Pia Thaning

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 (NG-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 ≈0.3 to ≈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.

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

Non‐technical summary  ATP has been proposed to contribute to the local regulation of skeletal muscle blood flow by inducing local vasodilatation. ATP is continuously released, degraded and taken up by cells and the physiological levels are therefore difficult to determine. In the present investigation, we used a novel technique involving microdialysis probes to determine plasma ATP levels in blood vessels supplying and draining resting and contracting human skeletal muscle and to investigate the stimuli for ATP release. The results show a local release of ATP into arterial and venous blood. In addition, we found that thigh compression is a stimulus for ATP release. Furthermore, we find that the half‐life of ATP in arterial blood 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.

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

•  Nitric oxide and prostanoids are substances that dilate the blood vessels. We examined the role of these vasodilators in the regulation of blood flow to contracting muscle and systemic blood pressure before and after a training intervention in subjects with essential hypertension and in healthy controls. •  We show that blood flow to the exercising leg is lower in essential hypertension. •  Surprisingly, this effect on blood flow is not the result of a reduced capacity of the nitric oxide and prostanoid systems to dilate the blood vessels; however, these systems do appear to play a role in the training induced reduction in blood pressure. •  These findings advance our understanding of vascular dysfunction associated with essential hypertension and the mechanisms underlying the blood pressure reducing effect of exercise.

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.

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. Exercise training can improve the ability to override sympathetic vasoconstrictor activity. Here we show that 8 weeks of exercise training reduces the vasoconstrictor response to sympathetic nerve activity (induced by tyramine) and improves the ability to override sympathetic vasoconstrictor activity. We found no difference in the ability to override sympathetic vasoconstrictor activity during exercise, the reduction in blood flow in response to increases in sympathetic nerve activity or the hyperaemic response to infused ATP between normo‐ and hypertensive subjects. These results help us to better understand how exercise training can reduce blood pressure and improve tissue perfusion.

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 P2Y2 and P2X1 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 (∼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/μmol ATP × kg, 494 ± 80 vs. 234 ± 39 ml/μmol UTP × kg, and 14.9 ± 2.7 vs. 7.5 ± 0.6 ml/μmol ADO × 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.

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.

Impaired formation of vasodilators in peripheral tissue in essential hypertension is normalized by exercise training: role of adenosine and prostacyclin.

Objectives: This study examined vascular function and the adenosine system in skeletal muscle of patients diagnosed with essential hypertension (n = 10) and of normotensive (n = 11) patients, before and after aerobic training. Methods: Before and after 8 weeks of aerobic training, the patients completed experiments in which leg blood flow was determined during infusion of adenosine, acetylcholine and during exercise (20 W); muscle interstitial fluid and femoral venous plasma were sampled via microdialysis probes during baseline conditions, exercise and adenosine infusion and resting muscle biopsies were obtained from muscle vastus lateralis. Results: Before training, leg vascular conductance in response to arterial adenosine infusion was similar in the hypertensive and normotensive groups and the individual vascular response was positively correlated to that of both acetylcholine infusion (r 2 = 0.66, P < 0.001) and exercise (r 2 = 0.72, P < 0.001). Before training, interstitial adenosine concentrations during exercise and prostacyclin (PGI2) concentrations after adenosine infusion were lower in the hypertensive than the normotensive group (P < 0.05). In the hypertensive group, training did not affect the vasodilatory response to arterially infused adenosine but increased the formation of interstitial adenosine and PGI2 and lowered blood pressure. In the normotensive group, training resulted in lower (P < 0.05) leg vascular conductance in response to arterial adenosine infusion. Conclusion: The present data suggest that essential hypertension is associated with a reduced capacity to form adenosine and PGI2 at the skeletal muscle microcirculatory level, which is likely to contribute to the increased peripheral vascular resistance related to the disease. This impairment in vasodilator formation can be normalized by aerobic training.

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

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.