Regulation of the microvasculature during small muscle mass exercise in chronic obstructive pulmonary disease vs. chronic heart failure

Aim: Skeletal muscle convective and diffusive oxygen (O₂) transport are peripheral determinants of exercise capacity in both patients with chronic obstructive pulmonary disease (COPD) and chronic heart failure (CHF). We hypothesised that differences in these peripheral determinants of performance between COPD and CHF patients are revealed during small muscle mass exercise, where the cardiorespiratory limitations to exercise are diminished.

Methods: Eight patients with moderate to severe COPD, eight patients with CHF (NYHA II), and eight age- and sex-matched controls were studied. We measured leg blood flow (Q̇_leg) by Doppler ultrasound during submaximal one-legged knee-extensor exercise (KEE), while sampling arterio-venous variables across the leg. The capillary oxyhaemoglobin dissociation curve was reconstructed from paired femoral arterial-venous oxygen tensions and saturations, which enabled the estimation of O₂ parameters at the microvascular level within skeletal muscle, so that skeletal muscle oxygen conductance (D_SMO₂) could be calculated and adjusted for flow (D_SMO₂/Q̇_leg) to distinguish convective from diffusive oxygen transport.

Results: During KEE, Q̇_leg increased to a similar extent in CHF (2.0 (0.4) L/min) and controls (2.3 (0.3) L/min), but less in COPD patients (1.8 (0.3) L/min) (p <0.03). There was no difference in resting D_SMO₂ between COPD and CHF and when adjusting for flow, the D_SMO₂ was higher in both groups compared to controls (COPD: 0.97 (0.23) vs. controls 0.63 (0.24) mM/kPa, p= 0.02; CHF 0.98 (0.11) mM/kPa vs. controls, p= 0.001). The Q̇-adjusted D_SMO₂ was not different in COPD and CHF during KEE (COPD: 1.19 (0.11) vs. CHF: 1.00 (0.18) mM/kPa; p= 0.24) but higher in COPD vs. controls: 0.87 (0.28) mM/kPa (p= 0.02), and only CHF did not increase Q̇-adjusted D_SMO₂ from rest (p= 0.2).

Conclusion: Disease-specific factors may play a role in peripheral exercise limitation in patients with COPD compared with CHF. Thus, low convective O₂ transport to contracting muscle seemed to predominate in COPD, whereas muscle diffusive O₂ transport was unresponsive in CHF.

Hospital admission for COVID-19 pneumonitis - long-term impairment in quality of life and lung function

Background and Aim

Knowledge of long-term consequences of severe COVID-19 pneumonitis is of outmost importance. Our aim was, therefore, to assess the long-term impact on quality of life and lung function in adults hospitalized with severe COVID-19.

Methods

All patients hospitalized with COVID-19 pneumonitis at Copenhagen University Hospital-Hvidovre, Denmark, were invited to participate in the study 4–5 months after discharge. Of the 160 invited 128 responded positively (80%). Medical history and symptoms were assessed, and patients rated impact on quality of life and functional status with EuroQol-5D-5L and Post Covid Functional Scale. Lung function was assessed by dynamic spirometry and measurement of diffusing capacity.

Results

Fatigue, dyspnea, cough and cognitive dysfunction were the most common symptoms. Of 128 patients, 85% had at least one symptom, and 51% reported two or more symptoms. Self-rated Quality of life was impaired assessed by EuroQol 5D-5L, with dimensions ‘Pain or discomfort’ (61%) and ‘Usual activities’ (54%) mostly affected. Functional status was significantly worse than before COVID-19 assessed by Post-COVID Functional Scale. Among lung function parameters, diffusing capacity was most affected, with 45% having diffusing capacity < 80% of predicted.

Conclusion

Fatigue, respiratory symptoms and cognitive symptoms are highly common months after hospitalization for severe COVID-19. Compared to pre-COVID-19, functional status and usual activities continued to be impaired. In line with this, almost half of the patients were found to have impaired diffusing capacity.

Cardiorespiratory responses to high-intensity skeletal muscle metaboreflex activation in chronic obstructive pulmonary disease

BACKGROUND

Augmented skeletal muscle metaboreflex activation may accompany chronic obstructive pulmonary disease (COPD). The maintained metaboreflex control of mean arterial pressure (MAP) that has been reported may reflect limited evaluation using only one moderate bout of static handgrip (HG) and following postexercise ischaemia (PEI).

OBJECTIVE

We tested the hypothesis that cardiovascular and respiratory responses to high-intensity static HG and isolated metaboreflex activation during PEI are augmented in COPD patients.

METHODS

Ten patients with moderate to severe COPD and eight healthy age- and BMI-matched controls performed two-minute static HG at moderate (30% maximal voluntary contraction; MVC) and high (40% MVC) intensity followed by PEI.

RESULTS

Despite similar ratings of perceived exertion, arm muscle mass and strength, COPD patients demonstrated lower MAP responses during both HG intensities compared with controls (time × group interaction, p < .05). Indeed, during high-intensity HG at 40% MVC, peak MAP responses were significantly lower in COPD patients (ΔMAP: COPD 41 ± 9 mmHg vs. controls 56 ± 14 mmHg, p < .05). Notably, no group differences in MAP were observed during PEI (e.g. 40% MVC PEI: ΔMAP COPD 33 ± 9 mmHg vs. controls 33 ± 6 mmHg, p > .05). We found no between-group differences in heart rate, respiratory rate, or estimated minute ventilation during HG or PEI.

CONCLUSION

These results suggest that the pressor response to high-intensity HG is blunted in COPD patients. Moreover, despite inducing a strong cardiovascular and respiratory stimulus, skeletal muscle metaboreflex activation evoked similar responses in COPD patients and controls.

Muscle α-adrenergic responsiveness during exercise and ATP-induced vasodilation in chronic obstructive pulmonary disease patients

Sympathetic vasoconstriction is blunted in exercising muscle (functional sympatholysis) but becomes attenuated with age. We tested the hypothesis that functional sympatholysis is further impaired in chronic obstructive pulmonary disease (COPD) patients. We determined leg blood flow and calculated leg vascular conductance (LVC) during 1) femoral-arterial Tyramine infusion (evokes endogenous norepinephrine release, 1 µmol·min^-1·kg leg mass^-1), 2) one-legged knee extensor exercise with and without Tyramine infusion [10 W and 20% of maximal workload (WL_max)], 3) ATP (0.05 µmol·min^-1·kg leg mass^-1) and Tyramine infusion, and 4) incremental ATP infusions (0.05, 0.3, and 3.0 µmol·min^-1·kg leg mass^-1). We included 10 patients with moderate to severe COPD and 8 age-matched healthy control subjects. Overall, leg blood flow and LVC were lower in COPD patients during exercise ( P < 0.05). Tyramine reduced LVC in both groups at 10-W exercise (COPD: -3 ± 1 ml·min^-1·mmHg^-1 and controls: -3 ± 1 ml·min^-1·mmHg^-1, P < 0.05) and 20% WL_max (COPD: -4 ± 1 ml·min^-1·mmHg^-1 and controls: -3 ± 1 ml·min^-1·mmHg^-1, P < 0.05) with no difference between groups. Incremental ATP infusions induced dose-dependent vasodilation with no difference between groups, and, in addition, the vasoconstrictor response to Tyramine infused together with ATP was not different between groups (COPD: -0.03 ± 0.01 l·min^-1·kg leg mass^-1 vs.

CONTROLS

-0.04 ± 0.01 l·min^-1·kg leg mass^-1, P > 0.05). Compared with age-matched healthy control subjects, the vasodilatory response to ATP is intact in COPD patients and their ability to blunt sympathetic vasoconstriction (functional sympatholysis) as evaluated by intra-arterial Tyramine during exercise or ATP infusion is maintained. NEW & NOTEWORTHY The ability to blunt sympathetic vasoconstriction in exercising muscle and ATP-induced dilation in chronic obstructive pulmonary disease patients remains unexplored. Chronic obstructive pulmonary disease patients demonstrated similar sympathetic vasoconstriction in response to intra-arterial Tyramine during exercise and ATP-induced vasodilation compared with age-matched healthy control subjects.

Leg blood flow is impaired during small muscle mass exercise in patients with COPD

Skeletal muscle blood flow is regulated to match the oxygen demand and dysregulation could contribute to exercise intolerance in patients with chronic obstructive pulmonary disease (COPD). We measured leg hemodynamics and metabolites from vasoactive compounds in muscle interstitial fluid and plasma at rest, during one-legged knee-extensor exercise, and during arterial infusions of sodium nitroprusside (SNP) and acetylcholine (ACh), respectively. Ten patients with moderate to severe COPD and eight age- and sex-matched healthy controls were studied. During knee-extensor exercise (10 W), leg blood flow was lower in the patients compared with the controls (1.82 ± 0.11 vs. 2.36 ± 0.14 l/min, respectively; P < 0.05), which compromised leg oxygen delivery (372 ± 26 vs. 453 ± 32 ml O2/min, respectively; P < 0.05). At rest, plasma endothelin-1 (vasoconstrictor) was higher in the patients with COPD ( P < 0.05) and also tended to be higher during exercise ( P = 0.07), whereas the formation of interstitial prostacyclin (vasodilator) was only increased in the controls. There was no difference between groups in the nitrite/nitrate levels (vasodilator) in plasma or interstitial fluid during exercise. Moreover, patients and controls showed similar vasodilatory capacity in response to both endothelium-independent (SNP) and endothelium-dependent (ACh) stimulation. The results suggest that leg muscle blood flow is impaired during small muscle mass exercise in patients with COPD possibly due to impaired formation of prostacyclin and increased levels of endothelin-1.

NEW & NOTEWORTHY This study demonstrates that chronic obstructive pulmonary disease (COPD) is associated with a reduced blood flow to skeletal muscle during small muscle mass exercise. In contrast to healthy individuals, interstitial prostacyclin levels did not increase during exercise and plasma endothelin-1 levels were higher in the patients with COPD.

Effect of endurance versus resistance training on quadriceps muscle dysfunction in COPD: a pilot study

Introduction

Exercise is an important countermeasure to limb muscle dysfunction in COPD. The two major training modalities in COPD rehabilitation, endurance training (ET) and resistance training (RT), may both be efficient in improving muscle strength, exercise capacity, and health-related quality of life, but the effects on quadriceps muscle characteristics have not been thoroughly described.

Methods

Thirty COPD patients (forced expiratory volume in 1 second: 56% of predicted, standard deviation [SD] 14) were randomized to 8 weeks of ET or RT. Vastus lateralis muscle biopsies were obtained before and after the training intervention to assess muscle morphology and metabolic and angiogenic factors. Symptom burden, exercise capacity (6-minute walking and cycle ergometer tests), and vascular function were also assessed.

Results

Both training modalities improved symptom burden and exercise capacity with no difference between the two groups. The mean (SD) proportion of glycolytic type IIa muscle fibers was reduced after ET (from 48% [SD 11] to 42% [SD 10], P<0.05), whereas there was no significant change in muscle fiber distribution with RT. There was no effect of either training modality on muscle capillarization, angiogenic factors, or vascular function. After ET the muscle protein content of phosphofructokinase was reduced (P<0.05) and the citrate synthase content tended increase (P=0.08) but no change was observed after RT.

Conclusion

Although both ET and RT improve symptoms and exercise capacity, ET induces a more oxidative quadriceps muscle phenotype, counteracting muscle dysfunction in COPD.

Capillary growth, ultrastructure remodelling and exercise training in skeletal muscle of essential hypertensive patients

AIM

The aim was to elucidate whether essential hypertension is associated with altered capillary morphology and density and to what extent exercise training can normalize these parameters.

METHODS

To investigate angiogenesis and capillary morphology in essential hypertension, muscle biopsies were obtained from m. vastus lateralis in subjects with essential hypertension (n = 10) and normotensive controls (n = 11) before and after 8 weeks of aerobic exercise training. Morphometry was performed after transmission electron microscopy, and protein levels of several angioregulatory factors were determined.

RESULTS

At baseline, capillary density and capillary-to-fibre ratio were not different between the two groups. However, the hypertensive subjects had 9% lower capillary area (12.7 ± 0.4 vs. 13.9 ± 0.2 μm(2)) and tended to have thicker capillary basement membranes (399 ± 16 vs. 358 ± 13 nm; P = 0.094) than controls. Protein expression of vascular endothelial growth factor (VEGF), VEGF receptor-2 and thrombospondin-1 were similar in normotensive and hypertensive subjects, but tissue inhibitor of matrix metalloproteinase was 69% lower in the hypertensive group. After training, angiogenesis was evident by 15% increased capillary-to-fibre ratio in the hypertensive subjects only. Capillary area and capillary lumen area were increased by 7 and 15% in the hypertensive patients, whereas capillary basement membrane thickness was decreased by 17% (P < 0.05). VEGF expression after training was increased in both groups, whereas VEGF receptor-2 was decreased by 25% in the hypertensive patients(P < 0.05).

CONCLUSION

Essential hypertension is associated with decreased lumen area and a tendency for increased basement membrane thickening in capillaries of skeletal muscle. Exercise training may improve the diffusion conditions in essential hypertension by altering capillary structure and capillary number.

Exercise training modulates functional sympatholysis and α-adrenergic vasoconstrictor responsiveness in hypertensive and normotensive individuals

Essential hypertension is linked to an increased sympathetic vasoconstrictor activity and reduced tissue perfusion. We investigated the role of exercise training on functional sympatholysis and postjunctional α-adrenergic responsiveness in individuals with essential hypertension. Leg haemodynamics were measured before and after 8 weeks of aerobic training (3-4 times per week) in eight hypertensive (47 ± 2 years) and eight normotensive untrained individuals (46 ± 1 years) during arterial tyramine infusion, arterial ATP infusion and/or one-legged knee extensions. Before training, exercise hyperaemia and leg vascular conductance (LVC) were lower in the hypertensive individuals (P < 0.05) and tyramine lowered exercise hyperaemia and LVC in both groups (P < 0.05). Training lowered blood pressure in the hypertensive individuals (P < 0.05) and exercise hyperaemia was similar to the normotensive individuals in the trained state. After training, tyramine did not reduce exercise hyperaemia or LVC in either group. When tyramine was infused at rest, the reduction in blood flow and LVC was similar between groups, but exercise training lowered the magnitude of the reduction in blood flow and LVC (P < 0.05). There was no difference in the vasodilatory response to infused ATP or in muscle P2Y2 receptor content between the groups before and after training. However, training lowered the vasodilatory response to ATP and increased skeletal muscle P2Y2 receptor content in both groups (P < 0.05). These results demonstrate that exercise training improves functional sympatholysis and reduces postjunctional α-adrenergic responsiveness in both normo- and hypertensive individuals. The ability for functional sympatholysis and the vasodilator and sympatholytic effect of intravascular ATP appear not to be altered in essential hypertension.

Adenosine diphosphate reduces infarct size and improves porcine heart function after myocardial infarct

Acute myocardial infarction continues to be a major cause of morbidity and mortality. Timely reperfusion can substantially improve outcomes and the administration of cardioprotective substances during reperfusion is therefore highly attractive. Adenosine diphosphate (ADP) and uridine-5-triphoshate (UTP) are both released during myocardial ischemia, influencing hemodynamics. Both mediate the release of tissue plasminogen activator (t-PA), which can reduce infarct size (IS). The objective of this study was to investigate whether exogenous ADP and UTP administration during reperfusion could reduce myocardial IS and whether this correlated to t-PA release or improvements in hemodynamic responses. Hemodynamic variables and t-PA were measured in 22 pigs before, during, and after 45 min of left anterior coronary artery occlusion. During reperfusion, the pigs were randomized to 240 min of intracoronary infusion of ADP, UTP, or control (no intervention). Ischemic area compared to the area at risk [IS/AAR] was measured. [IS/AAR] was 52 ± 11% in the control animals. ADP decreased [IS/AAR] by 19% (P < 0.05), while UTP increased [IS/AAR] by 15% (P < 0.05). Cardiac output (CO) increased from 3.4 to 3.5 L/min (P < 0.05) and mean arterial pressure (MAP) decreased from 87 to 73 mmHg in the ADP group (P < 0.05). t-PA concentration increased in the ADP and UTP group from 2.0 ng/mL to 2.5 and 2.4 ng/mL, respectively (P < 0.05) but remained unchanged in the control group. In conclusion, intracoronary ADP infusion during reperfusion reduces IS by ∼20% independently from systemic release of t-PA. ADP-induced reduction in both preload and afterload could account for the beneficial myocardial effect.

Two weeks of muscle immobilization impairs functional sympatholysis but increases exercise hyperemia and the vasodilatory responsiveness to infused ATP

During exercise, contracting muscles can override sympathetic vasoconstrictor activity (functional sympatholysis). ATP and adenosine have been proposed to play a role in skeletal muscle blood flow regulation. However, little is known about the role of muscle training status on functional sympatholysis and ATP- and adenosine-induced vasodilation. Eight male subjects (22 ± 2 yr, Vo(2max): 49 ± 2 ml O(2)·min(-1)·kg(-1)) were studied before and after 5 wk of one-legged knee-extensor training (3-4 times/wk) and 2 wk of immobilization of the other leg. Leg hemodynamics were measured at rest, during exercise (24 ± 4 watts), and during arterial ATP (0.94 ± 0.03 μmol/min) and adenosine (5.61 ± 0.03 μmol/min) infusion with and without coinfusion of tyramine (11.11 μmol/min). During exercise, leg blood flow (LBF) was lower in the trained leg (2.5 ± 0.1 l/min) compared with the control leg (2.6 ± 0.2 l/min; P < 0.05), and it was higher in the immobilized leg (2.9 ± 0.2 l/min; P < 0.05). Tyramine infusion lowers LBF similarly at rest, but, when tyramine was infused during exercise, LBF was blunted in the immobilized leg (2.5 ± 0.2 l/min; P < 0.05), whereas it was unchanged in the control and trained leg. Mean arterial pressure was lower during exercise with the trained leg compared with the immobilized leg (P < 0.05), and leg vascular conductance was similar. During ATP infusion, the LBF response was higher after immobilization (3.9 ± 0.3 and 4.5 ± 0.6 l/min in the control and immobilized leg, respectively; P < 0.05), whereas it did not change after training. When tyramine was coinfused with ATP, LBF was reduced in the immobilized leg (P < 0.05) but remained similar in the control and trained leg. Training increased skeletal muscle P2Y2 receptor content (P < 0.05), whereas it did not change with immobilization. These results suggest that muscle inactivity impairs functional sympatholysis and that the magnitude of hyperemia and blood pressure response to exercise is dependent on the training status of the muscle. Immobilization also increases the vasodilatory response to infused ATP.

Role of nitric oxide and prostanoids in the regulation of leg blood flow and blood pressure in humans with essential hypertension: effect of high-intensity aerobic training

We examined the role of nitric oxide (NO) and prostanoids in the regulation of leg blood flow and systemic blood pressure before and after 8 weeks of aerobic high-intensity training in individuals with essential hypertension (n = 10) and matched healthy control subjects (n = 11). Hypertensive subjects were found to have a lower (P < 0.05) blood flow to the exercising leg than normotensive subjects (30 W: 2.92 ± 0.16 vs. 3.39 ± 0.37 l min(−1)). Despite the lower exercise hyperaemia, pharmacological inhibition of the NO and prostanoid systems reduced leg blood flow to a similar extent during exercise in the two groups and vascular relaxation to the NO-dependent vasodilator acetylcholine was also similar between groups. High-intensity aerobic training lowered (P < 0.05) resting systolic (∼9 mmHg) and diastolic (∼12 mmHg) blood pressure in subjects with essential hypertension, but this effect of training was abolished when the NO and prostanoid systems were inhibited. Skeletal muscle vascular endothelial NO synthase uncoupling, expression and phosphorylation status were similar in the two groups before and after training. These data demonstrate that a reduction in exercise hyperaemia in hypertensive subjects is not associated with a reduced capacity of the NO and prostanoid systems to induce vasodilatation or with altered acetylcholine-induced response. However, our data suggest that the observed reduction in blood pressure is related to a training-induced change in the tonic effect of NO and/or prostanoids on vascular tone.

Purinergic receptors expressed in human skeletal muscle fibres

Purinergic receptors are present in most tissues and thought to be involved in various signalling pathways, including neural signalling, cell metabolism and local regulation of the microcirculation in skeletal muscles. The present study aims to determine the distribution and intracellular content of purinergic receptors in skeletal muscle fibres in patients with type 2 diabetes and age-matched controls. Muscle biopsies from vastus lateralis were obtained from six type 2 diabetic patients and seven age-matched controls. Purinergic receptors were analysed using light and confocal microscopy in immunolabelled transverse sections of muscle biopsies. The receptors P2Y(4), P2Y(11) and likely P2X(1) were present intracellularly or in the plasma membrane of muscle fibres and were thus selected for further detailed morphological analysis. P2X(1) receptors were expressed in intracellular vesicles and sarcolemma. P2Y(4) receptors were present in sarcolemma. P2Y(11) receptors were abundantly and diffusely expressed intracellularly and were more explicitly expressed in type I than in type II fibres, whereas P2X(1) and P2Y(4) showed no fibre-type specificity. Both diabetic patients and healthy controls showed similar distribution of receptors. The current study demonstrates that purinergic receptors are located intracellularly in human skeletal muscle fibres. The similar cellular localization of receptors in healthy and diabetic subjects suggests that diabetes is not associated with an altered distribution of purinergic receptors in skeletal muscle fibres. We speculate that the intracellular localization of purinergic receptors may reflect a role in regulation of muscle metabolism; further studies are nevertheless needed to determine the function of the purinergic system in skeletal muscle cells.

Functional sympatholysis during exercise in patients with type 2 diabetes with intact response to acetylcholine

OBJECTIVE

Sympathetic vasoconstriction is blunted in contracting human skeletal muscles (functional sympatholysis). In young subjects, infusion of adenosine and ATP increases blood flow, and the latter compound also attenuates α-adrenergic vasoconstriction. In patients with type 2 diabetes and age-matched healthy subjects, we tested 1) the sympatholytic capacity during one-legged exercise, 2) the vasodilatory capacity of adenosine and ATP, and 3) the ability to blunt α-adrenergic vasoconstriction during ATP infusion.

RESEARCH DESIGN AND METHODS

In 10 control subjects and 10 patients with diabetes and normal endothelial function, determined by leg blood flow (LBF) response to acetylcholine infusion, we measured LBF and venous NA, with and without tyramine-induced sympathetic vasoconstriction, during adenosine-, ATP-, and exercise-induced hyperemia.

RESULTS

LBF during acetylcholine did not differ significantly. LBF increased ninefold during exercise and during adenosine- and ATP-induced hyperemia. Infusion of tyramine during exercise did not reduce LBF in either the control or the patient group. During combined ATP and tyramine infusions, LBF decreased by 30% in both groups. Adenosine had no sympatholytic effect.

CONCLUSIONS

In patients with type 2 diabetes and normal endothelial function, functional sympatholysis was intact during moderate exercise. The vasodilatory response for adenosine and ATP did not differ between the patients with diabetes and the control subjects; however, the vasodilatory effect of adenosine and ATP and the sympatholytic effect of ATP seem to decline with age.

Local release of ATP into the arterial inflow and venous drainage of human skeletal muscle: insight from ATP determination with the intravascular microdialysis technique

Intraluminal ATP could play an important role in the local regulation of skeletal muscle blood flow, but the stimuli that cause ATP release and the levels of plasma ATP in vessels supplying and draining human skeletal muscle remain unclear. To gain insight into the mechanisms by which ATP is released into plasma, we measured plasma [ATP] with the intravascular microdialysis technique at rest and during dynamic exercise (normoxia and hypoxia), passive exercise, thigh compressions and arterial ATP, tyramine and ACh infusion in a total of 16 healthy young men. Femoral arterial and venous [ATP] values were 109 ± 34 and 147 ± 45 nmol l(−1) at rest and increased to 363 ± 83 and 560 ± 111 nmol l(−1), respectively, during exercise (P < 0.05), whereas these values did not increase when exercise was performed with the other leg. Hypoxia increased venous plasma [ATP] at rest compared to normoxia (P < 0.05), but not during exercise. Arterial ATP infusion (≤1.8 μmol min(−1) increased arterial plasma [ATP] from 74 ± 17 to 486 ± 82 nmol l(−1) (P < 0.05), whereas it remained unchanged in the femoral vein at ∼150 nmol l(−1). Both arterial and venous plasma [ATP] decreased during acetylcholine infusion (P < 0.05). Rhythmic thigh compressions increased arterial and venous plasma [ATP] compared to baseline conditions, whereas these values did not change during passive exercise or tyramine infusion. These results demonstrate that ATP is released locally into arterial and venous plasma during exercise and during hypoxia at rest. Compression of the vascular system could contribute to the increase during exercise whereas there appears to be little ATP release in response to increased blood flow, vascular stretch or sympathetic ATP release. Furthermore, the half-life of arterially infused ATP is <1 s.

Interstitial and plasma adenosine stimulate nitric oxide and prostacyclin formation in human skeletal muscle

One major unresolved issue in muscle blood flow regulation is that of the role of circulating versus interstitial vasodilatory compounds. The present study determined adenosine-induced formation of NO and prostacyclin in the human muscle interstitium versus in femoral venous plasma to elucidate the interaction and importance of these vasodilators in the 2 compartments. To this end, we performed experiments on humans using microdialysis technique in skeletal muscle tissue, as well as the femoral vein, combined with experiments on cultures of microvascular endothelial versus skeletal muscle cells. In young healthy humans, microdialysate was collected at rest, during arterial infusion of adenosine, and during interstitial infusion of adenosine through microdialysis probes inserted into musculus vastus lateralis. Muscle interstitial NO and prostacyclin increased with arterial and interstitial infusion of adenosine. The addition of adenosine to skeletal muscle cells increased NO formation (fluorochrome 4-amino-5-methylamino-2',7-difluorescein fluorescence), whereas prostacyclin levels remained unchanged. The addition of adenosine to microvascular endothelial cells induced an increase in NO and prostacyclin levels. These findings provide novel insight into the role of adenosine in skeletal muscle blood flow regulation and vascular function by revealing that both interstitial and plasma adenosine have a stimulatory effect on NO and prostacyclin formation. In addition, both skeletal muscle and microvascular endothelial cells are potential mediators of adenosine-induced formation of NO in vivo, whereas only endothelial cells appear to play a role in adenosine-induced formation of prostacyclin.

Attenuated purinergic receptor function in patients with type 2 diabetes

OBJECTIVE

Extracellular nucleotides and nucleosides are involved in regulation of skeletal muscle blood flow. Diabetes induces cardiovascular dysregulation, but the extent to which the vasodilatatory capacity of nucleotides and nucleosides is affected in type 2 diabetes is unknown. The present study investigated 1) the vasodilatatory effect of ATP, uridine-triphosphate (UTP), and adenosine (ADO) and 2) the expression and distribution of P2Y(2) and P2X(1) receptors in skeletal muscles of diabetic subjects.

RESEARCH DESIGN AND METHODS

In 10 diabetic patients and 10 age-matched control subjects, leg blood flow (LBF) was measured during intrafemoral artery infusion of ATP, UTP, and ADO, eliciting a blood flow equal to knee-extensor exercise at 12 W (approximately 2.6 l/min).

RESULTS

The vasodilatatory effect of the purinergic system was 50% lower in the diabetic group as exemplified by an LBF increase of 274 +/- 37 vs. 143 +/- 26 ml/micromol ATP x kg, 494 +/- 80 vs. 234 +/- 39 ml/micromol UTP x kg, and 14.9 +/- 2.7 vs. 7.5 +/- 0.6 ml/micromol ADO x kg in control and diabetic subjects, respectively, thus making the vasodilator potency as follows: UTP control subjects (100) > ATP control subjects (55) > UTP diabetic subjects (47) > ATP diabetic subjects (29) > ADO control subjects (3) > ADO diabetic subjects (1.5). The distribution and mRNA expression of receptors were similar in the two groups.

CONCLUSIONS

The vasodilatatory effect of the purinergic system is severely reduced in type 2 diabetic patients. The potency of nucleotides varies with the following rank order: UTP > ATP > ADO. This is not due to alterations in receptor distribution and mRNA expression, but may be due to differences in receptor sensitivity.

Adenosine contributes to blood flow regulation in the exercising human leg by increasing prostaglandin and nitric oxide formation

Adenosine can induce vasodilation in skeletal muscle, but to what extent adenosine exerts its effect via formation of other vasodilators and whether there is redundancy between adenosine and other vasodilators remain unclear. We tested the hypothesis that adenosine, prostaglandins, and NO act in synergy to regulate skeletal muscle hyperemia by determining the following: (1) the effect of adenosine receptor blockade on skeletal muscle exercise hyperemia with and without simultaneous inhibition of prostaglandins (indomethacin; 0.8 to 1.8 mg/min) and NO (N(G)-mono-methyl-l-arginine; 29 to 52 mg/min); (2) whether adenosine-induced vasodilation is mediated via formation of prostaglandins and/or NO; and (3) the femoral arterial and venous plasma adenosine concentrations during leg exercise with the microdialysis technique in a total of 24 healthy, male subjects. Inhibition of adenosine receptors (theophylline; 399+/-9 mg, mean +/- SEM) or combined inhibition of prostaglandins and NO formation inhibited the exercise-induced increase in leg blood flow by 14+/-1% and 29+/-2% (P<0.05), respectively, but combined inhibition of prostaglandins, NO, and adenosine receptors did not result in an additive reduction of leg blood flow (31+/-5%). Femoral arterial infusion of adenosine increased leg blood flow from approximately 0.3 to approximately 2.5 L/min. Inhibition of prostaglandins or NO, or prostaglandins and NO combined, inhibited the adenosine-induced increase in leg blood flow by 51+/-3%, 39+/-8%, and 66+/-8%, respectively (P<0.05). Arterial and venous plasma adenosine concentrations were similar at rest and during exercise. These results suggest that adenosine contributes to the regulation of skeletal muscle blood flow by stimulating prostaglandin and NO synthesis.