Assumption Of First-order Kinetics Research Articles

Driving under the influence of zopiclone: Elimination between two consecutive blood samples

AimZopiclone is a widely used hypnotic drug which is frequently detected in apprehended drivers. For assessments in forensic cases, the elimination half-life (t1/2) of a drug is sometimes important. A t1/2 of 3.5–6.5 h for zopiclone is previously reported in healthy individuals, but different factors like age and drug-interactions can affect the t1/2 of zopiclone. The aim of this study was to describe concentrations of zopiclone and co-ingestion of additional drugs in apprehended drivers, and to investigate the t1/2 of zopiclone based on two consecutive blood samples. MethodsData was collected from apprehended drivers in Norway between 2003 and 2021. All cases where zopiclone was detected were included. In a subset of the material, two consecutive whole blood samples were collected ≥ 20 and < 60 min apart. Concentrations of zopiclone in blood were determined by LC-MS or UHPLC-MS/MS. The elimination and t1/2 of zopiclone was estimated from the concentration change of zopiclone and the time interval between the two consecutive blood samples, under the assumption of first order kinetics. ResultsThe median concentration among all zopiclone positive cases was 0.044 mg/L (IQR 0.070 mg/L) (n = 2401). The most frequent additional drugs detected were ethanol (36%), diazepam (22%), amphetamine (14%) and THC (14%). In zopiclone-only cases (n = 364), the median concentration of zopiclone was 0.066 mg/L (IQR 0.115 mg/L). In 112 cases, two consecutive blood samples were collected. Of these, 28 cases showed increasing concentrations of zopiclone between the two sampling time points. Among the cases in which the concentration decreased (n = 84), the median C1 was 0.048 mg/L (IQR 0.062 mg/L) and the median C2 was 0.043 mg/L (IQR 0.056 mg/L). A Bayesian statistical model was used to obtain the posterior distribution of t1/2. The posterior median of t1/2 was estimated to 3.1 h (IQR=0.39 h) when including only the cases showing decreasing concentrations, and this increased to 3.8 h (IQR=0.52 h) when also including samples showing non outlying increase in concentrations. There was no statistically significant gender difference in the calculated half-lives (two-sided Mann-Whitney U test, p = .525). ConclusionsThis study showed that zopiclone is frequently detected in apprehended drivers in supra therapeutic concentrations and poly drug cases. The elimination of zopiclone in blood from two consecutive blood samples indicated an apparent t1/2 of between 3.1 and 3.8 h, which is within the lower range of what previous experimental studies on healthy individuals have reported.

ESR thermochronometry of Al and Ti centres in quartz: A case study of the Fergusons Hill-1 borehole from the Otway Basin, Australia

Electron spin resonance (ESR) thermochronometry can in principle be used for reconstructing thermal histories of the upper ~ 2 km of the Earth's crust. We have analysed the ESR ages of Al and Ti centres in quartz from Fergusons Hill-1 borehole in the Otway Basin, Australia, to assess their trap parameters under natural conditions. The apparent ESR ages of both centres decrease with increasing temperature (42.9–56.2 °C). A feasible sequential quadratic programming optimisation algorithm was applied to determine the optimal trap parameters based on the assumption of first order kinetics and thermally static condition. The estimated activation energy and frequency factor of the Al centre in quartz were 1.5 eV and 1.1×1011 s−1, respectively. This was in excellent agreement with a re-analysis of samples from the Eldzhurtinskiy granite, a highly dynamic geologic system, for which we obtained 1.5 eV and 1.0×1011 s−1. The trap parameters of the Ti centre could not be obtained from first order kinetics for the Fergusons Hill samples, but the samples from the Eldzhurtinskiy yielded 1.6 eV and 2.4×1011 s−1. Further studies on trap kinetics under better controlled laboratory conditions are required for ESR thermochronometry.

Pyrolysis kinetics of Hibiscus rosa sinensis and Nerium oleander

ABSTRACTPyrolysis is a thermal method in which volatile matter present in a biomass will be degraded to lower molecular substances. This study aims at evaluation of kinetics of lignocellulosic biomass of leaves and stems of Hibiscus rosa sinensis and Nerium oleander under pyrolytic conditions, their proximate and ultimate analyses and their calorific values. Pyrolysis experiments were carried out using a thermogravimetric balance, which combines heat flux type differential thermal analysis (DTA) with thermogravimetric analysis (TGA) at different heating rates under a nitrogen atmosphere. The results were analyzed using weight loss vs. temperature and derivative weight vs. temperature profiles obtained from DTA and TGA curves. Three kinetic stages apart from moisture evaporation were observed based on the type of biomass. Kinetic parameters such as the pre-exponential factor and activation energy for each degradation step were calculated using first-order kinetics. The assumption of first-order kinetics was also proved from the experimental data. The results showed that Nerium oleander leaves are more suitable for fuel production through pyrolysis due to their higher percentage of volatile matter, low sulfur content, lower ash content, higher calorific value and lower activation energy required to decompose constituents of biomass, namely hemicellulose, cellulose and lignin.

Effectiveness Factors and Conversion in a Biocatalytic Membrane Reactor.

Analytical expressions of the effectiveness factor of a biocatalytic membrane reactor, and its asymptote as the Thiele modulus becomes large, are presented. The evaluation of the effectiveness factor is based on the solution of the governing equations for solute transport in the two regions of the reactor, i.e. the lumen and the matrix (with the biofilm immobilized in the matrix). The lumen solution accounts for both axial diffusion and radial convective flow, while the matrix solution is based on Robin-type boundary conditions. The effectiveness factor is shown to be a function of the Thiele modulus, the partition coefficient, the Sherwood number, the Peclet number, and membrane thickness. Three regions of Thiele moduli are defined in the effectiveness factor graphs. These correspond with reaction rate limited, internal-diffusion limited, and external mass transfer limited solute transport. Radial convective flows were shown to only improve the effectiveness factor in the region of internal diffusion limitation. The assumption of first order kinetics is shown to be applicable only in the Thiele modulus regions of internal and external mass transfer limitation. An iteration scheme is also presented for estimating the effectiveness factor when the solute fractional conversion is known. The model is validated with experimental data from a membrane gradostat reactor immobilised with Phanerochaete chrysosporium for the production of lignin and manganese peroxidases. The developed model and experimental data allow for the determination of the Thiele modulus at which the effectiveness factor and fractional conversion are optimal.

Pyrolysis characteristics of a North Korean oil shale

Pyrolysis characteristics of a North Korean oil shale and its pyrolysates were investigated in this paper. The pyrolysis experiments were conducted below 600 °C at a heating rate of 10, 15, 20 and 25 °C/min, respectively. The kinetics data were calculated using both integral and differential methods with the assumption of first order kinetics. The results show that the averaged oil content of the North Korean oil shale is about 12.1 wt% and its heat value is 13,400 kJ/kg. The oil yields at different retorting temperatures show that the higher the retorting temperature the greater the oil and retorting gas yields. The optimal retorting temperature for the North Korean oil shale is about 500 °C. The properties of the North Korean shale oil including density, viscosity, flash point and freezing point are found to be relatively low compared with those of shale oil from FuShun, China. The gasoline fraction, diesel fraction and heavy oil fraction account for 11.5 wt%, 41.5 wt% and 47 wt%, respectively. The major pyrolysis gases are CH4 (the most abundant), H2, CO2, H2S, CO, and C2–C5 hydrocarbons. The heat value of retorting gas is more than 900 kJ/mol, and the retorting gas has high sulfur content.

Identification of bound nitro musk-protein adducts in fish liver by gas chromatography–mass spectrometry: Biotransformation, dose–response and toxicokinetics of nitro musk metabolites protein adducts in trout liver as biomarkers of exposure

Ubiquitous occurrences of synthetic nitro musks are evident in the literature. The in vivo analysis of musk xylene (MX) and musk ketone (MK)-protein adducts in trout liver has been performed by gas chromatography–mass spectrometry using selected ion monitoring (GC–SIM-MS). Biotransformation, dose–response and toxicokinetics studies of 2-amino-MX (2-AMX), 2-amino-MK (2-AMK) and 4-amino-MX (4-AMX) metabolites, covalently bound to cysteine amino acids in proteins in fish liver, formed by enzymatic nitro-reduction of MX and MK, have been described. Trouts were exposed to single exposures of 0.010, 0.030, 0.10, and 0.30 mg/g MX and/or MK. Forty-two fish liver samples were collected from exposed- and control-fish subsequent to exposure intervals of 1 day, 3 days, and 7 days and were composited as per exposure schedules and times. Alkaline hydrolysis released bound metabolites from exposed liver composites that were extracted into n-hexane and then concentrated and analyzed by GC–SIM-MS. The presence of the metabolites in liver extracts was confirmed based on agreement of similar mass spectral properties and retention times with standards. In the dose–response study, the maximum adduct formation was 492.0 ng/g for 2-AMX, 505.5 ng/g for 2-AMK and 12588.5 ng/g for 4-AMX in liver at 0.03 mg/g MX and MK fish in 1 day after exposure. For toxicokinetics investigation, the highest amount of the target metabolites was found to be the same concentration as observed in the dose–response study for 1 day after exposure with 0.03 mg/g MX and MK fish and the half-lives of the metabolites were estimated to be 2–9 days based on assumption of first-order kinetics. Average recoveries exceeded 95% with a relative standard deviation (RSD) around 9%, and the limit of detection (LOD) ranged from 0.91 to 3.8 ng/g based on a signal to noise ratio of 10 ( S/ N = 10) could be achieved for the metabolites. No metabolites were detected in the controls and exposed non-hydrolyzed liver extracts. This is the first report on dose–response and toxicokinetics of nitro musk-cysteine-protein adducts in fish liver.

Dissolution of Microcrystalline Cellulose in Phosphoric Acid—Molecular Changes and Kinetics

In this study, we aimed to dissolve microcrystalline cellulose (MCC) with phosphoric acid to obtain high-quality fermentable saccharides. MCC was directly dissolved in phosphoric acid (the concentration was 83%) for 10 hours at temperatures of 30, 50, and 70 °C. The structural changes of MCC were determined in detail with X-ray powder diffraction, solid-state cross-polarization magic angle spinning 13C-NMR, and X-ray photoelectron spectroscopy. The kinetics of MCC decrystallization during treatment with phosphoric acid was also compared at 30, 50, and 70 °C. With the assumption of first order kinetics, the Arrhenius parameters of K, A0 and Ea were calculated. The rate constants of decrystallization reaction (K) were 0.06, 0.17, and 0.12 h-1 respectively. The pre-exponential factor (A0) was 1.2 × 106 h-1, and the activation energy (Ea) was 42.4 k J/mol.

A kinetic model for predicting biodegradation

Biodegradation plays a key role in the environmental risk assessment of organic chemicals. The need to assess biodegradability of a chemical for regulatory purposes supports the development of a model for predicting the extent of biodegradation at different time frames, in particular the extent of ultimate biodegradation within a ‘10 day window’ criterion as well as estimating biodegradation half-lives. Conceptually this implies expressing the rate of catabolic transformations as a function of time. An attempt to correlate the kinetics of biodegradation with molecular structure of chemicals is presented. A simplified biodegradation kinetic model was formulated by combining the probabilistic approach of the original formulation of the CATABOL model with the assumption of first order kinetics of catabolic transformations. Nonlinear regression analysis was used to fit the model parameters to OECD 301F biodegradation kinetic data for a set of 208 chemicals. The new model allows the prediction of biodegradation multi-pathways, primary and ultimate half-lives and simulation of related kinetic biodegradation parameters such as biological oxygen demand (BOD), carbon dioxide production, and the nature and amount of metabolites as a function of time. The model may also be used for evaluating the OECD ready biodegradability potential of a chemical within the ‘10-day window’ criterion.

TL Dating Technique Based on a Trap Model and its Application as a Geochronometer for Granitic Quartz

A trap model is introduced to describe the behaviours of both thermally sensitive and radiation sensitive TL traps. The former are relatively shallow traps. The latter are deep traps, in which population increases with exposure to alpha dose. Thermal decay of both types of traps at ambient temperature is dependent on the trap lifetimes. A trap's population can be measured as TL sensitivity to a laboratory test dose. The trap model has been supported by observations of age dependent TL signals from granitic quartz samples with different crystallisation ages. The trap lifetimes are from 1.98 x 109 to 5.36 x 1015 years estimated using the isothermal decay experiment with the assumption of first order kinetics. Dating techniques are proposed based on the trap model. For old granites (>400 Ma), ages can be obtained by measuring the total exposed alpha dose using the additive alpha dose method, whereas for young granites (<400 Ma), ages can also be obtained by interpolating the TL sensitivity to a curve of TL sensitivities for known ages.

Isotopic Fractionation of Oxygen During Sediment Respiration

The apparent community respiration fractionation factor for deep-sea sediments has a predicted value of 9%0 (Bender, 1990). This value is based on a model of sediment oxygen consumption that satisfies the assumption of first order kinetics with respect to carbon (i.e. a pore water oxygen profile which decreases exponentially with depth below the sediment-water interface). This implies that the organic matter oxidation rate is carbon limited. Bender suggests the geometry of the sediments (a partially-closed system) prevents the expression of the full microbial respiration fractionation which has been determined to be 19.1%o (cz = 0.980) for freeliving bacteria (Kiddon et al., 1993). The apparent fractionation in sediments (9%0 or et = 0.991) has a square-root relation to the fractionation factor for bacterial cultures as a result of the assumption of an exponential oxygen decrease in the sediments. Brandes and Devol (1996) have presented oxygen isotope data for benthic chamber deployments in Puget Sound, Washington. Their results indicate an apparent sedimentary respiration fraction factor of 3%0 (~ = 0.997). The Puget Sound sediments are sites of rapid organic matter diagenesis and complete oxygen consumption within the top centimeter of the sediments. Brandes and Devol argued that the apparent fractionation is dramatically reduced from Bender's value of 9%0 (or from the free-living bacterial fractionation of 18%o) due to complete oxygen removal and due to the diffusive limitation present in a sedimentary system. Their model parameterizes intense diagenesis in discrete sediment micro-sites wherein oxygen is completely consumed. The apparent fractionation is small because oxygen concentrations go to zero at the micro-site. They suggest this small fractionation arises as a result of oxidant limitation. When the diffusive oxygen flux is insufficient to balance the respiratory demand all oxygen molecules are consumed and there is no distinction between the heavy and the light isotopes.

Thermal Inactivation Kinetics of Three Vegetative Bacteria as Influenced by Combined Temperature and pH in a Liquid Medium

The kinetics of thermal inactivation as affected by combined temperature and liquid pH for three vegetative bacteria— Escherichia coli (ATCC 25922), Listeria monocytogenes (SLCC 5764) and Pseudomonas fluorescens (172)— have been studied using published bench-scale data and additional experimentally determined data from the heating of samples in ampoules of thin-walled glass. These bacteria represent common micro-organisms known to grow in solid and liquid foods. Up to six levels of temperature (52, 54, 56, 58, 60 and 62° C) in combination with up to eight levels of pH (4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0 and 7.5) with exposure times ranging from 10 seconds to five minutes, were used in experimental designs to cover the wide biokinetic range of interest. The carrier liquid, a 2 kg m −3 mucilage of Carbopol ® 934 was selected because of the stability of its viscosity over the range of temperature and pH values, and its resistance to bacterial growth. The viscosity of this mucilage closely simulates that of a range of liquid foods. The effect of pH on the rate of thermal inactivation was significant for all three bacteria, especially at the lower exposure temperatures. However, because survivor data showed tailing with longer exposure times, the widely held assumption of first-order kinetics for thermal inactivation of vegetative bacteria is not supported. Concave-up tails appear in the data for P. fluorescens and E. coli and both concave-up and concave-down tails appear in the data for L. monocytogenes . Taken together, data for all three bacteria support the necessity for a model formulation for non-linear survivor kinetics as influenced by combined exposure temperature and liquid pH.

ELIMINATION OF β-HEXACHLOROCYCLOHEXANE IN OCCUPATIONALLY EXPOSED PERSONS

The elimination offi-hexachlorocyclohexane ($-HCH) in humans was investigated in a group of 40 former workers of a lindane-producing plant by analyzing at least 2 blood specimens (3 specimens in 3 workers) from different time points. Assuming a first-order kinetic model for excretion, the median half-life of$-HCH is 7.2 yr calculated by concentrations in whole blood and 7.6 yr calculated by concentrations in extractable lipids. In univariate analyses an influence of age, percent body fat, and liver disease (additionally in whole blood an influence of contents of extractable lipids) on clearance was observed. All factors show a positive correlation with half-life. According to a multiple regression model, influence of percent body fat calculated according to Deurenberg et al. (1991) is an important covariate in the description of the variations of the clearance rate (calculated on the basis of extractable lipids) of$-HCH. The data support the assumption of first-order kinetics.

A kinetic study of diffusion in the electrothermal atomizer with a graphite filter by laser excited atomic fluorescence

Laser excited atomic fluorescence-electrothermal atomizer (LEAFS-ETA) was used to study atomization and diffusion mechanisms in a novel diffusive graphite tube atomizer. The atomizer design included a hollow graphite cylinder mounted between two graphite rods which served as electrodes. One of the rods had a small graphite insert with a sampling hollow and could move backwards and forwards. After the sample was introduced into the hollow, the electrodes tightly sealed the graphite cylinder ensuring that the insert was directly in the center of the furnace. The furnace assembly was then heated and the vaporized sample diffused through the hot graphite wall. The atomic fraction of the sample vapor was excited by a laser beam which was directed along the graphite tube surface so that no gap remained between the beam and the tube surface. Fluorescence vs. time profiles for three elements — Cu, Ag and Ni — were obtained within the temperature range of 1400–2600 K. The rate constants of specific processes were measured from the decay portions of the fluorescence signals under the assumption of first-order kinetics. The Arrhenius plots were constructed and the activation energies, E a were evaluated from their slopes. The plots obtained for Cu and Ag consisted of two linear parts, the corresponding values of E a were: 195 kJ/mol and 77 kJ/mol for Cu (1550 K < T < 2600 K) and 238 kJ/mol and 97 kJ/mol for Ag (1430 K < T < 2280 K). The Arrhenius plot for Ni was linear within the temperature range of 1770–2530 K resulting in an E a equal to 161 kJ/mol. The diffusion coefficients were evaluated on the basis of a steady-state diffusion model out of a hollow cylinder. The values for the diffusion coefficients were: 3.7·10 −4−2.0·10 −3 cm 2/s (1750–2600 K) for Cu, 6.5·10 −3−1.4·10 −3 cm 2/s (1750–2280 K) for Ag and 5.6·10 −5−1.5·10 −3 cm 2/s (1770–2530 K) for Ni.

Non-reversible charge transfer and mass transfer: Methodology for experimental characterization without theoretical models

Abstract An extension of the methodology based on the concept of mass transfer operator to non-reversible (i.e. quasi-reversible or irreversible) charge transfer is proposed. We show how it is possible to characterize the charge and mass transfer steps, without postulating a modelling equation to describe the behaviour (except a very general assumption of first-order kinetics). The concept of mass transfer operator, which can be written using the convolution formalism, enables us to analyse separately both mass and charge transfer. Using this method, the characteristic parameters of electron transfer can be measured under known mass transport conditions. Conversely, a reverse approach based on the same concepts enables the characterization of any mass transfer associated with the same redox reaction. This new approach proves itself very attractive when no mathematical model is available to describe the electrochemical behaviour of a complex redox system. An example of an application given here is the study under galvanostatic conditions of the Fe ( CN ) 4− 6 Fe ( CN ) 3− 6 redox couple in aqueous solution; our results can be compared with the literature data.

Dechlorination Rates of Polyvinyl Chloride with Water and Aqueous Solutions of Organic Solvents.

Polyvinyl chloride (PVC) pellets were subjected to dechlorination in a batch reactor with, separately, water and aqueous solutions respectively of tetrahydrofuran, cyclohexanone and methanol. In the temperature range of 230-280°C, dechlorination rate constants were obtained under the assumption of first-order kinetics with respect to the chlorine content in the solid sample. The rate constant for dechlorination with water (1ml) + THE (1ml) was found to be nearly three times that with water, although they have almost the same activation energy, 205kJ/mol.

A pheromone-degrading aldehyde oxidase in the antennae of the moth Manduca sexta

Antennae of the tobacco hornworm moths Manduca sexta contain an aldehyde oxidase (AOX) that oxidizes aldehydes to carboxylic acids. The enzyme, which is distinguishable from aldehyde-oxidizing activities in other tissues, is secreted into the receptor lymph that bathes the primary olfactory dendrites. First detectable about 3 d before eclosion, AOX levels increase through the first day after eclosion. This parallels the development of the antennal responsiveness to bombykal (a male attractant aldehydic pheromone produced by female M. sexta) and trans-2-hexenal (an aldehyde commonly found in leaves). The AOX is about 60% more abundant in antennae of males than in antennae of females. The antennal AOX is a dimer with Mr of 295 kDa and is capable of oxidizing a variety of aldehydes. Of all aldehydes examined, the pheromone bombykal was the best substrate with an apparent Km of 5 microM, whereas the next best substrate, benzaldehyde, had an apparent Km of 255 microM. Using kinetic parameters estimated in vitro and the assumption of first-order kinetics, the half-life of bombykal in sensilla was estimated to be about 0.6 msec. The affinity of the antennal AOX for bombykal, its location in the receptor lymph, and its pattern of developmental expression all suggest that it plays a role in modulating the sensitivity of adult M. sexta to aldehyde odors and, in particular, the sensitivity of males to the pheromone bombykal.

On the identification of a middle-large dynamic system for IN VIVO drug distribution and metabolism: comparison of three optimization routines

To identify the fate of the allticancer drug daunomycin (DAU) in the rat the distribution of DAU and the formation of its metabolite daunomycinol (DOL) are described with first order processes in a double seven-compartment model. For DAU and DOL the compartments represent plasma; liver; spleen; well-perfused organs heart. kidneys. lungs. muscles in one “tissues.l” compartment; other tissues in a “tissues.2” compartment; and two excretion fluids. urine and bile. Anatomically determined pathways link the compartments. DAU and DOL concentrations are observed (high performance liquid chromatography (HPLC) method) in all compartments. except tissues.2. at various timepoints after a single i.v. injection of DAU. This input is modeled as an instantaneous loading of the plasma DAU compartment at time zero. Twenty-five rate constants that describe the dynamic system behaviour in terms of concentration-time histories are estimated by automatic optimization in the maximum likelihood (ML) sense. Three different optimization routines are described: 1) a modified Gauss-Newton method based on the solution of analytical sensivity differential equations (MGN); 2) a finite differences approach to the computation (FD); or 3) a direct determination of the gradient vector and the Hessian matrix of the log-likelihood function in the parameter space. again through finite differences (DEH). The performance of the three methods with respect to convergence characteristics. accuracy of results and speed of computation is compared. With proper precautions taken to prevent divergence the MGN method performs better than the DEH method. The FD approach may speed up computation only marginally. Revision of the assumption of first order kinetics seems necessary.

Kinetic models for plasma disappearance of insulin in normal subjects.

Three theoretical kinetic models for plasma disappearance of insulin were examined in six normal men. The models allowed for the existence of non-saturable and/or saturable mechanisms. Constant infusion of porcine insulin at different rates was used to achieve different levels of steady state plasma insulin concentrations, while normoglycaemia was secured by a glucose clamp technique. Appropriate validation procedures demonstrated that one of the three models was superior to the others in describing the relationship between the exogenous insulin infusion rate Iex and the steady state plasma insulin concentration C: Iex = -Iend + k2 X C/(k3 + C), where Iend is the endogenous post-hepatic insulin delivery rate. Thus, only saturable mechanism(s) could be demonstrated. The median value of k2 (the maximal insulin disappearance rate) and k3 (the plasma insulin concentration at which the insulin disappearance rate is half maximal) were 7.31 nmol X min.-1 and 3.89 nmol X 1-1. The median value of k2/k3 (the clearance rate of insulin for infinitesimal plasma insulin concentrations) was 25.0 ml X kg-1 X min.-1. Thus, at physiological levels of plasma insulin concentrations the metabolic clearance rate of insulin is higher than insulin clearance estimates previously reported in studies based on the assumption of first order kinetics.

CYTODYNAMICS IN THE THYMUS OF YOUNG ADULT MICE:

Cell proliferation and cell loss in the thymic blast cell population were studied in young adult mice by (1) stathmokinetic methods combined with an analysis of the PLMe-curve after a pulse 3H-TdR, and (2) nigrosin-dye exclusion combined with 3H-TdR-autoradiography. It was calculated that about 17 percent of the blast cells do not progress into mitosis within the period of an average cell cycle. The dye exclusion studies indicated a rate of blast cell death of about 2-5 percent/hr. The two methods of assessing blast cell loss (death) support each other very well. In spite of these findings scintillation countings on thymuses removed from 1 to 17 hr after 3H-TdR injection showed fairly constant levels of thymic radioactivity. This suggests a very extensive reutilization of 3H-labelled break-down products from dying blast cells. The very sparse labelling of pyknotic thymocytes strongly suggests that thymic blast cells do not become pyknotic. The rate of small thymocyte production and disappearance was studied by pulse and repeated 3H-TdR labelling techniques combined with dye exclusion studies and pyknotic counts. The data from the repeated labelling experiment were analysed by use of a model based on the assumption of first order kinetics of small viable, dead, and pyknotic thymocytes. The rate of cell production was estimated to 1-6 percent/hr whereas the rates of cell loss due to disintegration, i.e. supravital stainability and nuclear pyknosis, were calculated to 0-02 percent/hr and 0-0006 percent/hr respectively. Cell loss due to disintegration was less than 2 percent of the total loss of small thymocytes. It was concluded that pyknotic counts are a useless method of assessing the cell death in the population of thymic blast cells and small thymocytes. On the basis of a model for thymocyte proliferation, production and loss it is suggested that about 45 percent of the small viable thymocytes re-enter the generative cell pool, whereas about 55 percent disappear by emigration.

Effect of phenobarbital administration on the in vitro hydroxylation of cortisol and on over-all substrate and product metabolism in the guinea pig and rat.

Treatment of 4-month-old female strain 13 guinea pigs with phenobarbital (75 mg/kg daily ip) for 18 days stimulated the hepatic microsomal metabolism of cortisol, 2α- and 6β-hydroxycortisol 2.3-, 2.1- and 1.5-fold, respectively. The mean net formation of 2α- and 6β-hydroxycortisol increased 1.4- and 1.7-fold, respectively. The mean initial rates of 2α- and 6β-hydroxylation by hepatic microsomes (calculated on the assumption of first-order kinetics from the over-all rates of metabolism of substrate and product) were 3.6- and 3.8-fold higher in the drug-treated guinea pigs, suggesting induction of enzymatic activity as a possible mechanism. No significant effect of phenobarbital on the hy-droxylation of cortisol or the over-all metabolism of substrate or product by guinea pig adrenal homogenates was found. Treatment of 3-month-old female Sprague-Dawley rats with phenobarbital (75 mg/kg daily ip) for 6 days enhanced the hepatic microsomal 6β-hydroxylation of cortisol 3.6-fold, but did not cause a significant...