Kinetics of ternary complex formation with fusion proteins composed of the A(1)-adenosine receptor and G protein alpha-subunits

High affinity agonist binding to G protein-coupled receptors depends on the formation of a ternary complex between agonist, receptor, and G protein. This process is too slow to be accounted for by a simple diffusion-controlled mechanism. We have tested if the interaction between activated receptor and G protein is rate-limiting by fusing the coding sequence of the human A(1)-adenosine receptor to that of Galpha(i-1) (A(1)/Galpha(i-1)) and of Galpha(o) (A(1)/Galpha(o)). Fusion proteins of the expected molecular mass were detected following transfection of HEK293 cells. Ternary complex formation was monitored by determining the kinetics for binding of the high affinity agonist (-)-N(6)-3[(125)I](iodo-4-hydroxyphenylisopropyl)adenosine; these were similar in the wild-type receptor and the fusion proteins over the temperature range of 10 to 30 degrees C. Agonist dissociation may be limited by the stability of the ternary complex. This assumption was tested by creating fusion proteins in which the Cys(351) of Galpha(i-1) was replaced with glycine (A(1)/Galpha(i-1)C351G) or isoleucine (A(1)/Galpha(i-1)C351I) to lower the affinity of the receptor for the G protein. In these mutated fusion proteins, the dissociation rate of the ternary complex was accelerated; in contrast, the rate of the forward reaction was not affected. We therefore conclude that (i) receptor activation per se rather than its interaction with the G protein is rate-limiting in ternary complex formation; (ii) the stability of the ternary complex is determined by the dissociation rate of the G protein. These features provide for a kinetic proofreading mechanism that sustains the fidelity of receptor-G protein coupling.

Diimidazo[1,2-c:4',5'-e]pyrimidines: N6-N1 conformationally restricted adenosines

Tethering the N6-substituents of N6-substituted adenosines to N1 has resulted in a series of conformationally restricted adenosine analogues. The resultant diimidazo[1,2-c:4',5'-e]pyrimidines were shown to be adenosine A1 selective.

Diimidazo[1,2-c:4',5'-e]pyrimidines: adenosine agonist activity demonstrated by microphysiometry

Silicon-based microphysiometry, measuring extracellular acidification rate of cells in culture, demonstrated that a series of diimidazo[1,2-c:4',5'-e]pyrimidines were agonists at the human adenosine A1 receptor. 5-amino-7,8-dihydro-3-ribofuranose-8-(R)-(phenyl)-3H-diimidazo [1,2-c:4',5'-e]pyrimidine (2a) had an EC50 of 100 microM and reached 90% of the Emax produced by R-PIA.

Inhibition of receptor/G protein coupling by suramin analogues

Suramin analogues act as direct antagonists of heterotrimeric G proteins because they block the rate-limiting step of G protein activation (i.e., the dissociation of GDP prebound to the G protein alpha subunit). We have used the human brain A1 adenosine receptor and the rat striatal D2 dopamine receptor, two prototypical Gi/G(o)-coupled receptors, as a model system to test whether the following analogues suppress the receptor-dependent activation of G proteins: 8-(3-nitrobenzamido)-1,3,5-naphthalenetrisulfonic acid (NF007), 8-(3-(3-nitrobenzamido)-benzamido)-1,3,5-naphthalenetrisulfonic acid (NF018); 8,8'-(carbonylbis(imino-3,1-phenylene))bis-(1,3,5-naphthalenetr isulfonic acid) (NF023); 8,8'-(carbonylbis(imino-3,1-phenylene)carbonylimino-(3,1- phenylene)) bis(1,3,5-naphthalenetrisulfonic acid) (NF037); and suramin. Suramin and its analogues inhibit the formation of the agonist-specific ternary complex (agonist/receptor/G protein). This inhibition is (i) quasicompetitive with respect to agonist binding in that it can be overcome by increasing receptor occupancy but (ii) does not result from an interaction of the analogues with the ligand binding pocket of the receptors because the binding of antagonists or of agonists in the absence of functional receptor/G protein interaction is not affected. In addition to suppressing the spontaneous release of GDP from defined G protein alpha subunits, suramin and its analogues reduce receptor-catalyzed guanine nucleotide exchange. The site, to which suramin analogues bind, overlaps with the docking site for the receptor on the G protein alpha subunit. The structure-activity relationships for inhibition of agonist binding to the A1 adenosine receptor (suramin > NF037 > NF023) and of agonist binding to the inhibition D2 dopamine receptor (suramin = NF037 > NF023 > NF018) differ. Thus, NF037 discriminates between the ternary complexes formed by the agonist-liganded D2 dopamine receptors and those formed by the A1 adenosine receptor with > 10-fold selectivity. Therefore, our results also show that inhibitors can be identified that selectively uncouple specific receptor/G protein tandems.

Interactions of the bovine brain A1-adenosine receptor with recombinant G protein alpha-subunits. Selectivity for rGi alpha-3

The ability of the bovine brain A1-adenosine receptor to discriminate between different G protein subtypes was tested using G protein alpha-subunits synthesized in Escherichia coli (rG alpha-subunits). When combined with a 3-fold molar excess of beta gamma-subunit purified from bovine brain and used at high concentrations, all three subtypes of rGi alpha (rGi alpha-1, rGi alpha-2, and rGi alpha-3) and rGo alpha were capable of reconstituting guanine nucleotide-sensitive high-affinity binding of the agonist radioligand (-)-N6-3-[125I] (iodo-4-hydroxyphenylisopropyl) adenosine ([125I]HPIA) to the purified A1-adenosine receptor (Kd approximately 1.2 nM). Titration of the A1-adenosine receptor with increasing amounts of rG alpha revealed a approximately 10-fold higher affinity for rGi alpha-3 compared with rGi alpha-1, rGi alpha-2, and rGo alpha. This selectivity was also observed in the absence of beta gamma. Other alpha-subunits (rGs alpha-s, rGs alpha-L, rGs alpha PT, and rGz alpha) did not promote [125I]HPIA binding to the purified receptor. In N-ethylmaleimide-treated bovine brain membranes, rGi alpha-3 was the only rG alpha-subunit capable of reconstituting high-affinity agonist binding. Similarly, rGi alpha-3 competed potently with rGo alpha for activation by the agonist-liganded A1-adenosine receptor, whereas a approximately 50-fold molar excess of rGo alpha was required to quench the receptor-mediated release of [alpha-32P]GDP from rGi alpha-3. Hence, in spite of the extensive homology between alpha-subunits belonging to the Gi/Go group, the A1-adenosine receptor appears to discriminate between the subtypes. This specificity is likely to govern transmembrane signaling pathways in vivo.

Interactions of purified bovine brain A1-adenosine receptors with G-proteins. Reciprocal modulation of agonist and antagonist binding

The bovine brain A1-adenosine receptor was purified 8000-fold by affinity chromatography on xanthine-amine-congener (XAC)-Sepharose. Addition of a 120-fold molar excess of a purified bovine brain G-protein preparation (Go,i a mixture of Go and Gi, containing predominantly Go) decreases the Bmax of the binding of the antagonist radioligand [3H]XAC to the receptor. This decrease is observed not only after insertion into phospholipid vesicles but also in detergent solution, and is reversed by GTP analogues. In the presence of Go,i, about 20 and 40% of the receptors display guanine-nucleotide-sensitive high-affinity binding of the agonist radioligand (-)-N6-3-([125I]iodo-4-hydroxyphenylisopropyl)adenosine after reconstitution into lipid vesicles and in detergent solution, respectively. The ability of Go,i to enhance agonist binding and decrease antagonist binding is concentration-dependent, with a half-maximal effect occurring at approximately 10-fold molar excess of G-proteins over A1-adenosine receptors. In the presence of the receptor, the rate of guanosine 5'-[gamma-[35S]thio]triphosphate (GTP[35S]) binding to Go,i is accelerated. This rate is further enhanced if the receptor is activated by the agonist (-)(R)-N6-phenylisopropyladenosine, whereas the antagonist XAC decreases the association rate of GTP[35S] to levels observed in the absence of receptor. These results show (1) that detergent removal is not a prerequisite for the observation of coupling between the A1-adenosine receptor and Go,i, and (2) that the regulatory effect of G-proteins on antagonist binding to the A1-adenosine receptor can be reconstituted by using purified components.

Mechanism of A2 adenosine receptor activation. I. Blockade of A2 adenosine receptors by photoaffinity labeling

It has previously been shown that covalent incorporation of the photoreactive adenosine derivative (R)-2-azido-N6-p-hydroxy-phenylisopropyladenosine [(R)-AHPIA] into the A1 adenosine receptor of intact fat cells leads to a persistent activation of this receptor, resulting in a reduction of cellular cAMP levels [Mol. Pharmacol. 30:403-409 (1986)]. In contrast, covalent incorporation of (R)-AHPIA into human platelet membranes, which contain only stimulatory A2 adenosine receptors, reduces adenylate cyclase stimulation via these receptors. This effect of (R)-AHPIA is specific for the A2 receptor and can be prevented by the adenosine receptor antagonist theophylline. Binding studies indicate that up to 90% of A2 receptors can be blocked by photoincorporation of (R)-AHPIA. However, the remaining 10-20% of A2 receptors are sufficient to mediate an adenylate cyclase stimulation of up to 50% of the control value. Similarly, the activation via these 10-20% of receptors occurs with a half-life that is only 2 times longer than that in control membranes. This indicates the presence of a receptor reserve, with respect to both the extent and the rate of adenylate cyclase stimulation. These observations require a modification of the models of receptor-adenylate cyclase coupling, which is described in the accompanying paper [Mol. Pharmacol. 39:524-530 (1991)].

Mechanism of activation of A2 adenosine receptors. II. A restricted collision-coupling model of receptor-effector interaction

Existing models describing the kinetics of receptor-effector interaction were found to be insufficient to account for the experimental findings on adenylate cyclase activation by A2 adenosine receptors described in the preceding manuscript [Mol. Pharmacol. 39: 517-523 (1991)]. We have, therefore, chosen another approach and have developed discrete computer simulations of receptor-effector interactions taking place on a spherical membrane. These simulations were based on the following principles: (a) receptors activate effectors in a catalytic manner, and (b) diffusion of receptors and effectors is slow, so that receptors will only activate effectors that are in their vicinity at the time of agonist occupation. Using several experimentally determined parameters, these simulations could reproduce the experimental findings on adenylate cyclase activation by A2 adenosine receptors described in the preceding manuscript. In addition, by appropriate choice of the simulation parameters, they are shown to accommodate the behavior of several other models of receptor-effector interactions.

Role of adenosine receptors in caffeine tolerance

Caffeine is a competitive antagonist at adenosine receptors. Receptor up-regulation during chronic drug treatment has been proposed to be the mechanism of tolerance to the behavioral stimulant effects of caffeine. This study reassessed the role of adenosine receptors in caffeine tolerance. Separate groups of rats were given scheduled access to drinking bottles containing plain tap water or a 0.1% solution of caffeine. Daily drug intake averaged 60-75 mg/kg and resulted in complete tolerance to caffeine-induced stimulation of locomotor activity, which could not be surmounted by increasing the dose of caffeine. 5'-N-ethylcarboxamidoadenosine (0.001-1.0 mg/kg) dose dependently decreased the locomotor activity of caffeine-tolerant rats and their water-treated controls but was 8-fold more potent in the latter group. Caffeine (1.0-10 mg/kg) injected concurrently with 5-N-ethylcarboxamidoadenosine antagonized the decreases in locomotor activity comparably in both groups. Apparent pA2 values for tolerant and control rats also were comparable: 5.05 and 5.11. Thus, the adenosine-antagonist activity of caffeine was undiminished in tolerant rats. The effects of chronic caffeine administration on parameters of adenosine receptor binding and function were measured in cerebral cortex. There were no differences between brain tissue from control and caffeine-treated rats in number and affinity of adenosine binding sites or in receptor-mediated increases (A2 adenosine receptor) and decreases (A1 adenosine receptor) in cAMP accumulation. These results are consistent with theoretical arguments that changes in receptor density should not affect the potency of a competitive antagonist. Experimental evidence and theoretical considerations indicate that up-regulation of adenosine receptors is not the mechanism of tolerance to caffeine-induced stimulation of locomotor activity.

Decrease in A1 adenosine receptors in adipocytes from spontaneously hypertensive rats

We have investigated whether the insulin resistance reported to occur in hypertension is due to decreased insulin receptors or to adenosine receptors in adipocyte membranes. Membranes were isolated from adipocytes from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKy) rats and assayed for insulin receptors and A1 adenosine receptors. The two groups of membranes bound 125I-insulin equally, but in contrast the SHR membranes bound approximately 25% less 125I-HPIA ([(-)-N6-p-hydroxyphenylisopropyl adenosine], an A1 adenosine receptor agonist) than the WKy (P less than .005). Scatchard analysis demonstrated that this was due to a lower number of receptors in the SHR. The affinity of the receptor for HPIA was approximately 0.7 nmol/L in both groups. 5'-Nucleotidase activity was approximately 40% higher in membranes from SHR than WKy (P less than .001), indicating that adipocytes from SHR have a higher capacity for adenosine production. This may cause increased adenosine concentrations in the SHR adipose tissue, leading to adenosine receptor down-regulation. Since we have previously demonstrated that adenosine receptor down-regulation can lead to insulin resistance, these findings may partly account for the insulin resistance of hypertension.

Characterization of human adipocyte adenosine receptors

125I-Hydroxyphenylisopropyl adenosine (125I-HPIA) was used to characterize adenosine receptors in human adipocyte plasma membranes. Steady state binding was achieved after 6 h at 37 degrees. Scatchard plots were linear, with a KD of approx. 2.5 nM, and Bmax of 360-1800 fmol/mg protein. (-)N6-phenylisopropyl adenosine (PIA) was a more potent inhibitor of binding than N-ethyl carboxamido adenosine, and (+)PIA was more than 10-fold less potent than (-)PIA, consistent with A1 adenosine receptor binding. Theophylline was a potent inhibitor of binding (IC50 approx. 10 microM). Photoaffinity cross-linking studies demonstrated that the receptor is a single subunit, Mr approx. 43 kDa. The findings demonstrate that the human adipocyte adenosine receptor is similar to the A1 adenosine receptor of rat adipocytes, although its molecular weight is higher, and its affinity for HPIA is lower than that of the rat.

Characterization of agonist radioligand interactions with porcine atrial A1 adenosine receptors

The agonist radioligand (-)-N6-[125I]-p-hydroxyphenylisopropyl-adenosine ([ 125I]HPIA) was used to characterize adenosine recognition sites in porcine atrial membranes. [125I]HPIA showed saturable binding to an apparently homogeneous population of sites with a maximum binding capacity of 35 +/- 3 fmol/mg of protein and an equilibrium dissociation constant of 2.5 +/- 0.4 nM. Kinetic experiments were performed to address the molecular mechanism of [125I]HPIA binding in porcine atrial membranes. [125I]HPIA apparently interacts with the cardiac adenosine receptor in a simple bimolecular reaction. A kinetically derived [125I] HPIA dissociation constant (2.4 nM) was in good agreement with that parameter measured at equilibrium. Guanyl nucleotides negatively modulated [125I]HPIA binding by increasing its rate of dissociation. This finding is consonant with the formation of a ternary complex in porcine atrial membranes, consisting of ligand, receptor, and guanyl nucleotide-binding protein. Prototypic adenosine receptor agonists and antagonists inhibited specific binding in a manner consistent with the labeling of an A1 adenosine receptor. The results of these experiments suggest that the adenosine receptor present in porcine atrial membranes, as labeled by [125I]HPIA, is of the A1 subtype.

Autoradiographic visualization of A1-adenosine receptors in brain and peripheral tissues of rat and guinea pig using 125I-HPIA

A1-adenosine receptors were identified in sections of rat brain and guinea pig kidney with the radioiodinated agonist 125I-N6-p-hydroxyphenylisopropyladenosine (125I-HPIA) using in vitro autoradiography. The affinities of adenosine receptor ligands in competing with 125I-HPIA binding to tissue sections were in good agreement with those found in membranes, and indicate that the binding site represents an A1-adenosine receptor. The distribution of 125I-HPIA binding sites in rat brain sections was similar to the pattern of [3H]N6-cyclohexyladenosine ([3H]CHA) binding sites determined previously, with highest densities in the hippocampus and dentate gyrus, the cerebellar cortex, some thalamic nuclei and certain layers of the cerebral cortex. In the guinea pig kidney 125I-HPIA labelled longitudinal structures in the medulla. This study demonstrates that 125I-HPIA allows the autoradiographic detection of A1 adenosine receptors in the brain and peripheral organs and has the advantage of short exposure times.

Pharmacological characterization of A1 adenosine receptors in isolated rat ventricular myocytes

The aim of the present study was the characterization of adenosine receptors in isolated rat ventricular myocytes. The cAMP-levels of rat ventricular myocytes in the presence of 1 mumol/l isoprenaline were reduced by up to 48% by adenosine analogues; the rank order of potency was: R-N6-phenylisopropyladenosine (IC50 60 nmol/l), 5'-N-ethylcarboxamidoadenosine (IC50 360 nmol/l) and S-N6-phenylisopropyladenosine (IC50 16 mumol/l). The adenosine receptor antagonist XAC ("xanthine amine congener") antagonized the effect of R-N6-phenylisopropyladenosine in a concentration-dependent manner with a Ki-value of 20 nmol/l. The A1 receptor-selective radioligand R-N6-125I-p-hydroxyphenylisopropyladenosine bound to membranes prepared from rat ventricular myocytes in a saturable manner with a Bmax of 17.7 fmol/mg protein and a KD-value of 1.1 nmol/l. Adenosine analogues competed for the binding with the same rank order of potency as for the inhibition of the isoprenaline-induced cAMP-increase. GTP inhibited radioligand binding with an IC50-value of 73 mumol/l. These results suggest the presence of A1 adenosine receptors on rat ventricular myocytes, which mediate an inhibition of adenylate cyclase. The receptors may be responsible for the effects of adenosine and its analogues on the heart.

Synaptic and extrasynaptic localization of adenosine binding sites in the rat hippocampus

In vitro binding sites for [125I]iodohydroxyphenylisopropyladenosine, an A1 adenosine agonist, were visualized in the CA1 area of the rat hippocampus by electron microscopical autoradiography. By fixing hippocampal slices after incubation in paraformaldehyde and osmium tetroxide, the specifically bound radioactive ligands were preferentially retained and cross-linked to the tissue. Autoradiographic silver grains were localized by a statistical evaluation according to the '50% probability circle analysis' and by measuring the distance of the grains from neighbouring membrane structures. A significant association of silver grains, indicating the presence of A1 adenosine receptors, was found at synaptic complexes and in addition at extrasynaptic sites on dendritic membranes. This suggests that modulation of nerve cell activity by adenosine involves synaptic as well as non-synaptic mechanisms.

Glomeruli and microvessels of the rabbit kidney contain both A1- and A2-adenosine receptors

Rabbit renal cortices were fractionated by collagenase dispersion and glomeruli, microvessels and tubuli purified on a discontinuous sucrose gradient. Binding experiments with (-)[125I]N6-(4-hydroxyphenylisopropyl)-adenosine ([125I]HPIA) provided evidence for the presence of A1-adenosine receptors in the glomerular and microvascular fraction. With glomeruli, saturation isotherms for specific [125I]HPIA binding were mono-phasic with a KD of 1.3 nmol/l and a Bmax of 7.7 fmol/mg protein. In kinetic experiments, an association rate constant of 4.9 X 10(5) (mol/l-1 s-1 and a dissociation rate constant of 4.3 X 10(-4) s-1 were obtained, yielding a KD of 0.9 nmol/l. Adenosine analogs displaced [125I]HPIA binding with a rank order of potency typical of A1-adenosine receptors; furthermore, binding was inhibited by methylxanthines and modulated by GTP. Saturation experiments with the microvessels revealed a KD of 1.9 nmol/l and a Bmax of 13.4 fmol/mg protein. However, no inhibition of glomerular and microvascular adenylate cyclase activity could be demonstrated, but instead both 5'-N-ethylcarboxamido-adenosine (NECA) and N6-(R-phenylisopropyl)-adenosine (R-PIA) stimulated enzyme activity, with EC50 values of 0.14 mumol/l and 1.5 mumol/l, respectively. The concentration-response curve for NECA was shifted to the right (factor 9) by 10 mumol/l 8-phenyltheophylline. On the other hand, computer simulation of biphasic curves (adenylate cyclase inhibition in the presence of activation via a stimulatory receptor) indicates that the failure to observe an A1-adenosine receptor-mediated inhibition of adenylate cyclase activity in the presence of stimulatory adenosine receptors may be attributable to methodological constraints.(ABSTRACT TRUNCATED AT 250 WORDS)

The rat testicular A1 adenosine receptor-adenylate cyclase system

When adenosine interacts with membrane-bound A1 receptors, it is capable of inhibiting the enzyme adenylate cyclase in brain and fat tissue. In this paper we characterize the A1 adenosine receptor-adenylate cyclase system of the rat testes. The agonist radioligand (-)-N-[3-[125I]iodo-4-hydroxyphenyl-(isopropyl)adenosine binds with high affinity (Kd, congruent to 1 nM) in a saturable manner (maximum binding, congruent to 600 fmol/mg protein). The A1 adenosine receptor binding displays the appropriate pharmacology, stereospecificity, and sensitivity to guanine nucleotides. The testicular A1 receptor is also coupled in an inhibitory manner to the enzyme adenylate cyclase, as demonstrated by the ability of N6-R-phenyl-2-propyladenosine to inhibit isoproterenol- and forskolin-stimulated cAMP accumulation. The testicular A1 receptor can be solubilized in high yield and in an active form with the detergent digitonin. The solubilized receptor retains all of the pharmacological properties of the membrane-bound receptor. Although there are many similarities among the A1 receptor from testes, brain, and fat tissue, the testicular A1 receptor displays a larger apparent mol wt. By photoaffinity labeling, the A1 adenosine receptor-binding subunit of fat and brain are 38,000 mol wt proteins, while the testicular A1 receptor binding subunit is a 42,000 mol wt protein.

Agonist photoaffinity labeling of A1 adenosine receptors: persistent activation reveals spare receptors

This study describes experiments investigating the mechanism of activation of A1 adenosine receptors. Isolated rat fat cells were used as a cellular model. The A1 receptors of these cells were covalently labeled with the agonist photoaffinity label R-2-azido-N6-p-hydroxyphenylisopropyladenosine. The covalent incorporation of the label into the binding subunit of the receptor was verified by demonstration of specific labeling of a peptide with Mr = 35,000 by the radioiodinated label. Such covalent labeling followed by removal of label not covalently bound led to a concentration-dependent reduction of cellular cAMP levels. This persistent effect of covalent labeling occurred with an IC50 value of 9 nM compared to an IC50 value of 0.9 nM for the direct reduction of cAMP levels by the label. The affinity of the label was determined in binding experiments. The Ki value of 19 nM was about 20 times higher than the corresponding IC50 value of cAMP reduction. Finally, the comparison between covalent binding and its effects suggests that covalently labeled receptors were fully activated. The data are interpreted as evidence for a receptor activation according to the occupancy theory. The analysis of the various concentration-response curves reveals the presence of spare receptors, which can be demonstrated by the method of agonist photoaffinity labeling.

Photochemical cross-linking of 125I-hydroxyphenylisopropyl adenosine to the A1 adenosine receptor of rat adipocytes

Rat adipocyte plasma membranes were incubated with the A1 adenosine receptor agonist, 125I-hydroxyphenylisopropyl adenosine (1 nM) and then treated with the photoactive cross-linking agent, ANB-NOS. The membranes were solubilized and analyzed by SDS-PAGE and autoradiography. A single protein, Mr approx. 38,000, was specifically labeled. Reduction with 2-mercaptoethanol did not affect the apparent Mr of the labeled protein. Labeling was inhibited by the adenosine receptor agonists, HPIA, PIA and NECA, and by the antagonist, theophylline, but was unaffected by inosine. We conclude that the A1 adenosine receptor is a single protein of Mr approx. 38,000.

Two saturable recognition sites for (-) [125I]iodo-N6-(4-hydroxyphenyl-isopropyl)-adenosine binding on purified cardiac sarcolemma

Analysis of (-) [125]iodo-N6-(4-hydroxyphenylisopropyl)-adenosine [( 125I]HPIA) binding to purified sarcolemmal preparations of guinea pig and bovine hearts revealed two classes of binding sites when unlabeled iodo-HPIA (100 mumol/l) was used as non-specific binding marker. In the presence of 1 mmol/l theophylline, however, only the high affinity component was detected. Adenosine receptor agonists caused biphasic displacement of [125I]HPIA binding, with a high affinity potency rank order typical of interaction with A1-adenosine receptors. Biphasic competition curves were also observed with 8-phenyltheophylline and isobutylmethylxanthine, whereas the theophylline curve was monophasic up to 1 mmol/l. In brain membranes, specific binding of [125I]HPIA as well as of [3H]PIA was further reduced when unlabeled iodo-HPIA replaces theophylline as the non-specific binding marker. These results suggest the presence of two [125I]HPIA binding sites on cardiac sarcolemma and brain membranes, but receptor function can only be ascribed to the high affinity sites. The low affinity site probably represents an artefact, which is often observed when non-specific binding is defined with the unlabeled counterpart or a structurally related ligand of the radioligand used.

Adenosine-receptor-mediated stimulation of low-Km GTPase in guinea-pig cerebral cortex

Inhibition of receptor-coupled adenylate cyclase by hormones is proposed to be associated with GTP hydrolysis. Since adenosine inhibits cerebral-cortical adenylate cyclase via A1 adenosine receptors, the present study attempts to verify this mechanism for A1-selective adenosine derivatives. In guinea-pig cortical membranes N6-(phenylisopropyl)adenosine (PIA) increased the Vmax. of the low-Km GTPase, with an EC50 (concentration causing 50% of maximal stimulation) of about 0.1 microM, and the stimulatory effect was competitively antagonized by 5 microM-8-phenyltheophylline. The rank order of potency of the stereoisomers of PIA and of 5-(N-ethylcarboxamido)adenosine (NECA) to stimulate GTPase correlated with their ability to inhibit adenylate cyclase activity (R-PIA greater than NECA greater than S-PIA). Competition binding studies with (-)-N6- ([125I]iodo-4-hydroxyphenylisopropyl)adenosine suggest that adenylyl imidodiphosphate (p[NH]ppA), an essential component of the GTPase assay system, is a more potent A1-receptor agonist than ATP, with an IC50 (concentration giving half-maximal displacement of radioligand binding) of 7.9 microM. On the basis of the p[NH]ppA concentration used in the GTPase assay (1.25 mM), enzyme stimulation by adenosine seems to be highly underestimated. Nevertheless, adenosine-induced GTP hydrolysis reflects an increased turnover of guanine nucleotides at the Ni regulatory site and appears to be a crucial step in the sequence of events processing the inhibitory signal to adenylate cyclase.

Photoaffinity labeling of A1-adenosine receptors

The ligand-binding subunit of the A1-adenosine receptor has been identified by photoaffinity labeling. A photolabile derivative of R-N6-phenylisopropyladenosine, R-2-azido-N6-p-hydroxyphenylisopropyladenosine (R-AHPIA), has been synthesized as a covalent specific ligand for A1-adenosine receptors. In adenylate cyclase studies with membranes of rat fat cells and human platelets, R-AHPIA has adenosine receptor agonist activity with a more than 60-fold selectivity for the A1-subtype. It competes for [3H]N6-phenylisopropyladenosine binding to A1-receptors of rat brain membranes with a Ki value of 1.6 nM. After UV irradiation, R-AHPIA binds irreversibly to the receptor, as indicated by a loss of [3H]N6-phenylisopropyladenosine binding after extensive washing; the Ki value for this photoinactivation is 1.3 nM. The p-hydroxyphenyl substituent of R-AHPIA can be directly radioiodinated to give a photoaffinity label of high specific radioactivity (125I-AHPIA). This compound has a KD value of about 1.5 nM as assessed from saturation and kinetic experiments. Adenosine analogues compete for 125I-AHPIA binding to rat brain membranes with an order of potency characteristic for A1-adenosine receptors. Dissociation curves following UV irradiation at equilibrium demonstrate 30-40% irreversible specific binding. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that the probe is photoincorporated into a single peptide of Mr = 35,000. Labeling of this peptide can be blocked specifically and stereoselectively by adenosine receptor agonists and antagonists in a manner which is typical for the A1-subtype. The results indicate that 125I-AHPIA identifies the ligand-binding subunit of the A1-adenosine receptor, which is a peptide with Mr = 35,000.

Demonstration of Ri-type adenosine receptors in bovine myocardium by radioligand binding

Adenosine has been shown to have negative inotropic, chronotropic and dromotropic effects on the heart. The pharmacological profiles of these effects suggest that they are mediated via Ri (A1) adenosine receptors, but a direct demonstration of these receptors is still missing. In the present study we report direct labelling of these receptors with (-)N6-[125I]-p-hydroxyphenylisopropyladenosine [( 125I]HPIA)1. The radioligand bound in a saturable and reversible manner to a crude membrane preparation, the Bmax-value was 30.5 fmol/mg protein and the KD-value 1.1 nmol/l. A similar affinity of the ligand was obtained in kinetic and competition experiments. Competition experiments with a variety of adenosine analogues gave a pharmacological profile characteristic of Ri adenosine receptors with high affinities of N6-substituted derivatives and a marked stereospecificity for N6-phenylisopropyladenosine (PIA). Purification of the membrane preparation by density gradient centrifugation resulted in a 30-fold increase in the number of binding sites which was paralleled by a similar increase in the number of binding sites for [3H]ouabain. Guanine nucleotides decreased binding of [125I]HPIA in a dose-dependent manner, but the IC50-values were considerably higher than those reported in other tissues. Finally, binding of [125I]HPIA appeared to be entropy-driven which has been shown to be characteristic of agonist binding to Ri adenosine receptors. These results suggest the presence of Ri adenosine receptors in ventricular myocardium which may be responsible for the mediation of the effects of adenosine and its analogues.

Purification and characterization of (-)[125I]hydroxyphenylisopropyladenosine, an adenosine R-site agonist radioligand and theoretical analysis of mixed stereoisomer radioligand binding

(-)-N6-(R-4-Hydroxyphenylisopropyl)adenosine (HPIA) was iodinated with NaI and trace 125I. Mono- and diiodinated reaction products and the starting material were separated by high pressure liquid chromatography and the structures of the reaction products were verified by NMR. (-)-N6-(R-Phenylisopropyl)adenosine (PIA), IHPIA, and I2HPIA decreased rat atrial contractility with ED50 values of 24, 28, and 33 nM, respectively. The contractile effects of these compounds were competitively blocked by theophylline (KI = 7.9 microM), but were not affected by adenosine deaminase. IHPIA also inhibited (-)isoproterenol-stimulated cyclic AMP accumulation in adipocytes with an ED50 (10 nM) and to an extent (83%) nearly identical to PIA. [125I]HPIA prepared using carrier-free 125I bound to adenosine receptors on membranes from rat cerebral cortex, adipocyte ghosts, and heart ventricles. Binding was inhibited stereospecifically by PIA and by other adenosine analogues and alkylxanthines. The KD of [125I]HPIA determined kinetically using brain membranes at 21 degrees was 0.94 nM (K1 = 2.55 X 10(7) M-1 min-1; K-1 = 0.024 min-1) in good agreement with the equilibrium determination of 1.94 nM. The density of adenosine receptors in brain membranes was found to be 871 fmol/mg of protein. When normalized to protein, the density of receptors in heart membranes and adipocyte ghosts, respectively, was found to be 39- and 2.3-fold less than in brain membranes. We conclude that [125I]HPIA can be rapidly synthesized and purified, binds to adenosine R-sites and is an agonist radioligand resistant to adenosine deaminase. Computer modeling of the equilibrium binding resulting from the use of mixed stereoisomers of a radioligand indicates that the combined use of (-)[125I]HPIA and (+)[125I]HPIA would result in the generation of nonlinear Scatchard plots.

Labelling of Ri adenosine receptors in rat fat cell membranes with (-)-[125iodo]N6-hydroxyphenylisopropyladenosine

A new adenosine analogue, (-)-iodo-N6-phydroxyphenylisopropyladenosine [(-)-IHPIA], has been developed for radioligand binding studies of Ri adenosine receptors. In addition, the effects of (-)IHPIA on adenosine-mediated responses of rat fat cells have been characterized. (-)IHPIA is slightly less potent at Ri adenosine receptors than (-)N6-phenylisopropyladenosine [(-)PIA] as assessed by adenylate cyclase and lipolysis studies. (-)IHPIA inhibited basal adenylate cyclase activity with an IC50 of 60 nmol/l compared to an IC50 of 16.3 nmol/l for (-)PIA. (-)PIA and (-)IHPIA inhibited adenosine deaminase-stimulated lipolysis of intact rat fat cells with an IC50 of 0.55 and 3.6 nmol/l. The potency of (-)N6-phydroxyphenylisopropyladenosine [(-)HPIA] was intermediate. (-)HPIA has been labelled with carrier-free Na[125I] to very high specific activity (2,175 Ci/mmol) and used as agonist radioligand in binding studies of Ri adenosine receptors. The binding of (-)[125I]HPIA was saturable, reversible and stereospecific. Saturation analysis revealed two affinity states with dissociation constants (KD) of 0.7 and 7.6 nmol/l and maximal number of binding sites (Bmax) of 0.94 and 0.95 pmol/mg protein. The rate constant of association, k1, was 3.7 X 10(8) l X mol-1 X min-1. Binding was slowly reversible with a t1/2 of 88 min. In competition experiments specific binding was most potently inhibited by (-)PIA, N6-cyclohexyladenosine (CHA), (-)HPIA and (-)IHPIA, followed by 5'-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine. 1,3-Diethyl-8-phenylxanthine (DPX) and 8-phenyltheophylline were the most potent adenosine antagonists with Ki-values of 67 and 83 nmol/l, whereas the methylxanthines 3-isobutyl-1-methylxanthine, theophylline and caffeine had Ki-values between 1 and 21 mumol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

[125I] N6-p-Hydroxyphenylisopropyladenosine, a new ligand for Ri adenosine receptors

N6-p-Hydroxyphenylisopropyladenosine (HPIA) has been labelled with carrier-free Na[125I] to very high specific activity (2,175 Ci/mmol) and used as an agonist ligand to characterize Ri adenosine receptors in rat cerebral cortex membranes. The binding is saturable, reversible, stereospecific and dependent on protein concentration. The specific binding at 37 degrees C was of high affinity with an equilibrium dissociation constant KD of 0.48 nmol/l and was saturable with 0.23 pmol of [125I]HPIA per mg of protein. The rate constant of association, k1, was 3.25 x 10(8) l mol-1 min-1 and that of dissociation, k2 0.0110 min-1 yielding at t1/2 of 63 min. In competition experiments the (-)isomer of N6-phenylisopropyladenosine (PIA) was 16-fold more potent than the (+)isomer in competing for binding sites. Specific binding was most effectively displaced by N6-cyclohexyladenosine (CHA, ki=0.26 nmol/l), (-)PIA (ki= 0.33 nmol/l) and HPIA (ki=0.52 nmol/l), whereas 5'-N-ethylcarboxamidoadenosine (NECA, ki=1.42 nmol/l) was less effective. The methylxanthines 3-isobutyl-1-methylxanthine (IBMX), theophylline and caffeine which have been classified as adenosine antagonists had ki values between 5-43 mumol/l. Binding of [125I]HPIA was regulated by guanine nucleotides and divalent cations. The results indicate that [125I]HPIA labels Ri adenosine receptors in rat brain membranes.

Radioiodination of p-hydroxyphenylisopropyladenosine: development of a new ligand for adenosine receptors

dl-p-Hydroxyphenylisopropyladenosine has been iodinated with 125I and tested as a ligand of adenosine receptors in membranes from rat brain using a filtration assay. Binding studies using l-[3H]phenylisopropyladenosine as a ligand were carried out in parallel, and the binding of both ligands could be displaced by dl-p-hydroxyphenylisopropyladenosine and l-phenylisopropyladenosine in a similar fashion. These data establish the feasibility of using radioiodinated derivatives of adenosine as ligands of suitable types of adenosine receptors.