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Biomedical subjects

H Timmerman

Publications and source records attributed to H Timmerman.

At least 217 records · Page 12Linked to original sources
PubMed1988

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

In order to investigate the antimycoplasmal activity of compounds structurally related to 2,2'-bipyridyl, a series of both aliphatic and aromatic amides derived from 1-amino-3-(2-pyridyl)isoquinoline were synthesized. The most active compounds appeared to be as active as Tylosin, an antimycoplasmal therapeutic that is used in veterinary practice, in the presence of a small nontoxic amount of copper. Furthermore, it was found that antimycoplasmal activity depends on the hydrophobic fragmental value of amide residue. A quantitative structure-activity relationship established the optimal hydrophobic fragmental value of the amide residue to be 0.30.

2,2'-Dipyridyl↗
PubMed1988

Mapping of the beta 2-adrenoceptor on Chang liver cells. Differences between high- and low-affinity receptor states.

Receptor mapping procedures based on the methodology of Crippen are used to study the beta 2-adrenergic receptor system in intact Chang liver cells. In cases of agonists, the presence of both a low- and high-affinity receptor state is assumed, whereas antagonists bind to the low-affinity state only. The high-affinity state is considered to contain the "functional" binding site responsible for formation of the second messenger (cAMP), whereas the low-affinity state is assumed to be the "true" (physiological) low-affinity state. Both receptor states are taken into account in the receptor mapping process. Characterization of the high- and low-affinity states made it possible to identify features that make a state an antagonist or agonist. The receptor model found for the low-affinity state of the beta-adrenergic receptor present in an intact cell system is compared to the low-affinity state previously obtained for this receptor present on a membrane preparation of the bovine skeletal muscle in the presence of high amounts of Gpp(NH)p guanosine 5'-(beta, gamma-imidotriphosphate). Remarkable differences are found between the two receptor models. The tentative conclusion is drawn that these differences in low-affinity states most probably are artificial and are caused by the different pharmacological properties (e.g., intrinsic activity) of the labeled ligands used in displacement experiments for determining the affinities of the drugs.

Cell Membrane↗
PubMed1988

Frequency-dependent autoinhibition of histamine release from rat cortical slices: a possible role for H3 receptor reserve.

The inhibition of histamine release after depolarization of rat cerebral cortex slices by electrical stimulation and mediated by the postulated presynaptic autoreceptor (H3) depends strongly on the conditions of stimulation. Using electrically stimulated slices of the cortex a rightwards shift of the concentration-response curve of histamine (an H3 agonist) was observed on increasing the frequency of stimulation. The pA2 value of the H3 antagonist impromidine was, however, not altered at different stimulation frequencies; for a partial agonist only the maximal effect was influenced. These results indicate the existence of a receptor reserve at the H3 autoreceptor.

Animals↗
PubMed1988

The effect of hydrogen peroxide on beta-adrenoceptor function in the heart.

A membrane preparation of calf heart left ventricle has been used to study the effect of radical stress on the beta-adrenoceptor complex. To this end the membranes were incubated for 30 minutes with several concentrations of hydrogen peroxide. This resulted in a dose dependent peroxidation of the membrane lipids. Preincubation with hydrogen peroxide in the concentration range 10(-7)-10(-3) M caused an increase in specific (-)-[125I]-Iodocyanopindolol binding, possibly due to a decrease in membrane fluidity as a result of lipid peroxidation, thus making the receptor protein more accessible. Higher concentrations H2O2 reduced the specific (-)-[125I]-Iodocyanopindolol binding, which is most likely the effect of deterioration of the receptor protein by the more pronounced radical stress induced by these higher concentrations. Also adenylate cyclase activity was affected by radical stress. Basal cyclic-AMP production and cyclic-AMP production induced by NaF (10(-2) M) or guanylylimidodiphosphate (10(-4) M), was suppressed after pretreatment with concentrations of H2O2 above 10(-4) M. This indicates a higher sensitivity of the adenylate cyclase toward radical stress when compared to the receptor protein. Our results show that radical stress can perturb beta-adrenoceptor function considerably in the heart.

Adenylyl Cyclases↗
PubMed1988

Irreversible binding of the fluorescent beta-adrenoceptor probes alprenolol-NBD and pindolol-NBD to specific and non-specific binding sites.

The fluorescent 4-nitrobenzo-2-oxa-1,3-diazolyl (NBD) derivatives of alprenolol and pindolol bind irreversible to beta-adrenoceptors and non-receptor binding sites. This was established in functional experiments on the guinea pig right atrium and trachea smooth muscle, and by radioligand binding assay of beta-adrenoceptors on Chang liver cells in culture. The pD2'-values of alprenolol-NBD and pindolol-NBD for the beta-adrenoceptors on the right atrium were: 8.3 +/- 0.1 and 8.5 +/- 0.1; on the tracheal smooth muscle strip: 8.2 +/- 0.1 and 8.8 +/- 0.1; and its pKd on Chang liver cells: 8.5 +/- 0.1 and 8.9 +/- 0.1 respectively. The results indicated that no selectivity for the beta-adrenoceptor subtypes was introduced by the addition of NBD. The irreversible binding characteristic to beta-adrenoceptors and non-receptor binding sites of the fluorescent NBD derivatives of alprenolol and pindolol makes these drugs unsuitable to label beta-adrenoceptors specifically. As the irreversible binding is introduced by the coupling of the drug with NBD, it is concluded that NBD derivatives of beta-adrenoceptor antagonists are not suitable to visualize the two-dimensional motion of beta-adrenoceptors during challenge with agonists.

4-Chloro-7-nitrobenzofurazan↗
PubMed1987

The effect of ischemia and recirculation, hypoxia and recovery on anti-oxidant factors and beta-adrenoceptor density. Is the damage in the erythrocytes a reflection of brain damage caused by complete cerebral ischemia and by hypoxia?

This investigation was focussed on the gravity of tissue injury caused by complete ischemia (for five min) and hypoxia (for three weeks) in the cerebral cortex (homogenate) and the erythrocyte lysate or the erythrocyte membrane of the rat in order to investigate if the changes that occur in brain tissue are reflected in the erythrocyte. To this end, glutathione (GSH), superoxide dismutase (SOD) and catalase were measured, also alterations in beta-adrenoceptor density under these two conditions were examined. It was found that in ischemia partial parallelism in changes that occur in the central nervous system (cerebral cortex) and the erythrocyte exists. The SOD activity became higher and the beta-adrenoceptor density (measured as specific (-)-[125I] iodocyanopindolol binding) was decreased in both tissues. However after the hypoxic condition we established a decrease in the number of beta-adrenoceptors in the cerebral cortex but an increase in beta-adrenoceptor density in the erythrocyte.

Animals↗
PubMed1987

Investigation into the mechanism of copper uptake by Mycoplasma gallisepticum in the presence of 2,9-dimethyl-1,10-phenanthroline.

In the presence of copper certain 2,2'-bipyridyls show antimycoplasmal activity, whereas copper itself causes a toxic effect. In this paper results are presented to elucidate the mechanism of copper uptake in the presence of 2,9-dimethyl-1,10-phenanthroline. The time course of copper and/or ligand uptake under the applied conditions is consistent with a carrier transport mechanism in which 2,9-dimethyl-1,10-phenanthroline operates as a carrier for copper ions. The influence of valinomycin on copper uptake indicates that the transmembrane potential is not the driving force in the carrier process.

Biological Transport↗
PubMed1987

Inhibition of NADH oxidase and lactate dehydrogenase of Mycoplasma gallisepticum by copper complexes of 2,2'-bipyridyl analogues.

In the presence of copper, 2,2'-bipyridyl analogues possess growth-inhibitory activity against Mycoplasma gallisepticum. Inhibition of the energy yielding metabolism plays a role in the mechanism of action. We showed that probably inhibition of lactate dehydrogenase and NADH oxidase is involved. Both enzymes were inhibited in vitro and in vivo by several copper 2,2'-bipyridyl complexes. A two-step mechanism of action is proposed, i.e. first a copper complex enters the cell, then after dissociation of the complex the enzymes are inhibited by free copper.

2,2'-Dipyridyl↗
PubMed1987

HA autoreceptor assay with superfused slices of rat brain cortex and electrical stimulation.

Slices of rat brain cortex previously loaded with [3H]histamine ([3H]HA) via de novo synthesis from [3H]histidine released tritiated histamine ([3H]HA) Ca2+ dependently in a superfused system. Both electrical field stimulation and high levels of K+ ions elicited this release. The extent of release depended on stimulation intensity. Rather strong stimuli, either by high frequency or longer stimulation, were required to elicit sufficient HA release for proper assessment of the concentration-dependence of release inhibition by drugs. The system showed marked depletion (less response per pulse) upon long-continued or successive stimulations. HA added to the superfusion medium inhibited the release evoked by stimulation at frequencies up to 10 Hz or with 30 mM K+ but not the release at higher frequencies or with 45 mM K+. The inhibition was mediated by H3 receptors, was concentration-dependent (pD2 = 7.4) and was complete at 10(-6) M. The H2 agonist impromidine antagonized the inhibition competitively (pA2 = 7.1). It is concluded that this assay in a superfusion system with electrical stimulation is suitable for the assessment of H3 receptor activity of drugs.

Animals↗
PubMed1987

A disbalance between beta-adrenergic and muscarinic responses caused by hydrogen peroxide in rat airways in vitro.

The effect of hydrogen peroxide on adrenergic and muscarinic responses of rat airway smooth muscle was studied. The trachea muscle and the lung parenchymal strip were contracted with methacholine and relaxed with (-)-isoprenaline. Recording of three (-)-isoprenaline curves on the trachea muscle and the lung parenchymal strip was followed by treatment for 30 min with hydrogen peroxide (H2O2) (1mM) after which a new dose response curve for (-)-isoprenaline was constructed. Using the trachea muscle this treatment with H2O2 resulted in a decrease of 61% of the maximum contraction by methacholine compared with the control and a complete inhibition of the relaxation by (-)-isoprenaline. In the lung parenchymal strip preparation we found, after the same treatment no reduction of the contraction by methacholine and 61% reduction of the relaxation by (-)-isoprenaline, compared with the control. The results demonstrate that the adrenergic response in rat airways is more susceptible to hydrogen peroxide than the muscarinic response.

Animals↗
PubMed1987

Experimental conditions influence [3H]-dihydroalprenolol binding characteristics to living HeLa cells due to morphological changes: a warning.

Harvesting of plated growing HeLa cells, followed by incubation of these cells without any addition at 37 degrees C was found to cause changes in the cell shape. This phenomenon is accompanied by a diminished binding of the beta-adrenergic antagonist [3H]-dihydroalprenolol and the alpha-adrenergic antagonist phentolamine to a binding compartment not representing beta-adrenergic receptors. These binding sites have a high affinity for hydrophobic agents and most probably represent lipophilic structures in the cellular membrane. Changes in the cell shape obviously cause alterations in the physical properties of the plasma membrane. This might lead to misinterpretations of the results from experiments in which the redistribution of beta-adrenergic receptors is followed during incubation with agonists, as receptor occupation with subsequent receptor redistribution is possibly accompanied by effects on the membrane microviscosity. It is concluded that investigations performed in order to follow physiological events like receptor redistribution and desensitization processes, may be obfuscated by changes in the normal physical state of the living cells.

Adrenergic beta-Agonists↗
PubMed1987

Relation between pharmacological response and receptor binding with histamine blocking drugs. Irreversible antagonism of three analogues of mifentidine on right atrium and cerebral cortex of the guinea-pig.

The effects of the H2-receptor antagonists cimetidine, ranitidine, mifentidine and three analogues of mifentidine, were studied on the spontaneously beating right atrium (H2-antagonism) and membranes of the cerebral cortex (displacement of 3H-tiotidine), both obtained from the male guinea-pig. The choice of these compounds was based on preliminary experiments in which some mifentidine analogues were shown to displace tiotidine from the H2-receptor in a deviant manner. In the present study we investigated the relation between pharmacological response and receptor binding, also testing the degree of irreversible antagonism of these compounds in the atrium (functional) and cerebral cortex (binding) model. Our data indicate that a relation between the two different approaches for measuring the effect on the H2-receptor can be found, although some differences emerged as well.

Animals↗
PubMed1987

H3 receptor assay in electrically-stimulated superfused slices of rat brain cortex; effects of N alpha-alkylated histamines and impromidine analogues.

The release of the putative neurotransmitter histamine (HA) from rat brain cortex slices is under negative feedback control by an HA autoreceptor. This autoreceptor has been postulated to belong to a new class of HA receptors, H3. To verify this hypothesis we have developed an assay using superfused rat brain cortex slices. The HA transmitter pool is labelled by incubation of the slices with the precursor 3H-histidine; 3H-HA is estimated after separation by column chromatography. Release of HA was found both after K+-induced depolarization and electrical field stimulation. The latter resulted in higher and more reproducible HA release. Electrically induced HA release could be fully inhibited in a concentration dependent way by exogenous HA in the superfusion buffer. N alpha-alkylated histamines also showed agonistic activity. The action of exogenous HA was totally blocked by the potent H2 agonist impromidine and some of its analogues.

Animals↗
PubMed1987

4-Hydroxy-2,3-trans-nonenal stimulates microsomal lipid peroxidation by reducing the glutathione-dependent protection.

Glutathione (GSH) protects liver microsomes against lipid peroxidation. This is probably due to the reduction of vitamin E radicals by GSH, a reaction catalyzed by a membrane-bound protein. Pretreatment of liver microsomes with 0.1 or 1mM 4-hydroxy-2,3-trans-nonenal (HNE), a major product of lipid peroxidation, reduces the GSH-dependent protection. GSH and vitamin E concentrations are not affected by this pretreatment. Pretreatment with 0.1 mM N-ethyl maleimide (NEM), a synthetic sulfhydryl reagent, resulted in a reduction similar to that with HNE of the GSH-dependent protection against lipid peroxidation. The reduction of the GSH-dependent protection by HNE and NEM is probably the result of inactivation of the membrane-bound protein by covalent binding to an essential SH group on the protein. If the GSH-dependent protection would proceed via the microsomal GSH transferase, pretreatment with NEM, which activates the microsomal GSH transferase, should enhance the GSH-dependent protection. Actually a decrease in the GSH-dependent protection is found. Apparently the GSH-dependent protection does not proceed via the microsomal GSH transferase. Also the microsomal phospholipase A2 is not involved, since addition of 0.1 mM mepacrine, an inhibitor of phospholipase A2, did not preclude the GSH-dependent protection. Once the process of lipid peroxidation, either in vivo or in vitro, has started, the protection of liver microsomes by GSH is less effective. This might be the result of formed HNE. In this way an endproduct of lipid peroxidation stimulates the process that generates this product.

Aldehydes↗
PubMed1987

Studies on the active molecular species of the H2 receptor antagonists cimetidine and mifentidine.

The N'-(4-1H-imidazol-4-ylphenyl)formamidines were recently introduced as a new class of active H2 antagonists; the authors of the compounds (Donetti et al. of de Angeli, Italy) have suggested that these compounds interact with the H2 receptor through their monocations. This is at variance with the model proposed for cimetidine by the SK&F (Smith Kline & French, UK) group who proposed the neutral molecule as the species active at the H2 receptor. In the present study we have investigated the issue whether the neutral or charged species is the active one by measuring the pA2 values of mifentidine and cimetidine at different pH values. Changing the pH will influence the species equilibria of both compounds and thereby affect their activity. The activity changes measured for both compounds are consistent with the proposition that cimetidine as well as mifentidine elicit their activity through their neutral species.

Animals↗
PubMed1987

Cytochrome P450 metabolic intermediate complex of nefopam.

NADPH-catalysed biotransformation of nefopam in liver microsomes obtained from phenobarbitone-pretreated rats leads to the formation of an inactive cytochrome P450 metabolic intermediate (MI) complex. This complex can be detected spectrophotometrically by an absorbance maximum at 459 nm. The extent of the in-vitro MI complexation of 33 microM nefopam, a cyclic analogue of orphenadrine, was almost equal to the extent of the in-vitro MI complexation of 33 microM tofenacine, the mono-N-demethylated metabolite of orphenadrine. The time course of the MI complexation of nefopam and studies with two of its major metabolites suggest an initial biotransformation, which has to occur before MI complexation can take place. Maximal MI complexation of nefopam occurred at approximately 25 microM, whereas the MI complexation could not be detected at 100 microM nefopam.

Animals↗
PubMed1987

Calcium modulation and clinical effect. Profile of cyclandelate.

Since its original development as a vasodilator, cyclandelate has been shown to possess a pharmacological profile which reflects its primary mechanism of action-calcium modulation. Thus, the ability of the drug to improve the rheological properties of blood by maintaining red blood cell deformability and inhibiting platelet aggregation may be explained by its effects on the influx of extracellular calcium. Since elevated intracellular calcium concentrations are known to contribute to pathological changes in conditions of local cerebral ischaemia, the clinical implications for the use of cyclandelate in the treatment of such diseases are clear.

Calcium↗