepatic arterial vasodilatation to acetylcholine and ATP and enhanced vasoconstriction to ATP. Both these effects may be characteristic of damage to the microvascular endothelium and may be the result of decreased synthesis of nitric oxide.

Cellular mechanisms by which adenosine induces vasodilatation in rat skeletal muscle: significance for systemic hypoxia

1. In anaesthetized rats, we recorded arterial blood pressure (ABP), heart rate (HR), femoral blood flow (FBF) and femoral vascular conductance (FVC). We tested the effects of the nitric oxide (NO) synthesis inhibitor L-NAME (nitro-L-arginine methyl ester), or the ATP-sensitive K+ (KATP) channel inhibitor glibenclamide, on responses evoked by systemic hypoxia (breathing 8% O2 for 5 min) or i.a. infusion for 5 min of adenosine, the NO donor sodium nitroprusside (SNP), the adenosine A1 receptor agonist CCPA (2-chloro-N6-cyclopentyladenosine) or the adenosine A2A receptor agonist CGS 21680 (2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadeno sin e hydrochloride). 2. L-NAME (10 mg kg-1 i.v.) greatly reduced the increase in FVC induced by hypoxia or adenosine, as we have shown before, but had no effect on the increase in FVC evoked by SNP. In addition, L-NAME abolished the increase in FVC evoked by CCPA and greatly reduced that evoked by CGS 21680. These results substantiate the view that muscle vasodilatation induced by systemic hypoxia and infused adenosine are largely NO dependent. They also indicate that muscle dilatation induced by A1 receptor stimulation is entirely NO dependent while that induced by A2A receptors is largely NO dependent; dilatation may also be induced by direct stimulation of A2A receptors on the vascular smooth muscle. 3. Glibenclamide (10 or 20 mg kg-1 i.v.) reduced the increase in FVC induced by hypoxia, preferentially affecting the early part (< 1 min). In addition, glibenclamide greatly reduced the increase in FVC induced by adenosine, but it had no effect on that evoked by SNP. Further, glibenclamide abolished the increase in FVC evoked by CCPA and greatly reduced that evoked by CGS 21680. These results substantiate the view that hypoxia-induced muscle vasodilatation is initiated by KATP channel opening. They also indicate that NO does not induce muscle vasodilatation by opening KATP channels on the vascular smooth muscle, but indicate that the dilatation induced by adenosine and by A2A receptor stimulation is largely dependent on KATP channel opening, while that induced by A1 receptor stimulation is wholly dependent on KATP channel opening. 4. These results, together with previous evidence that hypoxia-induced vasodilatation in skeletal muscle is largely mediated by adenosine acting on A1 receptors, lead us to propose that adenosine is released from endothelium during systemic hypoxia and acts on endothelial A1 receptors to open KATP channels on the endothelial cells and cause synthesis of NO, which then acts on the vascular smooth muscle to cause dilatation. During severe systemic hypoxia we propose that adenosine may also act on A2A receptors on the endothelium to cause dilatation by a similar process and may act on A2A receptors on the vascular smooth muscle to cause dilatation by opening KATP channels.

Role of endothelial carbon monoxide in attenuated vasoreactivity following chronic hypoxia

Chronic hypoxic exposure has been previously demonstrated to attenuate systemic vasoconstrictor activity to a variety of agents. This attenuated responsiveness is observed not only in conscious animals but in isolated vascular preparations as well. Because hypoxia has been documented to increase heme oxygenase (HO) levels and the subsequent production of the vasodilator CO in vitro, we hypothesized that the blunted reactivity observed with chronic hypoxia (CH) may be in part due to increased HO activity. In thoracic aortic rings from CH rats, cumulative dose-response curves to phenylephrine (PE) in the presence of the nitric oxide (NO) synthase inhibitor Nomega-nitro-L-arginine (L-NNA) and the HO inhibitor zinc protoporphyrin 9 (ZnPPIX) elicited increased contractility compared with CH rings treated with only L-NNA. Similar results were observed in rings incubated overnight with the HO-inducing agent sodium m-arsenite. In contrast, contractile responses in rings from control rats were unaffected by the HO inhibitor. Furthermore, endothelium-denuded rings from either control or CH rats did not exhibit an increase in reactivity to PE following ZnPPIX incubation. ZnPPIX had no effect on relaxant responses to the NO donor S-nitroso-N-penicillamine, suggesting that its actions were specific to HO inhibition. Finally, aortic rings exhibited dose-dependent relaxant responses to exogenous CO that were endothelium independent and blocked by an inhibitor of soluble guanylyl cyclase. The other products of HO enzyme activity, iron and biliverdin, were without effect on vasoreactivity. Thus we conclude that the attenuated vasoreactivity to PE following CH is likely to involve the induction of endothelial HO and the subsequent enhanced production of CO.

Chemoreceptors and cardiovascular control in acute and chronic systemic hypoxia

This review describes the ways in which the primary bradycardia and peripheral vasoconstriction evoked by selective stimulation of peripheral chemoreceptors can be modified by the secondary effects of a chemoreceptor-induced increase in ventilation. The evidence that strong stimulation of peripheral chemoreceptors can evoke the behavioural and cardiovascular components of the alerting or defence response which is characteristically evoked by novel or noxious stimuli is considered. The functional significance of all these influences in systemic hypoxia is then discussed with emphasis on the fact that these reflex changes can be overcome by the local effects of hypoxia: central neural hypoxia depresses ventilation, hypoxia acting on the heart causes bradycardia and local hypoxia of skeletal muscle and brain induces vasodilatation. Further, it is proposed that these local influences can become interdependent, so generating a positive feedback loop that may explain sudden infant death syndrome (SIDS). It is also argued that a major contributor to these local influences is adenosine. The role of adenosine in determining the distribution of O2 in skeletal muscle microcirculation in hypoxia is discussed, together with its possible cellular mechanisms of action. Finally, evidence is presented that in chronic systemic hypoxia, the reflex vasoconstrictor influences of the sympathetic nervous system are reduced and/or the local dilator influences of hypoxia are enhanced. In vitro and in vivo findings suggest this is partly explained by upregulation of nitric oxide (NO) synthesis by the vascular endothelium which facilitates vasodilatation induced by adenosine and other NO-dependent dilators and attenuates noradrenaline-evoked vasoconstriction.

Studies on the roles of ATP, adenosine and nitric oxide in mediating muscle vasodilatation induced in the rat by acute systemic hypoxia

1. In Saffan-anaesthetized rats, we have further investigated the mechanisms underlying the vasodilatation induced by adenosine in skeletal muscle by acute systemic hypoxia (breathing 8% O2 for 5 min). 2. In eleven rats the nitric oxide (NO) synthesis inhibitor nitro-L-arginine methyl ester (L-NAME, 10 mg kg-1, i.v.) reduced the increase in femoral vascular conductance (FVC) induced by hypoxia by approximately 50%. L-NAME had similar effects on the increase in FVC induced by intra-arterial (I.A.) infusion of adenosine (at 1.2 mg kg-1 min-1 for 5 min via the tail artery) and by ATP (I.A., 1 mg kg-1 min-1 for 5 min). Subsequent administration of the adenosine receptor antagonist 8-sulphophenyl theophylline (8-SPT, 20 mg kg-1, i.v.) virtually abolished the adenosine- and ATP-induced increase in FVC. 3. In a further nine rats, 8-SPT reduced the increase in FVC induced by hypoxia by approximately 50%. This remaining increase in FVC was substantially reduced by L-NAME. 4. In an additional nine rats, alpha,beta-methyleneADP (160 micrograms kg-1, i.v.) which inhibits the 5'-ectonucleotidase that degrades AMP to adenosine, reduced the peripheral vasodilatation (fall in arterial blood pressure, ABP) induced by ATP infusion, but had no effect on the increase in FVC or decrease in ABP evoked by systemic hypoxia. 5. These results provide the first evidence that the muscle vasodilatation induced by adenosine during systemic hypoxia is mainly dependent on NO synthesis. They also suggest that adenosine is released as such rather than being formed extracellularly from AMP. Given evidence that extraluminal adenosine acts in an NO-independent fashion we propose that hypoxia releases adenosine from the endothelium. Our results also indicate that hypoxia induces muscle vasodilatation that is adenosine independent but NO dependent: they allow the possibility that this is partly mediated by ATP released from the endothelium.