Impact of Ramadan fasting on kidney function and related outcomes in patients with chronic kidney disease: a systematic review protocol

INTRODUCTION

Fasting during the month of Ramadan is a significant Islamic religious practice that involves abstinence from food, drink and medication from dawn to dusk. As just under a quarter of the world's population identifies as Muslim, the effect of fasting on chronic conditions, such as chronic kidney disease (CKD) is a topic of broad relevance. To date, the information in this area has been mixed, with many limitations of previous studies. This study aims to synthesise the evidence of the effect of Ramadan fasting on changes on kidney function, risk factors, episodes of acute kidney injury and impact on the quality of life in patients with CKD or kidney transplant.

METHODS AND ANALYSIS

A systematic review of the literature will be conducted, using electronic databases such as MEDLINE, Embase, Global Health, CINAHL and Scopus. Original research and grey literature on the effect of Ramadan fasting in adult patients with CKD or renal transplantation will be included. Two reviewers will independently screen articles for inclusion in the review and independently assess the methodology of included studies using a customised checklist. Mean difference or risk ratio will be reported for continuous or dichotomous outcomes and results will be pooled using a random-effects model where heterogeneity is reasonable. If possible, subgroups (CKD status, setting, season and risk of bias) will be analysed for effect modification with fasting and the outcomes of interest. Risk of bias will be assessed using the Downs and Black checklist.

ETHICS AND DISSEMINATION

The results will be disseminated using a multifaceted approach to engage all stakeholders (patients, practitioners and community leaders). Research ethics board approval is not required as this is a systematic review of previously published research.

PROSPERO REGISTRATION NUMBER

CRD42018088973.

Guidelines, policies, and barriers to kidney care: findings from a global survey

An international survey led by the International Society of Nephrology in 2016 assessed the current capacity of kidney care worldwide. To better understand how governance and leadership guide kidney care, items pertinent to government priority, advocacy, and guidelines, among others, were examined. Of the 116 responding countries, 36% (n = 42) reported CKD as a government health care priority, which was associated with having an advocacy group (χ2 = 11.57; P = 0.001). Nearly one-half (42%; 49 of 116) of countries reported an advocacy group for CKD, compared with only 19% (21 of 112) for AKI. Over one-half (59%; 68 of 116) of countries had a noncommunicable disease strategy. Similarly, 44% (48 of 109), 55% (57 of 104), and 47% (47 of 101) of countries had a strategy for nondialysis CKD, chronic dialysis, and kidney transplantation, respectively. Nearly one-half (49%; 57 of 116) reported a strategy for AKI. Most countries (79%; 92 of 116) had access to CKD guidelines and just over one-half (53%; 61 of 116) reported guidelines for AKI. Awareness and adoption of guidelines were low among nonnephrologist physicians. Identified barriers to kidney care were factors related to patients, such as knowledge and attitude (91%; 100 of 110), physicians (84%; 92 of 110), and geography (74%; 81 of 110). Specific to renal replacement therapy, patients and geography were similarly identified as a barrier in 78% (90 of 116) and 71% (82 of 116) of countries, respectively, with the addition of nephrologists (72%; 83 of 116) and the health care system (73%; 85 of 116). These findings inform how kidney care is currently governed globally. Ensuring that guidelines are feasible and distributed appropriately is important to enhancing their adoption, particularly in primary care. Furthermore, increasing advocacy and government priority, especially for AKI, may increase awareness and strategies to better guide kidney care.

Global access of patients with kidney disease to health technologies and medications: findings from the Global Kidney Health Atlas project

Access to essential medications and health products is critical to effective management of kidney disease. Using data from the ISN Global Kidney Health Atlas multinational cross-sectional survey, global access of patients with kidney disease to essential medications and health products was examined. Overall, 125 countries participated, with 118 countries, composing 91.5% of the world's population, providing data on this domain. Most countries were unable to access eGFR and albuminuria in their primary care settings. Only one-third of low-income countries (LICs) were able to measure serum creatinine and none were able to access eGFR or quantify proteinuria. The ability to monitor diabetes mellitus through serum glucose and glycated hemoglobin measurements was suboptimal. Pathology services were rarely available in tertiary care in LICs (12%) and lower middle-income countries (45%). While acute and chronic hemodialysis services were available in almost all countries, acute and chronic peritoneal dialysis services were rarely available in LICs (18% and 29%, respectively). Kidney transplantation was available in 79% of countries overall and in 12% of LICs. While over one-half of all countries publicly funded RRT and kidney medications with or without copayment, this was less common in LICs and lower middle-income countries. In conclusion, this study demonstrated significant gaps in services for kidney care and funding that were most apparent in LICs and lower middle-income countries.

The effect of socioeconomic status on access to primary care: an audit study

BACKGROUND

Health care office staff and providers may discriminate against people of low socioeconomic status, even in the absence of economic incentives to do so. We sought to determine whether socioeconomic status affects the response a patient receives when seeking a primary care appointment.

METHODS

In a single unannounced telephone call to a random sample of family physicians and general practices (n = 375) in Toronto, Ontario, a male and a female researcher each played the role of a patient seeking a primary care physician. Callers followed a script suggesting either high (i.e., bank employee transferred to the city) or low (i.e., recipient of social assistance) socioeconomic status, and either the presence or absence of chronic health conditions (diabetes and low back pain). We randomized the characteristics of the caller for each office. Our primary outcome was whether the caller was offered an appointment.

RESULTS

The proportion of calls resulting in an appointment being offered was significantly higher when the callers presented themselves as having high socioeconomic status than when they presented as having low socioeconomic status (22.6% v.14.3%, p = 0.04) and when the callers stated the presence of chronic health conditions than when they did not (23.5% v. 12.8%, p = 0.008). In a model adjusted for all independent variables significant at a p value of 0.10 or less (presence of chronic health conditions, time since graduation from medical school and membership in the College of Family Physicians of Canada), high socioeconomic status was associated with an odds ratio of 1.78 (95% confidence interval 1.02-3.08) for the offer of an appointment. Socioeconomic status and chronic health conditions had independent effects on the likelihood of obtaining an appointment.

INTERPRETATION

Within a universal health insurance system in which physician reimbursement is unaffected by patients' socioeconomic status, people presenting themselves as having high socioeconomic status received preferential access to primary care over those presenting themselves as having low socioeconomic status.

Ca2+-dependent induction of TRPM2 currents in hippocampal neurons

TRPM2 is a Ca(2+)-permeable member of the transient receptor potential melastatin family of cation channels whose activation by reactive oxygen/nitrogen species (ROS/RNS) and ADP-ribose (ADPR) is linked to cell death. While these channels are broadly expressed in the CNS, the presence of TRPM2 in neurons remains controversial and more specifically, whether they are expressed in neurons of the hippocampus is an open question. With this in mind, we examined whether functional TRPM2 channels are expressed in this neuronal population. Using a combination of molecular and biochemical approaches, we demonstrated the expression of TRPM2 transcripts and proteins in hippocampal pyramidal neurons. Whole-cell voltage-clamp recordings were subsequently carried out to assess the presence of TRPM2-mediated currents. Application of hydrogen peroxide or peroxynitrite to cultured hippocampal pyramidal neurons activated an inward current that was abolished upon removal of extracellular Ca(2+), a hallmark of TRPM2 activation. When ADPR (300 microM) was included in the patch pipette, a large inward current developed but only when depolarizing voltage ramps were continuously (1/10 s) applied to the membrane. This current exhibited a linear current-voltage relationship and was sensitive to block by TRPM2 antagonists (i.e. clotrimazole, flufenamic acid and N-(p-amylcinnamoyl)anthranilic acid (ACA)). The inductive effect of voltage ramps on the ADPR-dependent current required voltage-dependent Ca(2+) channels (VDCCs) and a rise in [Ca(2+)](i). Consistent with the need for a rise in [Ca(2+)](i), activation of NMDA receptors (NMDARs), which are highly permeable to Ca(2+), was also permissive for current development. Importantly, given the prominent vulnerability of CA1 neurons to free-radical-induced cell death, we confirmed that, with ADPR in the pipette, a brief application of NMDA could evoke a large inward current in CA1 pyramidal neurons from hippocampal slices that was abolished by the removal of extracellular Ca(2+), consistent with TRPM2 activation. Such a current was absent in interneurons of CA1 stratum radiatum. Finally, infection of cultured hippocampal neurons with a TRPM2-specific short hairpin RNA (shRNA(TRPM2)) significantly reduced both the expression of TRPM2 and the amplitude of the ADPR-dependent current. Taken together, these results indicate that hippocampal pyramidal neurons possess functional TRPM2 channels whose activation by ADPR is functionally coupled to VDCCs and NMDARs through a rise in [Ca(2+)](i).

Activation of pannexin-1 hemichannels augments aberrant bursting in the hippocampus

Pannexin-1 (Px1) is expressed at postsynaptic sites in pyramidal neurons, suggesting that these hemichannels contribute to dendritic signals associated with synaptic function. We found that, in pyramidal neurons, N-methyl-d-aspartate receptor (NMDAR) activation induced a secondary prolonged current and dye flux that were blocked with a specific inhibitory peptide against Px1 hemichannels; knockdown of Px1 by RNA interference blocked the current in cultured neurons. Enhancing endogenous NMDAR activation in brain slices by removing external magnesium ions (Mg2+) triggered epileptiform activity, which had decreased spike amplitude and prolonged interburst interval during application of the Px1 hemichannel blocking peptide. We conclude that Px1 hemichannel opening is triggered by NMDAR stimulation and can contribute to epileptiform seizure activity.

TRPM7 channels in hippocampal neurons detect levels of extracellular divalent cations

Exposure to low Ca(2+) and/or Mg(2+) is tolerated by cardiac myocytes, astrocytes, and neurons, but restoration to normal divalent cation levels paradoxically causes Ca(2+) overload and cell death. This phenomenon has been called the "Ca(2+) paradox" of ischemia-reperfusion. The mechanism by which a decrease in extracellular Ca(2+) and Mg(2+) is "detected" and triggers subsequent cell death is unknown. Transient periods of brain ischemia are characterized by substantial decreases in extracellular Ca(2+) and Mg(2+) that mimic the initial condition of the Ca(2+) paradox. In CA1 hippocampal neurons, lowering extracellular divalents stimulates a nonselective cation current. We show that this current resembles TRPM7 currents in several ways. Both (i) respond to transient decreases in extracellular divalents with inward currents and cell excitation, (ii) demonstrate outward rectification that depends on the presence of extracellular divalents, (iii) are inhibited by physiological concentrations of intracellular Mg(2+), (iv) are enhanced by intracellular phosphatidylinositol 4,5-bisphosphate (PIP(2)), and (v) can be inhibited by Galphaq-linked G protein-coupled receptors linked to phospholipase C beta1-induced hydrolysis of PIP(2). Furthermore, suppression of TRPM7 expression in hippocampal neurons strongly depressed the inward currents evoked by lowering extracellular divalents. Finally, we show that activation of TRPM7 channels by lowering divalents significantly contributes to cell death. Together, the results demonstrate that TRPM7 contributes to the mechanism by which hippocampal neurons "detect" reductions in extracellular divalents and provide a means by which TRPM7 contributes to neuronal death during transient brain ischemia.

Regulation of G protein-coupled receptor-adenylyl cyclase responsiveness in human airway smooth muscle by exogenous and autocrine adenosine

Adenosine is a mediator of bronchoconstriction in asthmatics and is believed to mediate its effects through adenosine receptor activation in inflammatory cells. In this study, we identify human airway smooth muscle (ASM) as a direct target of adenosine. Acute exposure of human ASM cultures to adenosine receptor (AR) agonists resulted in rapid accumulation of cyclic adenosine monophosphate (cAMP) with a pharmacologic profile consistent with A(2b)AR activation. Little or no evidence of A1AR or A3AR expression was suggested on acute addition of various AR ligands, although a low level of A1ARs was identified in radioligand binding studies. Treatment with adenosine deaminase suggested that human ASM cultures secrete adenosine that feeds back on A(2b)ARs and regulates basal cAMP levels as well as a small degree of A(2b)AR, beta(2)AR, and prostaglandin E(2) receptor desensitization. When subjected to chronic treatment with AR agonists or agents that enhance accumulation of endogenous, extracellular adenosine, a dual effect of A(2b)AR desensitization and adenylyl cyclase (AC) sensitization was observed. This AC sensitization was eliminated by pertussis toxin and partially reversed by the A1AR antagonist 8-cyclopentyl-1,3-dipropylxanthine, suggesting a contributory role for the A1AR. Overexpression of A1ARs and A(2b)ARs in human ASM cultures resulted in differential effects on basal, agonist-, and AC-mediated cAMP production. These data demonstrate that human ASM is a direct target of exogenous and autocrine adenosine, with effects determined by differential contributions of A(2b) and A1 adenosine receptors that are time-dependent. Accordingly, the relative distribution and activation of AR subtypes in ASM in vivo may influence airway function in diseases such as asthma and warrant consideration in therapeutic strategies that target ARs or alter nucleotide/ nucleoside levels in the airway.

Down-regulation of vascular endothelial growth factor expression after A(2A) adenosine receptor activation in PC12 pheochromocytoma cells

Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen that promotes angiogenesis during embryonic development and the progression of certain pathologies. This study examined the regulation of VEGF expression by adenosine receptor (AR) activation in PC12 rat pheochromocytoma cells. Treatment of cells with the AR agonist CGS21680 reduced the VEGF mRNA level to approximately 20% of that in control cells with an EC(50) value of 0.47 nM, indicative of mediation by the A(2A)AR. Down-regulation of VEGF mRNA by CGS21680 was abolished by pretreatment of cells with the AR antagonist ZM241385. Additionally, ZM241385 alone increased VEGF mRNA by 2.8-fold above basal. RNase protection assays indicated that CGS21680 down-regulated VEGF(121), VEGF(165), and VEGF(189) transcripts. VEGF protein secretion was similarly decreased by CGS21680. Under hypoxic conditions, VEGF mRNA expression was reduced by 85.7% after pretreatment with CGS21680. The down-regulation response appears to be mediated predominately by coupling of the A(2A)AR to G(s) because cholera toxin treatment also reduced VEGF expression. The decrease in VEGF mRNA steady-state levels after A(2A)AR activation is apparently due to a decrease in the VEGF gene transcription rate and not to a decrease in mRNA stability. Thus, depending on the cell type, adenosine may have an inhibitory effect on VEGF production, which may have implications in blood vessel development.

The role of receptor structure in determining adenosine receptor activity

Adenosine produces a wide variety of physiological effects through the activation of cell surface adenosine receptors (ARs). ARs are members of the G-protein-coupled receptor family, and currently, four subtypes, the A1AR, A2AAR, A2BAR, and A3AR, are recognized. This review focuses on the role of receptor structure in governing various facets of AR activity. Ligand-binding properties of ARs are primarily dictated by amino acids in the transmembrane domains of the receptors, although a role for extracellular domains of certain ARs has been suggested. Studies have identified certain amino acids conserved amongst AR subtypes that are critical for ligand recognition, as well as additional residues that may differentiate between agonist and antagonist ligands. Receptor regions responsible for activation of Gs have been identified for the A2AAR. The location of these intracellular sites is consistent with findings described for other G-protein-coupled receptors. Site-directed mutagenesis has been employed to analyze the structural basis for the differences in the kinetics of the desensitization response displayed by various AR subtypes. For the A2AAR and A3AR, agonist-stimulated phosphorylation of the AR, presumably via a G-protein receptor kinase, has been shown to occur. For these AR subtypes, intracellular regions or individual amino acids that may be targets for this phosphorylation have been identified. Finally, the role of A1AR gene structure in regulating the expression of this AR subtype is reviewed.

Selective A(3) adenosine receptor antagonists: water-soluble 3, 5-diacyl-1,2,4-trialkylpyridinium salts and their oxidative generation from dihydropyridine precursors

A(3) adenosine receptor antagonists are sought for their potential antiinflammatory, antiasthmatic, and antiischemic properties. We have found that 3,5-diacyl-1,2,4-trialkyl-6-phenylpyridinium derivatives constitute a novel class of selective A(3) adenosine receptor antagonists. The structure-activity relationships of this class of antagonists, incorporating the 3-thioester, have been explored. The most potent analogue in this group was 2, 4-diethyl-1-methyl-3-(ethylsulfanylcarbonyl)-5-ethyloxycarbonyl -6-phe nylpyridinium iodide (11), which had an equilibrium inhibition constant (K(i)) value of 219 nM at human A(3) receptors (binding of [(125)I]AB-MECA (N(6)-(4-amino-3-iodobenzyl)-5'-N-methylcarbamoyladenosine)) expressed in Chinese hamster ovary (CHO) cells and >10 microM at rat brain A(1) and A(2A) receptors and at recombinant human A(2B) receptors. Compound 11 could be generated through oxidation of the corresponding 3,5-diacyl-1,2,4-trialkyl-6-phenyl-1,4-dihydropyridine, 24, with iodine or in the presence of rat brain homogenates. A 6-cyclopentyl analogue was shown to increase affinity at human A(3) receptors upon oxidation from the 1-methyl-1,4-dihydropyridine analogue, 25, to the corresponding pyridinium derivative, 23 (K(i) 695 nM), suggesting a prodrug scheme. Homologation of the N-methylpyridinium derivatives to N-ethyl and N-propyl at the 1-position caused a progressive reduction in the affinity at A(3) receptors. Modifications of the alkyl groups at the 2-, 3-, 4-, and 5-positions failed to improve potency in binding at A(3) receptors. The pyridinium antagonists are not as potent as other recently reported, selective A(3) receptor antagonists; however, they display uniquely high water solubility (43 mM for 11). Compound 11 antagonized the inhibition of adenylate cyclase elicited by IB-MECA in CHO cells expressing the human A(3) adenosine receptor, with a K(B) value of 399 nM, and did not act as an agonist, demonstrating that the pyridinium salts are pure antagonists.

Pharmacological characterization of novel A3 adenosine receptor-selective antagonists

The effects of putative A3 adenosine receptor antagonists of three diverse chemical classes (the flavonoid MRS 1067, the 6-phenyl-1,4-dihydropyridines MRS 1097 and MRS 1191, and the triazoloquinazoline MRS 1220) were characterized in receptor binding and functional assays. MRS1067, MRS 1191 and MRS 1220 were found to be competitive in saturation binding studies using the agonist radioligand [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide) at cloned human brain A3 receptors expressed in HEK-293 cells. Antagonism was demonstrated in functional assays consisting of agonist-induced inhibition of adenylate cyclase and the stimulation of binding of [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTP-gamma-S) to the associated G-proteins. MRS 1220 and MRS 1191, with KB values of 1.7 and 92 nM, respectively, proved to be highly selective for human A3 receptor vs human A1 receptor-mediated effects on adenylate cyclase. In addition, MRS 1220 reversed the effect of A3 agonist-elicited inhibition of tumor necrosis factor-alpha formation in the human macrophage U-937 cell line, with an IC50 value of 0.3 microM.

Identification of A2a adenosine receptor domains involved in selective coupling to Gs. Analysis of chimeric A1/A2a adenosine receptors

Responses to adenosine are governed by selective activation of distinct G proteins by adenosine receptor (AR) subtypes. The A2aAR couples via Gs to adenylyl cyclase stimulation while the A1AR couples to Gi to inhibit adenylyl cyclase. To determine regions of the A2aAR that selectively couple to Gs, chimeric A1/A2aARs were expressed in Chinese hamster ovary cells and ligand binding and adenylyl cyclase activity analyzed. Replacement of the third intracellular loop of the A2aAR with that of the A1AR reduced maximal adenylyl cyclase stimulation and decreased agonist potency. Restricted chimeras indicated that the NH2-terminal portion of intracellular loop 3 was predominantly responsible for this impairment. Reciprocal chimeras composed primarily of A1AR sequence with limited A2aAR sequence substitution stimulated adenylyl cyclase and thus supported these findings. A lysine and glutamic acid residue were identified as necessary for efficient A2aAR-Gs coupling. Analysis of chimeric receptors in which sequence of intracellular loop 2 was substituted indicated that the nature of amino acids in this domain may indirectly modulate A2aAR-Gs coupling. Replacement of the cytoplasmic tail of the A2aAR with the A1AR tail did not affect adenylyl cyclase stimulation. Thus, selective activation of Gs is predominantly dictated by the NH2-terminal segment of the third intracellular loop of the A2aAR.

Novel N6-(substituted-phenylcarbamoyl)adenosine-5'-uronamides as potent agonists for A3 adenosine receptors

A series of adenosine-5'-uronamide derivatives bearing N6-phenylurea groups have been synthesized and tested for their affinity at A1 and A2A adenosine receptors in rat brain membranes and at cloned rat A3 receptors from stably transfected CHO cells. Some N6-arylcarbamoyl derivatives, N6-((2-chlorophenyl)carbamoyl)-, N6-((3-chlorophenyl)carbamoyl)-, and N6-((4-methoxyphenyl)carbamoyl)adenosine-5'-ethyluronamide (4l-n), were found to have affinity at A3 receptors in the low nanomolar range (Ki values < 10 nM). In CHO cells stably transfected with the rat A3 receptor, compound 4n was found to be a full agonist in inhibiting adenylate cyclase activity. The present study represents the first example of N6-acyl-substituted adenosine analogs having high affinity at adenosine receptors and, in particular, at the A3 receptor subtype.

Structure-activity relationships of 9-alkyladenine and ribose-modified adenosine derivatives at rat A3 adenosine receptors

9-Alkyladenine derivatives and ribose-modified N6-benzyladenosine derivatives were synthesized in an effort to identify selective ligands for the rat A3 adenosine receptor and leads for the development of antagonists. The derivatives contained structural features previously determined to be important for A3 selectivity in adenosine derivatives, such as an N6-(3-iodobenzyl) moiety, and were further substituted at the 2-position with halo, amino, or thio groups. Affinity was determined in radioligand binding assays at rat brain A3 receptors stably expressed in Chinese hamster ovary (CHO) cells, using [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)adenosine-5'-(N-methyluronamide)), and at rat brain A1 and A2a receptors using [3H]-N6-PIA ((R)-N6-phenylisopropyladenosine) and [3H]CGS 21680 (2-[[[4-(2-carboxyethyl)-phenyl]ethyl]amino]-5'- (N-ethylcarbamoyl)adenosine), respectively. A series of N6-(3-iodobenzyl) 2-amino derivatives indicated that a small 2-alkylamino group, e.g., methylamino, was favored at A3 receptors. N6-(3-Iodobenzyl)-9-methyl-2-(methylthio)adenine was 61-fold more potent than the corresponding 2-methoxy ether at A3 receptors and of comparable affinity at A1 and A2a receptors, resulting in a 3-6-fold selectivity for A3 receptors. A pair of chiral N6-(3-iodobenzyl) 9-(2,3-dihydroxypropyl) derivatives showed stereoselectivity, with the R-enantiomer favored at A3 receptors by 5.7-fold. 2-Chloro-9-(beta-D-erythrofuranosyl)-N6-(3-iodobenzyl)adenine had a Ki value at A3 receptors of 0.28 microM. 2-Chloro-9-[2-amino-2,3-dideoxy-beta-D-5-(methylcarbamoyl)- arabinofuranosyl]-N6-(3-iodobenzyl)adenine was moderately selective for A1 and A3 vs A2a receptors. A 3'-deoxy analogue of a highly A3-selective adenosine derivative retained selectivity in binding and was a full agonist in the inhibition of adenylyl cyclase mediated via cloned rat A3 receptors expressed in CHO cells. The 3'-OH and 4'-CH2OH groups of adenosine are not required for activation at A3 receptors. A number of 2',3'-dideoxyadenosines and 9-acyclic-substituted adenines appear to inhibit adenylyl cyclase at the allosteric "P" site.

Adenosine receptor subtypes: characterization and therapeutic regulation

Adenosine receptors (ARs) are members of the G protein-coupled receptor family and mediate the multiple physiological effects of adenosine. Currently, four AR subtypes have been cloned: A1AR, A2aAR, A2bAR, and A3AR. All subtypes are distinctly distributed throughout the body and AR agonists and antagonists have potential therapeutic utility. Knowledge of AR amino acid structure has been utilized in mutagenesis studies to identify specific receptor regions that interact with distinct classes of ligands. Cloning of ARs has also permitted receptor regulatory processes such as desensitization to be studied in greater detail, in particular, the molecular mechanisms underlying this event. Cloning of the human A1AR has revealed that alternate splicing generates distinct receptor transcripts. The existence of a particular transcript in a tissue or cell apparently regulates the level of A1AR expression in the tissue. This review focuses on these aspects of AR structure and function and their therapeutic regulation.

Search for new purine- and ribose-modified adenosine analogues as selective agonists and antagonists at adenosine receptors

The binding affinities at rat A1, A2a, and A3 adenosine receptors of a wide range of derivatives of adenosine have been determined. Sites of modification include the purine moiety (1-, 3-, and 7-deaza; halo, alkyne, and amino substitutions at the 2- and 8-positions; and N6-CH2-ring, -hydrazino, and -hydroxylamino) and the ribose moiety (2'-, 3'-, and 5'-deoxy; 2'- and 3'- O-methyl; 2'-deoxy 2'-fluoro; 6'-thio; 5'-uronamide; carbocyclic; 4'- or 3'-methyl; and inversion of configuration). (-)- and (+)-5'-Noraristeromycin were 48- and 21-fold selective, respectively, for A2a vs A1 receptors. 2-Chloro-6'-thioadenosine displayed a Ki value of 20 nM at A2a receptors (15-fold selective vs A1). 2-Chloroadenin-9-yl(beta-L-2'-deoxy-6'- thiolyxofuranoside) displayed a Ki value of 8 microM at A1 receptors and appeared to be an antagonist, on the basis of the absence of a GTP-induced shift in binding vs a radiolabeled antagonist (8-cyclopentyl-1,3-dipropyl-xanthine). 2-Chloro-2'-deoxyadenosine and 2-chloroadenin-9-yl(beta-D-6'-thioarabinoside) were putative partial agonists at A1 receptors, with Ki values of 7.4 and 5.4 microM, respectively. The A2a selective agonist 2-(1-hexynyl)-5'-(N-ethylcarbamoyl)adenosine displayed a Ki value of 26 nM at A3 receptors. The 4'-methyl substitution of adenosine was poorly tolerated, yet when combined with other favorable modifications, potency was restored. Thus, N6-benzyl-4'-methyladenosine-5'-(N-methyluronamide) displayed a Ki value of 604 nM at A3 receptors and was 103- and 88-fold selective vs A1 and A2a receptors, respectively. This compound was a full agonist in the A3-mediated inhibition of adenylate cyclase in transfected CHO cells. The carbocyclic analogue of N6-(3-iodobenzyl)adenosine-5'-(N-methyluronamide) was 2-fold selective for A3 vs A1 receptors and was nearly inactive at A2a receptors.

Adenosine receptors: protein and gene structure

Adenosine produces a wide variety of effects throughout the body via activation of cell surface adenosine receptors. Adenosine receptors belong to the family of seven transmembrane domain G protein-coupled receptors and four subtypes have been cloned from a variety of species: the A1AR, A2aAR, A2bAR and A3AR. With a knowledge of both the protein sequence of adenosine receptors and the structure of the A1AR gene, the function and regulation of these receptors can be further explored. Site-directed mutagenesis of the A1AR has resulted in the identification of amino acid residues in transmembrane domains 6 and 7 that are critical in both agonist and antagonist binding. The construction and analysis of A1/A3 chimeric receptors has also revealed regions of adenosine receptors important in ligand binding. These include the distal region of the second extracellular loop of adenosine receptors, which has a role in the binding of both agonist and antagonist ligands. A segment of the exofacial portion of the transmembrane domain 5 of adenosine receptors appears to be involved in the selective recognition of agonist ligands containing a substitution at the 5'-position of the ribose moiety. Isolation of the genomic sequence of the human A1AR, in combination with analysis of the transcript distribution in several tissues, indicates that alternative splicing of the human A1AR occurs in the 5'-untranslated region of the gene. Two distinct transcripts, containing either exons 3, 5 and 6 or exons 4, 5 and 6, exist with exons 3 and 4 apparently mutually exclusive. The exon 4, 5 and 6 transcript has been detected in all tissues that express the A1AR, while the exon 3, 5 and 6 mRNA is found in tissues that display a relatively high A1AR expression. Findings suggest that the presence of two ATG codons in exon 4, upstream of the translation start site, is involved in the repression of the A1AR expression in those tissues containing the exon 4, 5 and 6 transcript.

Selective ligands for rat A3 adenosine receptors: structure-activity relationships of 1,3-dialkylxanthine 7-riboside derivatives

1,3-Dibutylxanthine 7-riboside has been found to be a partial agonist at A3 adenosine receptors (van Galen et al. Mol. Pharmacol. 1994, 45, 1101-1111). 1,3-Dialkylxanthine 7-riboside analogues modified at the 1-, 3-, and 8-purine positions and at the ribose 5'-position were synthesized. The nucleoside analogues were examined for affinity in radioligand binding assays at rat brain A3 adenosine receptors stably expressed in CHO cells, using the radioligand [[125I]-4-amino-3-iodobenzyl]adenosine-5'-N-methyluronamide (AB-MECA). Affinity was assayed at rat brain A1 and A2a receptors using [3H]PIA and [3H]CGS 21680, respectively. The affinity of xanthine 7-ribosides at A3 receptors depended on the 1,3-dialkyl substituents in the order: Pent > or = Bu >> Hx > Pr approximately Me. 1,3-Dipentylxanthine 7-riboside was slightly selective for A3 receptors (2-fold vs A1 and 10-fold vs A2a). 8-Methoxy substitution was tolerated at A3 receptors. 2-Thio vs 2-oxo substitution increased potency at all three subtypes and slightly increased A3 vs A1 selectivity. The 5'-uronamide modification, which was previously found to enhance A3 selectivity in N6-benzyladenosine derivatives, was also incorporated into the xanthine 7-ribosides, with similar results. The affinity of 1,3-dialkylxanthine 7-riboside 5'-uronamides at A3 receptors depended on the N-alkyluronamide substituent in the order: MeNH > EtNH >> NH2 >> Me2N. Affinity of the 5'-uronamides at A3 receptors was dependent on the 1,3-dialkyl substitution in the order: Bu > Pent > Hex. 1,3-Dibutylxanthine 7-riboside 5'-N-methylcarboxamide, with a Ki value of 229 nM at A3 receptors, was 160-fold selective for rat A3 vs A1 receptors and > 400-fold selective vs A2a receptors. This derivative acted as a full agonist in the A3 receptor-mediated inhibition of adenylate cyclase.

2-Substitution of N6-benzyladenosine-5'-uronamides enhances selectivity for A3 adenosine receptors

Adenosine derivatives bearing an N6-(3-iodobenzyl) group, reported to enhance the affinity of adenosine-5'-uronamide analogues as agonists at A3 adenosine receptors (J. Med. Chem. 1994, 37, 636-646), were synthesized starting from methyl beta-D-ribofuranoside in 10 steps. Binding affinities at A1 and A2a receptors in rat brain membranes and at cloned rat A3 receptors from stably transfected CHO cells were compared. N6-(3-Iodobenzyl)adenosine was 2-fold selective for A3 vs A1 or A2a receptors; thus it is the first monosubstituted adenosine analogue having any A3 selectivity. The effects of 2-substitution in combination with modifications at the N6- and 5'-positions were explored. 2-Chloro-N6-(3-iodobenzyl)adenosine had a Ki value of 1.4 nM and moderate selectivity for A3 receptors. 2-Chloro-N6-(3-iodobenzyl)adenosine- 5'-N-methyluronamide, which displayed a Ki value of 0.33 nM, was selective for A3 vs A1 and A2a receptors by 2500- and 1400-fold, respectively. It was 46,000-fold selective for A3 receptors vs the Na(+)-independent adenosine transporter, as indicated in displacement of [3H]N6-(4- nitrobenzyl)-thioinosine binding in rat brain membranes. In a functional assay in CHO cells, it inhibited adenylate cyclase via rat A3 receptors with an IC50 of 67 nM. 2-(Methylthio)-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide and 2-(methylamino)-N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide were less potent, but nearly as selective for A3 receptors. Thus, 2-substitution (both small and sterically bulky) is well-tolerated at A3 receptors, and its A3 affinity-enhancing effects are additive with effects of uronamides at the 5'-position and a 3-iodobenzyl group at the N6-position.

Role of the second extracellular loop of adenosine receptors in agonist and antagonist binding. Analysis of chimeric A1/A3 adenosine receptors

Adenosine receptor (AR) agonists and antagonists are approximately 100-fold and 100,000-fold, respectively, more potent at the bovine A1AR as compared to the rat A3AR. To determine regions of ARs involved in ligand recognition, chimeric receptors composed of bovine A1AR and rat A3AR sequence were constructed and their ligand binding properties examined following expression in COS-7 cells. Substitutions of the second extracellular loop or a region encompassing transmembrane domains 6 and 7 of the A1AR into the A3AR resulted in enhanced affinities of both agonists and antagonists compared to wild-type A3AR. The region of the second extracellular loop of the A1AR responsible for this effect was identified as the distal eleven amino acids of the loop. Replacement of this segment of the A3AR with that of the A1AR in combination with the regions encompassing transmembrane domains 6 and 7 resulted in a 50,000-fold increase in the Kd for antagonist radioligand, [3H]1,3-dipropyl-8- cyclopentylxanthine. Agonist affinity at this chimeric was over 100-fold greater than that displayed by wild-type A3AR. Thus, multiple regions of ARs including a segment of the second extracellular loop are involved in ligand recognition, and considerable overlap exists in structural features required for agonist and antagonist binding.

Structure-activity relationships of 1,3-dialkylxanthine derivatives at rat A3 adenosine receptors

1,3-Dialkylxanthine analogues containing carboxylic acid and other charged groups on 8-position substituents were synthesized. These derivatives were examined for affinity in radioligand binding assays at rat brain A3 adenosine receptors stably expressed in CHO cells using the new radioligand [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide), and at rat brain A1 and A2a receptors using [3H]PIA and [3H]CGS 21680, respectively. A synthetic strategy for introducing multiple carboxylate groups at the 8-position using iminodiacetic acid derivatives was explored. The presence of a sulfonate, a carboxylate, or multiple carboxylate groups did not result in a significant enhancement of affinity at rat A3 receptors, although as previously observed an anionic group tended to diminish potency at A1 and A2a receptors. The rat A3 receptor affinity was not highly dependent on the distance of a carboxylate group from the xanthine pharmacophore. 2-Thio vs 2-oxo substitution favored A3 potency, and 8-alkyl vs 8-aryl substitution favored A3 selectivity, although few derivatives were truly selective for rat A3 receptors. 1,3-Dimethyl-8-(3-carboxypropyl)-2-thioxanthine was 7-fold selective for A3 vs A2a receptors. 1,3,7-Trimethyl-8-(trans-2-carboxyvinyl)xanthine was somewhat selective for A3 vs A1 receptors. For 8-arylxanthines affinity at A3 receptors was enhanced by 1,3-dialkyl substituents, in the order dibutyl > dipropyl > diallyl.

Identification of an adenosine receptor domain specifically involved in binding of 5'-substituted adenosine agonists

The bovine A1 adenosine receptor (A1AR) and rat A3 adenosine receptor (A3AR) display distinct agonist and antagonist binding properties. To identify regions involved in ligand recognition, A1AR/A3AR chimeric receptors were created, expressed in COS-7 cells, and analyzed by radioligand binding. A chimeric receptor in which the third intracellular loop of the A1AR was replaced with that of the A3AR bound agonists and the antagonist, [3H]xanthine amine congener, with affinities identical to wild-type A1AR. A chimeric receptor with the fifth transmembrane domain (TM5) and third intracellular loop of the A1AR replaced with that of the A3AR displayed antagonist affinity similar to wild-type A1AR. However, relative to the A1AR, this chimeric demonstrated much greater affinity for 5'-substituted adenosine analogs, whereas affinity for N6-substituted compounds was unaffected. Substitution of a 6-amino acid cassette of the exofacial half of TM5 of the A3AR into the A1AR produced enhanced binding of exclusively a 5'-substituted analog, indicating involvement of this specific region in ligand recognition. These findings suggest that the 5'- and N6-substituents of adenosine agonists bind to distinct regions of ARs and that TM5 of the A3AR interacts more favorably with 5'-substituted compounds than does that of the A1AR.

A binding site model and structure-activity relationships for the rat A3 adenosine receptor

A novel adenosine receptor, the A3 receptor, has recently been cloned. We have systematically investigated the hitherto largely unexplored structure-activity relationships (SARs) for binding at A3 receptors, using 125I-N6-2-(4-aminophenyl)ethyladenosine as a radioligand and membranes from Chinese hamster ovary cells stably transfected with the rat A3-cDNA. As is the case for A1 and A2a receptors, substitutions at the N6 and 5' positions of adenosine, the prototypic agonist ligand, may yield fairly potent compounds. However, the highest affinity and A3 selectivity is found for N6,5'-disubstituted compounds, in contrast to A1 and A2a receptors. Thus, N6-benzyladenosine-5'-N-ethylcarboxamide is highly potent (Ki, 6.8 nM) and moderately selective (13- and 14-fold versus A1 and A2a). The N6 region of the A3 receptor also appears to tolerate hydrophilic substitutions, in sharp contrast to the other subtypes. Potencies of N6,5'-disubstituted compounds in inhibition of adenylate cyclase via A3 receptors parallel their high affinity in the binding assay. None of the typical xanthine or nonxanthine (A1/A2) antagonists tested show any appreciable affinity for rat A3 receptors. 1,3-Dialkylxanthines did not antagonize the A3 agonist-induced inhibition of adenylate cyclase. A His residue in helix 6 that is absent in A3 receptors but present in A1/A2 receptors may be causal in this respect. In a molecular model for the rat A3 receptor, this mutation, together with an increased bulkiness of residues surrounding the ligand, make antagonist binding unfavorable when compared with a previously developed A1 receptor model. Second, this A3 receptor model predicted similarities with A1 and A2 receptors in the binding requirements for the ribose moiety and that xanthine-7-ribosides would bind to rat A3 receptors. This hypothesis was supported experimentally by the moderate affinity (Ki 6 microM) of 7-riboside of 1,3-dibutylxanthine, which appears to be a partial agonist at rat A3 receptors. The model presented here, which is consistent with the detailed SAR found in this study, may serve to suggest future chemical modification, site-directed mutagenesis, and SAR studies to further define essential characteristics of the ligand-receptor interaction and to develop even more potent and selective A3 receptor ligands.

125I-4-aminobenzyl-5'-N-methylcarboxamidoadenosine, a high affinity radioligand for the rat A3 adenosine receptor

The rat A3 adenosine receptor (AR) is a recently characterized AR subtype cloned from testis and brain cDNA libraries. N6-2-(4-Amino-3-[125I]iodophenyl)ethyladenosine, a high affinity A1AR agonist, has served as the only radioligand available for study of the A3AR. The relatively low affinity of N6-2-(4-amino-3-[125I] iodophenyl)ethyladenosine for the A3AR and its greater A1AR selectivity necessitate the development of more appropriate radioligands for A3AR analysis. This report characterizes 125I-4-aminobenzyl-5'-N-methylcarboxamidoadenosine (125I-AB-MECA), a high affinity radioligand for the A3AR, in two cell lines that express this AR subtype. Membranes from Chinese hamster ovary (CHO) cells expressing the rat A3AR and from the rat mast cell line RBL-2H3 bound 125I-AB-MECA with Kd values of 1.48 +/- 0.33 nM and 3.61 +/- 0.30 nM, respectively. As determined by 125I-AB-MECA binding, levels of A3AR expression in the A3AR-CHO cell line and RBL-2H3 cells were 3.06 +/- 0.21 pmol/mg and 1.02 +/- 0.13 pmol/mg, respectively. Binding of 125I-AB-MECA was characterized in competition assays. In the A3AR-CHO cell line a potency order of cyclohexyl-5'-N-ethylcarboxamidoadenosine (cyclohexyl-NECA) = benzyl-NECA > (-)-N6-[(R)-phenylisopropyl]adenosine = NECA was observed, and in RBL-2H3 cells (-)-N6-[(R)-phenylisopropyl]adenosine and NECA were equipotent. Xanthine amine congener (XAC) and 8-cyclopentyl-1,3-dipropylxanthine did not significantly inhibit 125I-AB-MECA binding. The parent compound, AB-MECA, dose-dependently inhibited forskolin-stimulated adenylyl cyclase activity in A3AR-CHO cell membranes. 125I-AB-MECA bound to the rat A1AR and canine A2aAR expressed in COS-7 cells with Kd values of 3.42 +/- 0.43 nM and 25.1 +/- 12.6 nM, respectively. This binding was significantly reduced in the presence of 1 microM XAC. In RBL-2H3 cells, XAC had no effect on 125I-AB-MECA affinity and reduced the level of radioligand binding by approximately 5%.

Structure-activity relationships of N6-benzyladenosine-5'-uronamides as A3-selective adenosine agonists

Adenosine analogues modified at the 5'-position as uronamides and/or as N6-benzyl derivatives were synthesized. These derivatives were examined for affinity in radioligand binding assays at the newly discovered rat brain A3 adenosine receptor and at rat brain A1 and A2a receptors. 5'-Uronamide substituents favored A3 selectivity in the order N-methyl > N-ethyl approximately unsubstituted carboxamide > N-cyclopropyl. 5'-(N-Methylcarboxamido)-N6-benzyladenosine was 37-56-fold more selective for A3 receptors. Potency at A3 receptors was enhanced upon substitution of the benzyl substituent with nitro and other groups. 5'-N-Methyluronamides and N6-(3-substituted-benzyl)adenosines are optimal for potency and selectivity at A3 receptors. A series of 3-(halobenzyl)-5'-N-ethyluronamide derivatives showed the order of potency at A1 and A2a receptors of I approximately Br > Cl > F. At A3 receptors the 3-F derivative was weaker than the other halo derivatives. 5'-N-Methyl-N6-(3-iodobenzyl)adenosine displayed a Ki value of 1.1 nM at A3 receptors and selectivity versus A1 and A2a receptors of 50-fold. A series of methoxybenzyl derivatives showed that a 4-methoxy group best favored A3 selectivity. A 4-sulfobenzyl derivative was a specific ligand at A3 receptors of moderate potency. An aryl amino derivative was prepared as a probe for radioiodination and receptor cross-linking.

A role for central A3-adenosine receptors. Mediation of behavioral depressant effects

The behavioral effects of a selective A3 adenosine receptor agonist 3-IB-MECA (N6-(3-iodobenzyl)-5'-N-methylcarboxamidoadenosine) in mice and the localization of radioligand binding sites in mouse brain were examined. Low levels of A3 adenosine receptors were detected in various regions of the mouse brain (hippocampus, cortex, cerebellum, striatum), using a radioiodinated, high-affinity A3-agonist radioligand [125I]AB-MECA (N6-(3-iodo-4-aminobenzyl)-5'-N-methylcarboxamidoadenosine). Scatchard analysis in the cerebellum showed that the Kd value for binding to A3 receptors was 1.39 +/- 0.04 nM with a Bmax of 14.8 +/- 2.1 fmol/mg protein. 3-IB-MECA at 0.1 mg/kg i.p. was a locomotor depressant with > 50% reduction in activity. Although selective A1 or A2a antagonists reversed locomotor depression elicited by selective A1 or A2a agonists, respectively, the behavioral depressant effects of 3-IB-MECA were unaffected. 3-IB-MECA also caused scratching in mice, which was prevented by coadministration of the histamine antagonist cyproheptadine. The demonstration of a marked behavioral effect of A3 receptor activation suggests that the A3 receptor represents a potential new therapeutic target.

Adenosine Receptors

Our knowledge of A1 and A2ARs has grown dramatically since they were first defined by Burnstock. With the recent purification of the A1AR and the cDNA cloning of the A2AR, an even more rapid expansion of information regarding their structure, function, and regulation should now ensue. We may then be able to return to the intact cell and organ system and better understand the physiological role of these ubiquitous receptors.

Molecular cloning and characterization of an adenosine receptor: the A3 adenosine receptor

We have previously reported the selective amplification of several rat striatal cDNA sequences that encode guanine nucleotide-binding regulatory protein (G protein)-coupled receptors. One of these sequences (R226) exhibited high sequence identity (58%) with the two previously cloned adenosine receptors. A full-length cDNA clone for R226 has been isolated from a rat brain cDNA library. The cDNA clone encodes a protein of 320 amino acids that can be organized into seven transmembrane stretches. R226 has been expressed in COS-7 and CHO cells and membranes from the transfected cells were screened with adenosine receptor radioligands. R226 could bind the nonselective adenosine agonist tritiated N-ethyladenosine 5'-uronic acid ([3H]NECA) and A1-selective agonist radioiodinated N6-2-(4-amino-3-iodophenyl)-ethyladenosine ([125I]APNEA) but not A1-selective antagonists tritiated 1,3-dipropyl-8-cyclopentylxanthine ([3H]DPCPX) and 8-(4-[([[(2-aminoethyl)amino]carbonyl]methyl)oxy]-phenyl)-1, 3-dipropylxanthine ([3H]XAC) or the A2-selective agonist ligands tritiated 2-[4-(2-carboxyethyl)phenyl]ethyl-amino 5'-N-ethylcarboxamidoadenosine ([3H]CGS21680) and radioiodinated 2-[4-([2-[(4-aminophenyl)methylcarbonylamino] ethylaminocarbonyl]ethyl)phenyl]ethylamino 5'-N-ethylcarboxamidoadenosine. Extensive characterization with [125I]APNEA showed that R226 binds [125I]APNEA with high affinity (Kd = 15.5 +/- 2.4 nM) and the specific [125I]APNEA binding could be inhibited by adenosine ligands with a potency order of (R)-N6-phenyl-2-propyladenosine (R-PIA) = NECA greater than S-PIA greater than adenosine greater than ATP = ADP but not by antagonists XAC, isobutylmethylxanthine, and DPCPX. In R226 stably transfected CHO cells, adenosine agonists R-PIA, NECA, and CGS21680 inhibited by 40-50% the forskolin-stimulated cAMP accumulation through a pertussis toxin-sensitive G protein with an EC50 of 18 +/- 5.6 nM, 23 +/- 3.5 nM, and 144 +/- 34 nM, respectively. Based on these observations we conclude that R226 encodes an adenosine receptor with non-A1 and non-A2 specificity, and we thus name it the A3 adenosine receptor. mRNA analyses revealed that the highest expression of R226 was in the testis and low-level mRNAs were also found in the lung, kidneys, heart, and some parts of the central nervous system such as cortex, striatum, and olfactory bulb. The high-expression level of the A3 receptor in the testis suggests a possible role for adenosine in reproduction.

Cloning, expression, and characterization of the unique bovine A1 adenosine receptor. Studies on the ligand binding site by site-directed mutagenesis

The bovine brain A1 adenosine receptor (A1AR) is distinct from other A1ARs in that it displays the unique agonist potency series of N6-R-phenylisopropyladenosine (R-PIA) greater than N6-S-phenylisopropyladenosine (S-PIA) greater than 5'-N-ethylcarboxamidoadenosine and has a 5-10-fold higher affinity for both agonists and antagonists. The cDNA for this receptor has been cloned from a size-selected (2-4-kb) bovine brain library and sequenced. The 2.0-kb cDNA encodes a protein of 326 amino acid residues with a molecular mass of 36,570 daltons. The amino acid sequence fits well into the seven-transmembrane domain motif typical of G protein-coupled receptors. Northern analysis in bovine tissue using the full length cDNA demonstrates mRNAs of 3.4 and 5.7 kb with a tissue distribution consistent with A1AR binding. Subcloning of the cDNA in a pCMV5 expression vector with subsequent transfection into both COS7 and Chinese hamster ovary cells revealed a fully functional A1AR which could inhibit adenylylcyclase and retained the unique pharmacologic properties of the bovine brain A1AR. The A1AR was found to have a single histidine residue in each of transmembrane domains 6 and 7. Histidine residues have been postulated by biochemical studies to be important for ligand binding. Mutation of His-278 to Leu-278 (seventh transmembrane domain) dramatically decreased both agonist and antagonist binding by greater than 90%. In contrast, mutation of His-251 to Leu-251 decreased antagonist affinity and the number of receptors recognized by an antagonist radioligand. In contrast, agonist affinity was not perturbed but the number of receptors detected by an agonist radioligand was also reduced. These data suggest that both histidines are important for both agonist and antagonist binding, but His-278 appears critical for ligand binding to occur.

Distinct pathways of desensitization of A1- and A2-adenosine receptors in DDT1 MF-2 cells

Desensitization of adenosine receptors (ARs) was studied in DDT1 MF-2 cells, which possess both A1- and A2AR, differentially coupled to adenylate cyclase. (-)-N6-(R)-Phenylisopropyladenosine (R-PIA), an A1AR-selective agonist at the appropriate concentrations, desensitized A1AR-mediated inhibition of adenylate cyclase activity in a time- (t1/2, 8 hr) and dose-dependent and reversible fashion. This was associated with significant decreases in total A1AR number and in the number of receptors possessing a high affinity for agonist in membrane preparations. The decrease in total A1AR in the membranes from the desensitized cells (approximately 40%) was associated with a 37% increase in A1AR measured in light vesicle preparations, compared with control cells. To test a possible role of phosphorylation in A1AR desensitization, cells were incubated with [32P]orthophosphate, followed by exposure to R-PIA for 18 hr. Subsequent purification of the A1AR indicated a 3-4-fold increase in phosphorylation of A1AR in cells treated with R-PIA, compared with control cells. Desensitization of the A1AR did not alter the levels of alpha s and alpha 12 proteins or affect the ability of stimulatory effectors, such as isoproterenol, sodium fluoride, and forskolin, to activate adenylate cyclase. These results suggest that uncoupling, down-regulation, and phosphorylation of the A1AR contribute, at least in part, to desensitization of this inhibitory receptor. Desensitization of the A2AR was characterized using an A2-selective agonist, 2-[4-(2-(4-aminophenyl]methylcarbonyl)ethyl)phenyl]ethylamino- 5'-N-ethylcarboxamidoadenosine (PAPA-APEC). Pretreatment of cells with PAPA-APEC (100 nM) resulted in a rapid loss of agonist stimulation of adenylate cyclase activity (t1/2 of this effect, 45 min). This effect was dose dependent (EC50, approximately 10 nM) and rapidly reversible. Interestingly, desensitization of the A2AR resulted in no change in receptor number, affinity, or mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Taken together, these data suggest distinct mechanisms of desensitization of A1- and A2ARs in a single cell type.

Purification and characterization of bovine cerebral cortex A1 adenosine receptor

A1 adenosine receptors (A1AR) acting via the inhibitory guanine nucleotide binding protein inhibit adenylate cyclase activity in brain, cardiac, and adipose tissue. We now report the purification of the A1AR from bovine cerebral cortex. This A1AR is distinct from other A1ARs in that it displays an agonist potency series of N6-R-phenylisopropyladenosine (R-PIA) greater than N6-S-phenylisopropyladenosine greater than (S-PIA) greater than 5'-N-ethylcarboxamidoadenosine (NECA) compared to the traditional potency series of R-PIA greater than NECA greater than S-PIA. The A1AR was solubilized in 1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps) and then purified by chromatography on an antagonist [xanthine amine congener (XAC)]-coupled Affi-Gel 10 followed by hydroxylapatite chromatography. Following purification, sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single protein of Mr 36,000 by silver staining, Na125I iodination with chloramine T and photoaffinity labeling with [125I]8-[4-[[[[2-(4-aminophenyl acetylamino) ethyl] carbonyl] methyl] oxy]-phenyl]-1,3- dipropylxanthine. This single protein displayed all the characteristics of the A1AR, including binding an antagonist radioligand [( 3H]XAC) with high affinity (Kd = 0.7 nM) and in a saturable manner (Bmax greater than 4500 pmol/mg). Agonist competition curves demonstrated the expected bovine brain A1AR pharmacology: R-PIA greater than S-PIA greater than NECA. The overall yield from soluble preparation was 7%. The glycoprotein nature of the purified A1AR was determined with endo- and exoglycosidases. Deglycosylation with endoglycosidase F increased the mobility of the A1AR from Mr 36,000 to Mr 32,000 in a single step. The A1AR was sensitive to neuraminidase but resistant to alpha-mannosidase, suggesting the single carbohydrate chain was of the complex type. This makes the bovine brain A1AR similar to rat brain and fat A1AR in terms of its carbohydrate chains yet the purified A1AR retains its unique agonist potency series observed in membranes.

Affinity chromatography of the bovine cerebral cortex A1 adenosine receptor

An approximate 140-fold purification of the A1 adenosine receptor of bovine cerebral cortex has been obtained via affinity chromatography. The affinity column consists of Affi-Gel 10 coupled through an amide linkage to XAC, a high-affinity A1 adenosine receptor antagonist. As assessed by [3H]XAC binding, bovine brain membranes solubilized with the detergent CHAPS had a specific binding activity of 1.1 pmol/mg protein. Interaction of solubilized A1 adenosine receptors with the XAC-Affi-Gel was biospecific and 30% of the receptor activity was bound by the gel. Demonstration of [3H]XAC binding in the material eluted from the column with R-PIA required insertion of receptor into phospholipid vesicles. The specific activity of the affinity column purified receptor was 146 +/- 22 pmol/mg protein with typically 5-15% of the bound receptor recovered. The purified receptor displayed high-affinity antagonist binding and bound agonists with the potency order expected of the bovine brain A1 adenosine receptor: R-PIA greater than S-PIA greater than NECA. In purified preparations, the photoaffinity probe [125I]PAPAXAC-SANPAH specifically labelled a protein of molecular mass 38,000 which has previously been shown to be the A1 adenosine receptor binding subunit.

Differential effects of calcium channel blockade and intracellular calcium antagonism on endothelium-dependent responses of the rat aorta to drugs

The actions of many vasoactive drugs are mediated through, or modified by, the endothelium-derived relaxing (EDRF) and constricting (EDCF) factors. While EDRF appears to be nitric oxide, EDCF is a peptide or cyclooxygenase product. Using verapamil (a calcium channel blocker), propyl methylenedioxyindene (pr-MDI; an intracellular calcium antagonist), and sodium nitroprusside (which liberates nitric oxide from its molecular structure) as EDRF-independent pharmacological probes in rat aortic rings with and without endothelium, we attempted to provide additional insight into the role of extracellular [( Ca]o) and intracellular [( Ca]i) calcium in EDRF and EDCF release and action, and to explain some mechanisms underlying the modulatory effects of these endothelial factors on the actions of vasoactive drugs. The findings suggest that (1) the [Ca]o required for evoked EDRF release does not enter endothelial cells through verapamil-sensitive calcium channels; (2) mobilization of endoplasmic reticular [Ca]i by [Ca]o entering the endothelial cell may be the trigger for evoked EDRF release; (3) spontaneous release of EDRF appears to depend more on mobilization of [Ca]i than on influx of [Ca]o; (4) the action of EDRF on smooth muscle either does not require Ca or does not involve the mobilization of [Ca]i by [Ca]o; (5) Both EDRF and EDCF can modulate the actions of vasoactive drugs; (6) the EDCF of the rat aorta is not a cyclooxygenase product, and (7) the action of EDCF on vascular smooth muscle, and possibly its release from endothelial cells, are Ca-dependent.

Age-related changes in responsiveness of the rat aorta to depolarizing and receptor-mediated contractile stimuli and to calcium antagonism

This study was undertaken to determine the relative age-dependent responsiveness of the rat aorta to depolarizing (potassium) and receptor-activating (norepinephrine) contractile stimulants, and to the calcium antagonists propyl-methylenedioxyindene (pr-MDI) and nifedipine. Pr-MDI exhibits intracellular calcium antagonistic and calcium channel blocking properties in this tissue, while nifedipine acts principally as a calcium channel blocker. Thoracic strips from young (4-6 months old) and senescent (22-23 months old) Fischer F344 rats were contracted with KCl (10-60 mmol/l) or norepinephrine (10(-9)-5 X 10(-6) mol/l). Aortae from old rats were significantly more sensitive to norepinephrine than aortae from young rats, while the reverse was observed for KCl. Pr-MDI (10(-5)-10(-4) mol/l) significantly relaxed the aortic contractions induced by norepinephrine (10(-7) mol/l, a nondepolarizing concentration producing 88% of maximum response in young and old aortae) and by KCl (50 mmol/l, a depolarizing concentration producing 96% of maximum response in young and old aortae). However, there were no age-related differences in sensitivity to the relaxant effects of pr-MDI against either stimulant. Pr-MDI was more effective in relaxing KCl-induced contractions than those induced by norepinephrine. Similar results were obtained with nifedipine (10(-10)-10(-6) mol/l). These results indicate that senescence of the rat aorta is accompanied by an enhanced responsiveness of adrenergic alpha-receptor-mediated contraction, a reduced responsiveness to depolarizing stimuli, and no change in sensitivity to calcium antagonism.

Evaluation of the antiabortifacient and embryotoxic effects of methylenedioxyindene and methylenedioxyindan calcium antagonists

Calcium channel blockers have been advocated as potential therapeutic agents in the management of premature labor. In the present study, the class of intracellular calcium antagonistic methylenedioxyindenes (MDIs) was investigated for potential antiabortifacient activity in mice. Pretreatment of pregnant mice from day 15 of gestation with the MDIs did not afford protection against the abortifacient effect of prostaglandin F2alpha administered from day 17 of gestation. The MDIs demonstrated embryotoxic and fetotoxic activity as shown by a significant increase in the incidence of resorptions and stillbirths. Similar embryotoxicity was previously reported for the calcium channel blockers. It appears doubtful that any of the calcium antagonists so far examined will be clinically useful in the management of premature labor.