ATP overflow in skeletal muscle 1A arterioles

Use of Enzymatic Biosensors to Quantify Endogenous ATP or H2O2 in the Kidney

Enzymatic microelectrode biosensors have been widely used to measure extracellular signaling in real-time. Most of their use has been limited to brain slices and neuronal cell cultures. Recently, this technology has been applied to the whole organs. Advances in sensor design have made possible the measuring of cell signaling in blood-perfused in vivo kidneys. The present protocols list the steps needed to measure ATP and H2O2 signaling in the rat kidney interstitium. Two separate sensor designs are used for the ex vivo and in vivo protocols. Both types of sensor are coated with a thin enzymatic biolayer on top of a permselectivity layer to give fast responding, sensitive and selective biosensors. The permselectivity layer protects the signal from the interferents in biological tissue, and the enzymatic layer utilizes the sequential catalytic reaction of glycerol kinase and glycerol-3-phosphate oxidase in the presence of ATP to produce H2O2. The set of sensors used for the ex vivo studies further detected analyte by oxidation of H2O2 on a platinum/iridium (Pt-Ir) wire electrode. The sensors for the in vivo studies are instead based on the reduction of H2O2 on a mediator coated gold electrode designed for blood-perfused tissue. Final concentration changes are detected by real-time amperometry followed by calibration to known concentrations of analyte. Additionally, the specificity of the amperometric signal can be confirmed by the addition of enzymes such as catalase and apyrase that break down H2O2 and ATP correspondingly. These sensors also rely heavily on accurate calibrations before and after each experiment. The following two protocols establish the study of real-time detection of ATP and H2O2 in kidney tissues, and can be further modified to extend the described method for use in other biological preparations or whole organs.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

ATP metabolism in skeletal muscle arterioles

The purpose of this study was to investigate the metabolism of Adenosine triphosphate (ATP) in skeletal muscle resistance arterioles and to determine whether this metabolism is altered during the rapid growth phase of the rat. We attempted to quantify ATP metabolism in gastrocnemius first‐order arterioles from 8‐, 10‐, and 12‐week‐old rats. We measured ATP metabolism using an ATPase/GTPase assay with whole vessel segments as well as using a real‐time adenosine biosensor following electric field stimulation. Our first method of measuring ATP metabolism allowed us to measure the amount of free phosphate produced with ATP as a substrate. When ecto‐nucleotidase activity was inhibited by ARL67156, pyridoxal phosphate‐6‐azophenly‐2′, 4′‐disulfonic acid (PPADS), or suramin prior to adding ATP, we found that the rate of phosphate production was significantly reduced by 27%, 21%, and 22%, respectively (P < 0.05). Our second method of measuring ATP metabolism allowed us to measure the amount of adenosine produced following electric field stimulation of the arteriole with and without nucleotidase inhibitors. Surprisingly, we found that adenosine overflow was not attenuated by nucleotidase inhibitors. We concluded that ecto‐phosphodieterase/phyrophophatase (E‐NPP), ecto‐diadenosine polyphosphatase (ApnA), NTPDase1 and 2, and E5NT may be present on the gastrocnemius 1A arteriole and do play a role in ATP metabolism. Between the ages of 8 weeks and 12 weeks, however, overall ATP metabolism may not change.

The purpose of this study was to investigate the metabolism of Adenosine triphosphate (ATP) in skeletal muscle resistance arterioles and whether it is altered during the rapid growth phase of the rat. We concluded that ecto‐phosphodieterase/phyrophophatase (E‐NPP), ecto‐diadenosine polyphosphatase (ApnA), NTPDase1 and 2, and E5NT may be present on the gastrocnemius 1A arteriole and do play a role in ATP metabolism. Between the ages of 8 weeks and 12 weeks, however, overall ATP metabolism may not change.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Sources of intravascular ATP during exercise in humans: critical role for skeletal muscle perfusion

Exercise hyperaemia is regulated by several factors, and one factor known to increase with exercise that evokes a powerful vasomotor action is extracellular ATP. The origin of ATP detected in plasma from exercising muscle of humans is, however, a matter of debate, and ATP has been suggested to arise from sympathetic nerves, blood sources (e.g. erythrocytes), endothelial cells and skeletal myocytes, among others. Therefore, we tested the hypothesis that acute augmentation of sympathetic nervous system activity (SNA) results in elevated plasma ATP draining skeletal muscle, and that SNA superimposition during exercise increases ATP more than exercise alone. We showed that increased SNA via -40 mmHg lower body negative pressure (LBNP) at rest did not increase plasma ATP (51±8 nmol l(-1) at rest versus 58±7 nmol l(-1) with LBNP), nor did it increase [ATP] above levels observed during rhythmic hand-grip exercise (79±11 nmol l(-1) with exercise alone versus 71±8 nmol l(-1) with LBNP). Next, we tested the hypothesis that active perfusion of skeletal muscle is essential to observe increased plasma ATP during exercise. We showed that complete obstruction of blood flow to contracting muscle abolished exercise-mediated increases in plasma ATP (from 90±19 to 49±12 nmol l(-1)), and that cessation of blood flow prior to exercise completely inhibited the typical rise in ATP (3 versus 61%, obstructed versus intact perfusion). The lack of change in ATP during occlusion occurred in the face of continued muscular work and elevated SNA, indicating that the rise of intravascular ATP did not result from these extravascular sources. Our collective observations indicated that the elevation in extracellular ATP observed in blood during exercise was unlikely to originate from sympathetic nerves or the contacting muscle itself, but rather was dependent on intact skeletal muscle perfusion. We conclude that an intravascular source for ATP is essential, which indicates an important role for blood sources (e.g. red blood cells) in augmenting and maintaining elevated plasma ATP during exercise.

Measurement of purine release with microelectrode biosensors

Purinergic signalling departs from traditional paradigms of neurotransmission in the variety of release mechanisms and routes of production of extracellular ATP and adenosine. Direct real-time measurements of these purinergic agents have been of great value in understanding the functional roles of this signalling system in a number of diverse contexts. Here, we review the methods for measuring purine release, introduce the concept of microelectrode biosensors for ATP and adenosine and explain how these have been used to provide new mechanistic insight in respiratory chemoreception, synaptic physiology, eye development and purine salvage. We finish by considering the association of purine release with pathological conditions and examine the possibilities that biosensors for purines may one day be a standard part of the clinical diagnostic tool chest.

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.