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

Anemia and Hypoxia Impact on Chronic Kidney Disease Onset and Progression: Review and Updates

Chronic kidney disease (CKD) is caused by hypoxia in the renal tissue, leading to inflammation and increased migration of pathogenic cells. Studies showed that leukocytes directly sense hypoxia and respond by initiating gene transcription, encoding the 2-integrin adhesion molecules. Moreover, other mechanisms participate in hypoxia, including anemia. CKD-associated anemia is common, which induces and worsens hypoxia, contributing to CKD progression. Anemia correction can slow CKD progression, but it should be cautiously approached. In this comprehensive review, the underlying pathophysiology mechanisms and the impact of renal tissue hypoxia and anemia in CKD onset and progression will be reviewed and discussed in detail. Searching for the latest updates in PubMed Central, Medline, PubMed database, Google Scholar, and Google search engines were conducted for original studies, including cross-sectional studies, cohort studies, clinical trials, and review articles using different keywords, phrases, and texts such as "CKD progression, anemia in CKD, CKD, anemia effect on CKD progression, anemia effect on CKD progression, and hypoxia and CKD progression". Kidney tissue hypoxia and anemia have an impact on CKD onset and progression. Hypoxia causes nephron cell death, enhancing fibrosis by increasing interstitium protein deposition, inflammatory cell activation, and apoptosis. Severe anemia correction improves life quality and may delay CKD progression. Detection and avoidance of the risk factors of hypoxia prevent recurrent acute kidney injury (AKI) and reduce the CKD rate. A better understanding of kidney hypoxia would prevent AKI and CKD and lead to new therapeutic strategies.

Significance of K(ATP) channels, L-type Ca²⁺ channels and CYP450-4A enzymes in oxygen sensing in mouse cremaster muscle arterioles in vivo

Background

ATP-sensitive K⁺ channels (K_ATP channels), NO, prostaglandins, 20-HETE and L-type Ca²⁺ channels have all been suggested to be involved in oxygen sensing in skeletal muscle arterioles, but the role of the individual mechanisms remain controversial. We aimed to establish the importance of these mechanisms for oxygen sensing in arterioles in an in vivo model of metabolically active skeletal muscle. For this purpose we utilized the exteriorized cremaster muscle of anesthetized mice, in which the cremaster muscle was exposed to controlled perturbation of tissue PO₂.

Results

Change from “high” oxygen tension (PO₂ = 153.4 ± 3.4 mmHg) to “low” oxygen tension (PO₂ = 13.8 ± 1.3 mmHg) dilated cremaster muscle arterioles from 11.0 ± 0.4 μm to 32.9 ± 0.9 μm (n = 28, P < 0.05). Glibenclamide (K_ATP channel blocker) caused maximal vasoconstriction, and abolished the dilation to low oxygen, whereas the K_ATP channel opener cromakalim caused maximal dilation and prevented the constriction to high oxygen. When adding cromakalim on top of glibenclamide or vice versa, the reactivity to oxygen was gradually restored. Inhibition of L-type Ca²⁺ channels using 3 μM nifedipine did not fully block basal tone in the arterioles, but rendered them unresponsive to changes in PO₂. Inhibition of the CYP450-4A enzyme using DDMS blocked vasoconstriction to an increase in PO₂, but had no effect on dilation to low PO₂.

Conclusions

We conclude that: 1) L-type Ca²⁺ channels are central to oxygen sensing, 2) K_ATP channels are permissive for the arteriolar response to oxygen, but are not directly involved in the oxygen sensing mechanism and 3) CYP450-4A mediated 20-HETE production is involved in vasoconstriction to high PO₂.

Factors that render the kidney susceptible to tissue hypoxia in hypoxemia

To better understand what makes the kidney susceptible to tissue hypoxia, we compared, in the rabbit kidney and hindlimb, the ability of feedback mechanisms governing oxygen consumption (V̇o2) and oxygen delivery (Do2) to attenuate tissue hypoxia during hypoxemia. In the kidney (cortex and medulla) and hindlimb (biceps femoris muscle), we determined responses of whole organ blood flow and V̇o2, and local perfusion and tissue Po2, to reductions in Do2 mediated by graded systemic hypoxemia. Progressive hypoxemia reduced tissue Po2 similarly in the renal cortex, renal medulla, and biceps femoris. Falls in tissue Po2 could be detected when arterial oxygen content was reduced by as little as 4–8%. V̇o2 remained stable during progressive hypoxemia, only tending to fall once arterial oxygen content was reduced by 55% for the kidney or 42% for the hindlimb. Even then, the fall in renal V̇o2 could be accounted for by reduced oxygen demand for sodium transport rather than limited oxygen availability. Hindlimb blood flow and local biceps femoris perfusion increased progressively during graded hypoxia. In contrast, neither total renal blood flow nor cortical or medullary perfusion was altered by hypoxemia. Our data suggest that the absence in the kidney of hyperemic responses to hypoxia, and the insensitivity of renal V̇o2 to limited oxygen availability, contribute to kidney hypoxia during hypoxemia. The susceptibility of the kidney to tissue hypoxia, even in relatively mild hypoxemia, may have important implications for the progression of kidney disease, particularly in patients at high altitude or with chronic obstructive pulmonary disease.

Multiple mechanisms act to maintain kidney oxygenation during renal ischemia in anesthetized rabbits

We examined the mechanisms that maintain stable renal tissue PO(2) during moderate renal ischemia, when changes in renal oxygen delivery (DO(2)) and consumption (VO(2)) are mismatched. When renal artery pressure (RAP) was reduced progressively from 80 to 40 mmHg, VO(2) (-38 ± 7%) was reduced more than DO(2) (-26 ± 4%). Electrical stimulation of the renal nerves (RNS) reduced DO(2) (-49 ± 4% at 2 Hz) more than VO(2) (-30 ± 7% at 2 Hz). Renal arterial infusion of angiotensin II reduced DO(2) (-38 ± 3%) but not VO(2) (+10 ± 10%). Despite mismatched changes in DO(2) and VO(2), renal tissue PO(2) remained remarkably stable at ≥40 mmHg RAP, during RNS at ≤2 Hz, and during angiotensin II infusion. The ratio of sodium reabsorption to VO(2) was reduced by all three ischemic stimuli. None of the stimuli significantly altered the gradients in PCO(2) or pH across the kidney. Fractional oxygen extraction increased and renal venous PO(2) fell during 2-Hz RNS and angiotensin II infusion, but not when RAP was reduced to 40 mmHg. Thus reduced renal VO(2) can help prevent tissue hypoxia during mild renal ischemia, but when renal VO(2) is reduced less than DO(2), other mechanisms prevent a fall in renal PO(2). These mechanisms do not include increased efficiency of renal oxygen utilization for sodium reabsorption or reduced washout of carbon dioxide from the kidney, leading to increased oxygen extraction. However, increased oxygen extraction could be driven by altered countercurrent exchange of carbon dioxide and/or oxygen between renal arteries and veins.

Comparison of exogenous adenosine and voluntary exercise on human skeletal muscle perfusion and perfusion heterogeneity

Adenosine is a widely used pharmacological agent to induce a “high-flow” control condition to study the mechanisms of exercise hyperemia, but it is not known how well an adenosine infusion depicts exercise-induced hyperemia, especially in terms of blood flow distribution at the capillary level in human muscle. Additionally, it remains to be determined what proportion of the adenosine-induced flow elevation is specifically directed to muscle only. In the present study, we measured thigh muscle capillary nutritive blood flow in nine healthy young men using PET at rest and during the femoral artery infusion of adenosine (1 mgmin−1l thigh volume−1), which has previously been shown to induce a maximal whole thigh blood flow of ∼8 l/min. This response was compared with the blood flow induced by moderate- to high-intensity one-leg dynamic knee extension exercise. Adenosine increased muscle blood flow on average to 40 ± 7 ml·min−1·100 g muscle−1 with an aggregate value of 2.3 ± 0.6 l/min for the whole thigh musculature. Adenosine also induced a substantial change in blood flow distribution within individuals. Muscle blood flow during the adenosine infusion was comparable with blood flow in moderate- to high-intensity exercise (36 ± 9 ml·min−1·100 g muscle−1), but flow heterogeneity was significantly higher during the adenosine infusion than during voluntary exercise. In conclusion, a substantial part of the flow increase in the whole limb blood flow induced by a high-dose adenosine infusion is conducted through the physiological non-nutritive shunt in muscle and/or also through tissues of the limb other than muscle. Additionally, an intra-arterial adenosine infusion does not mimic exercise hyperemia, especially in terms of muscle capillary flow heterogeneity, while the often-observed exercise-induced changes in capillary blood flow heterogeneity likely reflect true changes in nutritive flow linked to muscle fiber and vascular unit recruitment.

Intrarenal oxygenation: unique challenges and the biophysical basis of homeostasis

The kidney is faced with unique challenges for oxygen regulation, both because its function requires that perfusion greatly exceeds that required to meet metabolic demand and because vascular control in the kidney is dominated by mechanisms that regulate glomerular filtration and tubular reabsorption. Because tubular sodium reabsorption accounts for most oxygen consumption (Vo2) in the kidney, renal Vo2 varies with glomerular filtration rate. This provides an intrinsic mechanism to match changes in oxygen delivery due to changes in renal blood flow (RBF) with changes in oxygen demand. Renal Vo2 is low relative to supply of oxygen, but diffusional arterial-to-venous (AV) oxygen shunting provides a mechanism by which oxygen superfluous to metabolic demand can bypass the renal microcirculation. This mechanism prevents development of tissue hyperoxia and subsequent tissue oxidation that would otherwise result from the mismatch between renal Vo2 and RBF. Recent evidence suggests that RBF-dependent changes in AV oxygen shunting may also help maintain stable tissue oxygen tension when RBF changes within the physiological range. However, AV oxygen shunting also renders the kidney susceptible to hypoxia. Given that tissue hypoxia is a hallmark of both acute renal injury and chronic renal disease, understanding the causes of tissue hypoxia is of great clinical importance. The simplistic paradigm of oxygenation depending only on the balance between local perfusion and Vo2 is inadequate to achieve this goal. To fully understand the control of renal oxygenation, we must consider a triad of factors that regulate intrarenal oxygenation: local perfusion, local Vo2, and AV oxygen shunting.

Role of adenosine in regulating the heterogeneity of skeletal muscle blood flow during exercise in humans

Evidence from both animal and human studies suggests that adenosine plays a role in the regulation of exercise hyperemia in skeletal muscle. We tested whether adenosine also plays a role in the regulation of blood flow (BF) distribution and heterogeneity among and within quadriceps femoris (QF) muscles during exercise, measured using positron emission tomography. In six healthy young women, BF was measured at rest and then during three incremental low and moderate intermittent isometric one-legged knee-extension exercise intensities without and with theophylline-induced nonselective adenosine receptor blockade. BF heterogeneity within muscles was calculated from 16-mm3voxels in BF images and heterogeneity among the muscles from the mean values of the four QF compartments. Mean BF in the whole QF and its four parts increased, and heterogeneity decreased with workload both without and with theophylline ( P < 0.001). Adenosine receptor blockade did not have any effect on mean bulk BF or BF heterogeneity among the QF muscles, yet blockade increased within-muscle BF heterogeneity in all four QF muscles ( P = 0.03). Taken together, these results show that BF becomes less heterogeneous with increasing exercise intensity in the QF muscle group. Adenosine seems to play a role in muscle BF heterogeneity even in the absence of changes in bulk BF at low and moderate one-leg intermittent isometric exercise intensities.

Responses evoked in single sympathetic nerve fibres of the rat tail artery by systemic hypoxia are dependent on core temperature

No direct evidence exists of the changes evoked by systemic hypoxia in sympathetic nerves to the rat cutaneous circulation, and of the concomitant changes in cutaneous blood flow. Here we investigated responses evoked by two levels of systemic hypoxia (12% and 8% inspired O(2)) in single sympathetic units supplying tail caudal ventral artery (CVA) in spontaneously breathing anaesthetized rats, whilst simultaneously recording tail blood flow and vascular resistance (TVR) from the CVA, under conditions of modest hypothermia and hyperthermia. During modest hypothermia and normoxia, TVR was high and CVA unit activity was present, with marked respiratory modulation and a rhythmictiy (T-rhythm) that was often independent of respiration. Hypoxia evoked a graded fall in TVR indicating vasodilatation, but there were no consistent changes in CVA unit firing rate or T-rhythm frequency, although respiratory modulation increased. By contrast, during hyperthermia, TVR was low and CVA unit activity was absent. Systemic hypoxia evoked graded increases in TVR, indicating vasoconstriction, and in 8% O(2) there was recommencement of firing in some CVA units, at low discharge rate, with respiratory modulation but no T-rhythm. These results indicate that the changes evoked by systemic hypoxia in TVR and sympathetic nerve activity to CVA are dependent on core temperature. During modest hypothermia, hypoxia-induced cutaneous vasodilatation in the tail is independent of sympathetic activity, whereas during hyperthermia, when sympathetic activity is 'switched off', severe hypoxia initiates respiratory-related low level activity, causing cutaneous vasoconstriction.

Contribution of alpha2-adrenoceptors and Y1 neuropeptide Y receptors to the blunting of sympathetic vasoconstriction induced by systemic hypoxia in the rat

There is evidence that sympathetically evoked vasoconstriction in skeletal muscle is blunted in systemic hypoxia, but the mechanisms underlying this phenomenon are not clear. In Saffan-anaesthetized Wistar rats, we have studied the role of α₂-adrenoceptors and neuropeptide Y (NPY) Y₁ receptors in mediating vasoconstriction evoked by direct stimulation of the lumbar sympathetic chain by different patterns of impulses in normoxia (N) and systemic hypoxia (H: breathing 8% O₂). Patterns comprised 120 impulses delivered in bursts over a 1 min period at 40 or 20 Hz, or continuously at 2 Hz. Hypoxia attenuated the evoked increases in femoral vascular resistance (FVR) by all patterns, the response to 2 Hz being most affected (40 Hz bursts: N = 3.25 ± 0.75 arbitrary resistance units (RU); H = 1.14 ± 0.29 RU). Yohimbine (Yoh, α₂-adrenoceptor antagonist) or BIBP 3226 (Y₁-receptor antagonist) did not affect baseline FVR. In normoxia, Yoh attenuated the responses evoked by high frequency bursts and 2 Hz, whereas BIBP 3226 only attenuated the response to 40 Hz (40 Hz bursts: N + Yoh = 2.1 ± 0.59 RU; N + BIBP 3226 = 1.9 ± 0.4 RU). In hypoxia, Yoh did not further attenuate the evoked responses, but BIBP 3226 further attenuated the response to 40 Hz bursts. These results indicate that neither α₂-adrenoceptors nor Y₁ receptors contribute to basal vascular tone in skeletal muscle, but both contribute to constrictor responses evoked by high frequency bursts of sympathetic activity. We propose that in systemic hypoxia, the α₂-mediated component represents about 50% of the sympathetically evoked constriction that is blunted, whereas the contribution made by Y₁ receptors is resistant. Thus we suggest the importance of NPY in the regulation of FVR and blood pressure increases during challenges such as systemic hypoxia.

Spreading dilatation to luminal perfusion of ATP and UTP in rat isolated small mesenteric arteries

Levels of ATP achieved within the lumen of vessels suggest a key autacoid role. P2Y receptors on the endothelium may represent the target for ATP, leading to hyperpolarization and associated relaxation of vascular smooth muscle through the endothelium-dependent hyperpolarizing factor (EDHF) pathway. EDHF signals radially from the endothelium to cause dilatation, and appears mechanistically distinct from the axial spread of dilatation, which we showed occurs independently of a change in endothelial cell Ca2+ in rat mesenteric arteries. Here we have investigated the potential of P2Y receptor stimulation to evoke spreading dilatation in rat resistance small arteries under physiological pressure and flow. Triple cannulation of isolated arteries enables focal application of purine and pyrimidine nucleotides to the endothelium, avoiding potential complicating actions of these agents on the smooth muscle. Nucleotides were locally infused through one branch of a bifurcation, causing near maximal local dilatation attributable to EDHF. Dilatation then spread rapidly into the adjacent feed artery and upstream against the direction of luminal flow, sufficient to increase flow into the feed artery. The rate of decay of this spreading dilatation was identical between nucleotides, and matched that to ACh, which acts only on the endothelium. In contrast, focal abluminal application of either ATP or UTP at the downstream end of cannulated arteries evoked constriction, which only in the case of ATP was also associated with modest spread of dilatation. The non-hydrolysable ADP analogue, ADPbetaS, acting at P2Y1 receptors, caused robust local and spreading dilatation responses whether applied to the luminal or abluminal surface of pressurized arteries. Dilatation to nucleotides was sensitive to inhibition with apamin and TRAM-34, selective blockers of small- and intermediate-conductance Ca2+-activated K+ channels, respectively. These data demonstrate that direct luminal stimulation of P2Y receptor on the endothelium of rat mesenteric arteries leads to marked spreading dilatation and thus suggests that circulating purines and pyrimidines may act as important regulators of blood flow.

The role of adenosine in the early respiratory and cardiovascular changes evoked by chronic hypoxia in the rat

Experiments were performed on anaesthetized normoxic (N) rats and chronically hypoxic rats that had been exposed to 12% O2 for 1, 3 or 7 days (1, 3 or 7CH rats). The adenosine A1 receptor antagonist DPCPX did not affect the resting hyperventilation of 1-7CH rats breathing 12% O2 and increased resting heart rate (HR) in 1CH rats only. DPCPX partially restored the decreased baseline arterial pressure (ABP) and increased femoral vascular conductance (FVC) of 1 and 3CH rats, but had no effect in N or 7CH rats. DPCPX also attenuated the decrease in arterial blood pressure (ABP) and increase in FVC evoked by acute hypoxia in N and 1-7CH rats. The non-selective adenosine receptor antagonist 8-SPT had no further effect on baselines or cardiovascular responses to acute hypoxia, but attenuated the hypoxia-evoked increase in respiratory frequency in 1-7CH rats. In N, and 1 and 3CH rats, the inducible nitric oxide synthase (iNOS) inhibitor aminoguanidine had no effect on baselines or increases in FVC evoked by acetylcholine. We propose: (i) that tonically released adenosine acting on A1 receptors reduces HR in 1CH rats and stimulates endothelial NOS in 1 and 3CH rats to decrease ABP and increase FVC, the remaining NO-dependent tonic vasodilatation being independent of iNOS activity; (ii) that in 7CH rats, tonic adenosine release has waned; (iii) that in 1-7CH rats, adenosine released by acute hypoxia stimulates A1 but not A2 receptors to produce muscle vasodilatation, and stimulates carotid body A2 receptors to increase respiration.

Nitric oxide regulation of microvascular oxygen exchange during hypoxia and hyperoxia

The objective of this work was to test the hypothesis that the limitation of nitric oxide (NO) availability accentuates microvascular reactivity to oxygen. The awake hamster chamber window model was rendered hypoxic and hyperoxic by ventilation with 10 and 100% oxygen. Systemic and microvascular parameters were determined in the two conditions and compared with normoxia in a group receiving the NO scavenger nitronyl nitroxide and a control group receiving only the vehicle (saline). Mean arterial blood pressure did not change with different gas mixtures during infusion of the vehicle, but it increased significantly in the NO-depleted group. NO scavenging increased the reactivity of microvessels to the changed oxygen supply, causing the arteriolar wall to significantly increase oxygen consumption. Tissue Po2 was correspondingly significantly reduced during NO scavenger infusion. The present findings support the hypothesis that microvascular oxygen consumption is proportional to oxygen-induced vasoconstriction. The effect of oxygen on vascular tone is modulated by NO. As a consequence, NO acts as a regulator of the vessel wall oxygen consumption. The vessel wall consumes oxygen in proportion to the local Po2, and an impairment of NO availability renders the circulation more sensitive to changes in the oxygen supply.

Measurement of nitric oxide release evoked by systemic hypoxia and adenosine from rat skeletal muscle in vivo

It is accepted that NO plays a role in hypoxic vasodilatation in several tissues. For rat hindlimb muscle there is evidence that during systemic hypoxia endogenously released adenosine acts on endothelial A1 receptors to evoke dilatation in a NO-dependent fashion, implying requirement for, or mediation by, NO. We tested in vivo whether systemic hypoxia and adenosine release NO from muscle. In anaesthetized rats, arterial blood pressure (ABP) and femoral blood flow (FBF) were recorded allowing computation of femoral vascular conductance (FVC). Blood samples taken from femoral artery and vein allowed electrochemical measurement of plasma [NO] after reduction of NO3- and NO2-. Systemic hypoxia and adenosine infusion for 5 min each, evoked an increase in FVC that was attenuated by the NO synthase (NOS) inhibitor l-NAME (Group 1, n = 8) and adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, Group 2, n = 6). Concomitant systemic hypoxia and adenosine infusion evoked increases in venous-arterial [NO] difference ([NO](v-a)) from -1.4 +/- 0.85 to 6.6 +/- 1.6 and 2.3 +/- 0.78 to 8.4 +/- 1.8 nmol l(-1), respectively (mean +/- s.e.m), which were abolished by l-NAME (-0.72 +/- 0.90 to -0.87 +/- 0.74 and 0.72 +/- 0.85 to -0.97 +/- 1.1 nmol l(-1), respectively). DPCPX also abolished the hypoxia-evoked increase in [NO](v-a) (control -4.2 +/- 1.8 to 12.5 +/- 3.7 nmol l(-1), with DPCPX -0.63 +/- 2.6 to 3.3 +/- 2.9 nmol l(-1)) and decreased the adenosine-evoked increase in [NO](v-a) (control 1.1 +/- 1.5 to 24 +/- 14, with DPCPX -0.43 +/- 2.9 to 12 +/- 5.9 nmol l(-1)). These results allow the novel conclusion that the muscle vasodilatation of systemic hypoxia is partly mediated by adenosine acting at endothelial A1 receptors to stimulate synthesis and release of NO, which then induces dilatation.

Equilibrative Nucleoside Transporter 1 Expression Is Downregulated by Hypoxia in Human Umbilical Vein Endothelium

Reduced oxygen level (hypoxia) induces endothelial dysfunction and release of the endogenous nucleoside adenosine. Human umbilical vein endothelium (HUVEC) function in an environment with 3% to 5% O2 and exhibit efficient adenosine membrane transport via human equilibrative nucleoside transporters 1 (hENT1). We studied whether adenosine transport and hENT1 expression are altered by hypoxia in HUVEC. Hypoxia (0 to 24 hours, 2% and 1% O2) reduced maximal hENT1-adenosine transport velocity (V(max)) and maximal nitrobenzylthionosine (NBMPR, a high-affinity hENT1 protein ligand) binding, but increased extracellular adenosine concentration. Hypoxia also reduced hENT1 protein and mRNA levels, effects unaltered by N(omega)-nitro-l-arginine methyl ester (l-NAME, nitric oxide synthase [NOS] inhibitor) or PD-98059 (inhibitor of mitogen-activated protein kinase kinase 1 and 2 [MEK1/2]). Hypoxia reduced endothelial NOS (eNOS) activity and eNOS phosphorylation at Ser(1177), but increased eNOS protein level. Hypoxia increased (1 to 3 hours), but reduced (24 hours) p42/44(mapk) phosphorylation. Thus, hypoxia-increased extracellular adenosine may result from reduced hENT1-adenosine transport in HUVEC. Hypoxia effect seems not to involve NO, but p42/44(mapk) may be required for the relatively rapid effect (1 to 3 hours) of hypoxia. These results could be important in diseases where the fetus is exposed to intrauterine environments poor in oxygen, such as intrauterine growth restriction, or where adenosine transport is altered, such as gestational diabetes.

Effect of acute hypoxia on microcirculatory and tissue oxygen levels in rat cremaster muscle

Repeated exposure to brief periods of hypoxia leads to pathophysiological changes in experimental animals similar to those seen in sleep apnea. To determine the effects of such exposure on oxygen levels in vivo, we used an optical method to measure Po2in microcirculatory vessels and tissue of the rat cremaster muscle during a 1-min step reduction of inspired oxygen fraction from 0.21 to 0.07. Under control conditions, Po2was 98.1 ± 1.9 Torr in arterial blood, 52.2 ± 2.8 Torr in 29.0 ± 2.7-μm arterioles, 26.8 ± 1.7 Torr in the tissue interstitium near venous capillaries, and 35.1 ± 2.6 Torr in 29.7 ± 1.9-μm venules. The initial fall in Po2during hypoxia was significantly greater in arterial blood, being 93% complete in the first 10 s, whereas it was 68% complete in arterioles, 47% at the tissue sites, and 38% in venules. In the 10- to 30-s period, the fall in normalized tissue and venular Po2was significantly greater than in arterial Po2. At the end of hypoxic exposure, Po2at all measurement sites had fallen very nearly in proportion to that in the inspired gas, but tissue oxygen levels did not reach critical Po2. Significant differences in oxyhemoglobin desaturation rate were also observed between arterial and microcirculatory vessels during hypoxia. In conclusion, the fall in microcirculatory and tissue oxygen levels in resting skeletal muscle is significantly slower than in arterial blood during a step reduction to an inspired oxygen fraction of 0.07, and tissue Po2does not reach anaerobic levels.

Influence of endogenous nitric oxide on sympathetic vasoconstriction in normoxia, acute and chronic systemic hypoxia in the rat

We studied the role of nitric oxide (NO) in blunting sympathetically evoked muscle vasoconstriction during acute and chronic systemic hypoxia. Experiments were performed on anaesthetized normoxic (N) and chronically hypoxic (CH) rats that had been acclimated to 12% O(2) for 3-4 weeks. The lumbar sympathetic chain was stimulated for 1 min with bursts at 20 or 40 Hz and continuously at 2 Hz. In N rats, acute hypoxia (breathing 8% O(2)) reduced baseline femoral vascular resistance (FVR) and depressed increases in FVR evoked by all three patterns of stimulation, but infusion of the NO donor sodium nitroprusside (SNP), so as to similarly reduce baseline FVR, did not affect sympathetically evoked responses. Blockade of NO synthase (NOS) with L-NAME increased baseline FVR and facilitated the sympathetically evoked increases in FVR, but when baseline FVR was restored by SNP infusion, these evoked responses were restored. Acute hypoxia after L-NAME still reduced baseline FVR and depressed evoked responses. In CH rats breathing 12% O(2), baseline FVR was lower than in N rats breathing air, but L-NAME had qualitatively similar effects on baseline FVR and sympathetically evoked increases in FVR. SNP similarly restored baseline FVR and evoked responses. Inhibition of neuronal NOS or inducible NOS did not affect baselines, or evoked responses. We propose that in N and CH rats sympathetically evoked muscle vasoconstriction is modulated by tonically released NO, but not depressed by additional NO released on sympathetic activation. The present results suggest that hypoxia-induced blunting of sympathetic vasoconstriction in skeletal muscle is not mediated by NO.

Does nitric oxide allow endothelial cells to sense hypoxia and mediate hypoxic vasodilatation? In vivo and in vitro studies

Hypoxia-evoked vasodilatation is a fundamental regulatory mechanism that is often attributed to adenosine. The identity of the O(2) sensor is unknown. Nitric oxide (NO) inhibits endothelial mitochondrial respiration and ATP generation by competing with O(2) for its binding site on cytochrome oxidase. We proposed that in vivo this interaction allows endothelial cells to release adenosine when O(2) tension falls or NO concentration increases. Using anaesthetised rats, we confirmed that the increase in femoral vascular conductance (FVC, hindlimb vasodilatation) evoked by systemic hypoxia is attenuated by NO synthesis blockade with L-NAME, but restored when baseline FVC is restored by infusion of NO donor. This "restored" hypoxic response, like the control hypoxic response, is inhibited by the adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. Similarly, the FVC increase evoked by adenosine infusion was attenuated by L-NAME but restored by infusion of NO donor. However, when baseline FVC was restored after L-NAME with 8-bromo-cGMP, the FVC increase evoked by adenosine infusion was restored, but not in response to systemic hypoxia, suggesting that adenosine was no longer released by hypoxia. Infusion of NO donor at a given rate after treatment with L-NAME evoked a greater FVC increase during systemic hypoxia than during normoxia, both responses being reduced by 8-cyclopentyl-1,3-dipropylxanthine. Finally, both bradykinin and NO donor released adenosine from superfused endothelial cells in vitro; L-NAME attenuated only the former response. We propose that in vivo, shear-released NO increases the apparent K(m) of endothelial cytochrome oxidase for O(2), allowing the endothelium to act as an O(2) sensor, releasing adenosine in response to moderate falls in O(2).

Erythrocyte and the regulation of human skeletal muscle blood flow and oxygen delivery: role of circulating ATP

Blood flow to contracting skeletal muscle is tightly coupled to the oxygenation state of hemoglobin. To investigate if ATP could be a signal by which the erythrocyte contributes to the regulation of skeletal muscle blood flow and oxygen (O2) delivery, we measured circulating ATP in 8 young subjects during incremental one-legged knee-extensor exercise under conditions of normoxia, hypoxia, hyperoxia, and CO+normoxia, which produced reciprocal alterations in arterial O2 content and thigh blood flow (TBF), but equal thigh O2 delivery and thigh O2 uptake. With increasing exercise intensity, TBF, thigh vascular conductance (TVC), and femoral venous plasma [ATP] augmented significantly (P<0.05) in all conditions. However, with hypoxia, TBF, TVC, and femoral venous plasma [ATP] were (P<0.05) or tended (P=0.14) to be elevated compared with normoxia, whereas with hyperoxia they tended to be reduced. In CO+normoxia, where femoral venous O2Hb and (O2+CO)Hb were augmented compared with hypoxia despite equal arterial deoxygenation, TBF and TVC were elevated, whereas venous [ATP] was markedly reduced. At peak exercise, venous [ATP] in exercising and nonexercising limbs was tightly correlated to alterations in venous (O2+CO)Hb (r2=0.93 to 0.96; P<0.01). Intrafemoral artery infusion of ATP at rest in normoxia (n=5) evoked similar increases in TBF and TVC than those observed during exercise. Our results in humans support the hypothesis that the erythrocyte functions as an O2 sensor, contributing to the regulation of skeletal muscle blood flow and O2 delivery, by releasing ATP depending on the number of unoccupied O2 binding sites in the hemoglobin molecule.

Interactions of adenosine, prostaglandins and nitric oxide in hypoxia-induced vasodilatation: in vivo and in vitro studies

Adenosine, prostaglandins (PG) and nitric oxide (NO) have all been implicated in hypoxia-evoked vasodilatation. We investigated whether their actions are interdependent. In anaesthetised rats, the PG synthesis inhibitors diclofenac or indomethacin reduced muscle vasodilatation evoked by systemic hypoxia or adenosine, but not that evoked by iloprost, a stable analogue of prostacyclin (PGI(2)), or by an NO donor. After diclofenac, the A(1) receptor agonist CCPA evoked no vasodilatation: we previously showed that A(1), but not A(2A), receptors mediate the hypoxia-induced muscle vasodilatation. Further, in freshly excised rat aorta, adenosine evoked a release of NO, detected with an NO-sensitive electrode, that was abolished by NO synthesis inhibition, or endothelium removal, and reduced by ~50 % by the A(1) antagonist DPCPX, the remainder being attenuated by the A(2A) antagonist ZM241385. Diclofenac reduced adenosine-evoked NO release by ~50 % under control conditions, abolished that evoked in the presence of ZM241385, but did not affect that evoked in the presence of DPCPX. Adenosine-evoked NO release was also abolished by the adenyl cyclase inhibitor 2',5'-dideoxyadenosine, while dose-dependent NO release was evoked by iloprost. Finally, stimulation of A(1), but not A(2A), receptors caused a release of PGI(2) from rat aorta, assessed by radioimmunoassay of its stable metabolite, 6-keto PGF(1alpha), that was abolished by diclofenac. These results suggest that during systemic hypoxia, adenosine acts on endothelial A(1) receptors to increase PG synthesis, thereby generating cAMP, which increases the synthesis and release of NO and causes muscle vasodilatation. This pathway may be important in other situations involving these autocoids.

Roles of adenosine in skeletal muscle during systemic hypoxia

1. The present review is concerned with the effects of acute systemic hypoxia on the gross vascular conductance of skeletal muscle (MVC) and on the behaviour of muscle microcirculation. 2. On the basis of experiments performed in the rat, it is argued that adenosine released from the vascular endothelium plays a major role in dilating muscle vasculature by acting on adenosine A1 receptors. 3. The dilatation of the proximal arterioles is primarily important in increasing MVC and in limiting the fall in O(2) delivery to muscle. It is suggested that the action of adenosine on proximal arterioles is dependent on nitric oxide (NO) rather than mediated by NO, such that adenosine dilates the proximal arterioles via other mechanisms when synthesis of NO is blocked. 4. In contrast, dilatation of terminal arterioles, particularly in regions within muscle where the hypoxia is most severe, helps to improve the distribution of available O(2), allowing muscle O(2) consumption to be maintained by increased O(2) extraction. It is concluded that the action of adenosine on terminal arterioles is mainly mediated by NO arising from stimulation of endothelial A1 receptors. 5. Therefore, adenosine plays a major role in coordinating the behaviour of muscle vasculature such that the relationship between O(2) supply and O(2) demand can be optimized even when the O(2) content of the arterial blood is greatly reduced.