Biomedical subjects

M Monshouwer

Publications and source records attributed to M Monshouwer.

12 recordsLinked to original sources

PubMed1998

Congener specific PCB and polychlorinated camphene (toxaphene) levels in Svalbard ringed seals (Phoca hispida) in relation to sex, age, condition and cytochrome P450 enzyme activity.

Congener specific PCB and toxaphene (polychlorinated camphene, PCC) analyses were performed in seal blubber, collected in Svalbard, Norway. The concentration, body burden and metabolic index (PCB congener concentration in seal relative to their prey) were calculated. Multiple regression analyses were carried out to evaluate the influence of age, sex, blubber (as a percentage of total body weight) and cytochrome P450 activities on PCB and PCC levels. Levels of total PCBs found were five times higher than in ringed seals from the Canadian Arctic, corresponding with the relatively high contaminant levels in the European Arctic. The dominant PCB congeners (> 70% of the total PCBs measured) were 153, 138, 99, 180 and 101. The observed PCB and PCC accumulation patterns were very similar to patterns in seals from other studies, suggesting a large resemblance in contaminant metabolism. A decrease in the relative abundance of the lower chlorinated PCBs, was associated with higher concentrations of PCB 153. Since there was no indication for selective PCB excretion by lactating females, this suggests metabolism of these PCBs in ringed seals due to xenobiotic metabolising enzymes. The metabolic index confirmed the model of persistency of the different PCBs except for congener 128 and 138. These congeners, considered persistent in seals, could to some extent be metabolised in ringed seals. However, co-elution of PCB 138 with PCB 163 and of PCB 128 with TOX 50 possibly has resulted in an underestimation of the metabolic index for these congeners. Multiple regression analyses revealed a significant positive effect of age and a negative effect of the blubber content on the PCB concentrations. Since large fluctuations of body lipids occur between seasons in pinnipeds, PCB measurements should account for the total blubber content to avoid biased results. PCBs with vicinal H-atoms in the o, m or the m, p positions showed in addition a relation with cytochrome P450 enzyme activities. Surprisingly, no effect of sex on the PCB concentrations was observed, probably because female ringed seals, unlike other pinnipeds, continue feeding during lactation. This results in only small amounts of lipid and lipid-associated contaminants being mobilised from the blubber. Consequently, contaminant excretion with the milk will be low. Toxaphene concentrations found were low compared to levels found in the Canadian Arctic. Two congeners, TOX 26 and TOX 50 were predominant (15 and 18%, respectively of total toxaphene). There was no effect of sex, age, total blubber, or cytochrome P450 activities on the toxaphene levels. There was also no correlation between toxaphene and PCB levels, which may indicate differences in exposure and metabolism between these contaminants. Toxaphenes did not bioaccumulate to any substantial extent in ringed seals.

Adipose Tissue↗

PubMed1998

Kupffer cell depletion partially prevents hepatic heme oxygenase 1 messenger RNA accumulation in systemic inflammation in mice: role of interleukin 1beta.

The heme oxygenase 1 (HO-1) gene is rapidly activated in the liver after lipopolysaccharide (LPS) treatment. Ninety minutes after LPS treatment (0.1 mg/kg, intraperitoneally) hepatic HO-1 messenger RNA (mRNA) of mice was 40 times the control value. To investigate the hepatic cellular source of the increased HO-1 transcript, we treated mice with LPS and galactosamine (700 mg/kg, intraperitoneally), a selective transcriptional inhibitor of hepatocytes. Galactosamine prevented the LPS-mediated increase of HO-1 mRNA in the liver, indicating that hepatocytes are the main cell type in which HO-1 mRNA accumulates after LPS treatment. We then tested in vitro and in vivo the hypothesis that LPS-mediated hepatic accumulation of HO-1 mRNA is caused by intercellular communication between Kupffer cells and hepatocytes. Isolated rat hepatocytes showed an increase in HO-1 mRNA compared with controls after 90 minutes of exposure to a LPS stimulated Kupffer cell-conditioned medium. This suggests that soluble mediators from Kupffer cells were responsible for this effect. To study the role of Kupffer cells in vivo, we treated mice with Kupffer cell-inactivating or -depleting agents and LPS. Gadolinium chloride and liposome-encapsulated dichloromethylene diphosphonate lowered LPS-mediated HO-1 mRNA accumulation (by about 50%); in these groups hepatic levels of interleukin (IL)-1beta were decreased, by more than 75%. Methylpalmitate hardly affected hepatic HO-1 mRNA accumulation or IL-1beta content after LPS treatment. There was no relationship between HO-1 mRNA and serum TNF or IL-6 levels. These results suggest that LPS-mediated hepatic HO-1 mRNA accumulation is a hepatocyte response partly caused by soluble mediators, particularly IL-1beta, released from Kupffer cells.

Animals↗

PubMed1998

Novobiocin inhibits both UDP-glucuronosyltransferase and cytochrome P450-mediated enzyme activities in pig liver microsomes.

The effects of novobiocin (range 0.0125-2 mmol/L) on the hydroxylation of testosterone, the N-demethylation of erythromycin, and the glucuronidation of alpha-naphthol and paracetamol were studied using pig hepatic microsomes, pooled from five animals. The final concentrations of these substrates in the incubation mixtures were selected to meet Vmax conditions. Novobiocin caused a concentration-dependent inhibition of the glucuronidation of paracetamol; the formation of alpha-naphthol-glucuronide was reduced to a lesser degree. These results confirm and extend earlier findings in laboratory animal species that novobiocin inhibits UDP-glucuronosyltransferases (UDPGTs). Moreover, novobiocin strongly inhibited 6 beta-hydroxylation of testosterone. The microsomal N-demethylation of erythromycin and hydroxylation of testosterone at the 15 alpha position were less affected by novobiocin. These results suggest that novobiocin inhibits not only UDPGTs, but also cytochrome P450 (CYP) enzyme activities, probably those belonging to the CYP3A subfamily. More research is needed to reveal which CYPs and UDPGTs are affected by novobiocin in vivo, in order to improve the understanding the probably the predictability of potential drug interactions with this antibiotic.

Acetaminophen↗

PubMed1996

Differential effect of pentoxifylline on lipopolysaccharide-induced downregulation of cytochrome P450.

It is now established that inflammatory stimuli such as lipopolysaccharides (LPS) and polyinosinic acid:polycytidylic (polyIC) suppress hepatic expression of cytochrome P450 (P450) genes in rat liver. Previous studies have suggested that LPS- or polyIC-induced downregulation of P450 was due to endogenously released inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1, interleukin-6, and interferons (IFNs). To improve our understanding of the role of inflammatory cytokines in mediating P450 depression, we investigated the possibility of preventing P450 downregulation with pentoxifylline. Pentoxifylline has been shown to inhibit LPS-induced TNF-alpha production by suppression of TNF-alpha gene expression. The present study shows that in uninduced male rats pentoxifylline selectively prevents the downregulation of microsomal P4501A2 and P4502B caused by LPS. No protective effect of pentoxifylline on the downregulation of P4502E1 and P4503A1/2 was observed. PolyIC-induced downregulation of P4501A2, P4502B, P4502E1, and P4503A1/2 was not affected by pentoxifylline. These results suggest that the LPS-induced downregulation of P4501A2 and P4502B is mediated to a large extent by TNF-alpha. Other cytokines might be involved in the suppression of P4502E1 and P4503A1/2. The fact that polyIC-induced downregulation is not protected by pentoxifylline is further evidence that this agent acts via a selective induction of IFNs.

Animals↗

PubMed1996

Suppression of cytochrome P450- and UDP glucuronosyl transferase-dependent enzyme activities by proinflammatory cytokines and possible role of nitric oxide in primary cultures of pig hepatocytes.

Proinflammatory cytokines play an important role in the depression of cytochrome P450 (CYP450)-dependent drug metabolism in mammals during inflammation and infection. Although much has been learned concerning the effects and mechanisms of cytokine-mediated suppression of CYP450, there is limited knowledge about how cytokines affect UDP glucuronosyl transferases (UDPGT). The aim of the present study was to investigate the effects and dose dependency of recombinant human proinflammatory cytokines on both CYP450- and UDPGT-dependent enzyme activities in primary cultures of pig hepatocytes. A possible role of nitric oxide in cytokine-induced suppression of enzyme activities was studied by incubating hepatocytes in the presence of N G-nitro-L-arginine (L-NAME), a competitive inhibitor of nitric oxide (NO) biosynthesis. Incubation of hepatocytes with interleukin-1alpha (IL-1alpha), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha) decreased both oxidation and glucuronidation activities dose dependently, in which the effects on glucuronidation activities were even more pronounced. IL-6 differed from IL-1alpha and TNF-alpha by inhibiting CYP450 and UDPFT more effectively after 24 hr of incubation, whereas the inhibition by IL-1alpha and TNF-alpha was more pronounced after 12 hr. Only at a concentration of 500 U/ml did interferon-gamma (IFN-ganna) inhibit CYP450 and UDPGT. The inhibition of CYP450 enzyme activities by cytokines was probably not due to the production of NO, because L-NAME totally blocked NO production but had no effect on the cytokine-induced suppression of CYP450 enzyme activities. However, there might be a role for NO in the decrease of glucuronidation by cytokines, as L-NAME slightly though significantly prevented the inhibition of glucuronidation.

Acute-Phase Reaction↗

PubMed1996

A lipopolysaccharide-induced acute phase response in the pig is associated with a decrease in hepatic cytochrome P450-mediated drug metabolism.

Drug disposition, including hepatic drug metabolism, is markedly affected by infection, inflammation and other conditions that invoke the acute phase response. In the present study, an Escherichia coli lipopolysaccharide (LPS)-induced acute phase response model was developed in pigs. This model was used to study the effects of the acute phase response on drug disposition and hepatic drug metabolism in vivo and in microsomal preparations. The results obtained were compared with those from Actinobacillus pleuropneumoniae-infected pigs. Intermittent intravenous administration of LPS induced a mild acute phase response as evidenced by increased rectal body temperatures, anorexia and increased cytokine (TNF-alpha and IL-6) serum levels within 1-2 h after the first LPS injection. The acute phase response is associated with a pronounced decrease of antipyrine plasma clearance (control 8.5 +/- 0.8 vs. LPS 2.2 +/- 0.7 mL/min.kg). Furthermore, total cytochrome P450 content and microsomal cytochrome P450-dependent activities were significantly decreased after 24 h. The decrease in cytochrome P450 activities was accompanied by losses of cytochrome P4501A and P4503A apoproteins. The microsomal glucuronidation rate of 1-naphthol was not affected in LPS-treated pigs. Comparing the LPS model with our previous findings in the Actinobacillus pleuropneumoniae model showed a remarkable similarity with regard to the effects on hepatic drug metabolism.

Animals↗

PubMed1996

Fluoroquinolone antibiotics inhibit cytochrome P450-mediated microsomal drug metabolism in rat and human.

The fluoroquinolone antibacterial agents have gained widespread use in the treatment of a broad range of bacterial infections. We recently described a possible interaction concerning the concomitant use of cyclosporine A and norfloxacin in pediatric renal transplant patients. We examined the effect of two common fluoroquinolone antibiotics on cytochrome P450-mediated drug biotransformations in human and rat liver microsomes. Rats were pretreated with inducers, which increased the levels of the P450 isozymes CYP3A2, CYP1A, CYP2E1, and CYP4A1. Ciprofloxacin and norfloxacin significantly depressed the N-demethylation of erythromycin by CYP3A4 in human microsomes and by CYP3A2 in rat microsomes. The inhibition was determined to be competitive in nature in rat microsomes, with ciprofloxacin and norfloxacin both exhibiting similar Ki values of 2.0 and 2.3 mM, respectively. Ciprofloxacin and norfloxacin also inhibited ethoxyresorufin-O-dealkylase (CYP1A). In contrast, ciprofloxacin and norfloxacin did not inhibit the metabolism of substrates that are specific for the P450 isozymes CYP2E1 and CYP4A1. Rats treated chronically with norfloxacin revealed no alterations in hepatic CYP3A2 protein levels or activity. These studies in hepatic microsomes demonstrate that fluoroquinolones can decrease CYP3A- and CYP1A-mediated biotransformation by competitive inhibition and that they have the potential to cause drug interactions with agents metabolized by these enzymes.

Animals↗

PubMed1995

Selective effects of a bacterial infection (Actinobacillus pleuropneumoniae) on the hepatic clearances of caffeine, antipyrine, paracetamol, and indocyanine green in the pig.

1. In order to investigate the effect of a bacterial acute phase response model on drug disposition in vivo, plasma clearances of antipyrine, caffeine, paracetamol and indocyanine green were investigated in the healthy and Actinobacillus pleuropneumoniae-infected pig. 2. Indocyanine green plasma and endogenous creatinine clearance were not changed during the infection, which indicates that hepatic blood flow and renal function were not significantly affected. 3. In the A. pleuropneumoniae-infected pig, plasma clearances of antipyrine and caffeine, both marker substrates for hepatic oxidative biotransformation, were decreased by 72 and 68% respectively. The clearance of paracetamol, a drug mainly glucuronidated in the pig, was reduced by 39%. 4. It is concluded that the most important change in drug elimination during an acute phase response induced by A. pleuropneumoniae is a suppression of oxidative hepatic biotransformation.

Acetaminophen↗

PubMed1995

The antibiotic tiamulin is a potent inducer and inhibitor of cytochrome P4503A via the formation of a stable metabolic intermediate complex. Studies in primary hepatocyte cultures and liver microsomes of the pig.

Tiamulin is a semisynthetic antibiotic frequently used in agricultural animals. The drug has been shown to produce clinically important--often lethal--interactions with other compounds that are simultaneously administered. To explain this, it has been suggested that tiamulin selectively inhibits oxidative drug metabolism via the formation of a cytochrome P450 metabolic intermediate complex. The aim of the present study was to provide further support for this hypothesis. When hepatic microsomes and cultured primary pig hepatocytes were incubated with tiamulin, a maximum in the absorbance spectrum at 455 nm was observed, which disappeared after adding KFe(CN)6. When hepatocytes were incubated with tiamulin for 72 hr, cytochrome P450 content and cytochrome P4503A apoprotein levels were increased. Tiamulin strongly inhibited and concentration dependently inhibited the hydroxylation rate of testosterone at the 6 beta-position in both microsomes and hepatocytes, and the microsomal N-demethylation rate of ethylmorphine. Other testosterone hydroxylations were inhibited to a lesser extent or not affected. The relative inhibition of the hydroxylation of testosterone at the 6 beta-position was more pronounced in microsomes from rifampicin- and triacetyloleandomycin-treated pigs. The results indicate that cytochrome P450 complex formation can at least partly explain the interactions observed with tiamulin. Tiamulin seems to be a strong, probably selective, inhibitor of the cytochrome P4503A subfamily and an interesting tool for further research.

Animals↗

PubMed1995

Infection (Actinobacillus pleuropneumoniae)-mediated suppression of oxidative hepatic drug metabolism and cytochrome P4503A mRNA levels in pigs.

The effect of Actinobacillus pleuropneumoniae infection, a well-characterized pig infection model, on both phase I (oxidative) and phase II (conjugative) microsomal enzyme activities was investigated in castrated male conventional pigs. A. pleuropneumoniae infection resulted after 24 hr in a significant suppression of 33% or more of all oxidative enzyme activities determined. After 40 hr, the activities were still suppressed, but did not differ from the results after 24 hr. On the contrary, all glucuronosyltransferase activities measured were not affected by A. pleuropneumoniae infection after both 24 and 40 hr. To elucidate further the mechanism of the suppression of oxidative enzyme activities, analysis of mRNA were conducted by dot-blot analysis using a human cytochrome P4503A4 cDNA probe. The results indicated that A. pleuropneumoniae infection suppressed oxidative enzyme activities. The reduction in cytochrome P4503A activity, specific for 6 beta-hydroxylation of testosterone is at a pretranslational level as measured by a decrease in the amount of mRNA.

Actinobacillus Infections↗

PubMed1994

A reversed-phase high-performance liquid chromatographic method for the determination of Clanfenur in rat and human plasma.

A selective and specific high-pressure liquid chromatographic (HPLC) method for the simultaneous assay of Clanfenur and its metabolites in biological fluids of interest has been developed which is suitable for routine analysis, using micro volumes (0.1 ml) of plasma samples only. After protein precipitation the extract is analysed by reversed-phase HPLC with UV detection. Excellent recovery, linearity, accuracy and precision (less than 5% for plasma) are achieved by the assay which is able to quantify Clanfenur and its metabolites in plasma at concentrations between 0.025 and 5.0 mg l-1.

Animals↗

PubMed1994

Dose-dependent pharmacokinetic interaction between antipyrine and paracetamol in vivo and in vitro when administered as a cocktail in pig.

1. The pharmacokinetic interactions between paracetamol (PA) and antipyrine (AP) were studied in pigs in order to investigate the usefulness of this combination for the simultaneous assessment of oxidative and conjugative metabolism. 2. When both drugs were given at a dose of 5 mg/kg, AP plasma clearance was reduced from 2.22 to 0.96 lh-1 kg-1. PA clearance was not changed in comparison with control values. 3. At a dose of 2 mg/kg no pharmacokinetic interaction between the two drugs was observed. 4. The only oxidative AP metabolite found in urine was 4-hydroxyantipyrine (4-OHA). It accounted for 80% of the dose and, like PA, it was completely glucuronidated. 5. The glucuronidation of PA has been studied in vitro in pig liver microsomes. The apparent Km value for PA glucuronidation was 40 mM with a Vmax = 54 nmol min-1 mg protein-1. To determine if 4-OHA and PA competed for the same UDP-glucuronosyl-transferase, the effect of 4-OHA and AP on PA glucuronidation was studied. It appeared that 4-OHA was a competitive inhibitor with a Ki app = 0.07 microM, whereas AP had no effect. 5. Results suggest a dose-dependent interaction between AP and PA, which may be due to competition at the level of glucuronidation. Therefore, the usefulness of AP and PA in vivo in a cocktail for metabolism studies is limited.

Acetaminophen↗