Effects of adenosine on optic nerve head circulation in rabbits

Myocardial Bridging of Mid-left Anterior Descending Artery (LAD) Presenting As Transient Left Bundle Branch Block During Nuclear Stress Test

Transient left bundle branch block occurring during a nuclear stress test in the setting of myocardial bridging is a relatively rare finding. We report a case of a 75-year-old male who presented with typical stable angina. Serial troponins were negative, and the electrocardiogram revealed normal sinus rhythm with left ventricular hypertrophy and T-wave inversions in the lateral leads. The nuclear stress test was non-ischemic but showed a transient left bundle branch block associated with chest pain and shortness of breath that occurred right after the administration of regadenoson. Coronary angiography revealed non-obstructive coronary artery disease and a mid-LAD myocardial bridge.

Hyperoxia evokes pericyte-mediated capillary constriction

Oxygen supplementation is regularly prescribed to patients to treat or prevent hypoxia. However, excess oxygenation can lead to reduced cerebral blood flow (CBF) in healthy subjects and worsen the neurological outcome of critically ill patients. Most studies on the vascular effects of hyperoxia focus on arteries but there is no research on the effects on cerebral capillary pericytes, which are major regulators of CBF. Here, we used bright-field imaging of cerebral capillaries and modeling of CBF to show that hyperoxia (95% superfused O₂) led to an increase in intracellular calcium level in pericytes and a significant capillary constriction, sufficient to cause an estimated 25% decrease in CBF. Although hyperoxia is reported to cause vascular smooth muscle cell contraction via generation of reactive oxygen species (ROS), endothelin-1 and 20-HETE, we found that increased cytosolic and mitochondrial ROS levels and endothelin release were not involved in the pericyte-mediated capillary constriction. However, a 20-HETE synthesis blocker greatly reduced the hyperoxia-evoked capillary constriction. Our findings establish pericytes as regulators of CBF in hyperoxia and 20-HETE synthesis as an oxygen sensor in CBF regulation. The results also provide a mechanism by which clinically administered oxygen can lead to a worse neurological outcome.

CD73 controls ocular adenosine levels and protects retina from light-induced phototoxicity

ATP and adenosine have emerged as important signaling molecules involved in vascular remodeling, retinal functioning and neurovascular coupling in the mammalian eye. However, little is known about the regulatory mechanisms of purinergic signaling in the eye. Here, we used three-dimensional multiplexed imaging, in situ enzyme histochemistry, flow cytometric analysis, and single cell transcriptomics to characterize the whole pattern of purine metabolism in mouse and human eyes. This study identified ecto-nucleoside triphosphate diphosphohydrolase-1 (NTPDase1/CD39), NTPDase2, and ecto-5′-nucleotidase/CD73 as major ocular ecto-nucleotidases, which are selectively expressed in the photoreceptor layer (CD73), optic nerve head, retinal vasculature and microglia (CD39), as well as in neuronal processes and cornea (CD39, NTPDase2). Specifically, microglial cells can create a spatially arranged network in the retinal parenchyma by extending and retracting their branched CD39^high/CD73^low processes and forming local “purinergic junctions” with CD39^low/CD73⁻ neuronal cell bodies and CD39^high/CD73⁻ retinal blood vessels. The relevance of the CD73–adenosine pathway was confirmed by flash electroretinography showing that pharmacological inhibition of adenosine production by injection of highly selective CD73 inhibitor PSB-12489 in the vitreous cavity of dark-adapted mouse eyes rendered the animals hypersensitive to prolonged bright light, manifested as decreased a-wave and b-wave amplitudes. The impaired electrical responses of retinal cells in PSB-12489-treated mice were not accompanied by decrease in total thickness of the retina or death of photoreceptors and retinal ganglion cells. Our study thus defines ocular adenosine metabolism as a complex and spatially integrated network and further characterizes the critical role of CD73 in maintaining the functional activity of retinal cells.

Regadenoson Stress Testing: A Comprehensive Review With a Focused Update

Regadenoson is a pharmacological stress agent that has been widely used since its approval by the Food and Drug Administration (FDA) in 2008. For many years, dipyridamole and adenosine, which are non-selective adenosine receptor agonists, were more popular. However, these agents are less preferred now due to their undesirable adverse effects as compared to regadenoson. In the ADVANCE (ADenoscan Versus regAdenosoN Comparative Evaluation) phase 3 clinical trial, regadenoson demonstrated non-inferiority to adenosine for detecting reversible myocardial ischemia. This review summarizes the clinical utilities of regadenoson as the most widely used pharmacological stress agent. Moreover, the use of regadenoson has been documented in specific patient populations. Although regadenoson has established safety and efficacy in most patients with chronic diseases, there are equivocal results in the literature for other chronic diseases. It is warranted to highlight that the use of regadenoson has not been studied in patients of low socioeconomic class; it is a condition that carries a significant burden on the cardiovascular system.

The Diameter of Retinal Arterioles Is Unaffected by Intravascular Administration of the Adenosine A2A Receptor Agonist Regadenoson in Normal Persons

Background

The neurotransmitter adenosine has been proposed to be involved in the pathogenesis of diabetic retinopathy, which may be due to the vasoactive properties of the compound. Previous studies have shown that adenosine can affect the tone of retinal arterioles in vitro to induce dilatation mediated by A_2A and A_2Breceptors and constriction mediated by A₁ and A₃ receptors.

Purpose

To investigate effects of intravenous administration of the adenosine A_2A receptor agonist regadenoson on the diameter of retinal vessels in vivo.

Method

The diameter responses of larger retinal arterioles and venules were evaluated using the dynamic vessel analyser in 20 normal persons (age 22–31 years) after intravenous administration of the adenosine A_2A receptor agonist regadenoson during exposure to systemic normoxia and hypoxia.

Results

The diameter of retinal arterioles and venules increased significantly during stimulation with flickering light (p < 0.0001). Regadenoson reduced the flicker-induced dilatation of venules during normoxia (p = 0.0006), but otherwise had no effect on vessel diameters (p > 0.08 for all comparisons).

Conclusions

Intravenous administration of the adenosine A_2A receptor agonist regadenoson had no significant effect on the diameter of retinal arterioles. Future studies should investigate differential effects of intra- and extravascular administration of adenosine receptor agonists on retinal vessels.

Adenosine as a Marker and Mediator of Cardiovascular Homeostasis: A Translational Perspective

Adenosine, a purine nucleoside, is produced broadly and implicated in the homeostasis of many cells and tissues. It signals predominantly via 4 purinergic adenosine receptors (ADORs) - ADORA1, ADORA2A, ADORA2B and ADOosine signaling, both through design as specific ADOR agonists and antagonists and as offtarget effects of existing anti-platelet medications. Despite this, adenosine has yet to be firmly established as either a therapeutic or a prognostic tool in clinical medicine to date. Herein, we provide a bench-to-bedside review of adenosine biology, highlighting the key considerations for further translational development of this proRA3 in addition to non-ADOR mediated effects. Through these signaling mechanisms, adenosine exerts effects on numerous cell types crucial to maintaining vascular homeostasis, especially following vascular injury. Both in vitro and in vivo models have provided considerable insights into adenosine signaling and identified targets for therapeutic intervention. Numerous pharmacologic agents have been developed that modulate adenmising molecule.

SUR1-Associated Mechanisms Are Not Involved in Ischemic Optic Neuropathy 1 Day Post-Injury

Ischemia-reperfusion injury after central nervous system (CNS) injury presents a major health care challenge with few promising treatments. Recently, it has become possible to reduce edema after CNS injury by antagonizing a sulfonylurea receptor 1 (SUR1) regulated ion channel expressed after injury. SUR1 upregulation after injury is a necessary precondition for the formation of this channel, and has been implicated in white matter injury after clinical spinal cord trauma. Glibenclamide, an SUR1 antagonist, appears to have neuroprotective effect against cerebral stroke in an open-label small clinical trial and great effectiveness in reducing damage after varied experimental CNS injury models. Despite its importance in CNS injuries, SUR1 upregulation appears to play no part in rodent anterior ischemic optic neuropathy (rAION) injury as tested by real-time PCR and immunohistochemical staining of rAION-injured rat optic nerve (ON). Furthermore, the SUR1 antagonist glibenclamide administered immediately after rAION injury provided no protection to proximal ON microvasculature 1 day post-injury but may reduce optic nerve head edema in a manner unrelated to ON SUR1 expression. Our results suggest that there may be fundamental differences between rAION optic nerve ischemia and other CNS white matter injuries where SUR1 appears to play a role.

Increased Nanoparticle Delivery to Brain Tumors by Autocatalytic Priming for Improved Treatment and Imaging

The blood-brain barrier (BBB) is partially disrupted in brain tumors. Despite the gaps in the BBB, there is an inadequate amount of pharmacological agents delivered into the brain. Thus, the low delivery efficiency renders many of these agents ineffective in treating brain cancer. In this report, we proposed an "autocatalytic" approach for increasing the transport of nanoparticles into the brain. In this strategy, a small number of nanoparticles enter into the brain via transcytosis or through the BBB gaps. After penetrating the BBB, the nanoparticles release BBB modulators, which enables more nanoparticles to be transported, creating a positive feedback loop for increased delivery. Specifically, we demonstrated that these autocatalytic brain tumor-targeting poly(amine-co-ester) terpolymer nanoparticles (ABTT NPs) can readily cross the BBB and preferentially accumulate in brain tumors at a concentration of 4.3- and 94.0-fold greater than that in the liver and in brain regions without tumors, respectively. We further demonstrated that ABTT NPs were capable of mediating brain cancer gene therapy and chemotherapy. Our results suggest ABTT NPs can prime the brain to increase the systemic delivery of therapeutics for treating brain malignancies.

Discovery of Molecular Therapeutics for Glaucoma: Challenges, Successes, and Promising Directions

Glaucoma, a heterogeneous ocular disorder affecting ∼60 million people worldwide, is characterized by painless neurodegeneration of retinal ganglion cells (RGCs), resulting in irreversible vision loss. Available therapies, which decrease the common causal risk factor of elevated intraocular pressure, delay, but cannot prevent, RGC death and blindness. Notably, it is changes in the anterior segment of the eye, particularly in the drainage of aqueous humor fluid, which are believed to bring about changes in pressure. Thus, it is primarily this region whose properties are manipulated in current and emerging therapies for glaucoma. Here, we focus on the challenges associated with developing treatments, review the available experimental methods to evaluate the therapeutic potential of new drugs, describe the development and evaluation of emerging Rho-kinase inhibitors and adenosine receptor ligands that offer the potential to improve aqueous humor outflow and protect RGCs simultaneously, and present new targets and approaches on the horizon.

Functional hyperemia and mechanisms of neurovascular coupling in the retinal vasculature

The retinal vasculature supplies cells of the inner and middle layers of the retina with oxygen and nutrients. Photic stimulation dilates retinal arterioles producing blood flow increases, a response termed functional hyperemia. Despite recent advances, the neurovascular coupling mechanisms mediating the functional hyperemia response in the retina remain unclear. In this review, the retinal functional hyperemia response is described, and the cellular mechanisms that may mediate the response are assessed. These neurovascular coupling mechanisms include neuronal stimulation of glial cells, leading to the release of vasoactive arachidonic acid metabolites onto blood vessels, release of potassium from glial cells onto vessels, and production and release of nitric oxide (NO), lactate, and adenosine from neurons and glia. The modulation of neurovascular coupling by oxygen and NO are described, and changes in functional hyperemia that occur with aging and in diabetic retinopathy, glaucoma, and other pathologies, are reviewed. Finally, outstanding questions concerning retinal blood flow in health and disease are discussed.

Assessment of retinal and choroidal blood flow changes using laser Doppler flowmetry in rats

PURPOSE

A new noninvasive laser Doppler flowmetry (LDF) probe (one emitting fiber surrounded by a ring of eight collecting fibers, 1-mm interaxis distance) was tested for its sensitivity to assess the retinal/choroidal blood flow variations in response to hypercapnia, hyperoxia, diverse vasoactive agents and following retinal arteries photocoagulation in the rat.

MATERIALS AND METHODS

After pupil dilation, a LDF probe was placed in contact to the cornea of anesthetized rats in the optic axis. Hypercapnia and hyperoxia were induced by inhalation of CO(2) (8% in medical air) and O(2) (100%) while pharmacological agents were injected intravitreously. The relative contribution of the choroidal circulation to the LDF signal was estimated after retinal artery occlusion by photocoagulation.

RESULTS

Blood flow was significantly increased by hypercapnia (18%), adenosine (14%) and sodium nitroprusside (16%) as compared to baseline values while it was decreased by hyperoxia (-8%) and endothelin-1 (-11%). Photocoagulation of retinal arteries significantly decreased blood flow level (-45%).

CONCLUSIONS

Although choroidal circulation most likely contributes to the LDF signal in this setting, the results demonstrate that LDF represents a suitable in vivo noninvasive technique to monitor online relative reactivity of retinal perfusion to metabolic or pharmacological challenge. This technique could be used for repeatedly assessing blood flow reactivity in rodent models of ocular diseases.

Cellular and physiological mechanisms underlying blood flow regulation in the retina and choroid in health and disease

We review the cellular and physiological mechanisms responsible for the regulation of blood flow in the retina and choroid in health and disease. Due to the intrinsic light sensitivity of the retina and the direct visual accessibility of fundus blood vessels, the eye offers unique opportunities for the non-invasive investigation of mechanisms of blood flow regulation. The ability of the retinal vasculature to regulate its blood flow is contrasted with the far more restricted ability of the choroidal circulation to regulate its blood flow by virtue of the absence of glial cells, the markedly reduced pericyte ensheathment of the choroidal vasculature, and the lack of intermediate filaments in choroidal pericytes. We review the cellular and molecular components of the neurovascular unit in the retina and choroid, techniques for monitoring retinal and choroidal blood flow, responses of the retinal and choroidal circulation to light stimulation, the role of capillaries, astrocytes and pericytes in regulating blood flow, putative signaling mechanisms mediating neurovascular coupling in the retina, and changes that occur in the retinal and choroidal circulation during diabetic retinopathy, age-related macular degeneration, glaucoma, and Alzheimer's disease. We close by discussing issues that remain to be explored.

Dual effects of ATP on isolated arteries of the bovine eye

Although the presence of purinoreceptors has been shown in many human and animal arteries, there is few data yet about their role in the arteries of the eye. The purpose of the present study was to evaluate the effects of several agonists of purinoreceptors on isolated arteries of the bovine eye. Responses of isolated preparations of bovine ophthalmic (OA) and posterior ciliary arteries (PCA) to agonists of purinoreceptors (ATP, α,β-methylene-ATP-α,β-meATP, 2-methylthioATP-2meSATP, uridine-5'-triphosphate-UTP) as well as agonists of adreno-, cholino-, adenosine and histamine receptors were recorded by a standard organ bath method. ATP induced contractions of the intact vessels but caused relaxation of α,β-meATP-pretreated arteries. Contractile responses of PCA to high concentrations of ATP and α,β-meATP were significantly stronger than responses of OA, as well as relaxative responses to ATP and adenosine were significantly stronger in PCA than in OA. We suggest that there are several subtypes of functionally active purinoreceptors in both OA and PCA, although the potency of agonists of purinoreceptors to produce mechanical responses is higher in PCA than in OA. Purinoreceptors can be potential targets for new drugs, treating vascular pathology of the eye.

Integration of image exposure time into a modified laser speckle imaging method

Speckle-based methods have been developed to characterize tissue blood flow and perfusion. One such method, called modified laser speckle imaging (mLSI), enables computation of blood flow maps with relatively high spatial resolution. Although it is known that the sensitivity and noise in LSI measurements depend on image exposure time, a fundamental disadvantage of mLSI is that it does not take into account this parameter. In this work, we integrate the exposure time into the mLSI method and provide experimental support of our approach with measurements from an in vitro flow phantom.

The complex interaction between ocular perfusion pressure and ocular blood flow - relevance for glaucoma

Glaucoma is an optic neuropathy of unknown origin. The most important risk factor for the disease is an increased intraocular pressure (IOP). Reducing IOP is associated with reduced progression in glaucoma. Several recent large scale trials have indicated that low ocular perfusion pressure (OPP) is a risk factor for the incidence, prevalence and progression of the disease. This is a strong indicator that vascular factors are involved in the pathogenesis of the disease, a hypothesis that was formulated 150 years ago. The relation between OPP and blood flow to the posterior pole of the eye is, however, complex, because of a phenomenon called autoregulation. Autoregulatory processes attempt to keep blood flow constant despite changes in OPP. Although autoregulation has been observed in many experiments in the ocular vasculature the mechanisms underlying the vasodilator and vasoconstrictor responses in face of changes in OPP remain largely unknown. There is, however, recent evidence that the human choroid regulates its blood flow better during changes in blood pressure induced by isometric exercise than during changes in IOP induced by a suction cup. This may have consequences for our understanding of glaucoma, because it indicates that blood flow regulation is strongly dependent not only on OPP, but also on the level of IOP itself. Indeed there is data indicating that reduction of IOP by pharmacological intervention improves optic nerve head blood flow regulation independently of an ocular vasodilator effect.

Pericyte-mediated regulation of capillary diameter: a component of neurovascular coupling in health and disease

Because regional blood flow increases in association with the increased metabolic demand generated by localized increases in neural activity, functional imaging researchers often assume that changes in blood flow are an accurate read-out of changes in underlying neural activity. An understanding of the mechanisms that link changes in neural activity to changes in blood flow is crucial for assessing the validity of this assumption, and for understanding the processes that can go wrong during disease states such as ischaemic stroke. Many studies have investigated the mechanisms of neurovascular regulation in arterioles but other evidence suggests that blood flow regulation can also occur in capillaries, because of the presence of contractile cells, pericytes, on the capillary wall. Here we review the evidence that pericytes can modulate capillary diameter in response to neuronal activity and assess the likely importance of neurovascular regulation at the capillary level for functional imaging experiments. We also discuss evidence suggesting that pericytes are particularly sensitive to damage during pathological insults such as ischaemia, Alzheimer's disease and diabetic retinopathy, and consider the potential impact that pericyte dysfunction might have on the development of therapeutic interventions and on the interpretation of functional imaging data in these disorders.

Effect of 2-(6-cyano-1-hexyn-1-yl)adenosine on ocular blood flow in rabbits

Previously, we reported that a relatively selective adenosine A(2A) receptor agonist 2-(6-cyano-1-hexyn-1-yl)adenosine (2-CN-Ado) elicited ocular hypotension in rabbits (Journal of Pharmacological Sciences 2005;97:501-509). In the present study, we investigated the effect of 2-CN-Ado on ocular blood flow in rabbit eyes. An intravitreal injection of 2-CN-Ado increased ocular blood flow, measured by a non-contact laser flowmeter. 2-CN-Ado-induced increase in ocular blood flow was accompanied with the retinal vasodilation. The increase in ocular blood flow was inhibited by an adenosine A(2A) receptor antagonist 1,3,7-trimethyl-8-(3-chlorostyryl)xanthine, but not by an adenosine A(2B) receptor antagonist alloxazine or an adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. The repetitive applications of topical 2-CN-Ado twice a day for 7 days produced a persistent increase in ocular blood flow with ocular hypotension. These results suggest that 2-CN-Ado increases the ocular blood flow mainly via adenosine A(2A) receptor, and that the topical application of 2-CN-Ado for several days not only increases the ocular blood flow but also prolong ocular hypotension, indicating that 2-CN-Ado may be a useful lead compound for the treatment of ischemic retinal diseases such as glaucoma.

Linear response range characterization and in vivo application of laser speckle imaging of blood flow dynamics

Noninvasive blood flow imaging can provide critical information on the state of biological tissue and the efficacy of approaches to treat disease. With laser speckle imaging (LSI), relative changes in blood flow are typically reported, with the assumption that the measured values are on a linear scale. A linear relationship between the measured and actual flow rate values has been suggested. The actual flow rate range, over which this linear relationship is valid, is unknown. Herein we report the linear response range and velocity dynamic range (VDR) of our LSI instrument at two relevant camera integration times. For integration times of 1 and 10 ms, the best case VDR was 80 and 60 dB, respectively, and the worst case VDR was 20 and 50 dB. The best case VDR values were similar to those reported in the literature for optical Doppler tomography. We also demonstrate the potential of LSI for monitoring blood flow dynamics in the rodent dorsal skinfold chamber model. These findings imply that LSI can provide accurate wide-field maps of microvascular blood flow rate dynamics and highlight heterogeneities in flow response to the application of exogenous agents.