Effects of increased liver metabolism of nicotine on its uptake, elimination and toxicity in mice

Cotinine: Pharmacologically Active Metabolite of Nicotine and Neural Mechanisms for Its Actions

Tobacco use disorder continues to be a leading public health issue and cause of premature death in the United States. Nicotine is considered as the major tobacco alkaloid causing addiction through its actions on nicotinic acetylcholine receptors (nAChRs). Current pharmacotherapies targeting nicotine's effects produce only modest effectiveness in promoting cessation, highlighting the critical need for a better understanding of mechanisms of nicotine addiction to inform future treatments. There is growing interest in identifying potential contributions of non-nicotine components to tobacco reinforcement. Cotinine is a minor alkaloid, but the major metabolite of nicotine that can act as a weak agonist of nAChRs. Accumulating evidence indicates that cotinine produces diverse effects and may contribute to effects of nicotine. In this review, we summarize findings implicating cotinine as a neuroactive metabolite of nicotine and discuss available evidence regarding potential mechanisms underlying its effects. Preclinical findings reveal that cotinine crosses the blood brain barrier and interacts with both nAChRs and non-nAChRs in the nervous system, and produces neuropharmacological and behavioral effects. Clinical studies suggest that cotinine is psychoactive in humans. However, reviewing evidence regarding mechanisms underlying effects of cotinine provides a mixed picture with a lack of consensus. Therefore, more research is warranted in order to provide better insight into the actions of cotinine and its contribution to tobacco addiction.

Intravenous nicotine injection induces rapid, experience-dependent sensitization of glutamate release in the ventral tegmental area and nucleus accumbens

Although numerous data suggest that glutamate (GLU) is involved in mediating the neural effects of nicotine, direct data on nicotine-induced changes in GLU release are still lacking. Here, we used high-speed amperometry with enzyme-based GLU and enzyme-free GLU-null biosensors to examine changes in extracellular GLU levels in the ventral tegmental area (VTA) and nucleus accumbens shell (NAcc) induced by intravenous nicotine in a low, behaviorally active dose (30 μg/kg) in freely moving rats. Using this approach, we found that the initial nicotine injection in drug-naive conditions induces rapid, transient, and relatively small GLU release (~ 90 nM; latency ~ 15 s, duration ~ 60 s) that is correlative in the VTA and NAcc. Following subsequent nicotine injections within the same session, this phasic GLU release was supplemented by stronger tonic increases in GLU levels (100-300 nM) that paralleled increases in drug-induced locomotor activation. GLU responses induced by repeated nicotine injections were more phasic and stronger in the NAcc than in VTA. Therefore, GLU is phasically released within the brain's reinforcement circuit following intravenous nicotine administration. Robust enhancement of nicotine-induced GLU responses following repeated injections suggests this change as an important mediator of sensitized behavioral and neural effects of nicotine. By using high-speed amperometry with glutamate (GLU) biosensors, we show that i.v. nicotine at a low, behaviorally relevant dose induces rapid GLU release in the NAcc and VTA that is enhanced following repeated drug injections. This is the first study reporting second-scale fluctuations in extracellular GLU levels induced by nicotine in two critical structures of the motivation-reinforcement circuit and rapid sensitization of GLU responses coupled with locomotor sensitization.

Pharmacokinetic considerations in understanding nicotine dependence

The pharmacokinetic and pharmacodynamic characteristics of a drug are important determinants of whether users become dependent on it and of the temporal patterns of drug use. Characteristics of cigarette smoking, which produces a high degree of dependence, and the use of nicotine gum, which has a relatively low risk of dependence, are compared. Nicotine from tobacco smoke is rapidly absorbed and transferred into the brain. This results in high brain concentrations and intensive psychological effects, with relatively little development of tolerance. The smoker may titrate the level of drug and associated psychological effects of nicotine. Thus, smoking provides a nearly optimal situation for behavioural reinforcement. Chewing nicotine gum results in slow absorption of nicotine, leading to lower levels of nicotine in the brain and substantial time for development of tolerance. Thus, the intensity of effect is less and the onset of effect is delayed from the onset of dosing, providing less opportunity for behavioural reinforcement. Pharmacokinetic and pharmacodynamic modelling techniques have been applied to these processes and used to assess the implications for understanding the daily smoking cycle.

Application of nicotine enantiomers, derivatives and analogues in therapy of neurodegenerative disorders

This review gives a brief overview over the major aspects of application of the nicotine alkaloid and its close derivatives in the therapy of some neurodegenerative disorders and diseases (e.g. Alzheimer's disease, Parkinson's disease, Tourette's syndrome, schizophrenia etc.). The issues concerning methods of nicotine analysis and isolation, and some molecular aspects of nicotine pharmacology are included. The natural and synthetic analogues of nicotine that are considered for medical practice are also mentioned. The molecular properties of two naturally occurring nicotine enantiomers are compared--the less-common but less-toxic (R)-nicotine is suggested as a natural compound that may find its place in pharmaceutical practice.

Chronic nicotine administration in the drinking water affects the striatal dopamine in mice

Although tobacco contains a large variety of substances, its addictive properties are most probably due to the reinforcing actions of nicotine that motivates continued tobacco use. Animals and humans self-administer nicotine, a response that appears to involve the mesolimbic dopamine system and to be common to other abused drugs. The present article reviews animal models to administer nicotine chronically. We also describe a new animal model in which nicotine is given to mice in drinking water as their sole source of fluid. This treatment produced nicotine plasma concentrations comparable to or above those found in smokers. We found that mice withdrawn from nicotine were tolerant to the effects of nicotine challenge on striatal dopamine metabolism as well as on body temperature and locomotor activity. Furthermore, 3H-nicotine binding in the cortex and midbrain was significantly increased in mice withdrawn from nicotine. The last part of the article will focus on the effects of this chronic nicotine treatment on striatal dopamine. Dopamine and its metabolites and locomotor activity were increased in the forenoon in mice still drinking nicotine solutions. We also report recent data in which chronic nicotine administration in the drinking water enhanced the effect of dopamine receptor agonist, quinpirole, on striatal metabolism. The animal model described appears to be a relevant method for studying the mechanisms that are thought to be involved in nicotine dependence.

Effects of cotinine at cholinergic nicotinic receptors of the sympathetic superior cervical ganglion of the mouse

Nicotine, the principal alkaloid in tobacco, is generally accepted to be responsible for most neuropharmacological effects due to tobacco use. Little is known about the action of cotinine, the major metabolite from nicotine, at neuronal structures. To evaluate the mode of action of cotinine at neuronal receptors, its effect on the surface compound potential of the sympathetic superior cervical ganglion (SCG) of the mouse was studied. The modulation of nicotine induced surface potentials by cotinine was tested. It was found that 2-min applications of cotinine (0.1-30 mmol/l) induced concentration dependent depolarizations at the SCG (EC(50)=1.7 mmol/l) which were followed by hyperpolarizations and weak afterdepolarizations. The intrinsic activity of cotinine compares to that induced by much lower concentrations of nicotine (EC(50)=21 micromol/l). These cotinine effects may be mediated at least in part by nicotine impurities which were found by capillary electrophoresis to be 0.1 and 0.8% in different batches of cotinine. Continuous application of 300 micromol/l cotinine shifted the concentration-response curve of nicotine to the right and reduced (IC(50)=302 micromol/l) the effects of submaximal nicotine concentrations (30 micromol/l). This effect could not be mimicked by continuous application of a nicotine concentration (0.3 micromol/l) equivalent to the lower impurity in cotinine. Therefore, the antagonistic action of cotinine at peripheral neuronal nicotinic receptors is at least in part independent of nicotine impurity. The observed antagonistic effect of cotinine at nicotinic receptors likely contributes to the neuropharmacological effects of smoking.

Contribution of CNS nicotine metabolites to the neuropharmacological effects of nicotine and tobacco smoking

Nicotine, the principal alkaloid in tobacco products, is generally accepted to be the active pharmacological agent responsible for CNS effects resulting from tobacco use. Arguments are presented in this commentary which take issue with this popular dogma, by providing evidence that nicotine metabolites may also be responsible for the CNS effects commonly attributed to nicotine. CNS effects attributed to nicotine include reinforcing effects, mood elevation, arousal, locomotor stimulant effects, and learning and memory enhancement. The reinforcing and locomotor stimulant effects of nicotine have been suggested to be the result of activation of CNS dopaminergic systems, and nicotine-induced modulation of dopaminergic neurotransmission has been studied in detail. Nicotine acts at a family of nicotinic receptor subtypes composed of multiple subunits; however, the exact composition of the subunits in native nicotinic receptors and the functional significance of the receptor subtype diversity are currently unknown. This nicotinic subtype diversity increases the complexity of the potential mechanisms of action of nicotine and its metabolites. Although peripheral metabolism of nicotine has been studied extensively, metabolism in the CNS has not been investigated to any great extent. Recently, studies from our laboratory have demonstrated that several nicotine metabolites are present in the CNS after acute nicotine administration. Moreover, nicotine metabolites are pharmacologically active in neurochemical and behavioral assays. Thus, CNS effects resulting from nicotine exposure may not be due solely to nicotine, but may result, at least in part, from the actions of nicotine metabolites.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Nicotine metabolism in isolated perfused lung and liver of phenobarbital- and benzoflavone-treated rats

The kinetics of nicotine elimination was investigated in isolated perfused lung and liver of phenobarbital (PB)- and 5,6-benzoflavone (BF)-pretreated rats. The estimated kinetic parameters demonstrated a high nicotine elimination rate in rat lung approaching the capacity of liver when both organs were in an uninduced state. The concentration-time profiles of cotinine as the main metabolite were almost identical for isolated lung and liver. In both organs the cotinine plasma concentrations reached a plateau level after 60 min of perfusion. Pretreatment of rats with 5,6-benzoflavone did not affect the rate of nicotine elimination and cotinine formation either in the lung or in the liver. Phenobarbital treatment, however, induced nicotine clearance in lung approximately 2-fold. This effect is quantitatively lower than the PB-related 8-fold induction of hepatic nicotine elimination observed in a previous study. The present results also indicate that the turnover of cotinine is markedly enhanced after PB induction. The elimination half-lives and clearance values for cotinine as the substrate were approximately 10-fold increased in rat liver after PB pretreatment. Thus, an important contribution of extrahepatic tissues to nicotine metabolism in rats has to be assumed. Moreover, since cotinine elimination is significantly increased after PB induction it is questionable whether cotinine plasma concentrations can further be used as suitable parameter for nicotine consumption.

Increased hepatic nicotine elimination after phenobarbital induction in the conscious rat

Elimination parameters of [14C]nicotine in conscious rats receiving nicotine (0.3 mg/kg) either intravenously or orally were studied. The oral availability of unchanged nicotine, derived by comparison of the respective areas under the concentration vs time curves (AUC), was 89%, indicating low hepatic extraction ratios of about 10%. Pretreatment of rats with phenobarbital (PB) markedly increased hepatic first-pass extraction of nicotine. The oral availability of unchanged nicotine in plasma dropped to 1.4% of the corresponding values obtained from PB-treated rats receiving nicotine iv. After PB pretreatment, the clearance of iv nicotine was increased approximately twofold over controls, much less than the observed more than ninefold increase of hepatic first-pass extraction. It is assumed that extrahepatic metabolism contributed significantly to the rapid removal of nicotine from the plasma. The elimination of cotinine, originating from nicotine administered either po or iv, was significantly increased by PB pretreatment, as determined by the ratio of corresponding AUCs. The pattern of nicotine metabolites in urine also indicated an increase in the rate of cotinine metabolic turnover. The amount of norcotinine in the organic extract of urine paralleled PB microsomal enzyme induction. The ratio between urinary concentrations of the normetabolite and cotinine correlated strongly with the PB-induced state of rat liver. This may be a suitable indicator of PB-inducible hepatic cytochrome P450 isoenzyme(s). Since smoking habits in man are feedback-regulated by nicotine plasma concentrations, a similar increase of nicotine elimination by microsomal enzyme induction in man may be of relevance for tobacco consumption.

Uptake and regional distribution of (+)-(R)- and (-)-(S)-N-[methyl-11C]-nicotine in the brains of rhesus monkey. An attempt to study nicotinic receptors in vivo

N-[methyl-11C] nicotine (11C-nicotine) was given intravenously to monkeys and the uptake and regional distribution of radioactivity was followed in the brain using positron emission tomography (PET). The 11C-radioactivity in the brain peaked within 1-2 min and then rapidly declined. Pretreatment with unlabelled nicotine (10 micrograms/kg) reduced the uptake of 11C-radioactivity to the brain by 30%. The uptake of radioactivity was higher following (+)11C-nicotine than (-)11C-nicotine. Both enantiomers were distributed in a similar manner within the brain. When animals were infused with a peripheral nicotinic blocker (trimetaphan) the uptake of radioactivity to the brain was lower following (+)11C-nicotine compared to (-)11C-nicotine. The amount of radioactivity was high in the occipital cortex, thalamus, intermediate in the frontal cortex and low in white matter in (-)11C injected monkeys while no regional difference in distribution of 11C-radioactivity was observed after injection of (+)11C-nicotine.

Neuropharmacological effects of nicotine in relation to cholinergic mechanisms

Studies with nicotine have, from the outset, been of fundamental importance for the elucidation of physiological mechanisms. However, many nicotinic cholinoreceptors do not appear to subserve a physiological role, at least within the present framework of knowledge. The main challenge in the immediate future is to determine the functional role of nicotinic cholinoceptors in the CNS. This knowledge will not only add to the physiological mechanisms that have been unravelled as a result of studies with nicotine, but will also provide a major contribution to the understanding of tobacco use.

Failure of behavioral dependence induction and oral nicotine bioavailability in rats

As failure to induce behavioral dependence to oral nicotine (0.31 mM) might be caused by taste aversion. Sixteen rats were presented nicotine around the taste aversion threshold (0.025 mM then 0.05 mM) as only source of fluid for 10 weeks. Eight of them had undergone portacaval anastomosis (PCA) to increase bioavailability of nicotine by preventing liver first-pass. Weekly choice sessions between nicotine and water demonstrated neither aversion nor preference for nicotine. In 10 control and 11 PCA rats accustomed to drink 0.31 mM nicotine, plasma nicotine was determined after 3 ml/kg intragastric nicotine-solution. In both groups, 0.05 mM nicotine did not lead to detectable levels but 0.31 mM nicotine led to peak levels higher than seen in man after smoking. Similar levels were recorded after spontaneous nicotine drinking in 8 isolated and 18 grouped normal rats accustomed to 0.31 mM nicotine. Drinking nicotine for at least 10 weeks did not induce behavioral dependence in these rats. This cannot be explained by poor nicotine bioavailability by oral route.

Blood nicotine levels in hamsters after smoking and subcutaneous nicotine

Information is limited on blood nicotine levels among laboratory animals subjected to smoking or nicotine injections. This study was done to provide such information on blood nicotine levels in hamsters to better investigate nicotine-related pathology, using levels similar to those of human smokers. Blood nicotine levels were quantitated by gas chromatography and mass spectrophotometry among adult hamsters smoking one of three different strength cigarettes and hamsters injected with four different doses of subcutaneous nicotine, and their controls. The recorded levels rose dose dependently with increasing cigarette strengths and increasing doses of injected nicotine. Based upon regression equations, blood nicotine levels in the hamster model approximating human habitual ad lib. smoking are achieved by using a 1.56-mg nicotine cigarette or injecting 0.15 mg/kg nicotine. The Syrian golden hamster provides a good model for acute or chronic studies involving cigarette smoking.

Apparent tolerance to the acute effect of nicotine results in part from distribution kinetics

Persons exposed to nicotine develop tolerance to many of its effects. When heart rate and forearm venous blood concentration are plotted against time after intravenous administration of nicotine, a greater increase in heart rate is seen for a given nicotine concentration during the rising phase of nicotine concentrations than during the decreasing phase. This could be due to acute tolerance or to more rapid distribution of drug to effect site (brain) than to venous blood. To distinguish between these possibilities, six rabbits were given nicotine intravenously. Blood samples were taken from the internal jugular vein (reflecting brain concentration), and the femoral vein and artery. Brain concentrations peaked before femoral venous concentrations. Seven men received intravenous infusions of nicotine. Peripheral venous blood concentrations and cardiovascular responses were measured. Heart rate peaked before venous concentrations. A physiological kinetic model, fit to the rabbit data, was scaled to humans and used to predict "brain" concentrations in them. Heart rate and predicted brain concentrations peaked simultaneously. We conclude that the rapid development of tolerance to the cardioaccelerating effect of nicotine can be attributed, at least in part, to its distribution kinetics.

Eightfold induction of nicotine elimination in perfused rat liver by pretreatment with phenobarbital

Elimination of nicotine by isolated rat livers was increased eightfold after pretreatment with phenobarbital (PB) as an inducer of cytochrome P-450 while it was only marginally influenced after pretreatment with 5,6-benzoflavone (BF) as an inducer of cytochrome P-448. Initial rates of cotinine formation were enhanced in the same order of magnitude in PB-induced livers. The 14C-nicotine-derived radioactivity excreted into bile within 2 h ranged between 6 -17% of the dose with only 2.7 fold higher values after PB pretreatment compared to controls.

Clinical pharmacokinetics of nicotine

Nicotine intake is considered to be a major factor in sustaining tobacco addiction. For this reason, nicotine gum has recently been introduced as an adjuvant to smoking cessation. The introduction of nicotine as a 'therapeutic' entity necessitates a careful examination of its clinical pharmacokinetics. Insufficient data exist to quantitatively assess the absorption of nicotine after oral administration. Based upon physicochemical and pharmacokinetic principles, the oral bioavailability of nicotine would be expected to be less than 20%. The limited data available in the literature appear to support this conclusion. Absorption from the oral mucosa is the principal site of nicotine absorption in subjects who chew tobacco or nicotine gum. Absorption by this route is highly pH dependent. Nicotine is also readily absorbed from the nasal mucosa, and after topical administration. Nicotine distributes extensively into body tissues with a volume of distribution ranging from 1.0 to 3.0 L/kg. Nicotine has been shown to transfer across the placenta and into breast milk in humans. Plasma protein binding is negligible, ranging from 4.9 to 20%. The predominant route of nicotine elimination is hepatic metabolism. Although a number of metabolites of nicotine have been identified, it is unclear whether any of these compounds contribute to the pharmacological effect of nicotine. Nicotine is also excreted unchanged in urine in a pH-dependent fashion. With urinary pH less than 5, an average 23% of the nicotine dose is excreted unchanged. When urinary pH is maintained above 7.0, unchanged nicotine urinary excretion drops to 2%. After intravenous administration, nicotine exhibits biexponential decline in plasma. Total plasma clearance ranges from 0.92 to 2.43 L/min. During urinary acidification, renal clearance averages 0.20 L/min. Non-renal blood clearance averages 1.2 L/min, indicating that nicotine elimination is dependent on hepatic blood flow. The literature is devoid of information regarding the effect of disease on the pharmacokinetics of nicotine. Based upon the drug's pharmacokinetics in healthy smokers, it would be anticipated that disease states which alter hepatic blood flow may have the greatest impact on nicotine pharmacokinetics. In addition, drugs which alter hepatic blood flow may cause significant alterations in the systemic clearance of nicotine. Dependence on smoking appears to be related, at least in part, to the achievement of a rapid rise in plasma nicotine concentrations. If this assessment is correct, the most desirable adjuvant for smoking cessation would be one that closely mimics this pattern of plasma nicotine concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative analysis of nicotine-like receptor-ligand interactions in rodent brain homogenate

The effects of different variables such as incubation time, temperature, tissue protein content, and pH on the interactions of various labelled nicotinic ligands with nicotine-like binding sites in vitro were studied in rodent brain preparations. The ligands tested were alpha-[3H]bungarotoxin (alpha-[3H]BTX), [3H]tubocurarine ([3H]TC), and [3H]nicotine ([3H]NIC). The regional distribution of the labelled nicotinic ligand binding was also studied and affinity constants and maximal binding (Bmax) values for the equilibrium [3H]NIC binding are given. Association kinetics for [3H]NIC and [3H]TC binding to brain homogenate were similar, with maximal binding within 5-10 min of incubation, followed by a continuous decrease. In contrast, the binding of alpha-[3H]BTX to brain homogenate was much slower, reaching equilibrium after 30-60 min of incubation. Scatchard analysis of equilibrium binding data for [3H]NIC in the hippocampus indicated two binding sites: a high-affinity site (Bmax, 60 pmol/g protein; KD, 6 nM) and a low-affinity site (Bmax, 230 pmol/g protein; KD, 125 nM). The data for the high-affinity [3H]NIC binding site are very similar to previously found data for the high-affinity binding site of [3H]TC and the binding site of alpha-[3H]BTX. Each ligand showed regional differences in binding, and the binding pattern also differed between the ligands.

The effects of smoking on memory consolidation

The effects of smoking deprivation and of smoking a single .8 mg, 1.3 mg or 2 mg nicotine yield cigarette, immediately post acquisition on a paired-associate learning task, were studied in 54 male smokers and 15 male nonsmokers. Subjects were retested for retention of the memorized material at intervals of one-half hour, one day, one week, and one month. Nonsmokers showed superior recall to all smokers at one-half-hour retest, and to some of the smoking groups on later re-tests. At one-month retest the low- and middle-nicotine cigarette smokers outperformed high-nicotine cigarette smokers. Low/middle-nicotine smokers achieved superior recall to nonsmokers at one-month retest. Results are discussed in terms of smoker versus nonsmoker differences, in terms of the effects of nicotine on memory consolidation, and in terms of the PAL response measure adopted.

Pharmacodynamic effects of nicotine and acetylcholine biosynthesis in mouse brain

Nicotine (4; 8 mg/kg) given intraperitoneally to mice had a marked hypothermic effect with a duration of more than 2 hours. Mecamylamine (0.5; 2.5 mg/kg) prevented the hypothermic effect of 2 mg/kg nicotine but had no effect on 4 mg/kg nicotine. Atropine (5 mg/kg) did not affect the hypothermia produced by nicotine. Other symptoms induced by nicotine (0.5-4 mg/kg) were convulsions, rigidity, tremor and decrease in motor activity. Pretreatment with mecamylamine (0.5 mg/kg) prevented convulsions but had only a slight effect on the decreased motor activity. Atropine pretreatment was ineffective. It thus seems as if the effects of nicotine at least to some part are mediated by nicotine-like receptors. When a tracer dose of labelled choline (3H-Ch) was given to mice treated with nicotine (4 mg/kg) a marked increase (+100%) in the biosynthesis of labelled acetylcholine (ACh) in the striatum was found when the animals were killed by decapitation. If the mice were killed by a more rapid technique, microwave irradiation of the head, no change in 3H-ACh formation was observed in comparison to controls. The findings indicate that nicotine can preserve a very labile pool of newly synthesized ACh in the striatum.

Fertility, maternal care, and offspring behavior in mice prenatally treated with tobacco smoke

Female mice from lines selectively bred for differences in open-field activity were exposed to tobacco smoke during gestation. Smoke-treated females were less likely than controls to have produced litters by 23 days after observation of a vaginal plug. Within the high-active line, fewer pups of smoke-treated dams survived to weaning. Regardless of treatment, fewer high-active than low-active offspring survived to weaning. Results of a 4-day series of open-field activity tests administered to offspring beginning at 28 days of age indicated that tobacco smoke administered prenatally and/or during testing depresses open-field activity in both lines. Other activity tests administered at 50 days of age gave similar results. Tissue nicotine levels after nicotine injection tended to be higher in high-active and control groups than in low-active and smoke-treated groups, respectively. Liver weight expressed as percentage of body weight was 11.9% greater in smoke-treated animals than in controls.