Characterization of adenosine receptors in isolated cerebral arteries of cat

Impact of impaired cerebral blood flow autoregulation on cognitive impairment

Although the causes of cognitive impairment are multifactorial, emerging evidence indicates that cerebrovascular dysfunction plays an essential role in dementia. One of the most critical aspects of cerebrovascular dysfunction is autoregulation of cerebral blood flow (CBF), mainly mediated by the myogenic response, which is often impaired in dementia individuals with comorbidities, such as diabetes and hypertension. However, many unsolved questions remain. How do cerebrovascular networks coordinately modulate CBF autoregulation in health and disease? Does poor CBF autoregulation have an impact on cognitive impairment, and what are the underlying mechanisms? This review summarizes the cerebral vascular structure and myogenic (a three-phase model), metabolic (O2, CO2, adenosine, and H+), and endothelial (shear stress) factors in the regulation of CBF; and the consequences of CBF dysautoregulation. Other factors contributing to cerebrovascular dysfunction, such as impaired functional hyperemia and capillary abnormalities, are included as well. Moreover, this review highlights recent studies from our lab in terms of novel mechanisms involved in CBF autoregulation and addresses a hypothesis that there is a three-line of defense for CBF autoregulation in the cerebral vasculature.

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.

Neurovascular signaling in the brain and the pathological consequences of hypertension

The execution and maintenance of all brain functions are dependent on a continuous flow of blood to meet the metabolic needs of the tissue. To ensure the delivery of resources required for neural processing and the maintenance of neural homeostasis, the cerebral vasculature is elaborately and extensively regulated by signaling from neurons, glia, interneurons, and perivascular nerves. Hypertension is associated with impaired neurovascular regulation of the cerebral circulation and culminates in neurodegeneration and cognitive dysfunction. Here, we review the physiological processes of neurovascular signaling in the brain and discuss mechanisms of hypertensive neurovascular dysfunction.

Contribution of allergic inflammatory response to the pathogenesis of malaria disease

Plasmodium falciparum, the aetiological agent of human lethal malaria, is responsible for over 2 million deaths per year and malaria episodes may vary considerably in their severity and clinical manifestations. Dysregulated balance of the inflammatory response and a defect in the anti-Plasmodium parasite immune response represent the hallmarks of malaria disease. Among the many possible mechanisms, it is now widely recognized that the production of pro-inflammatory mediators and cytokines and upregulation of endothelial cell adhesion molecules play important roles in malaria pathogenesis. We and others provided evidence that some components of allergic inflammatory response to malaria parasites or elicited by by-products of parasite infection may contribute to malaria pathogenesis. This review provides some clue regarding these mechanisms where mast cells and histamine, an inflammatory mediator generated following IgE-independent or IgE-mediated immune response, were found to play a major role in parasite transmission and malaria pathogenesis, respectively. This article is part of a Special Issue entitled: Mast cells in inflammation.

Adenosine can mediate its actions through generation of reactive oxygen species

Adenosine is an important cerebral vasodilator, but mediating mechanisms are not understood. We investigated the expression of adenosine receptor subtypes in isolated cerebral arterial muscle cells (CAMCs), and their role in adenosine-induced superoxide (O(2)(-)) generation and reduction in cerebral arterial tone. Reverse transcriptase-PCR, western blotting, and immunofluorescence studies have shown that CAMCs express transcript and protein for A1, A(2A), A(2B), and A(3) adenosine receptors. Stimulation of CAMCs with adenosine or the A(2A) agonist CGS-21680 increased the generation of O(2)(-) that was attenuated by the inhibition of A(2A) and A(2B) adenosine receptor subtypes, or by the peptide inhibitor of nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase gp91ds-tat, or by the mitochondria uncoupler 2,4-dinitrophenol. Application of adenosine or CGS-21680 dilated pressure-constricted cerebral arterial segments that were prevented by the antioxidants superoxide dismutase (SOD) conjugated to polyethylene glycol (PEG) and PEG-catalase or by the A(2B) adenosine receptor antagonist MRS-1754, or by the mixed A(2A) and A(2B) antagonist ZM-241385. Antagonism of the A(2A) and A(2B) adenosine receptors had no effect on cerebral vasodilatation induced by nifedipine. These findings indicate that adenosine reduces pressure-induced cerebral arterial tone through stimulation of A(2A) and A(2B) adenosine receptors and generation of O(2)(-) from NADPH oxidase and mitochondrial sources. This signaling pathway could be one of the mediators of the cerebral vasodilatory actions of adenosine.

Inhibition of histamine-mediated signaling confers significant protection against severe malaria in mouse models of disease

From the inoculation of Plasmodium sporozoites via Anopheles mosquito bites to the development of blood-stage parasites, a hallmark of the host response is an inflammatory reaction characterized by elevated histamine levels in the serum and tissues. Given the proinflammatory and immunosuppressive activities associated with histamine, we postulated that this vasoactive amine participates in malaria pathogenesis. Combined genetic and pharmacologic approaches demonstrated that histamine binding to H1R and H2R but not H3R and H4R increases the susceptibility of mice to infection with Plasmodium. To further understand the role of histamine in malaria pathogenesis, we used histidine decarboxylase-deficient (HDC(-/-)) mice, which are free of histamine. HDC(-/-) mice were highly resistant to severe malaria whether infected by mosquito bites or via injection of infected erythrocytes. HDC(-/-) mice displayed resistance to two lethal strains: Plasmodium berghei (Pb) ANKA, which triggers cerebral malaria (CM), and Pb NK65, which causes death without neurological symptoms. The resistance of HDC(-/-) mice to CM was associated with preserved blood-brain barrier integrity, the absence of infected erythrocyte aggregation in the brain vessels, and a lack of sequestration of CD4 and CD8 T cells. We demonstrate that histamine-mediated signaling contributes to malaria pathogenesis. Understanding the basis for these biological effects of histamine during infection may lead to novel therapeutic strategies to alleviate the severity of malaria.

Intrathecal adenosine increases spinal cord blood flow in the rat: measurements with the laser-Doppler flowmetry technique

BACKGROUND

Adenosine and adenosine analogues induce antinociception both after systemic and intrathecal (i.t.) administration in animal models. Further, patients with neuropathic pain have been treated successfully with i.t. adenosine. Prior to introducing new analgesic drugs for regular spinal use in humans, experimental studies must be undertaken to evaluate the risks of neurotoxicity. It is important to evaluate the possibility of cytotoxic effects and that antinociception may be due to decreased spinal cord blood flow (SCBF) and neural ischaemia. The present study evaluates whether adenosine or isotonic mannitol induces changes in SCBF as assessed by laser-Doppler flowmetry (LDF).

METHODS

After laminectomy and insertion of i.t. catheters, seven rats received adenosine 50 microg in isotonic mannitol 500 microg, six rats received isotonic mannitol 500 microg and eight rats received saline 0.9%. SCBF was registered by the LDF technique continuously for 3 h after injection. Arterial blood pressure was also assessed.

RESULTS

In the adenosine in mannitol group, SCBF increased up to 230% of baseline levels for almost 40 min, P = 0.044 and then declined. In the mannitol group, SCBF increased up to 180% of baseline (P < 0.011) before declining. At 60 min, SCBF had returned to saline levels and remained stable during the rest of the experiment.

CONCLUSION

Intrathecal administration of adenosine in mannitol and of mannitol both increased SCBF in rats, compared with saline. It is unlikely that the effects on SCBF induced by adenosine and mannitol could result in neurotoxicity of the spinal cord.

Structure–affinity relationships of 5 ′ -aromatic ethers and 5 ′ -aromatic sulfides as partial A 1 adenosine agonists, potential supraventricular anti-arrhythmic agents

Atrial fibrillation (AF) is the most commonly encountered sustained clinical arrhythmia with an estimated 2.3 million cases in the US (2001). A(1) adenosine receptor agonists can slow the electrical impulse propagation through the atrioventricular (AV) node (i.e., negative dromotropic effect) resulting in prolongation of the stimulus-to-His bundle (S-H) interval to potentially reduce ventricular rate. Compounds that are full agonists of the A(1) adenosine receptor can cause high grade AV block. Therefore, it is envisioned that a compound that is a partial agonist of the A(1) adenosine receptor could avoid this deleterious effect. 5(') Phenyl sulfides (e.g., 17, EC(50)=1.26 microM) and phenyl ethers (e.g., 28, EC(50)=0.2 microM) are partial agonists with respect to their AV nodal effects in guinea pig isolated hearts. Additional affinity, GTPgammaS binding data suggesting partial activity of the A(1) adenosine receptor, and PK results for 5(') modified adenosine derivatives are shown.

Methods for isolation and characterization of intracerebral arterioles in the C57/BL6 wild-type mouse

Vascular control mechanisms have been studied extensively in mice. However, an in vitro characterization of penetrating intracerebral arterioles has not been reported. We describe methods for isolation and cannulation for mouse intracerebral arterioles. This technique allows analysis of mouse cerebral arteriolar physiology and pharmacology without the confounding influences of the surrounding brain elements. Penetrating intracerebral arterioles from adult C57/BL6 wild-type (WT) mice were isolated at 4 degrees C, transferred to an inverted microscope and cannulated at both ends using a dual glass micropipette system, wherein intraluminal flow (0.2 microl/min) and pressure (60 mmHg) were maintained. The arterioles developed spontaneous tone when the chamber was warmed to 37 degrees C, with the resulting diameter reaching 68.4+/-0.9% of passive diameter (29.8+/-1.1 microm). After the development of spontaneous tone, incremental changes in luminal pressure from 20 to 140 mmHg induced myogenic responses. Acidosis (pH 6.8) and alkalosis (pH 7.6) caused dilation (20.0+/-1.4%) and constriction (17.2+/-1.4%), respectively. Extraluminal adenosine (ADO (10 microM); 24.3+/-3.6%) and sodium nitroprusside (SNP (10 microM); 28.6+/-4.1%) and intraluminal adenosine 5'-triphosphate (ATP (10 microM); 20.0+/-3.9%) resulted in vasodilation similar in magnitude to that observed in rat arterioles. This information provides a foundation for elucidating cerebral vascular control mechanisms in genetically engineered mice.

Effect of adenosine receptor blockade on pial arteriolar dilation during sciatic nerve stimulation

In the present study, we report the effects of adenosine receptor antagonists on pial vasodilatation during contralateral sciatic nerve stimulation (SNS). The pial circulation was observed through a closed cranial window in alpha-chloralose-anesthetized rats. In artificial cerebrospinal fluid (CSF), SNS resulted in a 30.5 +/- 13.2% increase in pial arteriolar diameter in the hindlimb somatosensory cortex. Systemic administration of the selective adenosine A2A receptor antagonist, 4-(2-[7-amino-2-[2-furyl][3,2,4]triazolol[2,3-a][1,3,5]triazin-5-yl-amino] ethyl)phenol (ZM-241385), significantly (P < 0.05, n = 6) attenuated the SNS-induced vasodilatation. Systemic administration of 8-(p-sulfophenyl)theophylline (8SPT), a nonselective antagonist that is blood-brain barrier (BBB) impermeable, had no effect on vasodilatation to SNS. In contrast, systemic theophylline, which readily penetrates the BBB, nearly abolished the SNS-induced vasodilatation (P < 0.01; n = 7). Topical superfusion of 8SPT significantly (P < 0.01; n = 6) attenuated vasodilatation during SNS. Topical superfusion of 8- cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist, significantly potentiated SNS-induced vasodilatation (P < 0.01; n > or = 5). Hypercarbic vasodilatation and somatosensory-evoked potentials were not affected by any of the compounds tested. Our findings suggest that luminal endothelial adenosine receptors are not involved in the arteriolar response to SNS, as demonstrated by a lack of effect with systemic 8SPT. Furthermore, the adenosine A2A receptor subtype appears to be involved in the dilator response to SNS. Finally, the neuromodulatory action of adenosine, via the A1 receptor subtype, significantly influences SNS-induced vasodilatation. Thus the present study provides further evidence for a role of adenosine in the regulation of CBF during somatosensory stimulation.

Pharmacology of adenosine receptors in the vasculature

Adenosine is widely distributed in mammals. One of the primary roles of adenosine within the cardiovascular system is to directly control the functions of both cardiac and vascular tissues. Recently, there has been considerable interest in the subclassification of adenosine receptors. Characterization of a heterogeneous population of receptors for adenosine could provide an opportunity for the development of novel compounds of therapeutic value. Adenosine is released from cells as a result of metabolism, and its release can be increased dramatically from cells that are metabolically stressed. This implies that adenosine can be released from a variety of cells throughout the body, as a result of increased metabolic rates, in concentrations that can have a profound impact on blood vessel function and, consequently, blood flow. It is recognized that the actions of this nucleoside on the vasculature are most prominent when oxygen demand is high and there is a reduction in oxygen tension at the site in question. Therefore, it is not surprising that adenosine has been shown to be an important regulator of blood vessel tone under hypoxic conditions. Furthermore, the activation of adenosine receptors on blood vessels can result in relaxation and/or contractions. The nature of the response subsequent to the activation of adenosine receptors is primarily dependent on the type of blood vessel involved and basal tone. This review will focus on the characterization of subtypes of adenosine receptors in blood vessels, as well as the effect of the stimulation of adenosine receptors on the peripheral circulation.

Role of adenosine A(2B) receptors in vasodilation of rat pial artery and cerebral blood flow autoregulation

This study was aimed to investigate the underlying mechanism of vasodilation induced by the activation of A(2B) adenosine receptors in relation to cerebral blood flow (CBF) autoregulation. Changes in pial arterial diameters were observed directly through a closed cranial window. N(omega)-nitro-L-arginine methyl ester (L-NAME, nitric oxide synthase inhibitor) significantly suppressed the concentration-dependent vasodilations induced by adenosine and 5'-N-ethylcarboxamido-adenosine (NECA) but not the vasodilation by CGS-21680 (A(2A)-receptor agonist). Moreover, NECA-induced vasodilation was suppressed by alloxazine (1 micromol/l) but not by ZM-241385 (1 micromol/l, A(2A) antagonist), which suggests mediation by A(2B)- receptor activation. Otherwise, the level of nitrite/nitrate was concentration dependently increased in the artificial cerebrospinal fluid (CSF) when adenosine and NECA were suffused over the cortical surface. L-NAME and alloxazine, but not ZM-241385, largely inhibited their releases. The lower limit of CBF autoregulation was little affected following pretreatment with L-NAME or alloxazine. Thus it is suggested that adenosine-induced vasodilation via activation of A(2B)-adenosine receptors of the rat pial artery is coupled to the production of nitric oxide, which contributes little to CBF autoregulation.

The synthesis and biological activity of a highly selective adenosine A2a receptor agonist

Three novel nucleosides 1, 2, and 3 were prepared that contained side chains at the 2-position of adenosine. Compound 1 was shown to be the most selective A2a receptor agonist reported to date having an A1/A2 ratio of 2400. In addition, compound 1 was shown to reduce blood pressure in rats and dogs with only minimal effects on heart rate.

Iatrogenic seizures

This article focuses on the subject of iatrogenic seizures, particularly those that are potentially avoidable. Seizures due to medications, surgical therapy, medical procedures, and diagnostic tests are all examined. Withdrawing antiepileptic drug (AED) therapy from epileptic patients who are undergoing inpatient evaluation for epilepsy surgery is also discussed.

Adenosine and migraine

BACKGROUND

Adenosine is a powerful natural vasodilator that participates in the control of cerebral and meningeal blood flow. In this context, it could be involved in the pathophysiology of migraine, since it was previously reported that intravenous adenosine can precipitate crises in migraine patients.

METHODS

We have investigated circulating adenosine levels in 12 patients suffering from migraine without aura, during crises and in crisis-free periods, and have compared the levels noted to those of a population of 10 controls. To determine if there are interactions between adenosine and serotonin, we examined the effect of adenosine and antagonists on the uptake and the release of (14C) serotonin by platelets.

RESULTS AND CONCLUSION

We have reached a dual conclusion: 1) during migraine headaches there is an increase (mean 68%) in circulating adenosine levels and this increase may participate in cephalalgia; 2) activation of A2 receptors by adenosine causes a dose-dependent serotonin uptake by platelets. This inhibition of uptake could participate in the rapid elimination of serotonin in migraine sufferers. As a result of this, the use of adenosine antagonists could be an effective complementary treatment for migraine.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Hypercapnia-induced increases in cerebral blood flow: roles of adenosine, nitric oxide and cortical arousal

The roles of nitric oxide, adenosine and cortical arousal in the response to 7.5% CO2 inhalation were investigated by measuring cerebral blood flow bilaterally in the rat somatosensory cortices with laser-Doppler flow probes. Administration of N(omega)-nitro-L-arginine methyl ester (L-NAME; 20 mg/kg, i.v.) significantly attenuated the response to hypercapnia (mean decrease of 47%). This effect was partially reversed by a subsequent administration of L-arginine. Caffeine (10 mg/kg, i.v.) also significantly reduced hypercapnic responses (mean decrease of 44%). Caffeine administration was also associated with a tendency for animals to exhibit electrocorticographic signs of arousal; often associated with a reduction in the attenuation of the flow response to CO2 inhalation. 8-(3-Chlorostyryl) caffeine (CSC, 1.0 mg/kg), a selective antagonist at adenosine A2a striatal receptors failed to attenuate CO2-evoked responses, whereas CGS 15943, a less selective A2a receptor antagonist, significantly reduced CO2 responses. These data from the rat suggest (1) that both nitric oxide and adenosine may contribute to pial arteriolar vasodilatation during hypercapnia, and (2) that CO2 inhalation acts as a potent stimulus for cortical arousal, with enhanced neuronal activity contributing to the vascular response. The effects of administration of adenosine antagonists, such as the methylxanthines antagonists caffeine and theophylline, on CBF responses to hypercapnia can potentially be negated by the ability of these agents to facilitate CO2-induced cortical arousal.

Effect of i.v. dipyridamole on cerebral blood flow, blood pressure, plasma adenosine and cAMP levels in rabbits

In response to intravenous administration of dipyridamole, the quantitative and temporal changes in plasma adenosine and cyclic AMP (cAMP) levels in relation to the changes in cerebral blood flow (CBF) and mean arterial blood pressure (MABP) have not been studied. Therefore, we investigated simultaneously the changes in CBF (hydrogen and thermal clearance methods), MABP, plasma adenosine (HPLC) and cAMP (radioimmunoassay) levels for 1 h after intravenous injection of 0.7 and 1.4 mg/kg dipyridamole in rabbits. In separate experiments, only plasma adenosine concentrations were measured to determine how and for how long intravenous administration of 0.7 mg/kg dipyridamole is able to inhibit the removal of plasma adenosine. Dipyridamole decreased MABP, increased plasma adenosine and cAMP levels in a dose-dependent manner. The dose-dependency of increases in CBF could not be demonstrated owing to the marked hypotension. The increase in plasma adenosine concentrations was biphasic. The first peak could be detected at the end of the dipyridamole injection. The second peak occurred 20 min after drug administration, simultaneously with the maximal increases in plasma cAMP level and CBF, whereas the maximal fall in MABP developed earlier. Intravenous administration of 0.7 mg/kg dipyridamole inhibited adenosine uptake only by 25%, which lasted less than 10 min. We concluded that intravenously given dipyridamole is responsible only for the initial short-lasting elevation of plasma adenosine concentration, and is able to induce vasodilation without either dipyridamole itself or adenosine necessarily gaining access to the muscular layer.

Effects of a novel inhibitor of guanylyl cyclase on dilator responses of mouse cerebral arterioles

BACKGROUND AND PURPOSE

Nitric oxide-induced vasodilatation is mediated by both cGMP-dependent and -independent mechanisms. Previous studies that examined the role of soluble guanylyl cyclase in cerebral vessels have used methylene blue and LY-83583, compounds that generate superoxide anion and are not specific for inhibition of soluble guanylyl cyclase. We examined the effects of ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one), a novel and highly selective inhibitor of soluble guanylyl cyclase, on responses of cerebral arterioles.

METHODS

The effects of ODQ on responses of cerebral arterioles to acetylcholine, nitroprusside, 8-bromo-cGMP, and adenosine were examined in anesthetized mice by means of a cranial window. The effects of two concentrations of ODQ were examined in the absence and presence of superoxide dismutase. The effects of NG-nitro-L-arginine, an inhibitor of nitric oxide synthase, were also tested.

RESULTS

ODQ (3 and 10 mumol/L) produced concentration-dependent inhibition of dilatation of cerebral arterioles (control diameter = 29 +/- 1 microns) (mean +/- SE) in response to acetylcholine and nitroprusside. For example, 10 mumol/L acetylcholine and 1 mumol/L nitroprusside dilated cerebral arterioles by 28 +/- 3% and 44 +/- 2% in the absence and 6 +/- 2% and 7 +/- 1%, respectively, in the presence of 10 mumol/L ODQ (P < .05 versus control). The inhibitory effects of ODQ were not altered by superoxide dismutase. Vasodilatation in response to 8-bromo-cGMP and adenosine was not inhibited by ODQ. NG-Nitro-L-arginine (100 mumol/L), an inhibitor of nitric oxide synthase, inhibited responses to acetylcholine by approximately 80% but tended to enhance responses to nitroprusside.

CONCLUSIONS

Thus, nitric oxide-mediated dilatation of mouse cerebral arterioles is profoundly inhibited by ODQ, an inhibitor of activity of soluble guanylyl cyclase. Cerebral vasodilator responses to adenosine and 8-bromo-cGMP were preserved in the presence of ODQ, indicating that inhibition by ODQ was selective. In contrast to previously used inhibitors of soluble guanylyl cyclase (methylene blue and LY-83583), the effects of ODQ are not mediated by generation of superoxide anion.

Alteration of human placental vascular tone by antiarrhythmic medications in vitro

INTRODUCTION

Antiarrhythmic medications are commonly used during pregnancy for treatment of maternal or fetal arrhythmias, but little is known about their effect on human placental vascular tone and, consequently, placental blood flow. The objective of this study was to evaluate the tone responses caused by antiarrhythmic medications in human placental vessels from normal term pregnancies in vitro.

METHODS AND RESULTS

Isolated human placental arteries and veins from uncomplicated term pregnancies incubated in Krebs'-bicarbonate under 5% oxygen/5% carbon dioxide/balance nitrogen (PO2 35 to 38 torr) were exposed to cumulative doses of quinidine, procainamide, lidocaine, flecainide, propranolol, amiodarone, verapamil, digoxin, and adenosine after submaximal contraction with 5-hydroxytryptamine. The study was conducted both in the presence and absence of endothelium. The addition of the tested medications caused a significant, dose-dependent relaxation of human placental arteries and veins except for adenosine, which induced a sustained, dose-dependent contraction of human placental vessels regardless of the presence or absence of tone. Removal of the endothelium did not alter these responses.

CONCLUSIONS

Based on these results, the medications tested should have no decremental effect on placental blood flow, with the possible exception of adenosine, which causes significant, dose-dependent contraction of human placental vessels in vitro. Should similar contraction be present in vivo, it may have an adverse effect on the fetus when administering adenosine to pregnant women at term or during labor.

Role of Ca(2+)-dependent K+ channels in cerebral vasodilatation induced by increases in cyclic GMP and cyclic AMP in the rat

BACKGROUND AND PURPOSE

The mechanisms by which cAMP and cGMP produce vasorelaxation are not entirely clear. In this study we examined the hypothesis that relaxation of cerebral arterioles in response to receptor-mediated activation of adenylate cyclase (increase in cAMP) is mediated through Ca(2+)-dependent K+ channels.

METHODS

We measured the diameter of cerebral arterioles (basal diameter, 47 +/- 1 microns) using an open cranial window in anesthetized rats. Agonists and antagonists were applied locally in the cranial window.

RESULTS

Topical application of adenosine (0.1 and 1 mmol/L), a receptor-mediated activator of adenylate cyclase, and dibutyryl cAMP (60 and 200 mumol/L), a cell-permeable analogue of cAMP, dilated cerebral arterioles. Iberiotoxin (50 nmol/L), a selective inhibitor of Ca(2+)-dependent K+ channels, reduced vasodilatation in response to 0.1 and 1 mmol/L adenosine by 66% and 28%, respectively. Tetraethylammonium (TEA) (1 mmol/L), another inhibitor of Ca(2+)-dependent K+ channels, reduced vasodilatation to 0.1 and 1 mmol/L adenosine by 58% and 42%, respectively, and reduced vasodilatation in response to 60 and 200 mumol/L dibutyryl cAMP by 75% and 66%, respectively. Topical application of sodium nitroprusside (0.1 and 1 mumol/L), a direct activator of guanylate cyclase, and 8-bromo cGMP (200 and 600 mumol/L), a cell-permeable analogue, produced dilatation of cerebral arterioles that was inhibited by iberiotoxin (50 nmol/L) and TEA (1 and 3 mmol/L). In contrast, dilatation of cerebral arterioles in response to papaverine (which produces vasodilatation in large part by inhibition of Ca2+ channels) and aprikalim (which produces vasodilation by activation of ATP-sensitive K+ channels) was not inhibited by iberiotoxin or TEA.

CONCLUSIONS

These findings suggest that dilatation of cerebral arterioles by receptor-mediated activation of adenylate cyclase and by direct activation of guanylate cyclase in the rat is mediated in large part by activation of Ca(2+)-dependent K+ channels.

Adenosinergic mechanisms in anticonvulsant action of diazepam and sodium valproate

The effects of adenosine receptor agonists and antagonists were studied in pentylenetetrazole (PTZ)-induced seizures in rats. Animals were pretreated with the non-specific adenosine receptor antagonist, theophylline (50 and 100 mg/kg, i.p.), or the specific A(1) adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), in a dose of 1 mg/kg, i.p., followed by 100% anticonvulsant doses of diazepam (4 mg/kg)/sodium valproate (300 mg/kg, i.p.). Subsequently, they were challenged with convulsant doses of PTZ i.e. 60 mg/kg, i.p. It was seen that while DPCPX could not reverse the protection of both the antiepileptic drugs, theophylline significantly reversed this protection, as assessed by percent incidence of seizures and change in latency parameters. In another set of experiments, the rats were pretreated with a combination of subanticonvulsant doses of adenosine (500 mg/kg) or specific adenosine A(1) receptor agonist, cyclopentyladenosine (CPA) and diazepam (0.5 and 1 mg/kg)/sodium valproate (150 mg/kg), prior to PTZ challenge. We observed a decrease in incidence and increase in latency of seizures following either combination. The protection observed was independent of the hypothermic and hypotensive effects of adenosine and CPA. These results indicate that though A(1) agonist enhances the protection of diazepam and sodium valproate, a direct involvement of adenosine A(1) receptor in anticonvulsant action of these drugs is doubtful.

Effects of adenosine A2A receptor agonist, CGS 21680, on blood pressure, cardiac index and arterial conductance in anaesthetized rats

The effects of 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) on blood pressure, total peripheral resistance, cardiac index, heart rate and arterial conductance in different vascular beds in the presence and absence of hexamethonium (ganglionic blocker) and phenylephrine (alpha 1-adrenoceptor agonist) were investigated in pentobarbitone-anaesthetized rats using a radioactive microsphere technique. CGS 21680 (0.1, 0.3 and 1.0 microgram/kg/min) significantly decreased blood pressure and total peripheral resistance, and increased heart rate and cardiac index. In addition, after infusion with CGS 21680 (0.1, 0.3 and 1.0 microgram/kg/min) arterial conductance in coronary bed significantly increased. However, while CGS 21680 (0.3 and 1.0 microgram/kg/min) significantly increased conductance in skeletal muscle, it significantly decreased splenic arterial conductance. Moreover, CGS 21680 (1.0 microgram/kg/min) significantly increased conductance in cerebral arterial bed. Infusion with hexamethonium (200 micrograms/kg/min) resulted in significant reduction in blood pressure, heart rate and cardiac index whereas stroke volume and total peripheral resistance remained unchanged. In animals that were pretreated with hexamethonium (200 micrograms/kg/min), further administration of CGS 21680 (0.3 microgram/kg/min), compared to CGS 21680 alone, significantly reduced blood pressure, heart rate and cardiac index but did not affect total peripheral resistance or conductance in any vascular bed. Administration of phenylephrine (7 micrograms/kg/min) resulted in a significant increase in blood pressure and total peripheral resistance, and a significant reduction in cardiac index and heart rate. In animals infused with phenylephrine and CGS 21680 combined, in comparison to those animals that received CGS 21680 alone, no significant differences in blood pressure, heart rate, total peripheral resistance, cardiac index or conductance in any vascular beds were found. Our present findings suggest that CGS 21680 decreased blood pressure by decreasing total peripheral resistance, and increased cardiac index possibly through a reflex-mediated increase in heart rate. Moreover the coronary arterial bed is the most sensitive and cerebral arterial bed is the least sensitive to the effects of CGS 21680. In addition, the autonomic nervous system did not appear to play a major role in the actions of CGS 21680 on arterial conductance, and there was no difference in the action of this compound in the states of normal and raised vascular tone.

Distribution and physiological roles of ATP-sensitive K+ channels in the vertebrobasilar system of the rabbit

The effect of an opener (levcromakalim) and a blocker (glibenclamide) of ATP-sensitive K+ (KATP) channels was investigated in the vertebrobasilar system of the rabbit. Arterial tension and membrane potential were measured by the isometric tension recording method and the microelectrode technique, respectively. Glibenclamide (10(-6) mol/L) depolarized the membrane and potentiated the contraction to histamine in vertebral arteries. The sensitivity to the relaxant effects of levcromakalim was in the following descending order: vertebral > proximal basilar > distal basilar > superior cerebellar arteries. Vertebral arteries were approximately 50 times more sensitive to levcromakalim than were superior cerebellar arteries. The relaxation to levcromakalim was abolished by glibenclamide (10(-6) mol/L). Glibenclamide attenuated vasorelaxation to adenosine in proximal arteries (vertebral and proximal basilar) but not in superior cerebellar arteries. Levcromakalim (7 x 10(-8) mol/L) and adenosine (10(-5) mol/L) induced glibenclamide-sensitive membrane hyperpolarization in vertebral arteries but not in distal basilar arteries. These results suggest that KATP channels contribute to the determination of resting membrane potential and resting tone in vertebral arteries. Furthermore, there is a marked heterogeneity in the sensitivity to an opener of KATP channels, and the heterogeneity has a functional link to the mechanism underlying vasorelaxation to adenosine in the vertebrobasilar system of the rabbit.

Adenosine activates ATP-sensitive potassium channels in arterial myocytes via A2 receptors and cAMP-dependent protein kinase

The mechanism by which the endogenous vasodilator adenosine causes ATP-sensitive potassium (KATP) channels in arterial smooth muscle to open was investigated by the whole-cell patch-clamp technique. Adenosine induced voltage-independent, potassium-selective currents, which were inhibited by glibenclamide, a blocker of KATP currents. Glibenclamide-sensitive currents were also activated by the selective adenosine A2-receptor agonist 2-p-(2-carboxethyl)-phenethylamino-5'-N- ethylcarboxamidoadenosine hydrochloride (CGS-21680), whereas 2-chloro-N6-cyclopentyladenosine (CCPA), a selective adenosine A1-receptor agonist, failed to induce potassium currents. Glibenclamide-sensitive currents induced by adenosine and CGS-21680 were largely reduced by blockers of the cAMP-dependent protein kinase (Rp-cAMP[S], H-89, protein kinase A inhibitor peptide). Therefore, we conclude that adenosine can activate KATP currents in arterial smooth muscle through the following pathway: (i) Adenosine stimulates A2 receptors, which activates adenylyl cyclase; (ii) the resulting increase intracellular cAMP stimulates protein kinase A, which, probably through a phosphorylation step, opens KATP channels.

Role of potassium channels in cerebral blood vessels

BACKGROUND

Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. In cerebral blood vessels, four different potassium channels have been described: ATP-sensitive potassium channels, calcium-activated potassium channels, delayed rectifier potassium channels, and inward rectifier potassium channels.

SUMMARY OF REVIEW

Activation of ATP-sensitive and calcium activated potassium channels appears to play a major role in relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, and hypoxia. Both calcium-activated and delayed rectifier potassium channels may contribute to a negative feedback system that regulates tone in large cerebral arteries. The influence of ATP-sensitive and calcium-activated potassium channels is altered in disease states such as hypertension, diabetes, and atherosclerosis.

CONCLUSIONS

Activation of potassium channels is a major mechanism of cerebral vasodilatation. Alteration of activity of potassium channels and impairment of vasodilatation may contribute to the development or maintenance of cerebral ischemia or vasospasm.

A novel non-xanthine adenosine A1 receptor antagonist

FK453, (+)-(R)-[(E)-3-(2-phenylpyrazolo[1,5-alpha]pyridin-3-yl) acryloyl]-2-piperidine ethanol, was examined for adenosine receptor antagonistic activity using isolated guinea-pig atria and aorta and for affinity for adenosine receptors in the rat cerebral cortex and striatum in comparison with FR113452 (S enantiomer of FK453), PD116948 (1,3-dipropyl-8-cyclopentylxanthine), theophylline (1,3-dimethylxanthine) and CGS15943 ([1,2,4]triazolo[1,5-c]quinazolone). FK453 showed potent inhibition of the negative inotropic activity elicited by 10 microM adenosine with an IC50 of 560 pM in guinea-pig atria. However, FK453 was less potent in inhibiting the relaxation induced by 3.2 microM adenosine and had an IC50 of 1.18 microM in guinea-pig aorta. The IC50 values for FR113452, PD116948, theophylline and CGS15943 were 1.18 microM, 1.31 nM, 20.2 microM and 74.2 nM in atria and > 100 microM, 656 nM, 239 microM, 127 nM in aorta respectively. In the binding study, FK453 antagonized [3H]N6-cyclohexyladenosine binding to the rat cortical adenosine A1 receptor with an IC50 of 17.2 nM. The IC50 values for FR113452, PD116948, theophylline and CGS15943 were 10.1 microM, 4.7 nM, 67.7 microM and 241 nM respectively. FK453 inhibited [3H]5'-N-ethylcarboxamideadenosine binding to rat striatum adenosine A2 receptor with an IC50 of 11.3 microM. FK453 had no adenosine A1 receptor agonistic activity, since it had no negative inotropic activity up to 100 microM in isolated guinea-pig atria. These results demonstrate that FK453 is a novel non-xanthine adenosine receptor antagonist and is potent and selective for the adenosine A1 receptor subtype.

Dilatation of cerebral arterioles in response to activation of adenylate cyclase is dependent on activation of Ca(2+)-dependent K+ channels

The role of Ca(2+)-dependent potassium channels in mediating vascular responses to activation of adenylate cyclase in vivo is not known. The goal of this study was to examine the hypothesis that dilatation of cerebral arterioles in response to activation of adenylate cyclase is mediated by activation of Ca(2+)-dependent potassium channels. Diameters of cerebral arterioles were measured in vivo in anesthetized rabbits. Topical application of forskolin (1 and 10 mumol/L), a direct activator of adenylate cyclase, dilated cerebral arterioles by 40 +/- 8% (mean +/- SEM) and 71 +/- 9%, respectively, from a control diameter of 85 +/- 4 microns. Iberiotoxin (50 and 100 nmol/L), a selective inhibitor of Ca(2+)-dependent potassium channels, inhibited dilatation in response to both concentrations of forskolin by 45% to 60%. We obtained similar results by using charybdotoxin (50 nmol/L), another inhibitor of Ca(2+)-dependent potassium channels. Vasodilatation in response to dibutyryl cAMP (a cell-permeable cAMP analogue) was also inhibited by iberiotoxin. In contrast, dilatation of cerebral arterioles in response to sodium nitroprusside and acetylcholine (activators of guanylate cyclase) and aprikalim (activator of ATP-sensitive potassium channels) was not inhibited by iberiotoxin. These findings suggest that dilatation of cerebral arterioles in response to forskolin and increases in intracellular concentrations of cAMP are mediated by activation of Ca(2+)-dependent potassium channels. Thus, activation of Ca(2+)-dependent potassium channels may be a major mechanism of cerebral vasodilatation in response to activation of adenylate cyclase in vivo.

Nitric oxide promotes arteriolar dilation during cortical spreading depression in rabbits

BACKGROUND AND PURPOSE

Pial arterioles transiently dilate during cortical spreading depression (CSD), although the mechanisms are unclear. We tested the hypothesis that increased production of nitric oxide (NO) promotes arteriolar dilation.

METHODS

Urethane-anesthetized rabbits were equipped with cranial windows, and the diameter (reported in micrometers) of a pial arteriole was determined via intravital microscopy. In each rabbit, a baseline CSD was elicited by microapplication of KCl onto the cortex, and resultant pial arteriolar dilation was measured. Either 100 mumol/L N omega-nitro-L-arginine methyl ester (L-NAME) or 50 mumol/L NG-nitro-L-arginine (L-NA), both competitive NO synthase inhibitors, was then applied to the brain surface. A CSD was elicited as before. The L-NAME and L-NA were then removed by artificial cerebrospinal fluid washes. An additional CSD was induced with KCl as before.

RESULTS

Control CSD in the L-NAME group dilated pial arterioles; baseline diameter, 66 +/- 7 mm, with CSD = 106 +/- 8 mm (59% increase). After topically applied L-NAME, CSD dilated pial arterioles less: baseline diameter, 61 +/- 7 mm, with CSD = 77 +/- 6 mm (26% increase), P < .05 compared with control CSD diameter. Topical L-NA had similar effects on CSD: control CSD dilated pial arterioles 51%; after topical L-NA, only 14% (P < .05). After removal of L-NAME or L-NA, CSD-induced pial arteriolar dilation was similar to original control values.

CONCLUSIONS

The reversible inhibition of CSD-induced pial arteriolar dilation by either L-NAME or L-NA suggests that NO contributes to arteriolar dilation observed with CSD.

Interaction between histamine and adenosine in human cerebromicrovascular endothelial cells: modulation of second messengers

This study demonstrates the presence of histamine H1 and H2 receptors and purinoreceptors A1 and A2 on endothelial cells derived from human brain microvessels (HBEC). Histamine induced formation of both inositol triphosphate (IP3) (EC50 = 10.2 +/- 0.9 microM) and cyclic adenosine monophosphate (cAMP) (EC50 = 5.2 +/- 0.9 microM) in HBEC in a concentration-dependent fashion. IP3 formation was inhibited by H1 receptor antagonists mepyramine maleate and chlorphenyramine, but not by H2 receptor antagonist cimetidine. Production of cAMP was efficiently inhibited by cimetidine. Selective A1 receptor agonists decreased, whereas A2 receptor agonists increased cAMP production in HBEC. When added together with histamine to HBEC cultures, both A1 and A2 receptor agonists diminished histamine-induced IP3 stimulation. This effect was reversed in the presence of specific A1 and A2 receptor antagonists, respectively. Marked augmentation of histamine-induced cAMP production by HBEC was observed in the presence of A2 agonist. This response was dependent on H1 receptors, since it was reduced in the presence of H1-receptor antagonist. It is suggested that interaction between histamine and adenosine modulating induction of second messengers in HBEC may influence endothelium-dependent responses of brain microvascular compartments.

Use of HPLC to measure circulating adenosine levels in migrainous patients

Adenosine is a powerful natural vasodilator that could be involved in migraine. It is difficult to assay this nucleoside, however, because it has a short half-life. We have used HPLC to compare the concentrations of blood adenosine sampled in crisis-free intervals and during crisis periods in ten patients with common migraine and have compared these levels to those noted in a control population. Our sampling technique uses vacuum suction and enables rapid mixing of the blocking solution and whole venous blood. This results in reproducible HPLC assays. We also show that, during a migraine crisis, mean blood adenosine levels increase by 47%. However, the origin of this adenosine release is difficult to define.

Effects of adenosine and its analogues on isolated intracerebral arterioles. Extraluminal and intraluminal application

We evaluated the responses of brain parenchymal arterioles to intraluminal and extraluminal application of adenosine and its analogues. Intracerebral arterioles (28.4- to 60.3-microns diameter) were isolated from Sprague-Dawley rats, cannulated with micropipettes, and perfused in vitro. Both extraluminal and intraluminal adenosine, 5'-(N-ethylcarboxamido)adenosine (NECA), R-N6-(phenylisopropyl)adenosine (R-PIA), and S-N6-(phenylisopropyl)adenosine (S-PIA) elicited concentration-dependent dilation of these arterioles, but intraluminal application was less potent and efficacious than extraluminal application. Inosine was not vasoactive. A common order of agonist potency (NECA > adenosine > R-PIA > or = S-PIA) was determined for both extraluminal and intraluminal application. Theophylline (10 microM) caused a rightward shift of the adenosine concentration-response curve and a 50-fold reduction in potency. Intraluminal theophylline was one sixth as effective as extraluminal theophylline in antagonizing the extraluminal adenosine response, whereas intraluminal 8-sulfophenyltheophylline, a polar theophylline derivative, was ineffective. Polyadenylic acid (PolyA, 1 microM), an adenosine polymer that does not penetrate the endothelium, induced a dilation of 44.2 +/- 5.3% when applied extraluminally but had no effect when infused intraluminally. The dilator effect of PolyA was antagonized by theophylline. We conclude that: (1) intraluminal adenosine and its analogues are effective dilators of intracerebral arterioles, (2) the dilator effects of both intraluminally and extraluminally applied adenosine are predominantly mediated by A2-type receptors, and (3) adenosine receptors mediating vasodilation are not present on the luminal surface of the endothelium.

Vascular actions of purines in the foetal circulation of the human placenta

1. The vasoactive effects of adenosine triphosphate (ATP), adenosine and other purines in the foetal circulation of the human placenta were examined. Single lobules of the placenta were bilaterally perfused in vitro with Krebs buffer (maternal and foetal sides 5 ml min-1 each, 95% O2:5% CO2, 37 degrees C). Changes in foetal vascular tone were assessed by recording perfusion pressure during constant infusion of each purine. To allow recording of the vasodilator effects, submaximal vasoconstriction was induced by concomitant infusion of prostaglandin F2 alpha (0.7-2.0 mumol l-1). 2. ATP (1.0-100 mumol l-1) usually caused concentration-dependent reductions in perfusion pressure. However, biphasic with initial transient increases, or only increases in pressure were sometimes observed. Falls in pressure caused by ATP were significantly reduced by addition to the perfusate of NG-nitro-L-arginine (L-NOARG) (100 mumol l-1) but not NG-nitro-D-arginine (D-NOARG) (100 mumol l-1). They were not influenced by addition of indomethacin (10 mumol l-1) or L-arginine (100 mumol l-1). 3. Adenosine (0.01-1.0 mmol l-1) consistently caused concentration-dependent reductions in perfusion pressure, this effect not being influenced by indomethacin. L-NOARG, but not D-NOARG, reduced the potency of adenosine approximately three fold. L-Arginine, but not D-arginine enhanced its potency by a similar amount. 4. 2-Methylthio-ATP, a selective P2 gamma agonist was approximately 50 times more potent than ATP as a vasodilator agent, always causing decreases in perfusion pressure. 5. Beta-gamma-Methylene ATP, a selective P20 agonist, was approximately 100 times more potent than ATP as a vasoconstrictor, but only caused transient increases in perfusion pressure.6. The rank order of vasodilator potencies of a selection of adenosine receptor agonists was, 2-chloroadenosine>>5-(N-cyclopropyl)-carboxamidoadenosine, >5-N-ethylcarboxamidoadenosine, >2-chloro-N6-cyclopentyladenosine, >CGS-21680 > N6-cyclohexyladenosine = adenosine. Vasodilatation due to adenosine was inhibited by the PI-A2 receptor antagonist 3,7-dimethyl-l-propargylxanthine(DMPX).7. These results suggest that ATP may cause an endothelium-dependent vasodilatation in the foetal vessels of the human placenta via activation of a P2y receptor linked to the formation of nitric oxide(NO). Vasodilatation caused by ATP may mask an accompanying vasoconstrictor effect mediated, via a P2X receptor, in the villous vascular smooth muscle. Adenosine acting on P1-A2 receptors, which are also present in the foetal vasculature, may require synergistic interaction with NO to achieve a maximal vasodilator response.

Effects of dipyridamole in spontaneously hypertensive rabbits with diffuse chronic cerebral ischemia

The effect of intravenous dipyridamole (0.7 mg/kg) on cerebral blood flow (CBF), mean arterial blood pressure (MABP), heart rate, respiration rate, cerebral electrical activity, arterial blood gases, pH, and glucose was investigated in 14 normotensive and 14 stroke-prone spontaneously hypertensive anesthetized rabbits. CBF was measured by hydrogen and heat clearance. In both groups, MABP decreased (normotensive: -24 mm Hg, hypertensive: -47 mm Hg; ANOVA: P < 0.0001) and CBF increased (normotensive: +59 ml/100 g/min, hypertensive: +72 ml/100 g/min; ANOVA: P < 0.0002). CBF returned to the initial level 21 min later in hypertensive than in normotensive rabbits. Changes in other parameters were insignificant. In additional experiments, 30 mg/kg theophylline entirely prevented the cerebral vasodilator and systemic hypotensive effects of dipyridamole in both normotensive and hypertensive rabbits. We conclude that, in stroke-prone spontaneously hypertensive rabbits, the longer-lasting and larger CBF increase in response to dipyridamole may be attributed to reversible functional changes in the cerebral vasculature resulting from hypertension.

Mechanisms of drug actions against neuronal damage caused by ischemia--an overview

1. Oxygen and energy deficits induces a cascade of pathological processes which lead to neuronal dysfunction and cell death. 2. The pathogenesis of ischemia-induced neuronal damage includes a disturbed calcium homeostasis, an excessive release of EAA and an enhanced formation of free oxygen radicals. 3. Calcium antagonists inhibit Ca2+ influx into the neuronal cell via VSCC. 4. Glutamate antagonists reduce intracellular Ca2+ concentration by inactivation of NMDA receptor-associated calcium channels (NMDA antagonists) or AMPA/quisqualate receptor-linked sodium channels (non-NMDA antagonists). 5. Furthermore, oxygen radical scavengers can avoid neuronal damage. 6. Agonists of the adenosinergic and serotonergic transmitter systems contribute to neuroprotection by hyperpolarization of the neuronal membrane due to an increase of K+ permeability.

A neuroprotective effect of adenosine A1-receptor agonists on ischemia-induced decrease in 2-deoxyglucose uptake in rat hippocampal slices

The effects of adenosine (A) receptor agonists on ischemia-induced impairment of 2-deoxyglucose (2-DG) uptake by rat hippocampal slices was evaluated. Hippocampal slices were exposed to 20-min hypoxia + hypoglycemia (ischemia) and then returned to oxygenated and glucose-containing Krebs-Ringer solution for 6 h. Ischemia reduced 2-DG uptake in the hippocampal slices. The ischemia-induced reduction in 2-DG uptake was attenuated by pretreatment with A1 receptor agonists but not with A2 receptor agonists. 8-Phenyltheophylline, an A1 receptor antagonist, exacerbated the ischemia-induced decrease. The A1 receptor agonist-induced neuroprotective effect was blocked by co-treatment with 8-phenyltheophylline. The present study suggests that the A1 receptor-mediated function has a protective role in ischemia-induced decreases in glucose metabolism in hippocampal slices.

Effects of propentofylline on adenosine A1 and A2 receptors and nitrobenzylthioinosine-sensitive nucleoside transporters: quantitative autoradiographic analysis

Previous studies have demonstrated that the xanthine compound, propentofylline, has beneficial effects in models of cerebral ischemia and can enhance some and exhibit other effects of adenosine. We investigated the in vitro effects of propentofylline and its hydroxy metabolite, A72,0287, on the binding of [3H]cyclohexyladenosine ([3H]CHA), [3H]2-[p-(2-carbonyl-ethyl)-phenylethyl-amino]-5'-N- ethylcarboxamido adenosine ([3H]CGS 21680) and [3H]nitrobenzylthioinosine ([3H]NBMPR) to adenosine A1 and A2 receptors and NBMPR-sensitive nucleoside transporters, respectively, in 10-microns coronal rat brain sections. Both xanthines had micromolar affinity for each of these sites with approximately 10-fold lower affinity for A2 receptors than for A1 receptors and [3H]NBMPR binding sites. Saturation analysis of [3H]CHA or [3H]CGS 21680 binding in the presence of increasing concentrations of propentofylline produced significant increases in KD values without affecting Bmax values; thus propentofylline is a competitive inhibitor at A1 and A2 receptors. The effects on A2 receptors apparently require higher concentrations (Ki approximately 200 microM) than the effects on A1 receptors (Ki approximately 20 microM). Propentofylline was also found to be a competitive inhibitor of [3H]NBMPR binding. Therefore we conclude that propentofylline interacts with adenosine-responsive systems to increase interstitial adenosine concentrations and to selectively inhibit A1 receptors.

Cellular processes in atherogenesis: potential targets of Ca2+ channel blockers

Atherosclerosis is characterized by increased endothelial permeability, monocyte infiltration, intimal smooth muscle cell (SMC) proliferation, platelet aggregation and the accumulation of lipids, calcium and extracellular matrix components in the vessel wall. In various animal studies and recently in humans it could be established that Ca2+ channel blockers delayed the progression of the atherosclerotic process at the stage of early lesions. This review surveys the interaction of Ca2+ channel blockers with various membrane proteins (purinergic receptors, nucleoside transporter, peripheral benzodiazepine receptors, multi-drug resistance protein) which are involved in signal transduction and their potential impact on the observed antiatherosclerotic effects. Although the precise mechanisms have yet to be fully elucidated, it has been clearly shown that these drugs inhibit smooth muscle cell proliferation and migration, improve cellular lipoprotein metabolism in vascular cells, alter phospholipid turnover, decrease platelet adhesion in the vessel wall, reduce extracellular matrix synthesis and protect against radical induced cell damage. Most of these effects are independent of Ca2+ flux across voltage-operated Ca2+ channels. However, all these processes are relevant to the pathogenesis of atherosclerosis and therefore the elucidation of the antiatherogenic mechanisms of Ca2+ channel blockers at the cellular level is of great interest. The future development of Ca2+ channel blockers with altered molecular structures optimized for their antiatherosclerotic targets may provide a useful tool in the therapy of atherosclerosis and risk factor intervention. The protective mechanisms are related to a stabilization of cell membrane integrity, the modulation of secretory activities and cell/cell communication processes rather than to a lowering of plasma lipoprotein levels.

Role of protein kinase C in bradykinin-induced increases in microvascular permeability

The goal of this study was to determine whether protein kinase C mediates bradykinin-induced increases in microvascular permeability. Permeability of the hamster cheek pouch was evaluated using intravital fluorescent microscopy and fluorescein isothiocyanate (FITC)-dextran (MW 70,000). We examined effects of sphingosine, a protein kinase C inhibitor, on bradykinin-induced increases in permeability. Increases in permeability were quantitated by counting the number of leaky sites and calculating the clearance of FITC-dextran. During bradykinin (10(-6) M), leaky sites increased from 0 to 40 +/- 4 (mean +/- SEM) sites/0.11 cm2, and clearance increased from 1.7 +/- 1.0 to 22 +/- 9 ml/sec x 10(-6). The bradykinin type-2 receptor antagonist D-Arg,[Hyp3,Thi5,8,D-Phe7]-bradykinin virtually abolished formation of leaky sites in response to bradykinin. To determine whether changes in microvascular pressure contribute to the increase in leaky sites, venular pressure was measured using a micropipette and survo-null device. Increases in cheek pouch venular pressure were similar during application of bradykinin and adenosine, which increased permeability, and isoproterenol, which did not increase permeability in the cheek pouch. Thus, increases in permeability were not linked to changes in microvascular pressure. The protein kinase C inhibitor, sphingosine (10(-6) M), markedly attenuated responses to bradykinin. Leaky sites increased from 0 to only 2 +/- 1 sites/0.11 cm2, and clearance increased from 3.9 +/- 1.4 to only 6.7 +/- 2.2 ml/sec x 10(-6). To test the specificity of sphingosine, we examined effects of adenosine (10(-6) M). Sphingosine did not significantly alter increases in microvascular permeability in responses to adenosine. We also examined effects of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), another protein kinase C inhibitor, on responses to bradykinin and adenosine. H-7 greatly attenuated formation of leaky sites during stimulation with bradykinin and did not alter the number of leaky sites produced during adenosine. The findings suggest that protein kinase C may mediate increases in vascular permeability in response to bradykinin.

Adenosine-induced dilatation of the rabbit hepatic arterial bed is mediated by A2-purinoceptors

1. This study was carried out in order to identify the receptor responsible for adenosine-induced dilatation of the hepatic arterial vascular bed. 2. Livers of 10 New Zealand White rabbits were perfused in vitro with Krebs-Bülbring buffer via the hepatic artery and the portal vein at constant flows of 26 and 77 ml min-1 100 g-1 liver respectively. The tone of the preparation was raised by the presence of noradrenaline in the perfusate (concentration: 10(-5) M). 3. Dose-response curves for adenosine and its analogues 5'-N-ethyl-carboxamido-adenosine (NECA), the 2-substituted NECA analogue CGS 21680C, and R- and S-N6-phenyl-isopropyl-adenosine (R- and S-PIA) were obtained after their injection into the hepatic arterial supply. 4. The order of vasodilator potency of these agents was: NECA greater than CGS 21680C greater than adenosine greater than R-PIA greater than S-PIA. Their potency, expressed relative to that of adenosine, was in the approximate ratio 10:3:1:0.3:0.1, consistent with that resulting from activation of P1-purinoceptors of the A2 sub-type (which mediate vasodilatation due to adenosine). 5. The P1-purinoceptor antagonist 8-phenyltheophylline (10(-5) M) caused significant attenuation of the vasodilatation to adenosine and analogues. 6. It is concluded that adenosine-induced dilatation of the hepatic arterial vascular bed is mediated by P1-purinoceptors of the A2 sub-type.

Selective protection by adenosine receptor agonists against DMCM-induced seizures

The anticonvulsant actions of the adenosine receptor agonists, 1-phenylisopropyladenosine, 2-chloroadenosine and cyclohexyl-adenosine, against DMCM (methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate)-induced seizures in mice were studied with an infusion technique. 2-Chloroadenosine and cyclohexyladenosine were active at 1 mg/kg whereas 1-phenyl-isopropyladenosine was active at 0.03 mg/kg given i.p. At 10 mg/kg, 1-phenylisopropyladenosine was only weakly active against pentylenetetrazol-induced seizures and not active against bicuculline-induced seizures. The selective effect of 1-phenylisopropyladenosine against DMCM-induced seizures suggests that adenosine receptor agonists may allosterically counteract the negative modulating effect of DMCM on GABA coupling to the chloride channel. This indicates that adenosine receptors may have a physiological function within the GABA/benzodiazepine receptor complex in the brain.

Effects of topical adenosine analogs and forskolin on rat pial arterioles in vivo

We utilized the closed window technique to study the in vivo responses of rat pial arterioles to superfused adenosine agonists. Adenosine and its analogs dilated pial arterioles and exhibited the following order of potency: 5'N-ethylcarboxamide adenosine (NECA) greater than 2-chloroadenosine (2-CADO) greater than adenosine = R-N6-phenylisopropyladenosine (R-PIA) = S-PIA greater than N6-cyclohexyladenosine (CHA). This potency profile suggests that cerebral vasodilation is mediated through the A2 receptor. Forskolin (10(-9) M) potentiated the vasodilation caused by 10(-6) M NECA, thus implicating adenylate cyclase activation during NECA-induced vasodilation and providing further support for involvement of the A2 receptor.

Second messenger systems: functional role in cerebrovascular smooth muscle regulation

The role of two second messenger systems in alterations of cerebrovascular smooth muscle tone was examined in feline cerebral arteries using an in vitro preparation of vessel segments and cortical pial vessels in situ. Forskolin, which is known to activate adenylate cyclase, elicited a concentration-dependent relaxation of arteries preconstricted with prostaglandin F2 alpha (PGF2 alpha) (EC50 was approximately 300 nM). Microapplication of forskolin around individual cortical arteries and arterioles in situ elicited a dose-dependent dilatation. The maximum increase in arteriolar calibre was 54 +/- 4% from pre-injection calibre and EC50 was approximately 100 nM. Phorbol 12,13 dibutyrate (PDBu), which activates protein kinase C, elicited strong contractions of cerebral vessels. In vitro, PDBu contracted vessel segments in a concentration-dependent manner (EC50 was approximately 100 nM). Similarly, PDBu elicited potent dose-dependent constriction of pial arterioles in situ. The maximum response to PDBu was a 37 +/- 5% reduction in arteriolar calibre and the concentration eliciting EC50 was approximately 100 nM. These data provide an assessment to capacity of feline cerebral arteries to dilate and contract in response to adenylate cyclase and protein kinase C activation respectively.