Effect of acute and repeated chlordimeform treatment on rat hepatic microsomal drug metabolizing enzymes

The effect of chlordimeform treatment on the hepatic microsomal drug metabolizing enzymes was examined in male and female rats following either acute or repeated treatment. After acute administration of chlordimeform (100 mg/kg, ip one hr prior to sacrifice) differential effects were observed in various parameters of the hepatic microsomal mixed function oxidase system with significant decreases in ethylmorphine metabolism, cytochrome P-450 content, NADPH cytochrome c reductase, and in the spectral binding of hexobarbital and aniline while no changes were found in the metabolism of aniline or p-nitroanisole. Durations of zoxazolamine-induced paralysis and pentobarbital-induced hypnosis were increased significantly after acute chlordimeform administration. Following repeated administration of chlordimeform (75 mg/kg ip for four days) to adult male rats, a decrease was observed in zoxazolamine-induced paralysis time while pentobarbital-induced hypnosis was not altered. Metabolism studies using isolated hepatic microsomal fractions showed a decrease rate of biotransformation of ethylmorphine and aniline while the activity of p-nitroanisole O-demethylase was not changed. No differences were found in cytochrome P-450 levels whereas microsomal spectral binding of hexobarbital was reduced while that of aniline was not affected. Following acute or repeated administration of chlordimeform to adult female rats, decreases in the hepatic microsomal metabolism of aniline, but not ethylmorphine or p-nitroanisole, were observed. Addition of chlordimeform to microsomal suspensions yielded a Type I spectral binding curve.

Barbiturate shift as a tool for determination of efficacy of benzodiazepine-receptor ligands

The change in benzodiazepine(BZ)-receptor affinity for selected BZ receptor ligands, induced by pentobarbital at 30 degrees C in the presence of 200 mM NaCl (barbiturate shift) was investigated. The affinity for benzodiazepines (e.g. flunitrazepam) was increased approximately two-fold by the presence of pentobarbital (1 mM) whereas the affinity for convulsive BZ-receptor ligands (e.g. DMCM ) was reduced approximately two-fold. The affinity for BZ-receptor antagonists (e.g. Ro 15-1788) was unaltered by pentobarbital. The results obtained suggest that barbiturate shifts have predictive value in determining the pharmacological efficacies of BZ-receptor ligands. However, compounds such as CL 218.872 and ZK 93423 would not have been recognized as agonists, notwithstanding their clear agonistic profile in pharmacological tests.

Ocular distribution of sodium pentobarbital after injection of lethal and anesthetic doses and after transfer via corneal grafting

The distribution of sodium pentobarbital in the eye after lethal and anesthetic doses and the possibility of the drug's transfer via corneal grafting were studied in rabbits. The drug was found in all the ocular tissues and humours following the intravenous injection of a single lethal (60 mg/kg) or anesthetic (30 mg/kg) dose, and it was still present 24 hours after the anesthetic injection. In both instances the largest quantity was found in the cornea. After grafting of corneal tissue containing pentobarbital the drug dispersed into the recipients' corneas and other ocular tissues. In-vitro studies showed that 250 micrograms of the drug per millilitre of incubation medium reduced the rate of growth of the corneal fibroblasts to 50%, and 300 micrograms/mL reduced their protein synthesis to 66%. Calculations showed that these concentrations could be reached in the cornea or the aqueous humour in vivo following either the lethal or the anesthetic injection of the drug.

Influence of phencyclidine on the translocation of pentobarbital in mice

The influence of phencyclidine (PCP) on the tissue distribution of pentobarbital in male Swiss mice (20-25 g) was investigated. Animals pretreated with PCP (179 mumol/kg/day) for 4 days, i.p., were administered pentobarbital (60 mg/kg, i.p.) 24 h after the PCP injection, sacrificed at various time intervals, and tissues (serum, brain, liver and kidney) were collected. Pentobarbital levels in tissues were determined by the gas liquid chromatography. When compared to the control group, PCP treatment decreased the pentobarbital level by 68% in liver and increased the level by 97% in kidney at 45 min post-injection. No significant change in serum and brain pentobarbital level was noted with either group. These data indicate that PCP influences the translocation of pentobarbital.

Chronic phenytoin administration and the hepatic mixed function oxidase system in female rats

The long term effects of phenytoin administration on mixed function oxidase activities and serum estradiol in female rats were examined. No induction of hepatic mixed function oxidases was seen until 8 days after the administration of 100 mg phenytoin/kg/day either orally or intraperitoneally. Maximum increase in aniline hydroxylase, aryl hydrocarbon hydroxylase (AHH) and ethylmorphine-N-deethylase activities occurred between days 8 and 16 of treatment and decreased thereafter. No induction of lung or intestinal AHH activity was observed. Serum levels of estradiol were significantly decreased after 16 days of phenytoin treatment. Maximal increases in hepatic cytochrome P-450 and cytochrome c reductase occurred after 8-16 days of treatment. Furthermore, pentobarbital sleeping times were shortest after 16 days of phenytoin administration. The activities of all enzymes after 32 days of phenytoin treatment were less than at the peak activities at 8-16 days.

Binding characteristics and interactions of depressant drugs with [35S]t-butylbicyclophosphorothionate, a ligand that binds to the picrotoxinin site

[35S]t-Butylbicyclophosphorothionate (TBPT), a cage convulsant with picrotoxinin-like activity, binds to rat brain membranes to a single site with an apparent KD of 25.1 +/- 5.6 nM and a Bmax of 1.40 +/- 0.22 pmol/mg protein. TBPT binding to rat brain membranes was inhibited by a variety of convulsant, depressant, anxiolytic, and anticonvulsant drugs that had previously been shown to inhibit [3H]alpha-dihydropicrotoxinin binding. Depressant drugs such as pentobarbital and the nonbarbiturate (+)etomidate inhibited TBPT binding in an uncompetitive manner. Thus, pentobarbital and (+)etomidate decreased both the affinity and the number of binding sites of TBPT to whole brain membranes. The IC50 values of (+)etomidate (9 microM) and pentobarbital (90 microM) are similar to the EC50 values at which they enhance both [3H]gamma-aminobutyric acid and [3H]diazepam binding in cerebral cortex membranes. RO5-4864, which has recently been shown to be a convulsant, also inhibited TBPT binding (IC50 = 10 microM). These results suggest that TBPT binds to the picrotoxinin site and further supports the notion that the picrotoxinin site is an important modulatory site at the benzodiazepine-GABA receptor-ionophore complex.

Significance of ratios of different volumes of distribution in pharmacokinetics

Multicompartmental pharmacokinetics involves the four volumes: Vc = volume of the central compartment; Vss = volume of distribution steady-state; V beta = volume of distribution beta; and Vext = the extrapolated volume of distribution. The ratio Vc/Vext is indicative of the degree of multicompartmental character of a set of data. The quantity (Vext/V beta)--1 is the fractional error in the total clearance when one assumes a monoexponential rather than a polyexponential equation for disposition of a drug. The ratio Vss/V beta indicates how well the one-compartment body model predicts average amounts of drug in the body when a multicompartmental model is actually operative. The quantity (Vss/Vc)--1 is equal to either k12/k21 or k12/k21 + k13/k31 of the two- and three-compartmental mammillary models. Examples from the literature are reported and discussed.

Comparison of the effects of acute and subchronic administration of Aroclor 1254, a commercial mixture of polychlorinated biphenyls, on pentobarbital-induced sleep time and [14C]pentobarbital disposition in mice

We have reported previously that polychlorinated biphenyls (PCBs) alter neurochemistry and suppress spontaneous locomotor activity in mice. The present study was initiated to determine whether orally administered (Aroclor 1254) would potentiate pentobarbital-induced sleep time. Sleep time was enhanced significantly by Aroclor 1254 (500 mg/kg) given 0 to 8 h prior to pentobarbital, with the peak effect occurring at 2 h. This effect was demonstrated to be dose-responsive in the range of 5 to 25 mg/kg given 2 h prior to pentobarbital, but only slightly larger increments in sleep time were observed with higher doses of PCBs (50, 100, 250, and 500 mg/kg). Administration of vehicle or Aroclor 1254 (30 or 100 mg/kg) for 14 successive days reduced sleep time when pentobarbital was given 45 min after the last dose of vehicle or Aroclor 1254, with a further reduction when pentobarbital was given 24 h after the last dose. As a correlate to the sleep-time studies, levels of pentobarbital and metabolites were measured in brain, liver, and plasma of mice that had received varying doses of Aroclor 1254 2 h prior to [14C]pentobarbital. Elevated levels of pentobarbital and decreased levels of metabolites were found after acute administration of Aroclor 1254 during a period of time when Aroclor 1254-treated mice were still asleep. These effects of Aroclor 1254 on pentobarbital disposition were found to be dose-dependent. Brain levels of pentobarbital in mice after 14 d of Aroclor 1254 treatment (30 mg/kg) were less than those in vehicle-treated animals, and these levels were consistent with the reduced sleep times. Thus, a correlation between pentobarbital brain levels and sleep time in both Aroclor 1254-treated and nontreated animals suggests that Aroclor 1254 does not alter pentobarbital narcosis by a direct action on the brain. Rather, acutely administered Aroclor 1254 may be augmenting sleep time by competing with pentobarbital for metabolic sites in the liver, while chronically administered Aroclor 1254 induces pentobarbital metabolism.

Drug metabolism and chemosensitization. Nitroimidazoles as inhibitors of drug metabolism

The nitroimidazole misonidazole (MISO) and related compounds have been shown to enhance the response of tumours to cytotoxic agents, and often to improve their therapeutic indices. Previous experiments suggested inhibition of cytotoxic drug metabolism as a mechanism. We have now investigated the effects of MISO and related compounds on drug metabolism in mice, and the results can be summarised as follows. (1) MISO and related compounds inhibit drug-metabolising enzymes, as measured by pentobarbitone sleep-time and zoxazolamine paralysis-time. (2) Enzyme inhibition is primarily dependent on lipophilicity, with maximum inhibition exhibited by the most active chemosensitizers. (3) MISO significantly slowed the clearance of pentobarbitone, aminopyrine and the cytotoxic agent chlorambucil, but had no effect on renal function or protein binding. These data support the view that inhibition of cytotoxic drug metabolism may be an important factor in chemosensitization.

4'-Hydroxylated derivatives as urinary metabolites of two barbiturates

4'-Hydroxybutobarbitone and 4'-hydroxypentobarbitone have been synthesized. These 4'-hydroxy derivatives have been quantified in the urine of volunteers following single doses of butobarbitone, and single and multiple doses of pentobarbitone. The corresponding aldehydes have also been synthesized, and shown to be excreted in minor quantities. The excretion of an ingested dose of 4'-hydroxybutobarbitone, and its oxidation products, has also been followed over 24 h.

Effect of doxapram on the action of other drugs and the hepatic drug-metabolizing system in mice

Effects of doxapram, a respiratory stimulant, on the action of other drugs and the activity of the hepatic drug-metabolizing enzyme were studied in mice. The hypothermic effect induced by aminopyrine and the muscle relaxant effect induced by meprobamate were potentiated by the pretreatment with doxapram 60 min before. Furthermore, doxapram significantly enhanced the lethalities of picrotoxin and strychnine and the analgesic actions of aminopyrine and morphine. The plasma concentration of aminopyrine or pentobarbital in doxapram-treated mice was higher than those in untreated mice, and the plasma concentration of normustard related to an active metabolite of cyclophosphamide after the administration of cyclophosphamide was lower in doxapram-treated mice. On the other hand, doxapram (50 mg/kg, i.p.) reduced remarkably the activities of aminopyrine N-demethylase and aniline hydroxylase in the hepatic 9,000xg supernatant fraction, and also reduced the cytochrome P-450 contents in hepatic microsomes. However, no significant alteration by doxapram was observed on the activities of NADH-ferricyanide reductase and NADPH-cytochrome c reductase and cytochrome b5 contents. It seems likely that the mechanisms of the interaction between doxapram and combined drugs involved the depression of the hepatic drug-metabolizing system in microsomes and a subsequent variation of drug level in the plasma.

Microsomal hydroxylation as measured by pentobarbital elimination in patients with idiopathic systemic lupus erythematosus

A mechanism postulated for drug- or chemical-induced systemic lupus erythematosus (SLE) is that the chemical is covalently bound to nuclear macromolecules increasing the immunogenicity of the macromolecule. This may require metabolic activation by oxidation. There are many similarities between drug-induced and idiopathic SLE. Twelve patients with idiopathic SLE and 12 normal subjects were given 100 mg pentobarbital orally to evaluate their microsomal hydroxylating activity. Plasma pentobarbital concentration was measured by gas-liquid chromatography. Mean plasma pentobarbital half-life was 24 +/- 10 (mean +/- SD) hr in the SLE patients, which is only slightly shorter than the 26 +/- 12 hr in the control subjects. The mean apparent volume of distribution in the patients was 1.28 +/- 0.30 l/kg, which is slightly above the 1.00 +/- 0.37 l/kg in the normal subject (P less than 0.05). Mean metabolic clearance rate in the SLE patients was 0.045 +/- 0.022 l/hr/kg, which is more than the 0.028 +/- 0.008 l/hr/kg in the normal control subjects (P less than 0.02). Since the metabolic clearance rate of a drug is the proper value for evaluating metabolism rate, we conclude that patients with SLE hve an increased elimination rate for drugs or other foreign compounds that are biotransformed by microsomal oxidation and may more rapidly bioactivate chemicals to reactive compounds.

Histamine-induced arousal in the conscious and pentobarbital-pretreated rat

Histamine has been shown to possess many neurotransmitter-like properties, and a variety of studies indicate that central histamine may function in modulating behavioral arousal. To examine this possibility further, histamine was administered into the lateral cerebral ventricles of the conscious and pentobarbital-anesthetized rat. In the conscious animal, histamine induced a significant increase in spontaneous motor activity which consisted of increased grooming and exploratory behaviors (sniffing, rearing and locomotion) as compared to saline-treated controls. In the pentobarbital-pretreated rat, histamine caused a dose-related decrease in narcosis duration and hypothermia without altering the disposition of pentobarbital in brain or plasma. Administration of compounds structurally related to histamine did not alter spontaneous activity or shorten narcosis duration. While pretreatment with the H2-histamine antagonist, cimetidine, was no effective, H1-histamine antagonists were found to abolish histamine-induced arousal. Administration of haloperidol in doses that significantly attenuated increased spontaneous motor activity by amphetamine did not alter histamine-induced hyperactivity. Likewise, atropine did not significantly alter histamine-induced arousal. These data support the hypothesis that histamine may function in modulating behavioral arousal.

Binding of a radiolabeled triazolopyridazine to a subtype of benzodiazepine receptor in the rat cerebellum

The triazolopyridazine (TPZ) drugs, typified by CL218,872 (CL), have a relatively higher affinity for a subpopulation of benzodiazepine (BZ) receptors. The binding of radiolabeled CL to membranes from rat cerebellum, a region enriched in the TPZ-preferring ("Type 1") BZ receptor, was characterized and compared with that of [3H]flunitrazepam ([3H]FLU) in the same preparation. [3H]CL had clonazepam displaceable binding which was saturable. The Kd was approximately 21 nM and the Bmax was approximately 600 fmol/mg of protein. [3H]CL binding was similar to that for [3H]FLU in that exogenous gamma-aminobutyric acid (GABA) enhanced the binding; however, [3H]CL binding differed from that for [3H]FLU in that anions, cartazolate and pentobarbital did not enhance [3H]CL binding. These data suggest that [3H]CL binds to the Type 1 BZ receptor in a manner different from that of a BZ drug such as FLU. Inasmuch as GABA enhances [3H]CL binding, but anions, cartazolate and pentobarbital do not, [3H]CL may bind to the Type 1 BZ receptor in such a way that it interacts with the GABA site, but perhaps not directly with the ionophore or the postulated pyrazolopyridine-barbiturate site. Thus, TPZ drugs may affect the GABA receptor complex in a different or perhaps less extensive way than the BZs. This, in addition to the regional localization of the Type 1 receptor, may be an important part of the mechanism of action of the TPZs.

Effects of liver congestion on hepatic drug metabolism in the rat

Pharmacokinetic alterations in drug disposition have been demonstrated for a variety of hepatic disease states, but there is little information concerning the effects of elevated hepatic venous pressure on hepatic drug metabolism. A rat model of hepatic congestion which was characterized by significantly elevated hepatic venous pressure, marked prolongation of prothrombin time, reduced total hepatic blood flow and histological changes of marked pericentral fibrosis and central vein dilation was used to study the effects of liver congestion on hepatic microsomal biotransformation and in vivo disposition of morphine and pentobarbital. No significant differences in levels of microsomal cytochrome P-450 or NADPH-dependent cytochrome c reductase were seen between the two groups. Total hepatic microsomal capacity for glucuronidation of morphine and hydroxylation of pentobarbital was not altered by elevated hepatic vein pressure and no change in in vivo systemic clearance was seen for either drug in response to hepatic venous congestion. These animal data demonstrate that hepatic congestion produces minimal alterations in hepatic metabolism of morphine and pentobarbital and may not have the severe detrimental effects on drug biotransformation and disposition which would be predicted.

Effects of ethinyl estradiol on pharmacokinetics of meperidine and pentobarbital in the rat

Effects of estrogen administration on in vivo hepatic microsomal drug metabolism have been frequently assessed, but there has been little study of the effects of estrogen treatment on in vivo drug pharmacokinetics. After administration of ethinyl estradiol (5 mg/kg daily for 5 days), meperidine and pentobarbital pharmacokinetics were determined both in vivo and in the isolated perfused rat liver. Estrogen pretreatment caused a 45% reduction in systemic meperidine clearance in vivo and perfusate disappearance of both meperidine and its major metabolite, normeperidine, was slower in isolated liver experiments from ethinyl estradiol-treated animals as compared with propylene glycol-treated controls. In contrast to meperidine, clearance of pentobarbital was not decreased by estrogen treatment. These studies demonstrate that estrogen treatment singularly can alter drug pharmacokinetics in the rat and raise the question as to whether clinically important alterations in drug pharmacokinetics occur in humans receiving estrogen therapy.

Interaction of barbiturates with benzodiazepine receptors in the central nervous system

The interaction of barbiturates with benzodiazepine receptors was studied in extensively washed membrane preparations from rat brain. Sedative/hypnotic and anesthetic barbiturates such as pentobarbital, and convulsant barbiturates such as DMBB, enhanced [3H]diazepam binding in a stereospecific fashion. Freeze-thawing of membranes resulted in a decrease in the potency of barbiturates to enhance [3H]diazepam binding, while the maximum response to barbiturates remained unchanged. Significant differences in both the potency and maximum enhancement of [3H]diazepam binding by pentobarbital was observed among brain regions. The rank order potency of pentobarbital in different brain regions was: cerebellum greater than cortex greater than hippocampus, while the rank order efficacy of pentobarbital in these brain regions was reversed. The effects of a combination of anesthetic and/or convulsant barbiturates on [3H]diazepam binding suggested that these compounds function as partial agonists while a combination of anesthetic or convulsant barbiturates with phenobarbital suggested that latter compound antagonized the actions of both anesthetic and convulsant barbiturates. The convulsant benzodiazepine Ro-5-3663 and inosine were more potent as inhibitors of pentobarbital-enhanced than basal (non-pentobarbital enhanced) [3H]diazepam binding. Solubilization of benzodiazepine receptors with Lubrol-PX resulted in a complete loss of barbiturate enhanced [3H]diazepam binding, and greater than a 75% loss in efficacy in the remaining (insoluble receptor) tissue. These data, coupled with recent observations from this and other laboratories, suggests that the site(s) at which barbiturates act to enhance [3H]diazepam binding to benzodiazepine receptors is distinct from the site at which GABA acts to enhance [3H]diazepam binding. The phenomenon of enhanced benzodiazepine binding by barbiturates may be related to the depressant actions of the barbiturates, that is, their direct effects to increase chloride conductance. Although it is premature to assign a pharmacologic correlate to this neurochemical phenomenon, it appears that this action may be related to the anesthetic effects of the barbiturates. However, the definitive assignment of either the electrophysiologic or pharmacologic sequelae to this neurochemical action will require further investigation.

Effect of antibiotics in the prevention of jejunoileal bypass-induced liver dysfunction

Administration of antibiotics has been reported to prevent or minimize liver dysfunction in experimental animals having been subjected to jejunoileal bypass, suggesting that jejunoileal bypass-induced liver dysfunction results from production of toxic substances by bacteria in the defunctionalized bowel. However, improved absorption will also prevent bypass-induced liver injury. We studied the effects of tetracycline on the development of bypass-induced liver dysfunction and compared it to the mucosal adaptation of the intact bowel after bypass. After 6 weeks, rats subjected to bypass but not given antibiotics had decreased levels of serum triglycerides, hepatic cytochrome P-450, and hepatic pentobarbital hydroxylase. Evaluation of intestinal mucosal hyperplasia after bypass indicated that animals given antibiotics after bypass developed greater increases in mucosal DNA content, mucosal protein, and mucosal weight than bypassed animals not receiving antibiotics. We speculate that the beneficial effects of antibiotic administration on liver function after bypass may be a result of improved absorption.

Effects of potassium ethylxanthogenate and 2,3-dimercaptopropane sulphonate sodium on the pentobarbital pharmacokinetics and metabolism in male mice

The effects of potassium ethylxanthogenate (PEX) and 2,3-Dimercaptopropane sulphonate sodium (Unithiol) on the pentobarbital (40 mg/kg, body weight i.v.) (PB) sleeping time, pharmacokinetics, and metabolism, were studied in comparative experiments on male albino mice. It was established that the pretreatment of animals with PEX (80 mg/kg of body weight s.c.) potentiated the PB sleeping time. Unithiol in an equimolar dose (105.2 mg/kg of body weight s.c.) had no effect. The pharmacokinetics of PB could be fitted to be a biexponential equation of the type Cp(t) = A.e-alpha t + B.e-beta t. PEX caused an almost two-fold decrease in the elimination rate constant of plasma PB, which led to a higher half-time and a lower total clearance compared with the controls. The curve for the plasma PB metabolites in the PEX-pretreated mice was significantly lower than of the controls. A higher PB level and decreased rate of elimination in PEX-pretreated animals was observed also in the: liver, lungs, kidney, and brain. A conclusion was drawn that the potentiating effect of PEX on the PB sleeping time is mainly due to inhibition of PB liver metabolism. It was suggested that the differences in the biological effects of the two thiol compounds are due to the differences in their chemical structures: PEX possesses a C = S group but Unithiol lacks this group.