Biomedical subjects

W Soudijn

Publications and source records attributed to W Soudijn.

At least 37 records · Page 2Linked to original sources

PubMed1994

Pharmacokinetic-pharmacodynamic relationship of the cardiovascular effects of adenosine A1 receptor agonist N6-cyclopentyladenosine in the rat.

The purpose of the investigation was to develop a pharmacokinetic-pharmacodynamic model for the characterization of the cardiovascular effects of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) in individual normotensive rats. After the i.v. administration of 200 micrograms/kg (0.60 mumol/kg) of CPA, the time course of heart rate and arterial blood pressure was monitored in conjunction with serial blood sampling. Potential interference of the concentration-cardiovascular effect relationship by the development of acute tolerance or the formation of (inter)active metabolites was investigated by infusion of CPA with different rates and by determination of blood concentrations both by high-performance liquid chromatography and radioreceptor assay. In the individual rats the concentration-hemodynamic effect relationships were satisfactorily modeled according to the sigmoidal Emax pharmacodynamic model. For the negative chronotropic effect, the pharmacodynamic parameters proved to be independent of the infusion rate, indicating the absence of development of acute tolerance during the experiment. Potency (EC50) and intrinsic efficacy (Emax) were 2.7 +/- 0.5 ng/ml and -209 +/- 10 bpm, respectively (mean +/- S.E., n = 17). The concentrations of CPA as determined by the radioreceptor assay were identical to those determined by high-performance liquid chromatography, thereby excluding the formation of (inter)active metabolites. It is concluded that on the basis of this integrated pharmacokinetic-pharmacodynamic model, with the negative chronotropic effect as a pharmacodynamic endpoint, estimates of the potency and the intrinsic efficacy of adenosine A1 receptor agonists in vivo can be obtained after the administration of a single dose.

Adenosine↗

PubMed1994

Molecular modelling of CCK-A receptors.

Recently, the primary structure of the cholecystokinin A-type (CCK-A) receptor has been determined. From the Kyte-Doolittle-predicted hydrophobic stretches of this sequence and the transmembrane domains of bacteriorhodopsin, a membrane-bound protein of known tertiary structure, a three-dimensional model of the membrane-embedded part of this receptor was built. Subsequently, the modelled receptor pore was searched for a binding site that matches the structural and conformational characteristics of the parent classes of the antagonists devazepide and lorglumide. In addition, the binding mode of hybrid analogues of these reference compounds was examined. The proposed antagonist, binding site includes regions in which hydrophobic, hydrogen-bonding and aromatic interactions stabilize the receptor-ligand complex.

Amino Acid Sequence↗

PubMed1993

Characterization of ecto-ATPase on human blood cells. A physiological role in platelet aggregation?

Ecto-ATPase (EC 3.6.1.15) is a plasma membrane-bound enzyme which degrades extracellular triphosphate nucleotides. Although its physiological function is still unclear, the enzyme obscures the study of P2 purinoceptors (i.e. receptors for ATP and other di- and triphosphate nucleotides), since it is capable of metabolizing the pharmacological ligands, such as ATP, for these receptors. We characterized the ecto-ATPase activity on human blood cells with a [gamma 32P]ATP assay and HPLC measurements. We also determined whether ecto-ATPase activity could affect the anti-aggregatory role of ATP in whole human blood. The Km for ATP of the ecto-ATPase on human blood cells was 8.5 +/- 2.3 microM and the maximum degradation rate, at 37 degrees, was 2.7 +/- 1.1 nmol ATP/(min x mL whole blood). In whole blood the major part of ATP was broken down by the blood cells, predominantly by the leukocytes. ATP and UTP were broken down equally well, mainly yielding the corresponding di- and monophosphates. In search of inhibitors for the ecto-ATPase, we studied several analogs of ATP. 8-Bromo-ATP as well as 2'- and 3'-deoxy-ATP were substrates for the enzyme. In contrast, modification of the phosphate side chain yielded inhibitors. Subsequently, a possible role of the ecto-ATPase in platelet aggregation was verified. To assess the role of the plasma membrane-bound enzyme, platelet aggregation was determined in whole blood instead of platelet-rich plasma. In the presence of ATP alone, an antagonist of ADP-induced platelet aggregation, some aggregation was still observed. As breakdown of ATP by the ecto-ATPase leads to gradual formation of ADP, as mentioned above, we compared the effects of a stepwise versus bolus addition of ADP. Subsequent dosing of ADP (1.5, 2.5, 5 and 10 microM) resulted in platelet aggregation but to a much smaller extent, at most approximately 60%, compared to the amount of platelet aggregation obtained with a bolus addition of ADP (10 microM). In conclusion, human blood cells possess a high affinity ecto-ATPase which degrades ATP as well as ATP analogs with modified base and ribose moieties. ATP analogs with a modified phosphate chain are inhibitors of the ecto-ATPase. A direct role of the ecto-ATPase activity on platelet aggregation is probably small, as degradation of ATP to ADP proceeds slowly and cumulative addition of ADP to platelets in whole blood results in a modest amount of aggregation.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Diphosphate↗

PubMed1993

High-performance liquid chromatography of the adenosine A1 agonist N6-cyclopentyladenosine and the A1 antagonist 8-cyclopentyltheophylline and its application in a pharmacokinetic study in rats.

This report describes a rapid and sensitive analysis for the simultaneous detection of the adenosine A1 receptor ligands N6-cyclopentyladenosine (CPA) and 8-cyclopentyltheophylline (CPT) in rat blood. The method involved alkaline extraction of the compounds and internal standard N6-cyclohexyladenosine (CHA) with ethyl acetate, followed by isocratic reversed-phase high-performance liquid chromatography on a 3-microns MicroSphere C18 column with UV detection at 269 nm. The mobile phase consisted of a mixture of 10 mM acetate buffer (pH 4.0)-methanol-acetonitrile (56:40:4, v/v/v) with a flow-rate of 0.50 ml/min. The total run time was ca. 19 min. For CPA and CPT extraction yields were greater than 77 and 66% in the concentration range of 0.010-0.75 microgram/ml and 0.025-15 micrograms/ml, respectively, with intra- and inter-assay variations less than 9%. In 100 microliter blood samples the corresponding limits of detection were 3.3 and 6.2 ng/ml (signal-to-noise ratio = 3). CPA was found to be degraded in rat blood in vitro with a half-life of 24 min at 37 degrees C. The utility of the analytical method was established by analyzing blood samples from rats which had received an intravenous administration of 200 micrograms/kg CPA or 12 mg/kg CPT. Due to its rapidity and sensitivity this method is concluded to be particularly useful in pharmacokinetic studies with CPA and CPT.

Adenosine↗

PubMed1993

Structure-affinity relationship studies on 5-HT1A receptor ligands. 1. Heterobicyclic phenylpiperazines with N4-alkyl substituents.

Structure-affinity relationship (SAR) studies for 5-HT1A receptor site are presented for two series of heterobicyclic phenylpiperazines with N4-alkyl substituents: 4-alkyl derivatives of 1-(2,3-dihydro-1,4-benzodioxin-5-yl)piperazine (3) and 1-(benzo[b]furan-7-yl)piperazine (4). The linear and branched hydrocarbon chain derivatives up to n-decyl were synthesized and evaluated for their ability to displace [3H]-2-(di-n-propylamino)-8-hydroxytetralin from its specific binding sites in rat frontal cortex homogenates. All compounds displayed a nanomolar affinity for the 5-HT1A receptor. In both series the N-ethyl and N-n-propyl substituted derivatives have similar affinities, being slightly but statistically significantly less active than the N-methyl-substituted derivatives. Elongation of the hydrocarbon chain increases the affinity for the central 5-HT1A receptor site, reaching a local maximum for the N-n-hexyl-substituted phenylpiperazines 23 (Ki = 0.50 nM) and 39 (Ki = 0.54 nM). Assuming that the arylpiperazine derivatives at the 5-HT1A binding site are in the ionic state, ionization constants were determined in order to evaluate the use of the local inhibition constant, Ki+, as a more convenient parameter to study the structure-affinity relationships. However, the Ki+ could not account for the specific N4-substituent effects found.

Animals↗

PubMed1993

Binding of the radioligand [35S]adenosine 5'-O-(2-thiodiphosphate) and intracellular calcium response in rat liver parenchymal cells.

The use of the radioligand [35S]adenosine 5'-O-(2-thiodiphosphate) (ADP beta 35S) for the determination of P2y-purinoceptors on turkey erythrocyte membranes has recently been described. In the present study, we were able to demonstrate specific binding of this radioligand in intact rat liver parenchymal cells. Within 10 min a thermodynamic equilibrium was obtained which lasted for 25 min with a subsequent decline. Displacement studies with several nucleotides were performed yielding Ki values of 1.5 +/- 0.47 microM for UTP, 1.8 +/- 0.35 microM for adenosine 5'-O-(2-thiodiphosphate) (ADP beta S), 31 +/- 6.2 microM for ATP and 35 +/- 6.1 microM for GTP. In addition, we showed that ADP beta 35S is highly resistant to degradation by ecto-nucleotidases, with only 14.5 +/- 1.4% of total ADP beta 35S present being degraded after 1 hr, and that the binding of ADP beta 35S to its binding sites was modulated by EDTA. The Ki value of ATP shifted to 8.1 +/- 1.2 microM upon the addition of 1 mM EDTA to the incubation medium. In these rat liver parenchymal cells all nucleotides promoted calcium entry in a dose-dependent manner with EC50 values of 3.5 +/- 0.22 microM for UTP, 20.7 +/- 3.1 microM for ATP, 38.3 +/- 6.4 microM for ADP beta S and 73.6 +/- 13.7 microM for GTP, with GTP being a partial agonist. Based on the data derived from the present study we discuss the possible correlation between binding and functional experiments and conclude that the described receptor resembles most closely the P2u-purinoceptor and/or "nucleotide receptor", in that UTP is at least as active as ATP.

Adenosine Diphosphate↗

PubMed1992

Molecular modelling of asperlicin derived cholecystokinin A receptor antagonists.

The C3-substituted benzodiazepines derived from asperlicin, e.g. devazepide (L-364,718, MK-329), constitute the most potent class of cholecystokinin A-type (CCKA) receptor antagonists. In order to gain insight into the prerequisites for binding, we examined the conformational properties of both potent and weak representatives of this class with computer assisted molecular modelling (CAMM) techniques. The CAMM results indicate that the binding site for the C3-substituents is a planar slot on the CCKA receptor surface and, in addition, allow the proposal of a model which describes the relative binding mode of the less potent R isomers versus that of the S isomers. The latter model illustrates the unique spatial properties of the benzodiazepine moiety, which we suggest functions primarily as an invertible core which assures an optimal arrangement of attached substituents.

Benzodiazepinones↗

PubMed1992

Hybrid cholecystokinin-A antagonists based on molecular modeling of lorglumide and L-364,718.

A series of novel nonpeptide cholecystokinin-A (CCK-A) antagonists have been synthesized. Designed on the basis of the structural homology between lorglumide and L-364,718, as investigated with molecular modeling, these compounds constitute a link between the N-acylglutamic acid and 3-amino-5-phenyl-1,4-benzodiazepin-2-one derived antagonists. The prepared compounds were tested in vitro as antagonists of the binding of [3H]-(+/-)-L-364,718 and [3H]-CCK-8(S) to rat pancreas and guinea pig brain membranes, respectively. All compounds proved to be selective for the (peripheral) CCK-A receptor, the most potent analogue, 6, having a Ki value of 90 nM. The structure-activity profile of the series of hybrid compounds relates closest to that of the N-acylglutamic acid derived antagonists.

Animals↗

PubMed1992

A steric and electrostatic comparison of three models for the agonist/antagonist binding site on the adenosine A1 receptor.

Several models have been described in the literature to explain the similarity of interaction of adenosine receptor agonists and antagonists with the binding site of the receptor. Besides the superposition of the nitrogen atoms of adenosine and xanthine (the "standard" model), two other models have been described: one in which xanthine is rotated around its "horizontal" axis before superposition ("flipped") and one in which the adenosine N6-region and the xanthine C8-region are superimposed ("N6-C8"). In this study we compared the steric and electrostatic properties of these models. The flipped model tends to show higher percentages of overlap for the positive electrostatic potentials and the N6-C8 model yielded predominantly a slightly higher overlap for the negative electrostatic potentials, although these differences were rather small. Since the N6-region in adenosine and the C8-region in xanthine are coinciding in this model, the N6-C8 model yielded a much larger overlap in van der Waals volume than the other two models. The N6-C8 model seems therefore to be the more probable model, also because the interactive groups point in the same direction for both adenosine and xanthine analogues. We determined the geometries of both the adenosine N6-substituents and the xanthine 8-substituents in earlier studies. The N6-C8 model causes a coincidence of these separately determined conformations.

Adenosine↗

PubMed1992

The mode of interaction of amiloride and some of its analogues with the adenosine A1 receptor.

Amiloride, a potassium sparing diuretic, inhibits adenosine A1 receptor-radioligand binding in calf and rat brain membranes in the low micromolar range. The drug interacted with the A1 receptor in a manner different from classical A1 ligands, but structure-activity relationship studies indicated that this inhibitory effect is not related to the ion transport inhibiting properties of amiloride (Garritsen et al., 1990a,b) In the present study, the question is addressed how amiloride interacts with the adenosine A1 receptor. Amiloride and two of its analogues, in concentrations equivalent to their Ki values in displacement studies, decrease the affinity of the A1 antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine, but not the maximal binding capacity of the radioligand. Furthermore, the dissociation rate of the receptor-ligand complex is unaltered in the presence of amiloride or its analogues in a concentration exceeding the Ki value 10-fold. These characteristics argue for a purely competitive mode of interaction. The functional consequences of the interaction between amiloride analogues and the A1 receptor were investigated at the level of cyclic adenosine 3',5'-monophosphate (cAMP) formation. The amiloride analogue 5-(N-butyl-N-methyl) amiloride (MBA) reversed A1-receptor mediated inhibition of forskolin-stimulated cAMP formation in rat fat cell membranes. In this model, the antagonist potency of MBA is ca 5 microM. This value is in fair agreement with a Ki value of 3.5 microM in binding assays under similar conditions. In conclusion, amiloride inhibits A1 receptor binding in an apparently competitive manner. This suggests that the binding sites of amiloride and the classic A1 receptor ligands may at least partially overlap.(ABSTRACT TRUNCATED AT 250 WORDS)

Amiloride↗

PubMed1991

The role of medicinal chemistry in drug research.

Classical medicinal chemistry, that is molecular modifications of existing bioactive compounds, leading to me-too drugs with value added and often to me-too drugs which will never reach the market or to drugs in another pharmacological field than that originally intended, will be with us for a long time to come. The ultimate goal, however, is of course rational or at least semi-rational drug design. In some instances this goal seems to have already been reached as in the case of angiotensin-converting enzyme inhibitors and H2 antagonists. In the (near) future results from the steady progress in molecular biology in combination with computer-assisted drug modelling--possibly coupled with artificial intelligence techniques--will help the medicinal chemist in his efforts to attain this goal.

Animals↗

PubMed1991

Mapping the xanthine C8-region of the adenosine A1 receptor with computer graphics.

Substitution at the 8-position of 1,3-dipropylxanthines can lead to very potent and selective adenosine A1 antagonists. The xanthine C8-region was investigated in this study, using CAMM (computer-assisted molecular modeling). This region can be divided into two subregions with a considerable overlap in volume: a phenyl region which binds the flat substituents and a cycloalkyl region which binds the other substituents. The 8-phenyl-substituted derivatives bind with an N9-C8-Cl'-C2' dihedral angle of 220 degrees; this dihedral angle is 330 degrees for the 8-cycloalkyl-substituted derivatives. The lower affinity of C8-substituted 7-methyl-1,3-dipropylxanthines can be explained quantitatively with steric hindrance, which C8-substituents experience from the 7-methyl group in these conformations. The substitution pattern determines the affinity for 8-phenyl-substituted compounds for which the energy cost to reach the dihedral angle of 220 degrees is low, but has little influence otherwise. The affinity of the 8-cycloalkyl-1,3-dipropylxanthines is mainly volume dependent, because of a forbidden area near the cycloalkyl region.

Animals↗

PubMed1991

1H-imidazo[4,5-c]quinolin-4-amines: novel non-xanthine adenosine antagonists.

On the basis of a model we recently developed for the antagonist binding site of the adenosine A1 receptor (J. Med. Chem. 1990, 33, 1708-1713), it was predicted that 1H-imidazo[4,5-c]quinolin-4-amines would be antagonists of the A1 receptor. Furthermore, it was expected that certain hydrophobic substitutions at the 2- and 4-positions would enhance affinity. Here, we report on the synthesis and the adenosine A1 and A2 receptor affinity of substituted 1H-imidazo[4,5-c]quinolin-4-amines. Some of these compounds have nanomolar affinity for the A1 receptor. The structure-activity relationships (SAR) of these compounds are discussed in relation to SAR for other adenosine receptor ligands. The 1H-imidazo[4,5-c]quinolin-4-amines constitute a novel class of non-xanthine adenosine antagonists.

Adenosine↗

PubMed1991

Receptor binding profiles of amiloride analogues provide no evidence for a link between receptors and the Na+/H+ exchanger, but indicate a common structure on receptor proteins.

Amiloride and its analogues affect radioligand binding to the adenosine-A1 receptor. In this paper, the specificity of this effect is investigated by generating receptor binding profiles for amiloride and two of its analogues. A limited structure-activity relationships study is performed to probe the relationship between inhibition of receptor binding by amiloride analogues and the effects of these compounds on Na+ transport, in particular Na+/H+ exchange. The receptor binding profiles of amiloride, benzamil and 5'-(N,N-hexamethylene)amiloride (HMA) indicate that the compounds affect a variety of receptors and that none of the compounds is highly selective for any of these. The SAR study indicates that it is very unlikely that a direct coupling between receptors and Na+/H+ exchange or another amiloride-sensitive ion transport system is responsible for the inhibition of receptor binding. A correlation between the signal transduction systems coupled to the receptors involved and the potency of the amiloride analogues is also absent. The varying nature of the receptors, affected by amiloride or its analogues, suggests a wide-spread presence of an amiloride binding site on receptors and other membrane proteins.

Amiloride↗

PubMed1990

Interaction of amiloride and its analogues with adenosine A1 receptors in calf brain.

Amiloride, a potassium sparing diuretic, is known to interact with a number of ion transport systems, receptors and enzymes. Here, we report on the interaction between this drug and the adenosine A1 receptor as present in calf brain membranes. Adenosine A1 receptors are characterized by a subnanomolar affinity for the antagonists [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) and the agonist [3H]N6-R-1-phenyl-2-propyladenosine ([3H]PIA). Amiloride displaces both agonist and antagonist binding with a Ki value in the low micromolar range. This inhibition is counteracted by NaCl and protons, in contrast to the binding of [3H]PIA and [3H]DPCPX. The results suggest that amiloride interacts with the adenosine A1 receptor at a site distinct from the ligand binding site. In order to elucidate the role of one of the ion transport systems known to be inhibited by amiloride, eight amiloride analogues with different sensitivities for these systems were tested. The potency and order of potency of these compounds towards adenosine A1 receptors excludes the involvement of the epithelial Na+ channel, Na+/H+ exchanger or Na+/Ca2+ exchanger.

Adenosine↗

PubMed1990

Chemical modification of adenosine A1 receptors. Implications for the interaction with R-PIA, DPCPX and amiloride.

Amiloride, a potassium sparing diuretic, inhibits the specific binding of [3H]8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and [3H]N6-R-1-phenyl-2-propyladenosine (PIA) to adenosine A1 receptors in calf brain. This interaction is different from the agonist-receptor or the antagonist-receptor interaction as Na+ and H+ counteract the inhibitory effect of amiloride whereas these ions hardly affect the binding of the classic A1 receptor ligands. In the present study, the effects of protein modifiers on the equilibrium inhibition constant of amiloride are compared with effects of these reagents on the affinities of DPCPX and PIA. It is demonstrated that the affinities of amiloride and [3H]DPCPX are changed after treatment with a carboxyl-modifying reagent but unaffected by modification of histidyl, arginyl and cystein residues. The maximal binding capacity of [3H]DPCPX is enhanced by sulfhydryl modification, whereas the number of [3H]DPCPX binding sites is reduced by treatment with a histidine-modifying reagent. The histidyl residues of the [3H]DPCPX binding site can be partially protected against modification by 300 microM amiloride, present during treatment of the membranes. An equivalent concentration of 8-phenyltheophylline results in complete protection. The apparent affinity of PIA is altered by modification of histidyl, carboxyl, arginyl and cystein residues. In the latter two cases, uncoupling of the G protein seems to be the major reason for the decrease in affinity of PIA. The results suggest that amiloride is an A1 antagonist with binding characteristics that differ from the classic A1 antagonists such as DPCPX.

Adenosine↗

PubMed1990

The concept of selectivity in 5-HT receptor research.

Since the demonstration that serotonin (5-hydroxytryptamine, 5-HT) interacts with different (sub)types of membrane receptors, several compounds have been proposed as potent and selective ligands for one of these 5-HT subtypes. Unfortunately, specific and highly selective ligands (selectivity ratios greater than or equal to 1000) for the majority of 5-HT subtypes are still lacking. A few compounds are selective (ratios greater than or equal to 100), but most of the reputed 'selective' tools display affinities for other 5-HT subtypes and/or other (neuro-) transmitter receptors. Mainly due to different interpretations of the concept of selectivity, many of these nonselective compounds are still used to characterize 5-HT receptors. In this paper, we present the affinities (obtained by radioligand binding studies) of the most selective tools known today for each of the 5-HT subtypes and discuss the structure-activity relationships of some interesting series. The potential use of several of these selective ligands as pharmacological tools and therapeutics will be briefly reviewed.

Animals↗

PubMed1990

A model for the antagonist binding site on the adenosine A1 receptor, based on steric, electrostatic, and hydrophobic properties.

With the aid of molecular modeling, both adenosine and adenosine A1 receptor antagonists belonging to various chemical classes were compared with respect to their minimum-energy conformations and molecular electrostatic potentials, as computed by the semiempirical molecular orbital program MOPAC. Distinct steric and electrostatic similarities between adenosine and the prototypic adenosine antagonist theophylline are evident when both compounds are superimposed, with theophylline in a so-called flipped orientation. Similar patterns were found for all other A1 antagonists investigated in this study. A model for the antagonist binding site on the adenosine A1 receptor, based on steric, electrostatic, and hydrophobic properties contributing to potency, is proposed.

Adenosine↗