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H Timmerman

Publications and source records attributed to H Timmerman.

At least 199 records · Page 11Linked to original sources
PubMed1990

Evidence for cell type dependent mechanisms in agonist-induced down-regulation of beta-adrenoceptors.

Processing of the beta-adrenoceptors by mammalian tumor cells, Chang liver cells and HeLa cells both containing beta-2-adrenoceptors, was studied using 125I-labeled iodocyanopindolol and 3H-labeled CGP12177. No difference could be detected between the results obtained with these two ligands. During desensitization of these cells by the beta-adrenergic agonist isoproterenol a time dependent decrease of intracellular cyclic-AMP was accompanied by loss of beta-adrenoceptors. The loss of receptors could not be prevented by the cytoskeleton disrupting agents colchicine and cytochalasin B. The effect of the lysosomotropic agent chloroquine is dependent upon the cell type. It showed no effect on the agonist-induced down-regulation of beta-adrenoceptors on Chang liver cells in contrast to a prominent inhibiting effect on HeLa cells. These results highly suggest that lysosomal enzymes are involved in the process of down-regulation of beta-adrenoceptors on certain cell types only. Colchicine and cytochalasin B, as well as Gpp(NH)p were able to uncouple the guanine nucleotide binding protein and the beta-adrenoceptor. This uncoupling however, did not inhibit the loss of receptors measured by radioligand binding techniques during challenge with agonists. From experiments performed to separate membrane fractions on non-linear sucrose gradients it cannot be concluded that receptors are redistributed intracellular or in the plane of the membrane. Withdrawal of the agonist during receptor down-regulation resulted in the reappearance of measurable receptors within 20 hr for receptors on HeLa cells and within 60 hr for Chang liver cells. The reappearance of receptors could be totally blocked by cycloheximide, indicating that these receptors were newly synthesized and not internalized receptors. From the results of our experiments it is concluded that beta-adrenoceptors, present on Chang liver cells and HeLa cells are down-regulated during desensitization by isoproterenol without uptake in intracellular structures of the cells. The fate of these down-regulated receptors remains unclear. It is also concluded that no universal mechanism can be proposed for beta-adrenoceptor down-regulation on mammalian cells.

Adrenergic beta-Agonists↗
PubMed1989

Histamine receptors in the central nervous system.

The role of histamine in brain function is discussed. A brief review is presented on the three types of histamine receptors with regard to their biochemistry and functions, as well as on specific ligands, both agonists and antagonists. It is concluded that several aspects of the pharmacology of histamine in the central nervous system are still a matter of empirism and speculations. The recent publications on the brain-penetrating H2 antagonist zolantidine and the selective H3 agonists and antagonists are expected to contribute to the knowledge of the histaminergic pathways in the brain. Therapeutical areas within the central nervous system are related to behaviour (including wake-sleep), neuroendocrinal processes, the control of muscle activity and cerebral circulation.

Central Nervous System↗
PubMed1989

High affinity, saturable [3H]mepyramine binding sites on rat liver plasma membrane do not represent histamine H1-receptors. A warning.

Rat liver plasma membrane contains a saturable, high affinity binding site for the labelled histamine H1-antagonist [3H]mepyramine. Nonlinear regression analysis of the performed saturation experiments revealed an equilibrium dissociation constant (Kd) of 7.7 +/- 0.4 nM and a maximal binding capacity (Bmax) of 70.4 +/- 9.5 pmol/mg protein. Specific binding could be inhibited completely by several histaminergic ligands. However, the affinities of the tested H1-antagonists other than mepyramine for this binding site were quite low and the known stereospecificity displayed by the histamine H1-receptor for the enantiomers of chlorpheniramine and pheniramine was not found. Moreover, the H2-selective agonist 4-methylhistamine (Ki = 412 microM) was even more potent than its H1-selective 2-methylderivative (Ki = 772 microM). Since several ethylenediamines were also very potent in displacing [3H]mepyramine we suggest the presence of an ethylenediamine recognition site on rat liver plasma membrane which is unrelated to the histamine H1-receptor. It is stressed that a proper pharmacological characterization of a reported binding site is needed, since we show in this study that [3H]mepyramine, which is frequently used in studies concerning the H1-receptor, labels non-H1-receptor binding sites in rat liver plasma membrane.

Aminopyridines↗
PubMed1989

Pharmacochemical aspects of leprosy. Recent developments and prospects for new drugs.

From a pharmacochemical point of view the existing anti-leprotics as well as possible innovations in the chemotherapy of leprosy are discussed. Of the main anti-leprotics, which are used nowadays--dapsone, rifampicin, clofazimine, isoniazide, ethionamide and prothionamide--the mechanism of action, the main problems in their application and possibilities to develop improved variants are reviewed. Based on the chemistry of Mycobacterium leprae, the target systems for new anti-leprotics are identified. These systems include the cell wall, the catabolism of reactive oxygen species, the metabolisms of carbon sources, the amino acid metabolism and the uptake of iron. Two possible new lead structures from other fields, 4-quinolones and mycobacterial ribonucleotide reductase inhibitors are presented.

Dapsone↗
PubMed1989

Involvement of protein kinase C in the histamine H1-receptor mediated contraction of guinea-pig lung parenchymal strips.

In this study we investigated the role of protein kinase C (PKC) in the histamine H1-receptor mediated contraction of guinea-pig parenchymal lung strips. Lung strips contract after administration of the PKC activators phorbol-12-myristate-13-acetate (PMA) and dioctanoylglycerol. Prolonged stimulation (45 min.) with a high concentration of PMA abolished subsequent responses to PMA whereas H1-responses were only partially reduced. Administration of a PKC inhibitor (H-7) also led to an inhibition of H1-responses. Experiments with combined incubations with the PKC inhibitor H-7 and the Ca2+-entry blocker verapamil suggest that the PKC contribution to the response is possibly mediated via regulation of Ca2+ influx.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1989

The effects of cimetidine, ranitidine and famotidine on rat hepatic microsomal cytochrome P-450 activities.

It has been questioned whether the interaction of H2-antagonists with cytochrome P-450 that is observed in vitro is also relevant for the in vivo situation. Until now the possibility that cytochrome P-450 may function with different modes of action has been neglected in this respect. We studied the effect of cimetidine, ranitidine and famotidine on the monoxygenase, the oxidase and the peroxidase action of cytochrome P-450. Biotransformation catalyzed by the monoxygenase and oxidase action of cytochrome P-450 was affected by cimetidine (probably via its ligand interaction with cytochrome P-450), whereas metabolism by the peroxidase mode of action of cytochrome P-450 was hardly influenced. Ranitidine and famotidine (both pharmacodynamically more potent than cimetidine) only slightly affected cytochrome P-450 activities.

Animals↗
PubMed1989

Effect of thiols on lipid peroxidation in rat liver microsomes.

The stimulatory or inhibitory effects of various thiol compounds on in vitro lipid peroxidation by iron-ascorbate in rat liver microsomes were determined. Glutathione had no measurable pro-oxidant capacity, in contrast, it protected against lipid peroxidation. N-Acetyl l-cysteine and S-methyl-glutathione had no effect on in vitro lipid peroxidation. l-Cysteine stimulated lipid peroxidation and also of d-penicillamine and dl-dithiothreitol the pre-oxidant capacity predominated the anti-oxidant capacity. Cysteamine afforded a pronounced protection against in vitro lipid peroxidation. In contrast to the labile character of the glutathione dependent protection, the protection by cysteamine was not affected by heat-pretreatment of the liver microsomes or alkylating protein sulfhydryl groups by N-ethyl maleimide. Again in contrast to glutathione, the protection against in vitro microsomal lipid peroxidation by cysteamine was not reduced after in vivo lipid peroxidation induced by CC14. This suggests that even after the process of lipid peroxidation has been started, administration of cysteamine might still be beneficial.

Acetylcysteine↗
PubMed1989

Contribution of 4-hydroxy-2,3-trans-nonenal to the reduction of beta-adrenoceptor function in the heart by oxidative stress.

Oxidative stress reduces adenylate cyclase activity and also the maximal response to beta-adrenoceptor stimulation in the rat heart, while beta-adrenoceptor density is not affected or increased. Since free sulfhydryl groups are essential to beta-adrenoceptor function and the sulfhydryl reactive substance 4-hydroxy-2,3-trans-nonenal (HNE) is responsible for part of the effects of oxidative stress, the effect of HNE on beta-adrenoceptor function in field stimulated left atria of the rat was determined. To this end field stimulated atria were incubated with 10 microM, 100 microM and 1 mM HNE for 25 min. After removing the excess of HNE, beta-adrenoceptor function was determined by measuring the positive inotropic response to (-)-isoproterenol. It was found that 10 microM HNE had no effect on beta-adrenoceptor function, whereas 100 microM HNE reduced the maximal effect to (-)-isoproterenol without affecting the pD2 (-log EC50). At these concentrations, HNE had no effect on either beta-adrenoceptor density or on c-AMP production. After 1 mM HNE, the atria stopped contracting. Since the effects of the synthetic thiol inactivator N-ethyl maleimide were similar to those of HNE, it was concluded that the reduction of beta-adrenoceptor function by HNE is probably the result of alkylation of free sulfhydryl groups. Our results indicate that the reduction of adenylate cyclase activity by oxidative stress is not mediated by the production of HNE, however oxidative stress and HNE both reduce the maximal response to beta-adrenoceptor stimulation.

Adenylyl Cyclases↗
PubMed1989

Synthesis and copper-dependent antimycoplasmal activity of 1-amino-3-(2-pyridyl)isoquinoline derivatives. 2. Amidines.

In our search for new compounds with antimycoplasmal activity, a series of aromatic amidines derived from 1-amino-3-(2-pyridyl)isoquinoline (1) was synthesized. In the presence of 40 microM copper the most active compounds show growth inhibition of Mycoplasma gallisepticum in the nanomolar range. These compounds are 3 times as active as tylosin, an antimycoplasmal therapeutic agent that is used in veterinary practice. In the presence of copper, amidines derived from 1 are 2-3 times more active than the corresponding amides. Furthermore it was established that for these compounds too, the presence of a 2,2'-bipyridyl moiety is a necessary prerequisite for antimycoplasmal activity. As for the amides, antimycoplasmal activity of amidines derived from 1 is dependent on the hydrophobic fragmental value of the aromatic nucleus of the amidine moiety. A quantitative structure-activity relationship established the optimal hydrophobic fragmental value of this part of the molecule to be zero.

Amidines↗
PubMed1989

Interaction of nefopam and orphenadrine with the cytochrome P-450 and the glutathione system in rat liver.

Nefopam, a cyclic analogue of orphenadrine, exhibits a type I (substrate) and a type II (ligand) interaction with ferri-cytochrome P-450 in control and phenobarbitone induced rat hepatic microsomes respectively. In-vitro metabolism of nefopam in phenobarbitone-induced microsomes leads to the production of a reactive metabolite which complexes with cytochrome P-450. In contrast to the known complexation of orphenadrine, complexation by nefopam can be inhibited by glutathione (GSH, 0.1-1.0 mM). However, in-vivo administration of nefopam to rats does not diminish the GSH content of liver cytosol nor increase oxidized glutathione levels nor alter the activities of GSH transferase and GSH peroxidase. In-vivo administration does not lead to cytochrome P-450 induction nor cytochrome P-450 complexation as has been shown for orphenadrine. Finally, nefopam inhibits the NADPH dependent endogenous H2O2 production in both control and phenobarbitone-induced microsomes.

Aminopyrine N-Demethylase↗
PubMed1989

Inhibition of superoxide anion radical production by ebselen (PZ51) and its sulfur analogue (PZ25) in guinea pig alveolar macrophages.

The production of superoxide anion radicals by guinea pig alveolar macrophages is stimulated by the chemotactic peptide N-formylmethionyl-leucylphenylalanine and the protein kinase C activator phorbol-12-myristate-13-acetate. Both stimulations are completely and equipotently (IC50 = 20 microM) inhibited by the seleno-organic compound ebselen. As the sulfur containing analogue, which is devoid of glutathione peroxidase-like activity, shows the same inhibitory activity towards superoxide anion radical production the observed effect of ebselen can not be explained by the described glutathione peroxidase-like activity. The observed inhibition is probably caused by an inhibition of protein kinase C or inhibition at a level distal to protein kinase C activation.

Animals↗
PubMed1988

Vitamin E and selenium regulate balance between beta-adrenergic and muscarinic responses in rat lungs.

The effects of hydrogen peroxide on the beta-adrenergic and muscarinic responses of the rat trachea muscle were studied in vitro, after feeding rats, for 6 weeks, either a diet deficient in vitamin E and selenium or a control diet. In the control situation after incubation with 1 mM hydrogen peroxide for 30 min, a reduction of the maximal response to methacholine of 39% occurred whereas no pD2 shift could be demonstrated. Moreover, no response to isoprenaline after precontraction with 3 x 10(-7) M methacholine was left. In the deficient situation, we found a reduction to 64% of the response to methacholine after incubation with 1 mM hydrogen peroxide. Again isoprenaline became inactive, i.e. no relaxation with isoprenaline was observed after precontraction with 3 x 10(-7) M methacholine. We therefore conclude that vitamin E and selenium protect against oxidative stress in lung tissue and thus regulate the (patho-) physiological balance between adrenergic and muscarinic responses.

Animals↗
PubMed1988

Activation of the microsomal glutathione-S-transferase and reduction of the glutathione dependent protection against lipid peroxidation by acrolein.

Allyl alcohol is hepatotoxic. It is generally believed that acrolein, generated out of allyl alcohol by cytosolic alcohol dehydrogenase, is responsible for this toxicity. The effect of acrolein in vitro and in vivo on the glutathione (GSH) dependent protection of liver microsomes against lipid peroxidation, and on the microsomal GSH-S-transferase (GSH-tr) in the rat was determined. In vitro incubation of liver microsomes with 5 mM acrolein for 30 sec resulted in a 2-fold activation of the GSH-tr. This activation probably proceeds via alkylation of the thiol group of the GSH-tr. In vivo administration of 1.1 mmol allyl alcohol/kg to rats did also result in a 2-fold stimulation of the GSH-tr activity. Administration of 375 mg pyrazole/kg, an inhibitor of the alcohol dehydrogenase, thus reducing the acrolein formation, prevented the in vivo stimulation of GSH-tr by allyl alcohol. This indicates that the activation of GSH-tr in vivo by allyl alcohol probably also proceeds via alkylation of the thiol group of the GSH-tr by acrolein. GSH protects liver microsomes against lipid peroxidation, probably via a free radical reductase that reduces vitamin E radicals at the expense of GSH. Incubating liver microsomes for 30 min with 0.1 mM acrolein reduced the GSH dependent protection against lipid peroxidation, probably because an essential thiol group(s) on the free radical reductase is alkylated. In vivo administration of allyl alcohol did not reduce the GSH dependent protection of the microsomes. Probably the thiol group(s) located on the free radical reductase is less accessible or less reactive than the thiol group on the GSH-tr. After administration of allyl alcohol we found no evidence for in vivo lipid peroxidation. Therefore we could not evaluate the importance of the GSH dependent protection against lipid peroxidation in vivo.

1-Propanol↗
PubMed1988

The effects of beta-adrenergic receptor agonists on the H2O2 formation in alveolar macrophage suspensions are not mediated by beta-receptors.

In the present study, the effects of beta-adrenergic receptor agonists on the hydrogen peroxide (H2O2) production in phorbol myristate acetate stimulated guinea pig alveolar macrophage suspensions have been investigated. In contrast to suggestions described in literature, our results indicate that the H2O2 production in alveolar macrophage suspensions is not mediated by beta-adrenergic receptors. Rather, it is suggested that the inhibitory effect of catecholamines in the H2O2 formation which is found after 5 minutes is the result of inhibition of the horseradish peroxidase catalysed phenol red oxidation, used as H2O2 measurement, by the catecholamines.

Adrenergic beta-Agonists↗
PubMed1988

Identification of structural characteristics of some potential H2-receptor antagonists that determine the interaction with rat hepatic P-450.

Several potential H2-receptor antagonists have been tested in vitro, using liver microsomal preparations from untreated rats, in order to study their interaction with P-450. The aim of this investigation was to establish structure-activity relationships for the P-450-inhibition developed by cimetidine and related drugs. Most of the compounds tested demonstrate an inhibitory activity and a binding ability to P-450, via type II (ligand type) binding. Our results strongly indicate that the cyano-guanidine moiety is an essential structural feature for both the inhibition of a ferrocytochrome P-450-metabolic intermediate complex formation occurring during the metabolism of tofenacine, and the binding of the compounds to the heme iron of P-450. The presence of an imidazole group is not necessary for these activities. Furthermore, it is pointed out that the lipophilic character of the cyano-guanidine side chain contributes to the interaction of the test compounds with P-450, since a trend for a parabolic relationship between lipophilicity and inhibitory activity or binding ability is observed. Finally, under the experimental conditions used, no increase of the inhibitory activity of cimetidine on the metabolism of tofenacine and 7-ethylresorufin is observed after preincubation of rat liver microsomes with cimetidine, confirming earlier results in similar studies.

Animals↗