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Biomedical subjects

A Bast

Publications and source records attributed to A Bast.

At least 235 records · Page 13Linked to original sources
PubMed1983

Product inhibition in orphenadrine metabolism as a result of a stable cytochrome P-450-metabolic intermediate complex formed during the disposition of mono-N-desmethylorphenadrine (tofenacine) in the rat.

Product inhibition is thought to be involved in unexpected accumulation of orphenadrine, which occurs during chronic medication with this anti-Parkinson drug in man. In previous studies (Biochem. Pharmacol. 31, 2745-2753 (1982) we established the formation of reactive metabolic intermediates (MI) during metabolism of orphenadrine and its mono-N-demethylated metabolite tofenacine, which may block cytochrome P-450 (MI-complex). In this study we investigated the role of MI-complexation in product inhibition. Three different assays were used to establish the amount of cytochrome P-450 involved in MI-complexation, which was induced by tofenacine (30 mg/kg i.p.) in phenobarbital pretreated rats. If liver microsomes were prepared 3 hours after tofenacine injection, both spectral titration of oxidized cytochrome P-450, determination of loss of metyrapone binding sites at reduced cytochrome P-450 as well as ferricyanide oxidation of the MI-complex revealed 8-13% complexation of cytochrome P-450. Our data also suggest that MI-complexation is generated on phenobarbital induced cytochrome P-450 species. Phenobarbital induction was also shown to activate orphenadrine metabolism in vitro. Moreover, with a newly developed capillary GLC method, using nitrogen detection, we showed inhibition of orphenadrine- and tofenacine metabolism in vitro, by MI-complexation. This study therefore showed that MI-complexation may produce product inhibition.

Animals↗
PubMed1982

Dose-dependent kinetics of aminopyrine metabolism in the rat caused by product inhibition and determined by capillary GLC.

Aminopyrine (DMAP) and its metabolites were estimated in plasma and urine with a newly developed capillary GLC method, using nitrogen detection. Dose-dependent kinetics for DMAP was observed in the rat. An intravenous administration of 30 or 100 mg/kg of DMAP resulted in total body clearance values of 20 +/- 8 and 15 +/- 5 ml/min/kg (mean +/- SD), respectively. These values were associated with terminal half-lives of 45 +/- 10 and 79 +/- 23 min, respectively. Moreover, kinetic values for the primary metabolite monomethyl-4-aminoantipyrine (MMAP) were determined, as MMAP originated from DMAP (both doses). The terminal half-life of MMAP was significantly longer than that of DMAP (123 +/- 93 and 204 +/- 90 min after DMAP doses of 30 and 100 mg/kg, respectively). MMAP is known to inhibit DMAP-N-demethylation in vitro. Furthermore, terminal 14CO2 exhalation measured after in vivo 14C-DMAP administration originates largely from 14C-MMAP. Pretreatment of rats with MMAP decreased DMAP clearance. These observations indicate that in the rat DMAP metabolism exhibits product inhibition. Since dose-dependent kinetics of DMAP has previously also been observed in man, it is suggested that product inhibition is a determinant for DMAP metabolism both in man and in the rat.

Aminopyrine↗
PubMed1981

Comparison of the effects of inhibitors of cytochrome P-450-mediated reactions on human platelet aggregation and arachidonic acid metabolism.

Metyrapone and SKF-525A, together with amphenone B, a structural analogue of metyrapone, which are all inhibitors of cytochrome P-450-mediated reactions, were shown to inhibit the arachidonic acid-induced aggregation of human platelets. Amphenone B, like metyrapone, exhibited a type II (ligand) binding spectrum with rat liver microsomal cytochrome P-450, in contrast to SKF 525A which is a type I (substrate) binding agent. Independently of their type of binding spectra and of their maximum spectral change, however, the affinity of the three compounds for rat liver cytochrome P-450 showed a close proportional correlation with their platelet aggregation inhibitory potency. All three compounds inhibited the formation of [1-14C]thromboxane B2 from [1-14C]arachidonic acid by human platelets aggregated with collagen. The effect of metyrapone on the remaining labelled products suggested that it is a selective thromboxane synthesis inhibitor, while amphenone B exhibited activity reminiscent of cyclo-oxygenase inhibitors. SKF 525A produced complex effects possibly attributable to cyclo-oxygenase inhibition and enhanced lipid peroxidation, since it also enhanced platelet malonaldehyde formation, which the other two compounds inhibited. These data provide further support for a role of cytochrome P-450 in thromboxane synthesis and platelet aggregation.

Animals↗
PubMed1981

Evaluation and comparison of colorimetric, radiometric and high performance liquid chromatographic assays for aminopyrine-N-demethylation by rat liver microsomes.

Aminopyrine (DMAP) is metabolized by two successive N-demethylations to monomethyl-4-aminoantipyrine (MMAP) and 4-aminoantipyrine (AAP). Separation and quantification of DMAP and its metabolites in microsomal incubation mixtures by h.p.l.c. showed that other reactions also occur. In the radiometric method where [Me2-14C]DMAP is used as substrate, [14C]formaldehyde is formed during the N-demethylation. However since commercial [14C]DMAP, is not completely double labelled, and both MMAP and AAP are formed, it is impossible to calculate the formaldehyde formation accurately from the specific activity of [14C]DMAP. Moreover, it was shown that DMAP, AAP and particularly MMAP may all develop considerable colour intensity with the Nash reagent, which is used to determine formaldehyde. Despite this difficulty, DMAP may still be used as a model substrate in vitro, with the Nash assay being used to determine formaldehyde if low substrate concentrations and a short incubation time are used. Thus the interference of DMAP or its metabolites with the Nash assay is negligible. As the same Km and Vmax values were obtained from both the radiometric and the relatively precise colorimetric assay it is suggested that there is little wrong with either method, at least under our experimental conditions. The h.p.l.c. method however, underestimates formaldehyde formation, probably because metabolites other than MMAP and AAP are formed. The latter method however may be used to analyse the aminopyrine metabolism in more detail.

Aminopyrine↗
PubMed1980

beta-Adrenoceptor studies. 6. Further investigations on the hybrid nature of the rat adipocyte beta-adrenoceptor.

The nature of the rat adipocyte beta-adrenoceptor was studied in further detail using a selected series of tolamol-type beta-adrenoceptor antagonists. Isoprenaline antagonism by these compounds was evaluated on adipocytes, right atrium, left atrium and left hemidiaphragm of the rat. Adipocyte pA2 values were corrected for binding to bovine serum albumin determined separately for each antagonist. A strong correlation between adipocyte and diaphragm pA2 values was found with those antagonists which had an identical N-substituent but a different substitution pattern in the phenoxypropanolamine ring. This relationship was absent with those compounds which had the same 2-methylphenoxypropanolamine moiety but a different N-substituent. With the latter antagonists, however, adipocyte pA2 values correlated significantly with the two cardiac pA2 values. These results support the idea that the interaction site of the rat adipocyte beta-adrenoceptor for the aromatic moiety of arylethanolamines and aryloxypropanolamines has beta 2-characteristics whereas the alkanolamine side-chain interaction site has a beta 1-nature. Finally, it was ascertained that the apparently dualistic (beta 1 and beta 2) character of the rat adipocyte adrenoceptor is not due to the presence of both a beta 1- and a beta 2-receptor population on the fat cell plasma membrane.

Adipose Tissue↗
PubMed1977

Contamination of rat urine with gut flora using all-glass metabolism cages for collection of urine and faeces.

In rat urine collected in all-glass metabolism cages, at least four strains of intestinal microflora were found: two types of E. coli, Enterobacter cloaceae and Proteus vulgaris. The number of bacteria of each strain increased with time. 2. The pH of the urine increased from 6-9 after 24 h to 8-95 after 120 h. The pH of the urine of neomycin-treated rats remained nearly constant over a period of two days. 3. Nitroreductase activity was present in the rat urine. Added p-nitrobenzoic acid was reduced within the first 24 h. Nitroreductase activity in the urine of neomycin-treated rats was significantly lower than in the urine of normal rats, during the second 24-h period only. 4. Collection of urine at -10 degrees prevented the consequences of contamination.

Animals↗
PubMedDate not supplied

Inhibition of diazepam metabolism in microsomal- and perfused liver preparations of the rat by desmethyldiazepam, N-methyloxazepam and oxazepam.

Hydroxylated metabolites of diazepam can be conjugated and are therefore generally thought not to affect the metabolism of diazepam. Liver microsomes, obtained from phenobarbital-pretreated rats, showed an inhibition of diazepam (10(-5) M) metabolism by desmethyldiazepam as well as by N-methyloxazepam or oxazepam (5 X 10(-5) M). In a single-pass perfusion of the rat liver an inhibition of diazepam disposition by exogenously administered desmethyldiazepam and by hydroxylated diazepam metabolites was also demonstrated. No oxazepam glucuronides were found after oxazepam infusion. However, infusion with N-methyloxazepam resulted in large amounts of oxazepam-glucuronides. The results indicate that administration of N-demethylated as well as hydroxylated metabolites may result in inhibition of the metabolism of their precursor. If hydroxylated metabolites are formed in situ they become more easily conjugated in comparison with administered hydroxylated metabolites and are therefore less effective as inhibitor.

Animals↗
PubMedDate not supplied

Dependence of hydrogen peroxide formation in rat liver microsomes on the molecular structure of cytochrome P-450 substrates: a study with barbiturates and beta-adrenoceptor antagonists.

In the present study, the molecular structure of xenobiotics has been successfully linked to their effect on the oxidase activity of cytochrome P-450, determined as microsomal hydrogen peroxide formation. A homologous series of 5-alkyl-5-ethyl barbiturates and a heterologous series of beta-adrenoceptor antagonists was used. The logarithm of the rate of microsomal hydrogen peroxide formation could be correlated with the logarithm of the apparent partition (n-octanol/buffer) coefficient for the barbiturate derivatives according to a parabolic function. The statistics of the correlation improved considerably by applying a bilinear model in order to fit the data. This probably indicates that both transport of the substrate to cytochrome P-450 and interaction with hydrophobic substrate binding sites of cytochrome P-450 are involved in the modulating effect of substrates on the oxidase function of cytochrome P-450. With the series of beta-adrenoceptor antagonists no clear-cut structure activity relationship with regard to the oxidase activity was apparent at first sight. However, when the inhibitory effect of the beta-antagonists on the 'cytochrome P-450 metabolic intermediate (MI) complex' formation that occurs during the microsomal biotransformation of 33 microM tofenacine was studied a relationship with the lipophilicity could be demonstrated. It is known that MI complex formation occurs with specific subforms of cytochrome P-450. By using this inhibitory activity of the beta-adrenoceptor antagonists, the interaction of the compounds becomes restricted to these specific subforms of cytochrome P-450. In both the oxidase activity as well as the MI complex formation phenobarbital induced cytochrome P-450 is involved.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic beta-Antagonists↗
PubMedDate not supplied

Quantification of the nonenzymatic fast and slow TRAP in a postaddition assay in human seminal plasma and the antioxidant contributions of various seminal compounds.

Total radical-trapping antioxidant potential (TRAP) measurements of human seminal plasma (N = 25) were performed by using a post-addition assay based on trapping 2,2' Azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radicals. This method enables the antioxidant capacity of human seminal plasma and its constituents to be quantified. The standard procedure consisted of determination of the Trolox equivalent antioxidant capacity (TEAC) after incubating the test sample in the ABTS radical solution for 10 seconds (fast TRAP) and 300 s (total TRAP). Interestingly, seminal plasma showed a fast TRAP and a high slow TRAP (Total TRAP - Fast TRAP). The final total TRAP of seminal plasma is about 10 times higher than that of blood plasma. Various components of seminal plasma contribute to its fast TRAP; 37% can be attributed to vitamin C, uric acid, and tyrosine; proteins and polyphenolic compounds contribute a further 57%. In contrast, the slow TRAP was attributed to vitamin C (1%), uric acid (2%), and tyrosine (15%) and to proteins and polyphenolic compounds (33%). It was not possible to account for the remaining 49%. Neither known putative antioxidants, such as spermine, pyruvate, and taurine, nor other seminal compounds, such as carnitine, sialic acid, fructose, spermidine, glycerophosphorylcholine, and hyaluronic acid, contributed to any significant radical-trapping activity at a standard concentration of 1 mM. Of the amino acids, only tyrosine possessed a slow TRAP, and it is present at a high concentration in seminal plasma. Glutathione and hypotaurine show high fast and slow TRAPs, respectively. However, because of their low concentration in seminal plasma, their contribution to the TRAP is negligible. In conclusion, seminal plasma possesses a high antioxidant buffer capacity that protects spermatozoa from oxidative stress. Moreover, these findings suggest that the fast and slow TRAPs may have an important role as infertility markers and treatment targets in future antioxidant therapies.

Amino Acids↗
PubMedDate not supplied

In vitro screening of antitumour agents for cardiotoxicity by means of isolated mouse left atria.

Cardiotoxicity, a side-effect that can occur after treatment with an anticancer drug, has severe clinical implications. Therefore, a model is desired to screen new anticancer drugs or drug combinations for possible cardiotoxic side-effects. In the present study we tested the applicability of the electrically stimulated isolated mouse left atrium model using a wide range of anticancer drugs with known cardiotoxicity. It appeared that the cardiotoxicity observed in our model, i.e. the negative or positive inotropic effects of the drugs on the isolated atrium, corresponded with the observed cardiotoxicity in animals and/or humans. It is therefore concluded that our model can be used to wam for possible cardiotoxic side-effects of anticancer drugs in vivo.

Aminoglycosides↗
PubMedDate not supplied

The use of vitamin supplements in self-medication.

Supplements, frequently containing vitamins, are often taken to try to compensate for an unhealthy life style or in an attempt to maximise health or performance. As illustrated by vitamin E, fundamental issues on both the efficacy and safety of the chronic use of relatively high dosages of vitamins still need to be resolved. To recommend the application of vitamin supplements, firstly the gaps in our knowledge on these fundamental issues should be filled. Secondly, objective and scientifically sound information should be given to the public. Via diverse sources there is an uncontrolled supply of vitamins. Educating the consumer on vitamins is therefore the only way to come to a rational use of vitamin supplements in self medication.

Abnormalities, Drug-Induced↗
PubMedDate not supplied

Desensitization of guinea-pig parenchymal lung strips after prolonged histamine H1-receptor stimulation.

In this study we examined the desensitization of guinea-pig parenchymal lung strips after excessive stimulation of the histamine H1-receptor. After stimulation with maximal effective concentrations of the selective H1-receptor agonist 2-pyridylethylamine (1 mM) for 30 min, subsequent 2-pyridylethylamine responses were inhibited as shown in a depression of the maximal response with approximately 40%. This desensitization was time- and concentration-dependent. Besides H1-receptor responses, potassium chloride (50 mM)-induced responses were also affected. Treatment of lung strips with 1 mM of 2-pyridylethylamine for 30 min resulted in an inhibition of the potassium-induced contraction with 26.1 +/- 8.7% of the control response. Desensitization of the lung strip preparation could not be prevented by the cyclooxygenase inhibitor indomethacin (20 microM), indicating that the observed inhibition is not due to an elevated production of relaxing prostaglandins. Desensitization was also not dependent on the influx of extracellular calcium. Stimulation of guinea-pig parenchymal lung strips with 1 mM of 2-pyridylethylamine in a calcium-free buffer, supplemented with the calcium chelator EGTA, did not lead to observable contractions. However, desensitization still developed under these conditions. These data indicate that guinea-pig parenchymal lung strips exhibit a desensitized state after prolonged H1-receptor stimulation. Because this effect is not only limited to H1-receptor responses, as KCl effects are reduced as well, it should, at least partly, be explained by an interference with processes which occur distally to the H1-receptor itself.

Animals↗