Effect of extraluminal ATP application on vascular tone and blood flow in skeletal muscle: implications for exercise hyperemia

Diazoxide and moderate-intensity exercise improve skeletal muscle function by decreasing oxidants and enhancing antioxidant defenses in hypertensive male rats

High sodium intake is decisive in the incidence increase and prevalence of hypertension, which has an impact on skeletal muscle functionality. Diazoxide is an antihypertensive agent that inhibits insulin secretion and is an opener of KATP channels (adosine triphosphate sensitive potasium channels). For this reason, it is hypothesized that moderate-intensity exercise and diazoxide improve skeletal muscle function by reducing the oxidants in hypertensive rats. Male Wistar rats were assigned into eight groups: control (CTRL), diazoxide (DZX), exercise (EX), exercise + diazoxide (EX + DZX), hypertension (HTN), hypertension + diazoxide (HTN + DZX), hypertension + exercise (HTN + EX), and hypertension + exercise + diazoxide (HTN + EX + DZX). To induce hypertension, the rats received 8% NaCl dissolved in water orally for 30 days; in the following 8 weeks, 4% NaCl was supplied to maintain the pathology. The treatment with physical exercise of moderate intensity lasted 8 weeks. The administration dose of diazoxide was 35 mg/kg intraperitoneally for 14 days. Tension recording was performed on the extensor digitorum longus and the soleus muscle. Muscle homogenates were used to measure oxidants using fluorescent probe and the activity of antioxidant systems. Diazoxide and moderate-intensity exercise reduced oxidants and increased antioxidant defenses.

Probenecid affects muscle Ca2+ homeostasis and contraction independently from pannexin channel block

Tight control of skeletal muscle contractile activation is secured by the excitation-contraction (EC) coupling protein complex, a molecular machinery allowing the plasma membrane voltage to control the activity of the ryanodine receptor Ca2+ release channel in the sarcoplasmic reticulum (SR) membrane. This machinery has been shown to be intimately linked to the plasma membrane protein pannexin-1 (Panx1). We investigated whether the prescription drug probenecid, a widely used Panx1 blocker, affects Ca2+ signaling, EC coupling, and muscle force. The effect of probenecid was tested on membrane current, resting Ca2+, and SR Ca2+ release in isolated mouse muscle fibers, using a combination of whole-cell voltage-clamp and Ca2+ imaging, and on electrically triggered contraction of isolated muscles. Probenecid (1 mM) induces SR Ca2+ leak at rest and reduces peak voltage-activated SR Ca2+ release and contractile force by 40%. Carbenoxolone, another Panx1 blocker, also reduces Ca2+ release, but neither a Panx1 channel inhibitory peptide nor a purinergic antagonist affected Ca2+ release, suggesting that probenecid and carbenoxolone do not act through inhibition of Panx1-mediated ATP release and consequently altered purinergic signaling. Probenecid may act by altering Panx1 interaction with the EC coupling machinery, yet the implication of another molecular target cannot be excluded. Since probenecid has been used both in the clinic and as a masking agent for doping in sports, these results should encourage evaluation of possible effects on muscle function in treated individuals. In addition, they also raise the question of whether probenecid-induced altered Ca2+ homeostasis may be shared by other tissues.

Thermal dysregulation in patients with multiple sclerosis during SARS-CoV-2 infection. The potential therapeutic role of exercise

Thermoregulation is a homeostatic mechanism that is disrupted in some neurological diseases. Patients with multiple sclerosis (MS) are susceptible to increases in body temperature, especially with more severe neurological signs. This condition can become intolerable when these patients suffer febrile infections such as coronavirus disease-2019 (COVID-19). We review the mechanisms of hyperthermia in patients with MS, and they may encounter when infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Finally, the thermoregulatory role and relevant adaptation to regular physical exercise are summarized.

Purinergic signaling as a new mechanism underlying physical exercise benefits: a narrative review

In the last years, it has become evident that both acute and chronic physical exercise trigger responses/adaptations in the purinergic signaling and these adaptations can be considered one important mechanism related to the exercise benefits for health improvement. Purinergic system is composed of enzymes (ectonucleotidases), receptors (P1 and P2 families), and molecules (ATP, ADP, adenosine) that are able to activate these receptors. These components are widely distributed in almost all cell types, and they respond/act in a specific manner depending on the exercise types and/or intensities as well as the cell type (organ/tissue analyzed). For example, while acute intense exercise can be associated with tissue damage, inflammation, and platelet aggregation, chronic exercise exerts anti-inflammatory and anti-aggregant effects, promoting health and/or treating diseases. All of these effects are dependent on the purinergic signaling. Thus, this review was designed to cover the aspects related to the relationship between physical exercise and purinergic signaling, with emphasis on the modulation of ectonucleotidases and receptors. Here, we discuss the impact of different exercise protocols as well as the differences between acute and chronic effects of exercise on the extracellular signaling exerted by purinergic system components. We also reinforce the concept that purinergic signaling must be understood/considered as a mechanism by which exercise exerts its effects.

Development and validation of a novel microfluidic device for the manipulation of skeletal muscle microvascular blood flow in vivo

OBJECTIVE

To develop and validate a novel liquid microfluidic approach to deliver drugs to microscale regions of tissue while simultaneously allowing for visualization and quantification of microvascular blood flow.

METHODS

Microfluidic devices were fabricated using soft lithographic techniques, molded in polydimethylsiloxane, and bound to a coverslip with a 600 × 300 μm micro-outlet. Sprague-Dawley rats, anesthetized with pentobarbital, were instrumented to monitor systemic parameters. The extensor digitorum longus muscle was dissected, externalized, and reflected across the device mounted on the stage of an inverted microscope. Doses (10^-8 to 10^-3  M) of adenosine triphosphate (ATP), acetylcholine, and phenylephrine (PE) were administered to the muscle via perfusion through the device. Microvascular blood flow directly overlying the micro-outlet was recorded at multiple focal depths. Red blood cell (RBC) velocity, supply rate, and hematocrit were measured from recordings.

RESULTS

ATP significantly increased RBC velocity and supply rate. Increasing concentrations of PE caused a decrease in RBC velocity and supply rate. Perfusion changes were restricted to areas directly overlying the micro-outlet and within 500 μm.

CONCLUSIONS

This novel microfluidic device allows for a controlled delivery of dissolved substances to constrained regions of microvasculature while simultaneously allowing for visualization and measurement of blood flow within discrete vessels and networks.

The effect of swimming training on adrenomedullin levels, oxidative stress variables, and gastrocnemius muscle contractile properties in hypertensive rats

Introduction/Aim: Regular exercise may have beneficial effects on high blood-pressure, as shown in different types of experimental hypertension models in rats. The present study aims to investigate the effects of 6-week swimming training on blood pressure, oxidative stress variables of selected tissues, serum adrenomedullin (ADM) levels, and in situ muscle contraction in rats with hypertension induced by Nω-nitro-L-arginine methyl ester hydrochloride (L-NAME), an inhibitor of endothelial nitric oxide synthases (eNOs). Materials and Methods: Twenty-six male Sprague Dawley, 8 weeks of age, rats were randomly divided into four groups: (I) normotensive (C), (II) normotensive + exercise (E), (III) hypertensive (L), and (IV) hypertensive + exercise (LE). Hypertension was induced by the oral administration of L-NAME (60 mg/kg) for 6 weeks. Exercise was performed 5 times (1-h each) per week for 6 weeks. At the end of the experiment, blood and tissue samples (the gastrocnemius muscle, heart, kidney, and thoracic aorta) were collected following contractile properties of the gastrocnemius muscle in situ weredetermined. In the collected tissues, oxidative stress (e.g., lipid oxidation and antioxidant enzyme activity) and serum ADM levels were measured. 6-week L-NAME administration per se (Group L) led to a significant increase in systolic and diastolic blood pressure compared to other groups.  Results: Importantly, 6-week exercise caused a protective effect of high blood pressure in the rats received L-NAME (Group LE). The level of ADM was lower in the rats received L-NAME than that of the control group. L-NAME increased lipid peroxidation in the thoracic aorta and decreased superoxide dismutase in the heart, kidney and muscle, and decreased catalase and glutathione in the heart. However, the exercise intervention did not have protective effect on the L-NAME-mediated oxidative damage in the collected tissues.  Conclusion: In conclusion, 6-week exercise intervention rescued rats from high blood pressure, but did not have ameliorative effect on the decreased ADM levels.

Prostaglandin contribution to postexercise hyperemia is dependent on tissue oxygenation during rhythmic and isometric contractions

The role of prostaglandins (PGs) in exercise hyperemia is controversial. We tested their contributions in moderate intensity forearm exercise, whether their release is oxygen (O₂)‐dependent or affected by aging. A total of 12 young (21 ± 1 years) and 11 older (66 ± 2 years) recreationally active men performed rhythmic and isometric handgrip contractions at 60% maximum voluntary contraction for 3 min during air breathing after placebo, after cyclooxygenase (COX) inhibition with aspirin, while breathing 40% O₂ and during their combination (aspirin + 40% O₂). Forearm blood flow (FBF) was recorded with venous occlusion plethysmography (forearm vascular conductance (FVC): FBF/mean arterial pressure). Venous efflux of PGI₂ and PGE₂ were assessed by immunoassay. Postcontraction increases in FVC were similar for rhythmic and isometric contractions in young and older men, and accompanied by similar increases in efflux of PGI₂ and PGE₂. Aspirin attenuated the efflux of PGI₂ by 75%–85%, PGE₂ by 50%–70%, (p < .05 within group; p > .05 young versus. older), and postcontraction increases in FVC by 22%–27% and 17%–21% in young and older men, respectively (p < .05 within group and young versus. older). In both age groups, 40% O₂ and aspirin + 40% O₂ caused similar inhibition of the increases in FVC and efflux of PGs as aspirin alone (p < .05 within group). These results indicate that PGs make substantial contributions to the postcontraction hyperemia of rhythmic and isometric contractions at moderate intensities in recreationally active young and older men. Given PGI₂ is mainly released by endothelium and PGE₂ by muscle fibers, we propose PG generation is dependent on the contraction‐induced falls in O₂ at these sites.

Insulin-induced membrane permeability to glucose in human muscles at rest and following exercise

KEY POINTS

Increased insulin action is an important component of the health benefits of exercise, but its regulation is complex and not fully elucidated. Previous studies of insulin-stimulated GLUT4 translocation to the skeletal muscle membrane found insufficient increases to explain the increases in glucose uptake. By determination of leg glucose uptake and interstitial muscle glucose concentration, insulin-induced muscle membrane permeability to glucose was calculated 4 h after one-legged knee-extensor exercise during a submaximal euglycaemic-hyperinsulinaemic clamp. It was found that during submaximal insulin stimulation, muscle membrane permeability to glucose in humans increases twice as much in previously exercised vs. rested muscle and outstrips the supply of glucose, which then becomes limiting for glucose uptake. This methodology can now be employed to determine muscle membrane permeability to glucose in people with diabetes, who have reduced insulin action, and in principle can also be used to determine membrane permeability to other substrates or metabolites.

ABSTRACT

Increased insulin action is an important component of the health benefits of exercise, but the regulation of insulin action in vivo is complex and not fully elucidated. Previously determined increases in skeletal muscle insulin-stimulated GLUT4 translocation are inconsistent and mostly cannot explain the increases in insulin action in humans. Here we used leg glucose uptake (LGU) and interstitial muscle glucose concentration to calculate insulin-induced muscle membrane permeability to glucose, a variable not previously possible to quantify in humans. Muscle membrane permeability to glucose, measured 4 h after one-legged knee-extensor exercise, increased ∼17-fold during a submaximal euglycaemic-hyperinsulinaemic clamp in rested muscle (R) and ∼36-fold in exercised muscle (EX). Femoral arterial infusion of N^G -monomethyl l-arginine acetate or ATP decreased and increased, respectively, leg blood flow (LBF) in both legs but did not affect membrane glucose permeability. Decreasing LBF reduced interstitial glucose concentrations to ∼2 mM in the exercised but only to ∼3.5 mM in non-exercised muscle and abrogated the augmented effect of insulin on LGU in the EX leg. Increasing LBF by ATP infusion increased LGU in both legs with uptake higher in the EX leg. We conclude that it is possible to measure functional muscle membrane permeability to glucose in humans and it increases twice as much in exercised vs. rested muscle during submaximal insulin stimulation. We also show that muscle perfusion is an important regulator of muscle glucose uptake when membrane permeability to glucose is high and we show that the capillary wall can be a significant barrier for glucose transport.

Contribution of prostaglandins to exercise hyperaemia: workload, ethnicity and sex matter!

The contribution of prostaglandins (PGs) to exercise hyperaemia is controversial. In this review, we argue this is partly explained by differences in exercise intensity between studies. The effects of cyclooxygenase (COX) inhibition and PG assays indicate that PGs contribute more at moderate to heavy than at light workloads and are mainly released by low tissue O₂ . But, the release and actions of PGs also depend on other O₂ -dependent dilators including ATP, adenosine and NO. K⁺ may inhibit the action of PGs and other mediators by causing hyperpolarization, but contributes to the hyperaemia. Thus, at lighter loads, the influence of PGs may be blunted by K⁺ , while COX inhibition leads to compensatory increases in other O₂ -dependent dilators. In addition, we show that other sources of variability are sex and ethnicity. Our findings indicate that exercise hyperaemia following rhythmic contractions at 60% maximum voluntary contraction, is smaller in young black African (BA) men and women than in their white European (WE) counterparts, but larger in men than in women of both ethnicities. We propose the larger absolute force in men causes greater vascular occlusion and accumulation of dilators, while blunted hyperaemia in BAs may reflect lower oxidative capacity and O₂ requirement. Nevertheless, COX inhibition attenuated peak hyperaemia by ∼30% in WE, BA men and WE women, indicating PGs make a substantial contribution in all three groups. There was no effect in BA women. Lack of PG involvement may provide early evidence of endothelial dysfunction, consistent in BA women with their greater risk of cardiovascular disease.

Neuropeptide Y1 and alpha-1 adrenergic receptor-mediated decreases in functional vasodilation in gluteus maximus microvascular networks of prediabetic mice

Prediabetes is associated with impaired contraction‐evoked dilation of skeletal muscle arterioles, which may be due to increased sympathetic activity accompanying this early stage of diabetes disease. Herein, we sought to determine whether blunted contraction‐evoked vasodilation resulted from enhanced sympathetic neuropeptide Y1 receptor (Y1R) and alpha‐1 adrenergic receptor (α1R) activation. Using intravital video microscopy, second‐, third‐, and fourth‐order (2A, 3A, and 4A) arteriolar diameters were measured before and following electrical field stimulation of the gluteus maximus muscle (GM) in prediabetic (PD, Pound Mouse) and control (CTRL, c57bl6, CTRL) mice. Baseline diameter was similar between groups; however, single tetanic contraction (100 Hz; 400 and 800 msec) and sustained rhythmic contraction (2 and 8 Hz, 30 sec) evoked rapid onset vasodilation and steady‐state vasodilatory responses that were blunted by 50% or greater in PD versus CTRL. Following Y1R and α1R blockade with sympathetic antagonists BIBP3226 and prazosin, contraction‐evoked arteriolar dilation in PD was restored to levels observed in CTRL. Furthermore, arteriolar vasoconstrictor responses to NPY (10⁻¹³–10⁻⁸ mol/L) and PE (10⁻⁹–10⁻⁵ mol/L) were greater in PD versus CTRL at higher concentrations, especially at 3A and 4A. These findings suggest that contraction‐evoked vasodilation in PD is blunted by Y1R and α1R receptor activation throughout skeletal muscle arteriolar networks.

Mechanisms for the control of local tissue blood flow during thermal interventions: influence of temperature-dependent ATP release from human blood and endothelial cells

New Findings

What is the central question of this study?

Skin and muscle blood flow increases with heating and decreases with cooling, but the temperature‐sensitive mechanisms underlying these responses are not fully elucidated.

What is the main finding and its importance?

We found that local tissue hyperaemia was related to elevations in ATP release from erythrocytes. Increasing intravascular ATP augmented skin and tissue perfusion to levels equal or above thermal hyperaemia. ATP release from isolated erythrocytes was altered by heating and cooling. Our findings suggest that erythrocytes are involved in thermal regulation of blood flow via modulation of ATP release.

Local tissue perfusion changes with alterations in temperature during heating and cooling, but the thermosensitivity of the vascular ATP signalling mechanisms for control of blood flow during thermal interventions remains unknown. Here, we tested the hypotheses that the release of the vasodilator mediator ATP from human erythrocytes, but not from endothelial cells or other blood constituents, is sensitive to both increases and reductions in temperature and that increasing intravascular ATP availability with ATP infusion would potentiate thermal hyperaemia in limb tissues. We first measured blood temperature, brachial artery blood flow and plasma [ATP] during passive arm heating and cooling in healthy men and found that they increased by 3.0 ± 1.2°C, 105 ± 25 ml min⁻¹ °C⁻¹ and twofold, respectively, (all P < 0.05) with heating, but decreased or remained unchanged with cooling. In additional men, infusion of ATP into the brachial artery increased skin and deep tissue perfusion to levels equal or above thermal hyperaemia. In isolated erythrocyte samples exposed to different temperatures, ATP release increased 1.9‐fold from 33 to 39°C (P < 0.05) and declined by ∼50% at 20°C (P < 0.05), but no changes were observed in cultured human endothelial cells, plasma or serum samples. In conclusion, increases in plasma [ATP] and skin and deep tissue perfusion with limb heating are associated with elevations in ATP release from erythrocytes, but not from endothelial cells or other blood constituents. Erythrocyte ATP release is also sensitive to temperature reductions, suggesting that erythrocytes may function as thermal sensors and ATP signalling generators for control of tissue perfusion during thermal interventions.

From one generation to the next: a comprehensive account of sympathetic receptor control in branching arteriolar trees

The effect of the sympathetic nervous system on blood flow distribution within skeletal muscle microvasculature is conditional upon regional activation of receptors for sympathetic neurotransmitters. Previous studies have shown that proximal arterioles are largely governed by adrenergic activation, whereas it is speculated that distal branches are controlled by peptidergic and purinergic activation. However, no study has systematically evaluated the activation of adrenergic, peptidergic and purinergic receptors in continuously branching arteriolar trees of an individual skeletal muscle model. Therefore, in the present study, sympathetic agonists were used to evaluate the constriction responses along first to fifth order arterioles in continuously branching arteriolar trees of a in vivo rat gluteus maximus muscle preparation with respect to specific activation of receptors for sympathetic neurotransmitters (α1R, α2R, NPY1R and P2X1R). Constriction responses were incorporated into a mathematical blood flow model to estimate the total flow, resistance and red blood cell flow heterogeneity within a computationally reconstructed gluteus maximus arteriolar network. For the first time, the effects of activating receptors for sympathetic neurotransmitters on vasoconstrictor responses and the ensuing haemodynamics in continuously branching arteriolar trees of skeletal muscle were characterized, where proximal arterioles responded most to α1R and α2R adrenergic activation, whereas distal arterioles responded most to Y1R and P2X1R activation. Total flow and resistance changed with activation of all receptors, whereas red blood cell flow heterogeneity was largely affected by peptidergic and purinergic activation in distal arterioles. The reported data highlight the functional consequences of topologically-dependent sympathetic control and may serve as novel input parameters in computational modelling of network flow.

Cardiovascular Adaptations to Exercise Training

Aerobic exercise training leads to cardiovascular changes that markedly increase aerobic power and lead to improved endurance performance. The functionally most important adaptation is the improvement in maximal cardiac output which is the result of an enlargement in cardiac dimension, improved contractility, and an increase in blood volume, allowing for greater filling of the ventricles and a consequent larger stroke volume. In parallel with the greater maximal cardiac output, the perfusion capacity of the muscle is increased, permitting for greater oxygen delivery. To accommodate the higher aerobic demands and perfusion levels, arteries, arterioles, and capillaries adapt in structure and number. The diameters of the larger conduit and resistance arteries are increased minimizing resistance to flow as the cardiac output is distributed in the body and the wall thickness of the conduit and resistance arteries is reduced, a factor contributing to increased arterial compliance. Endurance training may also induce alterations in the vasodilator capacity, although such adaptations are more pronounced in individuals with reduced vascular function. The microvascular net increases in size within the muscle allowing for an improved capacity for oxygen extraction by the muscle through a greater area for diffusion, a shorter diffusion distance, and a longer mean transit time for the erythrocyte to pass through the smallest blood vessels. The present article addresses the effect of endurance training on systemic and peripheral cardiovascular adaptations with a focus on humans, but also covers animal data.

Effect of PDE5 inhibition on the modulation of sympathetic α-adrenergic vasoconstriction in contracting skeletal muscle of young and older recreationally active humans

Aging is associated with an altered regulation of blood flow to contracting skeletal muscle; however, the precise mechanisms remain unclear. We recently demonstrated that inhibition of cGMP-binding phosphodiesterase 5 (PDE5) increased blood flow to contracting skeletal muscle of older but not young human subjects. Here we examined whether this effect of PDE5 inhibition was related to an improved ability to blunt α-adrenergic vasoconstriction (functional sympatholysis) and/or improved efficacy of local vasodilator pathways. A group of young (23 ± 1 yr) and a group of older (72 ± 1 yr) male subjects performed knee-extensor exercise in a control setting and following intake of the highly selective PDE5 inhibitor sildenafil. During both conditions, exercise was performed without and with arterial tyramine infusion to evoke endogenous norepinephrine release and consequently stimulation of α1- and α2-adrenergic receptors. The level of the sympatholytic compound ATP was measured in venous plasma by use of the microdialysis technique. Sildenafil increased (P < 0.05) vascular conductance during exercise in the older group, but tyramine infusion reduced (P < 0.05) this effect by 38 ± 9%. Similarly, tyramine reduced (P < 0.05) the vasodilation induced by arterial infusion of a nitric oxide (NO) donor by 54 ± 9% in the older group, and this effect was not altered by sildenafil. Venous plasma [ATP] did not change with PDE5 inhibition in the older subjects during exercise. Collectively, PDE5 inhibition in older humans was not associated with an improved ability for functional sympatholysis. An improved efficacy of the NO system may be one mechanism underlying the effect of PDE5 inhibition on exercise hyperemia in aging.

Metabolic control of microvascular networks: oxygen sensing and beyond

The metabolic regulation of blood flow is central to guaranteeing an adequate supply of blood to the tissues and microvascular network stability. It is assumed that vascular reactions to local oxygenation match blood supply to tissue demand via negative-feedback regulation. Low oxygen (O2) levels evoke vasodilatation, and thus an increase of blood flow and oxygen supply, by increasing (decreasing) the release of vasodilatory (vasoconstricting) metabolic signal substances with decreasing partial pressure of O2. This review analyses the principles of metabolic vascular control with a focus on the prevailing feedback regulations. We propose the following hypotheses with respect to vessel diameter adaptation. (1) In addition to O2-dependent signaling, metabolic vascular regulation can be effected by signal substances produced independently of local oxygenation (reflecting the presence of cells) due to the dilution effect. (2) Control of resting vessel tone, and thus perfusion reserve, could be explained by a vascular activity/hypoxia memory. (3) Vasodilator but not vasoconstrictor signaling can prevent shunt perfusion via signal conduction upstream to feeding arterioles. (4) For low perfusion heterogeneity in the steady state, metabolic signaling from the vessel wall or a perivascular tissue sleeve is optimal. (5) For amplification of perfusion during transient increases of tissue demand, red blood cell-derived vasodilators or vasoconstrictors diluted in flowing blood may be relevant.

Regulation of the skeletal muscle blood flow in humans

In humans, skeletal muscle blood flow is regulated by an interaction between several locally formed vasodilators, including NO and prostaglandins. In plasma, ATP is a potent vasodilator that stimulates the formation of NO and prostaglandins and, very importantly, can offset local sympathetic vasoconstriction. Adenosine triphosphate is released into plasma from erythrocytes and endothelial cells, and the plasma concentration increases in both the feed artery and the vein draining the contracting skeletal muscle. Adenosine also stimulates the formation of NO and prostaglandins, but the plasma adenosine concentration does not increase during exercise. In the skeletal muscle interstitium, there is a marked increase in the concentration of ATP and adenosine, and this increase is tightly coupled to the increase in blood flow. The sources of interstitial ATP and adenosine are thought to be skeletal muscle cells and endothelial cells. In the interstitium, both ATP and adenosine stimulate the formation of NO and prostaglandins, but ATP has also been suggested to induce vasoconstriction and stimulate afferent nerves that signal to increase sympathetic nerve activity. Adenosine has been shown to contribute to exercise hyperaemia, whereas the role of ATP remains uncertain due to lack of specific purinergic receptor blockers for human use. The purpose of this review is to address the interaction between vasodilator systems and to discuss the multiple proposed roles of ATP in human skeletal muscle blood flow regulation.

The structural basis of ATP as an allosteric modulator

Adenosine-5'-triphosphate (ATP) is generally regarded as a substrate for energy currency and protein modification. Recent findings uncovered the allosteric function of ATP in cellular signal transduction but little is understood about this critical behavior of ATP. Through extensive analysis of ATP in solution and proteins, we found that the free ATP can exist in the compact and extended conformations in solution, and the two different conformational characteristics may be responsible for ATP to exert distinct biological functions: ATP molecules adopt both compact and extended conformations in the allosteric binding sites but conserve extended conformations in the substrate binding sites. Nudged elastic band simulations unveiled the distinct dynamic processes of ATP binding to the corresponding allosteric and substrate binding sites of uridine monophosphate kinase, and suggested that in solution ATP preferentially binds to the substrate binding sites of proteins. When the ATP molecules occupy the allosteric binding sites, the allosteric trigger from ATP to fuel allosteric communication between allosteric and functional sites is stemmed mainly from the triphosphate part of ATP, with a small number from the adenine part of ATP. Taken together, our results provide overall understanding of ATP allosteric functions responsible for regulation in biological systems.