Effect of diphenylhydantoin on the uptake and catabolism of l-[3H]norepinephrine in vitro in rat cerebral cortex tissue

The effect of diphenylhydantoin on the accumulation of [³H]norepinephrine in vitro was examined in brain slices prepared from rat cerebral cortex. High concentrations of diphenylhydantoin (10^?3 M) caused a significant reduction in the 5-min accumulation of [³H]norepinephrine. On the other hand, 10^?5–10^?4 M diphenylhydantoin facilitated the 20-min accumulation of [³H]norepinephrine. This facilitative action of diphenylhydantoin was (1) associated with a reduction in oxidative catabolism of [³H]norepinephrine and (2) abolished by the 2-hr pretreatment of rats with 100 mg/kg of nialamide (i.p.). The inhibitory action of diphenylhydantoin on the oxidative catabolism of [³H]norepinephrine was observed in both whole and lyzed crude synaptosomal preparations. When diphenylhydantoin and pargyline were compared, it was found that pargyline $\text{(}\text{id}{}_{50}\text{= 1.5 × 10}{}^{\mathrm{?6}}\text{M}\text{)}$ was 37 times more effective than diphenylhydantoin $\text{(}\text{id}{}_{50}\text{= 5.5 × 10}{}^{\mathrm{?5}}\text{M}\text{)}$ in inhibiting the oxidative deamination of [³H]norepinephrine. These results suggest that diphenylhydantoin alters norepinephrine metabolism in cerebral cortex slices by an inhibitory action on (1) monoamine oxidase activity and (2) the neuronal uptake system.     

The inhibitory action of ketamine HC1 on [3H]5-hydroxytryptamine accumulation by rat brain synaptosomal-rich fractions: comparison with [3H]catecholamine and [3H]gamma-aminobutyric acid uptake.

Ketamine HO was incubated with synaptosomal-rich fractions prepared from rat cerebral cortex to evaluate the effect of this agent upon the synaptosomal accumulation of [³H]5-HT. Accumulation of [³H]5-HT was shown to be reduced by ketamine in a concentration-related fashion. This action of ketamine was also found in synaptosomal rich fractions prepared from hypothalamus, corpus striatum, medulla oblongata and midbrain. Accumulation studies carried-out in the presence of reserpine and pargyline indicated that ketamine reduced the accumulation of [³H]5-HT through a competitive action on the synaptosomal membrane high affinity transport system (neuronal reuptake system). The effect of ketamine on the high affinity transport of [³H]NE, [³H]DA and [³H]GABA was also examined. The order of inhibition of transport by ketamine was [³H]5-HT >$\text{[}{}^3\text{H}\text{]}\text{DA}\text{= [}{}^3\text{H}\text{]}\text{NE}\text{> [}{}^3\text{H}\text{]}\text{GABA}$. These results show that ketamine is a potent and preferential inhibitor of the 5-HT neuronal reuptake system. The possible role of this action of ketamine, in the post anaesthetic excitatory response seen following the administration of ketamine, is discussed.     

Mechanisms by which quipazine,a putative serotonin receptor agonist,alters brain 5-hydroxyindole metabolism

Quipazine (2-[1-piperazinyl] quinoline maleate) was shown to increase serotonin and decrease 5-hydroxyindoleacetic acid concentrations in whole brain, several brain regions, and the spinal cord of rats 1 hr after its administration (10 mg/kg, i.p.). In animals with transected spinal cords, quipazine induced stronger activation of extensor reflexes than 5-hydroxytryptophan, chlorimipramine, or Lilly 110140. This response could be blocked by methiothepin. In slices of rat cerebral cortex, quipazine inhibited the uptakes of [³H]-serotonin (EC₅₀ = 10^?6 M) and [³H]-norepinephrine ($\text{EC}{}_{50}\text{= 2 × 10}{}^{\mathrm{?6}}\text{m}$); it was equipotent with Lilly 110140 in inhibiting serotonin uptake, but less potent than chlorimipramine ($\text{EC}{}_{50}\text{= 10}{}^{\mathrm{?7}}\text{m}$). Quipazine administration to rats did not inhibit monoamine oxidase activity, and actually elevated brain tryptophan levels. These observations suggest that the effects of quipazine on brain serotonin and 5-hydroxyindoleacetic acid concentrations could have been caused by direct activation of central serotonin receptors (which would secondarily decrease impulse flow along serotonergic neurones), or by the inhibition of serotonin reuptake, or by both mechanisms.     

The Ah regulatory gene product. Survey of nineteen polycyclic aromatic compounds' and fifteen benzo[a]pyrene metabolites' capacity to bind to the cytosolic receptor

The capacity of 19 polycyclic aromatic compounds and 15 benzo[a]pyrene metabolites to displace $\text{[1,6-}{}^3\text{H}\text{]2,3,7,8-}\text{tetrachlorodibenzo}\text{-p-}\text{dioxine}\text{([}{}^3\text{H]Tcdd}\text{)}$ from the mouse liver cytosolic Ah receptor was examined. We compared our data with various parameters taken from previously published results: the capacity of seven polycyclic hydrocarbons to induce aryl hydrocarbon hydroxylase (AHH) activity in human cell cultures, the capacity of 10 polycyclic hydrocarbons to induce azo dye N-demethylase activity in rat liver, the capacity of 6 polycyclic hydrocarbons to shorten zoxazolamine paralysis times in the intact rat, and the capacity of 15 benzo[a]pyrene metabolites to induce AHH activity in rat hepatoma H-4-II-E cultures. An excellent correlation is seen between the capacity to displace the radioligand from the Ah receptor and the capacity to induce these monooxygenase activities. Differences in the rate of cellular uptake and formation of alkali-extractable metabolites of dibenzo[a,h]anthracene, 3-methylcholanthrene, and benzo[a]anthracene in Hepa-1 mouse hepatoma cell cultures do not account for differences in the capacity of these three polycyclic hydrocarbons to displace [³H]TCDD from the Ah receptor.     

The effects of cadmium, manganese and aluminium on sodium-potassium-activated and magnesium-activated adenosine triphosphatase activity and choline uptake in rat brain synaptosomes

The effects of Cd²⁺, Mn²⁺ and Al³⁺ on rat brain synaptosomal sodium-potassium-activated and magnesium-activated adenosine triphosphatase (Na-K-ATPase and Mg-ATPase) activity and choline uptake were studied. All three types of metal ions inhibited Na-K-ATPase activity more markedly than Mg-ATPase activity. The rank order of inhibition of Na-K-ATPase was: $\text{Cd}{}^{\mathrm{2+}}\text{(}\text{ic}{}_{50}\text{= 5.4 μ}\text{M}\text{) >}\text{Mn}{}_{\mathrm{2+}}\text{(}\text{ic}{}_{50}\text{= 955 μ}\text{m}\text{) >}\text{Al}{}^{\mathrm{3+}}\text{(}\text{ic}{}_{50}\text{= 8.3}\text{mM}$). The rank order of inhibition of Mg- was:$\text{Cd}{}^{\mathrm{2+}}\text{(}\text{ic}{}_{50}\text{= 316 μ}\text{M}\text{>}\text{Mn}{}^{\mathrm{2+}}\text{(}\text{ic}{}_{50}\text{= 5.5}\text{mM}\text{>}\text{Al}{}^{\mathrm{3+}}\text{(}\text{ic}{}_{50}\text{= 21.9}\text{mM}\text{).}\text{Al}{}^{\mathrm{3+}}$ was most potent in inhibiting synaptosomal choline uptake ($\text{ic}{}_{50}\text{= 24μ}\text{M}$ in the absence of Ca²⁺ and 123 μ.M in the presence of 1 mM Ca²⁺). $\text{Cd}{}^{\mathrm{2+}}\text{(}\text{ic}{}_{50}\text{= 363 μ}\text{M}\text{)}$ was a more effective inhibitor of choline uptake than $\text{Mn}{}^{\mathrm{2+}}\text{(}\text{ic}{}_{50}\text{= 1.2?1.5}\text{mM}\text{)}$ . The presence of 1 mM Ca²⁺ did not alter choline uptake, nor did it antagonize the inhibitory actions of the three metals. Our observations that Cd²⁺ and Al³⁺ inhibited synaptosomal choline uptake, but did not show parallel inhibitory effects on Na-K-ATPase activity directly contradicts the ionic gradient hypothesis. These results are also discussed in relation to the in vivo neurotoxicity of cadmium, manganese and aluminium.     

Determination of extraction constant,true partition coefficient and formation constant of ion-pair complexes of quaternary ammonium salts,tetrabutylammonium bromide and isopropamide iodide.with some organic anions by a solvent extraction technique

A new method of determining the extraction constant (K_e, the true partition coefficient (TPC) and the formation constant (K_f) of ion-pairs, was developed by the solvent extraction technique. K_e and TPC were estimated from the reciprocals of the intercept and the slope of the regression line obtained by plotting

$\text{B}{}^{\mathrm{T}}{}_{\mathrm{W}}\text{APC ? dA}\text{vs}\text{B}\text{B}{}^{\mathrm{T}}{}_{\mathrm{W}}\text{dA}\text{APC ? dA}\text{+}\text{A}{}^{\mathrm{T}}{}_{\mathrm{W}}\text{+}\text{B}{}^{\mathrm{T}}{}_{\mathrm{W}}$

in the following equation.

$\text{B}{}^{\mathrm{T}}{}_{\mathrm{W}}\text{APC ? dA}\text{=}\text{1}\text{K}{}_{\mathrm{e}}\text{+}\text{B}\text{B}{}^{\mathrm{T}}{}_{\mathrm{W}}\text{dA}\text{APC ? dA}\text{+}\text{A}{}^{\mathrm{T}}{}_{\mathrm{W}}\text{+}\text{B}{}^{\mathrm{T}}{}_{\mathrm{W}}\text{x}\text{1}\text{TPC}$

where [A^T_W] and [B^T_W] are the total concentrations of the cationic compound A and that of the anionic compound B in the aqueous phase respectively, APC is the apparent partition coefficient of A, dA is the partition coefficient of cation A⁺. K_f, which is expressed by K_e/TPC, was then calculated. These constants were determined for the ion-pair extraction of tetrabutylammonium bromide and isopropamide iodide with 4 organic anions, i.e. benzoic acid, p-toluenesulfonic acid, salicylic acid and taurodeoxycholic acid. This new method might be applicable to other ion-pairs without further assumptions except that the molar ratio of the ion-pair formation be 1 : 1.     

Mechanisms of accumulation of tyramine,metaraminol, and isoproterenol in isolated chromaffin granules and ghosts

The effects of the transmembrane pH gradient (ΔpH) and the transmembrane potential gradient (ΔΨ) on the uptake of several sympathomimetic amines were investigated, using bovine adrenal chromaffin granules isolated in isotonic sucrose. As previously described [R. Johnson and A. Scarpa, J. biol. Chem.254, 3750 (1979)], freshly isolated chromaffin granules maintain an intragranular pH of 5.5 as measured by [¹⁴C]methylamine distribution and, in the presence of ATP, generate a ΔΨ of 80 mV, positive inside, as measured by [¹⁴C]thiocyanate distribution. When tryamine, metaraminol, and isoproterenol (1–50 mM) were added to well-buffered suspensions of granules at pH 7.0, a dose-related alkalinization of the granule interior was observed. Study of the time-resolved influx of the same amines labeled radiochemically (5–21 μM) revealed that all the amines were accumulated against an apparent concentration gradient. However, while accumulation of [¹⁴C]serotonin and [³H]isoproterenol was totally inhibited by reserpine, [¹⁴C]tryramine accumulation was inhibited by only 60% and [¹⁴C]metaraminol uptake was unaffected. The ATP-dependent generation of a ΔΨ produced a stimulation of amine uptake in the order: serotonin > isoproterenol > tyramine; metaraminol accumulation was not enhanced by ATP addition. The relationship between the electrochemical proton gradient $\text{(Δ}\text{}\text{?}\text{gm}{}_{\mathrm{<mtext>H</mtext><mtext>+</mtext>}}\text{)}$ and the electrochemical gradient for each of the sympathomimetic amines $\text{(Δ}\text{}\text{?}\text{gm}{}_{\mathrm{<mtext>A</mtext>}}\text{)}$ was investigated utilizing chromaffin ghosts devoid of endogenous matrix gradients or components. All amines were accumulated in the presence of ΔpH alone. In the presence of ΔΨ alone, [¹⁴serotonin, [¹⁴C]tyramine, and [³H]isoproterenol were accumulated, but no [³H]metaraminol uptake was demonstrable. The results indicate that serotonin and isoproterenol accumulated in isolated chromaffin granules and ghosts via a reserpine-sensitive mechanism, driven by the magnitude of the electrochemical proton gradient. Conversely, metaraminol permeated the membrane of the chromaffin granule through the apolar lipid phase and distributed according to the ΔpH alone. Tyramine uptake proceeded by both mechanisms. The implications of the mechanism of accumulation of these potent physiologic and pharmacologie agents for their in vivo action are discussed.     

Effects of haloperidol and apomorphine on catecholamine metabolism in brain slices: Reserpine-like effects of haloperidol

The accumulation, release and catabolism of [³H]dopamine (DA) and [³H]norepinephrine (NE) synthesized from [³H]tyrosine were measured in mouse striatal and substantia nigral slices. Apomorphine inhibited both [³H]NE and [³H]DA accumulation $\text{(}\text{ic}{}_{50}\text{< 10}{}^{\mathrm{?6}}\text{M}\text{)}$, presumably by acting on a presynaptic receptor. Haloperidol (10^?8 M) caused a small, but significant increase in [³H]DA accumulation from [³H]tyrosine in the presence of 26 mM K⁺, possibly reflecting blockade of presynaptic receptors activated by released DA. However, at higher concentrations (10^?6 to 10^?5 M), haloperidol inhibited [³H]DA and [³H]NE accumulation. Reserpine also potently inhibited catecholamine synthesis; chlorpromazine had only a weak effect, and fluphenazine was ineffective. Both haloperidol (10^?5 M) and reserpine (10^?7 M), but not chlorpromazine and fluphenazine, markedly increased the formation of labeled dihydroxyphenylacetic acid (DOPAC) and increased the spontaneous release of labeled DA from striatal slices preloaded with [³H]tyrosine or [¹⁴C]DA. These data suggest that haloperidol has some direct effects on DA metabolism that are unrelated to DA-receptor blockade. Because the effects of haloperidol are apparently independent of DA release, haloperidol may elevate cytoplasmic DA by altering its vesicular storage. This would, in turn, increase the spontaneous release of labeled DA by diffusion, the oxidation of DA to DOPAC by monoamine oxidase, and the end-product inhibition of tyrosine hydroxylase.     

Effects of catecholamine releasing agents on synaptosomal dopamine biosynthesis: Multiple pools of dopamine or multiple forms of tyrosine hydroxylase?

The effects of agents, which are known to induce release of catecholamines from synaptosomes, were assessed on the synthesis of dopamine from tyrosine, as reflected in the evolution of ¹⁴CO₂ from L-[1-¹⁴C]-tyrosine, in intact rat striatal synaptosomes. At a time when release had occured, whereas reserpine inhibited the synthesis of dopamine from tyrosine, with an ED₅₀ of $\text{1 × 10}{}^{\mathrm{?8}}\text{M}$, tyramine (ED₅₀ of $\text{1 × 10}{}^{\mathrm{?5}}\text{M}$) and (+)-amphetamine (ED₅₀ of $\text{1·4 × 10}{}^{\mathrm{?6}}\text{M}$) enhanced the rate of synthesis. The presence of nialamide (10^?4M) or pargyline (10^?3M) had no effect on synaptosomal dopamine synthesis in the absence or presence of amphetamine, tyramine, or reserpine. Neither reserpine, tyramine, nor amphetamine effected the activity of tyrosine hydroxylase or DOPA decarboxylase in the absence of synaptosomal structural integrity. Nor did these drugs effect the accumulation of [³H]-tyrosine into synaptosomes. The data are consistent with the existence of at least two pools of synaptosomal dopamine, one of which can interact with tyrosine hydroxylase. Two hours after pretreatment of rats with 5 mg/kg (+)-amphetamine, the level of synaptosomal dopamine biosynthesis was decreased by 39%. The rate of dopamine synthesis in synaptosomes from amphetamine-pretreated rats was assessed in the presence of reserpine and tyramine. The data are not consistent with alterations in pool size being the only mechanism affecting synaptosomal dopamine synthesis. A mechanism is discussed involving an equilibrium of tyrosine hydroxylase between active and inactive conformers in the presence of an inhibitory pool of dopamine.     

Inhibition of neuronal uptake of 3H-biogenic amines into rat cerebral cortex by partially and fully saturated derivatives of imipramine and desipramine. The importance of the aromatic ring in adrenergic amines--part 3.

In order to assess the importance of the aromatic rings in inhibition of neuronal amine uptake produced by tricyclic antidepressant drugs, derivatives of imipramine (IMI) and dcsipramine (DMI) were prepared in which either one (IMIH, DMIH) or both (IMIH2, DMIH2) of the aromatic rings were fully reduced to cyclohexane rings. The relative abilities of these compounds to inhibit the uptake of $\text{l-[}{}^3\text{H}\text{]-}\text{norepinephrine}$ (NE), [³H]-dopamine (DA) and [³H]-5-hydroxytryptamine (5-HT) into chopped tissue of rat cerebral cortex were determined. Reduction of one or both aromatic rings did not alter significantly the inhibition of uptake of [³H]-DA or [³H]5-HT produced by either IMI or DMI (IC₅₀ values 25–80 μM). However, saturation of one or both rings abolished the selectivitv of DMI for inhibition of NE uptake (IC₅₀ 0.12 μM). decreasing potency 150-fold (IC₅₀ 18.3 μM) and 250-fold (IC₅₀ 29.4 μM) respectively. The effect of aromatic ring reduction on the IMI-induced inhibition of NE uptake was much less pronounced. The results suggest that hydrophobic rather than π-electron attractive forces are involved in the interaction of DMI or IMI with DA or 5-HT uptake sites. However, the loss in selectivity for inhibition of NE uptake upon reduction of DMI may reflect loss of π-electron interactions in the binding of DMI to the NE uptake site, or may reflect increased sensitivity to spatial disposition of the hydrophobic binding areas of the drug relative to that found at the DA or 5-HT amine uptake sites.     

Adenosine receptor interactions and anxiolytics

$\text{[}{}^3\text{H}\text{]-N}{}^6\text{-}\text{cyclohexyladenosine}$ and [³H]-1,3-diethyl-8-phenylxanthine label the A₁ subtype of adenosine receptor in brain membranes. The affinities of methylxanthines in competing for A₁ adenosine receptors parallel their potencies as locomotor stimulants. The adenosine agonist N⁶-(-phenylisopropyl) adenosine is a potent locomotor depressant. Both diazepam and $\text{N}{}^6\text{-(}\text{l}\text{-}\text{phenylisopropyl}\text{)}\text{adenosine}$ cause locomotor stimulation in a narrow range of subdepressant doses. Combined stimulant doses of the two agents depress motor activity, as do larger doses of either one, given separately.Evidence supporting and against the hypothesis that some of the actions of benzodiazepines are mediated via the adenosine system is reviewed. A number of compounds interact with both systems, probably because of physico-chemical similarities between adenosine and diazepam. It is concluded that of the four classic actions of benzodiazepines, the sedative and muscle relaxant (but not anxiolytic or anticonvulsant) actions could possibly be mediated by adenosine.     

Kinetic studies on the metabolism of ethylmorphine by isolated hepatocytes from adult rats

Hepatocytes of adult rats were isolated by infusion of a hyaluronidase collagenase mixture. High yields of cells excluding trypan blue were obtained. These cells, in Hank's buffer containing rat serum and 0.1% glucose, $\text{N-}\text{demethylate}\text{[}{}^3\text{H-CH}{}_3\text{-N]}$ethylmorphine. The formaldehyde initially formed is further metabolized to tritiated water. Fifteen per cent of the original metabolic activity was observed after 21 hr at 37° in 5% CO₂-air, and cumulative metabolism is linear for up to 90 min under these conditions. The K_m for the N-demethylation of [³H-CH₃-N]ethylmorphine is 50 μM, 20 per cent of the value observed for this reaction by musomal preparations. An active transport of the substrate into the cell is postulated to account for this difference.     

A fluorescent analogue of methotrexate as a probe for folate antagonist molecular receptors

A dansyl-l-lysine analogue of methotrexate, $\text{N}{}^{\mathrm{\alpha }}\text{-(}\text{4-amino-4-deoxy-10-methylpteroyl}\text{)-N}{}^{\mathrm{\epsilon }}\text{-(5-[N,N-}\text{dimethylamino]-1-naphthalenesulfonyl]-1-naphthalenesulfonyl)-}\text{l}\text{-lysine}$, is a potent inhibitor of murine L1210 dihydrofolate reductase. The dansyl fluorescence emission was enhanced approximately 3-fold with a 10 nm blue shift upon binding to L1210 dihydrofolate reductase. The fluorescent analogue was only 10-fold less potent than methotrexate in inhibiting the growth of methotrexate-sensitive and -resistant L1210 cells and competes effectively for [³H]methotrexate transport with a K_i of 7.02 μM, a value virtually identical to the K_t for methotrexate in both cell lines. In addition, strong dansyl fluorescence was found to be associated with dihydrofolate reductase from methotrexate-resistant, dihydrofolate reductase-overproducing L1210 cells following incubation of viable cells with the fluorescent methotrexate analogue for 4 hr. The results demonstrate that the dansyl-l-lysine analogue of methotrexate was rapidly transported into L1210 cells where it formed a high-affinity, fluorescent complex with intracellular dihydrofolate reductase.     

Extrapineal amine N-acetylation in rat brain: Regional and subcellular distribution and enzyme kinetics

Analysis by thin-layer chromatography showed that $\text{[}{}^{14}\text{C}\text{]N-}\text{acetyltryptamine and}\text{[}{}^{14}\text{C}\text{]N-}\text{acetyl}\text{-β-}\text{phenylethylamine}$ are formed from the incubation of [¹⁴C]acetyl-CoA with tryptamine or β-phenylethylamine. respectively, in the presence of rat brain extracts. The specific activity of the N-acetyltransferase in fifteen discrete regions of rat CNS ranged from 1.64 ± 0.05 nmoles of product formed/mg of protein/hr in cerebellum to 0.57 ± 0.05 nmole in occipital cortex with tryptamine as substrate, and from 2.80 ± 0.30 nmoles in cerebellum to 0.91 ± 0.13 nmole in cervical cord with β- phenylethylamine as substrate. Comparison of the regional specific activities in the presence of the respective substrates yielded a correlation coefficient of 0.83 (P < 0.01). In cerebellum N-acetyltransferase activity appears exclusively in cytosol. At two stages of purification (i.e. after Bio-Gel fractionation as well as after ammonium sulfate precipitation), the enzyme exhibited biphasic kinetics with respect to acetyl-CoA in the presence of tryptamine or β-phenylelhylamine and with respect to either substrate in the presence of acetyl-CoA.     

Dansylarginine N-(3-ethyl-1,5-pentanediyl)amide: A potent and selective fluorescent inhibitor of butyrylcholinesterase

Interactions between dansylarginine N-(3-ethyl-1,5-pentanediyl)amide (DAPA) and the cholinesterases were examined by the techniques of enzyme kinetics and fluorescence spectroscopy. When tested with partially purified enzyme preparations, DAPA was a potent inhibitor of butyrylcholinesterase $\text{(}\text{IC}{}_{50}\text{= 2 × 10}{}^{\mathrm{?7}}\text{M}\text{)}$ but not of acetylcholinesterase $\text{(}\text{IC}{}_{50}\text{= 4 × 10}{}^{\mathrm{?4}}\text{M}\text{)}$. For a detailed study of the effects of DAPA on butyrylcholinesterase (BuChE), the enzyme was purified to homogeneity from horse serum, with the aid of affinity chromatography on N-methyl acridinium. The kinetics of the inhibition of purified BuChE by DAPA were complex, having both competitive and non-competitive features, and it was not possible to estimate K_i unambiguously. Spectroscopic measurements showed that the fluorescence of the dansyl moiety was strongly affected by the binding to BuChE. With excitation at 330 nm, total fluorescence emission from bound DAPA (at 450 nm and above) was 21-fold greater than from free DAPA. In a titration experiment, this enhancement of fluorescence intensity was used to calculate that each monomer of BuChE has two apparently independent DAPA-binding sites with a K_d of 4.5 × 10^?7 M. Further measurements showed that the fluorescence emission of bound DAPA was markedly blue-shifted (to 502 nm from 570 nm in free solution) and that the fluorescence lifetime of this form was greatly prolonged (to 24 nsec from 2.7 nsec). These observations indicate that the high affinity binding sites on BuChE lock DAPA in a highly non-polar environment.     

Effects of exposure to ozone on susceptibility to experimental tuberculosis

Exposure of mice to 1.96 $\text{mg}\text{m}{}^3$ ozone (O₃) 3 $\text{h}\text{day}$, 5 days/week, for up to 8 weeks beginning at 1 or 2 weeks after challenge with Mycobacterium tuberculosis R 1Rv resulted in significant enhancement of bacterial titers in the lungs at 5 through 8 weeks after challenge when compared to mice exposed to filtered air. Exposure to lower concentrations of O₃ did not produce any significant changes compared to controls.Exposure of guinea pigs to 2.9 $\text{mg}\text{m}{}^3$ O₃ for 3 h immediately after challenge with M. tuberculosis resulted in a suppression of the cutaneous delayed hypersensitivity response, without affecting the serum hemagglutination antibody titers. However, exposure of guinea pigs to 0.98 $\text{mg}\text{m}{}^3$ O³ 3 $\text{h}\text{day}$ for 5 days, initiated within 3 h after the infectious challenge, enhanced hemagglutination antibody titers initially, but the delayed hypersensitivity reaction did not differ from controls.