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Biomedical subjects

H Timmerman

Publications and source records attributed to H Timmerman.

At least 163 records · Page 9Linked to original sources
PubMed1992

A theoretical study concerning the mode of interaction of the histamine H2-agonist dimaprit.

A theoretical study was performed to elucidate the mode of interaction of the histamine H2-agonist dimaprit with the histamine H2-receptor. For this purpose receptor mapping techniques, including ab initio energy calculations, geometry optimizations and molecular electrostatic potential calculations (MEPs), have been used. The characteristics of dimaprit were compared to those of histamine for which the points of interaction with the H2-receptor are known, as well as its bioactive conformation. In this comparative study two possible models for the interaction of dimaprit with the H2-receptor were considered. In one model the two nitrogen atoms of the isothiourea moiety of dimaprit play an essential role in the recognition of the ligand by the receptor and have the same function as the nitrogen atoms of the imidazole ring of histamine; in the second model this role is fulfilled by a sulphur and a nitrogen atom of the same isothiourea moiety. The comparison to histamine was based on geometrical resemblance as well as on similarity in MEPs. Also the conformational energy of dimaprit in the two interaction models was considered. Results of the investigations reveal that the isothiourea moiety of dimaprit most probably interacts with the histamine H2-receptor through the sulphur and nitrogen atom, the first atom acting as a proton acceptor and the second one as a proton donor. Subsequently, three analogues of dimaprit, namely SK&F 91487, SK&F 91488 and SK&F 92054, were studied. It was possible to explain their pharmacological behavior within the proposed model.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer Simulation↗
PubMed1992

Glutathione mobilization during cerebral ischemia and reperfusion in the rat.

1. Cerebral ischemia applied for 15 min and followed by a 30 min reperfusion did not change the glutathione (GSH) levels and beta-adrenoceptor density (Bmax) in brain cortex. 2. A significant increase in erythrocyte-lysate GSH concentration (vs control) and a significant decrease of Bmax values in erythrocyte membranes (vs control) was found at the same time. 3. Pretreatment with the alpha-adrenoceptor antagonist phentolamine (5 mg/kg i.p.) prevented the erythrocyte GSH increase but not the decrease of Bmax value. Pretreatment with the beta-antagonist propranolol (2 mg/kg i.p.) did not influence the increase in erythrocyte GSH but circumvented the decrease of Bmax.

Animals↗
PubMed1992

Short-term desensitization of the histamine H1 receptor in human HeLa cells: involvement of protein kinase C dependent and independent pathways.

1. In this study we have investigated the effects of short-term exposure of cells to histamine on the subsequent H1 receptor responsiveness in HeLa cells, using Ca2+ fluorescence microscopy and video digital imaging. 2. In HeLa cells, histamine (100 microM) induces an immediate H1 receptor-mediated biphasic elevation of the intracellular Ca2+ concentration ([Ca2+]i) (basal [Ca2+]i: 81 +/- 30 nM, histamine-induced Ca2+ response: first phase: 1135 +/- 79 nM; second phase: 601 +/- 52 nM, n = 11). 3. The histamine H1 receptors on HeLa cells are readily susceptible to desensitization since repetitive exposure of the same group of cells to histamine (100 microM) markedly affected the release and influx component of the induced Ca2+ response (second application of histamine: first phase: 590 +/- 92 nM, second phase: 279 +/- 47 nM; third application of histamine: first phase: 454 +/- 127 nM, second phase: 240 +/- 45 nM, n = 6). Video digital imaging revealed an increase in the lag time between stimulation and monitoring of the Ca2+ response and a reduced increase in [Ca2+]i after desensitization with histamine. 4. Neither the release component of the ATP response (50 microM) nor the caffeine (3 mM)-induced Ca2+ release were found to be affected by desensitization with 100 microM histamine. However, the second phase of the ATP response was significantly reduced after desensitization with histamine (control cells: 516 +/- 33 nM; desensitized cells: 331 +/- 96 nM, n = 4, P < 0.05).5. Activation of protein kinase C (PKC) by phorbol-12-myristate-1 3-acetate was found to inhibit the histamine as well as ATP-induced Ca2" response in a dose-dependent manner.6. In PKC downregulated cells the second phase of the histamine-induced Ca2+ response was significantly elevated, indicating the involvement of PKC in the negative feedback on the Ca2+ influx(control cells: second phase: 601 +/- 52 nM (n = 11); PKC downregulated cells: second phase:890 +/- 90nM, n = I0, P<0.05).7. Homologous desensitization of H, receptor responsiveness was still observed in PKC downregulated cells, implying the rapid activation of a regulatory mechanism other than PKC.8. Based on our experimental data we suggest that short-term desensitization of the histamine H,receptor evolves from two different processes: a selective reduction of the histamine-induced Ca2+ release, mediated by a PKC-independent pathway, and a non-selective inhibition of the receptormediated Ca2+ influx activated by a PKC-dependent pathway.

Adenosine Triphosphate↗
PubMed1992

Radiolabelled calmodulin ligands: their low affinity and high lipophilicity may lead to artefacts in binding studies.

In order to characterize the interaction site of a series of putative calmodulin antagonists of the diphenylalkylamine type with calmodulin (CaM), a representative member of this chemical class was radiolabelled. The binding of the selected compound, [3H]-VUF 4576, to calmodulin was studied according to a recently described technique using CaM agarose. However, some peculiar results were obtained: the tight binding of [3H]-VUF 4576 increased in presence of cold VUF 4576, resulting in a high non-specific binding. The unexpected results could readily be explained by a high binding capacity of the labelled compound and the cold ligands to the walls of the test tubes used. Such results were also found when [3H]-chlorpromazine ([3H]-CPZ) was applied. In literature comparable findings have been published. To explain such results the influence of positive cooperativity or irreversible binding has been suggested. We suppose that not only in our study, but also in other published investigations, binding to glass of the radioligand and/or the cold compounds may have had a strong influence. We suggest, therefore, that care should be taken in interpreting non-classical displacement data obtained with ligands which combine a rather low affinity and a high degree of lipophilicity, not only for binding to calmodulin, but for other systems as well.

Benzhydryl Compounds↗
PubMed1992

A simple and rapid in vitro test system for the screening of histamine H3 ligands.

A simple and rapid functional test system for the screening of histamine H3 ligands is described. It is based on the inhibitory effect of histamine H3 agonists on electrically-evoked contractile response of isolated guinea pig intestine. Whole jejunum segments are continuously stimulated maximally (15 V) by electrical pulses with a frequency of 0.1 Hz and a duration of 0.5 msec. The resulting twitches are recorded isotonically (1.0 g) and can be completely abolished by atropine (0.1 mcM).

Acetylcholine↗
PubMed1991

Animal models for testing anti-inflammatory drugs for treatment of bronchial hyperreactivity in asthma.

In the first part of this review the important role played by the bronchial hyperreactivity caused by chronic bronchopulmonary inflammation in asthma is described. Deliberately, more emphasis is placed on the role of pro-inflammatory eosinophils, alveolar macrophages, lymphocytes and platelets rather than on mast cells and neutrophils or the numerous mediators. The reason for this is that, on account of the large number of mediators and their multitude of functions and interactions in asthma, antagonism of a specific mediator will probably not be clinically relevant for optimally effective curative treatment of asthma. Inhibition of the infiltration and activation of pro-inflammatory cells is likely to be a more successful approach. In the second part, various animal models of bronchial hyperreactivity, which could be suitable for testing anti-asthmatic drugs, are discussed. Most animal models pay too little attention to chronic bronchopulmonary inflammation as the cause of bronchial hyperreactivity in asthma. In various models the bronchial hyperreactivity is provoked by a single mediator and this leads to selection of specific antagonists which are unlikely to be of clinical benefit. Rats appear to have certain advantages over guinea-pigs as experimental animals for bronchial hyperreactivity.

Animals↗
PubMed1991

Histamine H1-receptor-mediated cyclic GMP production in guinea-pig lung tissue is an L-arginine-dependent process.

Histamine produces a rapid and massive increase of the c-GMP level of guinea-pig lung tissue. The EC50 value for this in vitro response is found to be 27 microM and the c-GMP level is maximally 9-fold elevated by 100 microM histamine. The response is stereoselectively inhibited by the enantiomers of chlorpheniramine, indicating H1-receptor involvement. Preincubation of lung tissue with 200 microM NCDC, a phospholipase C inhibitor, reduces the histamine (100 microM) responses to 16 +/- 3% (N = 6) of the control c-GMP production. Inhibition of protein kinase C by 50 microM H-7 does not significantly attenuate the H1-receptor response, whereas omittance of extracellular Ca2+ results in almost complete inhibition of the c-GMP production. The histamine-induced c-GMP response is inhibited by hemoglobin, methylene blue and the antioxidants butylated hydroxytoluene and nordihydroguaretic acid, indicating the involvement of a nitric oxide-dependent activation of soluble guanylate cyclase. This suggestion is supported by the concentration-dependent inhibition of the c-GMP production by NG-monomethyl-L-arginine (NMA). At a concentration of 20 microM NMA the histamine (100 microM) response is inhibited to 34 +/- 8% (N = 6) of the control response. This inhibition is reversed to 127 +/- 20% (N = 6) by the exogenous addition of 1 mM L-arginine. These findings show that after an initial H1-receptor-mediated, phospholipase C-dependent, Ca(2+)-mobilization the enzymatic conversion of L-arginine to nitric oxide is stimulated. This nitric oxide production is finally responsible for the activation of soluble guanylate cyclase, leading to the production of c-GMP.

Animals↗
PubMed1991

Homologous histamine H1 receptor desensitization results in reduction of H1 receptor agonist efficacy.

Prolonged exposure of the guinea-pig intestinal longitudinal smooth muscle to histamine caused homologous desensitization of the H1 receptor, which led to reduced H1 receptor-mediated production of [3H]inositol phosphates as well as to reduced H1 agonist-induced contractions. [3H]Mepyramine binding studies showed that desensitization affected neither the agonist affinity nor the number of H1 receptors. Combining the data from the binding studies and the contraction measurements it was found that desensitization results in a selective reduction of agonist efficacy.

Animals↗
PubMed1991

4- or 5- (omega-aminoalkyl) thiazoles and derivatives; new selective H2-receptor agonists.

It is well known that both histamine and dimaprit show moderate H2-receptor agonistic activity (guinea pig right atrium). Quantum chemical calculations indicated that 2-aminothiazole derivatives that might be regarded as cyclic dimaprit analogues, should possess H2-receptor agonistic activity as well. In the present study a series of 4- or 5-(omega-aminoalkyl) thiazoles has been synthesized, showing a moderate to strong H2-receptor agonistic activity as compared to histamine whereas no activity on H1- and H3-receptors could be detected. In contrast to histamine and derivatives, which are supposed to "trigger" the H2-receptor via a tautomeric shift involving two protons viz. one proton of the active site of the receptor and one proton of the heteroaromatic ring system, the thiazole derivatives seem to stimulate the H2-receptor via a one proton mechanism. In a series of impromidine analogues the 3- [4(5)-imidazolyl]propyl moiety was replaced by the more H2-receptor specific 3-(4- or 5-thiazolyl)propyl fragment resulting in potent and selective full H2-receptor agonists.

Amines↗
PubMed1991

Is protein kinase C involved in histamine H1-receptor desensitization?

Histamine H1-receptor mediated effects in guinea-pig lung and intestine appear to desensitize homologously rather rapidly. Within a few minutes of exposure to a high concentration of histamine (30-100 microM) the c-GMP production in guinea-pig lung and the contraction of guinea-pig jejunum are markedly attenuated. In both tissues the responses to other stimulating agents (e.g. muscarinic agent, calcium ionophore) are not affected. The protein kinase C (PKC) activating phorbolester phorbol-12,13- dibutyrate (PDB) concentration-dependently depresses H1-receptor responses in both tissues. Yet, PDB does not only attenuate the H1-receptor responses but also affects responses to other stimulating agents. In the guinea-pig ileum muscarinic receptor mediated contractions are inhibited equipotently by PDB, whereas in lung tissue the c-GMP formation after calcium-ionophore addition is affected too. In view of these findings the possible role of PKC in H1-receptor desensitization is discussed.

Animals↗
PubMed1991

The histamine H1-receptor antagonist binding site. Part I: Active conformation of cyproheptadine.

The active conformation of several histamine H1-antagonists is investigated. As a template molecule we used the antagonist cyproheptadine, which consists of a piperidylene ring connected to a tricyclic system. The piperidylene moiety is shown to be flexible. The global minimum is a chair conformation but, additionally, a second chair and various boat conformations have to be considered, as their energies are less than 5 kcal/mol above the energy of the global minimum. Two semi-rigid histamine H1-antagonists, phenindamine and triprolidine, were fitted onto the various conformations of cyproheptadine in order to derive the pharmacologically active conformation of cyproheptadine. At the same time, the active conformation of both phenindamine and triprolidine was derived. It is demonstrated that, within the receptor-bound conformation of cyproheptadine, the piperidylene ring most probably exists in a boat form.

Binding Sites↗
PubMed1991

Fluoride is a contractile agent of guinea pig airway smooth muscle.

1. Guinea pig parenchymal lung strips and tracheal smooth muscle contract potently after NaF-addition. Maximal contractions of lung strips and tracheal rings induced by NaF were 208 +/- 17% (n = 6) and 151 +/- 8% (n = 4) of the maximal histamine response respectively. 2. The -log EC50-value for NaF on lung strips and tracheal rings was 2.38 +/- 0.01 (n = 6) and 2.28 +/- 0.01 (n = 4) respectively. 3. Contractions induced by NaF were augmented after Al3+ pretreatment, suggesting the involvement of a G-protein. NaF responses were not affected by blockade of H1-, muscarinic-, leukotriene C4- or leukotriene D4-receptors, indicating that mast cell degranulation or nerve activation is most probably not implicated. 4. Contractions after NaF-addition were relatively insensitive to removal of extracellular calcium and were reversed via cAMP- and cGMP-mediated pathways. 5. Relaxation studies with (-)isoprenaline and 8-bromo-cGMP on lung strips, precontracted to similar levels with either a H1-agonist, KCl or NaF, showed that the level of relaxation depends on the contractile agent that is used. 6. After precontraction with KCl (-)isoprenaline relaxes lung strips only to 58 +/- 9% (n = 5) of the initial contraction, whereas lung strips precontracted with NaF or a H1-agonist relax 114 +/- 8% (n = 4) and 120 +/- 7% (n = 5) respectively with (-)isoprenaline. 7. Similar results were obtained with relaxation induced with 8-bromo-cGMP. 8. These findings suggest that NaF-induced contractions are elicited via a mechanism, that is probably similar to that of the H1-receptor. The involvement of a G-protein in the observed NaF-responses is therefore likely.

Aluminum↗
PubMed1991

Synthesis and copper-dependent antimycoplasmal activity of amides and amidines derived from 2-amino-1,10-phenanthroline.

A series of both aliphatic and aromatic amides and aromatic amidines derived from 2-amino-1,10-phenanthroline (3) according to the Topliss scheme were synthesized and subsequently tested for antimycoplasmal potency. Although the compounds themselves showed no activity, in the presence of a nontoxic copper concentration of 40 microM all compounds appeared to be very active against Mycoplasma gallisepticum K154. The most active compounds were found in the amide series and show growth inhibition in the nanomolar range. These compounds are 4 times more active than tylosin, a macrolide antibiotic, which is used therapeutically in veterinary practice. In the presence of copper, amides derived from 3 are more active than corresponding amidines. Increased activity following derivatization of 3 may be due to the presence of a third coordination site for copper in the title compounds. Evaluation of biological data revealed that antimycoplasmal activity of amides derived from 3 is dependent on lipophilicity. For these amides a good linear correlation was found between antimycoplasmal activity and hydrophobic fragmental values for substituents considered. This quantitative structure-activity relationship study indicated that antimycoplasmal activity was increased upon a decrease of these hydrophobic fragmental values.

Amides↗
PubMed1991

Substituent effect on the stereochemistry of H2-receptor antagonists of the phenylformamidine series. A conformation-dependent mode of interaction with the H2 receptor.

The influence of alkyl substitution on the stereoisomerism of the formamidine cation (E,E vs E,Z) of several N-substituted (imidazolylphenyl)formamidines (1-10) was investigated. As (imidazolylphenyl)formamidines having alkyl substituents of more than three carbon atoms bind to H2-receptor preparations in a pseudoirreversible mode causing unsurmountable antagonism, the four isomeric butylformamidines (5-7 and 9) having comparable lipophilic character but different E,E/E,Z composition were investigated in H2-receptor assays to determine quantitatively any difference in their pseudoirreversible inhibitory pattern. It was found that the geometry of the formamidine cation is affected by the steric bulk of the substituent on the formamidine nitrogen. A relationship between the percentage of the E,E conformation of the formamidine cation and degree of pseudoirreversible antagonism was also found. The present studies support the hypothesis that bidentate hydrogen bonding plays an important role in the interaction of (imidazolylphenyl)formamidines with the H2 receptor.

Amidines↗