Exponential Release Research Articles

An assessment of the effects of sample ischaemia and spinning time on the metabolic profile of brain tumour biopsy specimens as determined by high‐resolution magic angle spinning 1H NMR

High-resolution magic angle spinning (HRMAS) (1)H NMR of biopsy tissue provides a biochemical profile that has potential diagnostic and prognostic value, and can aid interpretation of the lower-resolution (1)H-NMR spectra obtained in vivo. However, the biochemical profile obtained may be affected by experimental factors such as a period of ischaemia before snap-freezing of the biopsy tissue for subsequent analysis and the mechanical stress of the spinning procedure of HRMAS itself. We have used normal rat brain cortex as a 'gold standard', either funnel-frozen or deliberately allowed to become ischaemic for set periods of time before snap-freezing, to quantitatively investigate these two effects. In addition, we have compared biochemical changes that occur in normal rat brain during HRMAS (spun continuously at 5 kHz for 4 h at 4 degrees C as could be required for a two-dimensional acquisition) with those that occur in biopsy samples from low-grade and high-grade adult human astrocytomas, during the same HRMAS procedure. Significant changes due to delayed initial sample freezing were noted in metabolites associated with glycolysis (alanine, glucose and lactate), as expected. However, for the funnel-frozen rat tissue at 4 degrees C, there were even more significant changes, which appear to be the result of extended spinning at 5 kHz. In particular, the 18% total creatine increase observed is unlikely to be de novo synthesis of creatine. More likely, the asymptotic exponential increase in creatine suggests an exponential release of an NMR-invisible bound creatine store as a result of tissue damage from mechanical stress of sample spinning. Overall, it appears that tissue ischaemia during biopsy excision and delays in snap-freezing may have less significant effects on metabolite profile than the prolonged spinning times required for two-dimensional HRMAS, and this must be accounted for when results are being interpreted.

Formulation and Evaluation of a Salted-out Isoniazid-loaded Nanosystem

The purpose of this study was to develop a drug-loaded nanosystem that has the ability to achieve flexible yet rate-controlled release of model drug isoniazid (INH) employing either an aqueous or emulsion-based salting-out approach. Formulation conditions were aimed at reducing the polymeric size with subsequent rate-modulated INH release patterns from the polymeric nanosystem. The emulsion-based salted-out nanosystems had particle sizes ranging from 77-414 nm and a zeta potential of -24 mV. The dispersant dielectric constant was set at 78.5 and a conductivity of 3.99 mS/cm achieved. The reduced nanosystem size of the aqueous-based approach has demonstrated an intrinsically enhanced exposure of methacrylic acid-ethyl acrylate to zinc sulphate which was employed as a crosslinking reagent. This resulted in robustly interconnected polymeric supports in which INH was efficiently embedded and subsequently released. The multi-layer perceptron data obtained showed that the aqueous and emulsion-based salting out approaches had Power (law) (MSE = 0.020) and Linear (MSE = 0.038) relationships, respectively. Drug release from the nanosystems occurred in two phases with an initial burst-release in aqueous-based nanosystems (30-100%) and significantly lower bursts observed in emulsion-based nanosystems (20-65%) within the first 2 h. This was followed by a gradual exponential release phase over the remaining 12 h. The nanosystems developed demonstrated the ability to control the release of INH depending on the formulation approach adopted.

PLGA Nanoparticles for the Oral Delivery of 5-Fluorouracil Using High Pressure Homogenization-Emulsification as the Preparation Method and In Vitro/In Vivo Studies

The objective of the present study was to incorporate the hydrophilic anti-cancer drug 5-Fluorouracil(5-FU) into poly(lactide-co-glycolide) (PLGA) nanoparticles(NP) to improve the oral bioavailability. Owing to the high solubility of 5-FU in basic water, the water-in-oil-in-water (w/o/w) emulsification process has been chosen as one of the most appropriate method for the encapsulation of 5-FU, and the ammonia solution was used as the inner aqueous phase solvent to increase the solubility of 5-FU. In order to reach submicron size as well as increasing the grade of monodispersity compared to previous preparation techniques, we prepared 5-FU loaded PLGA-NP by a high-pressure emulsification-solvent evaporation process. The PLGA-NPs were characterized with respect to their morphology, particle size, size distribution, 5-FU encapsulation efficiency, in vitro and in vivo studies in rats. In vitro release of 5-FU from nanoparticles appeared to have two components with an initial rapid release due to the surface associated drug and followed by a slower exponential release of 5-FU, which was dissolved in the core. The in vivo research was studied in male Sprague-Dawley rats after an oral 5-FU dose of 45 mg/kg. Single oral administration of 5-FU loaded PLGA-NP to rats produced bioavailability, which was statistically higher than 5-FU solution as negative control. And the MRT (mean residence time) of 5-FU loaded PLGA-NP was significantly (P < 0.05) modified. Thus, it is possible to design a controlled drug delivery system for oral 5-FU delivery, improving therapy efficiency by possible reduction of time intervals between peroral administrations and reduction of local gastrointestinal side effects.

Colon-specific delivery of 5-aminosalicylic acid from chitosan-Ca-alginate microparticles

Chitosan-Ca-alginate microparticles for colon-specific delivery and controlled release of 5-aminosalicylic acid after peroral administration were prepared using spray drying method followed by ionotropic gelation/polyelectrolyte complexation. Physicochemical characterization pointed to the negatively charged particles with spherical morphology having a mean diameter less than 9 μm. Chitosan was localized dominantly in the particle wall, while for alginate, a homogeneous distribution throughout the particles was observed. 1H NMR, FTIR, X-ray and DSC studies indicated molecularly dispersed drug within the particles with preserved stability during microencapsulation and in simulated in vivo drug release conditions. In vitro drug release studies carried out in simulated in vivo conditions in respect to pH, enzymatic and salt content confirmed the potential of the particles to release the drug in a controlled manner. The diffusional exponents according to the general exponential release equation indicated anomalous (non-Fickian) transport in 5-ASA release controlled by a polymer relaxation, erosion and degradation. Biodistribution studies of [ 131I]-5-ASA loaded chitosan-Ca-alginate microparticles, carried out within 2 days after peroral administration to Wistar male rats in which TNBS colitis was induced, confirmed the dominant localization of 5-ASA in the colon with low systemic bioavailability.

In vivo release of rhBMP-2 loaded porous calcium phosphate cement pretreated with albumin

In this study, the release of rhBMP-2 loaded porous Ca-P cement was studied in vitro and in vivo. We hypothesized that adsorption sites of Ca-P ceramic with high affinity for rhBMP-2 can be blocked by pretreatment of the ceramic with albumin prior to rhBMP-2 loading, which would result in weaker rhBMP-2 binding and enhanced release of rhBMP-2. Preset porous Ca-P cement discs with a diameter of 6.35 mm (volume: 75 mm3) were pretreated by incubation in a solution of 10% rat serum albumin for 24 h or in ddH2O (control group) prior to administration of 5 mug radiolabeled 131I-rhBMP-2. Release was assessed in vitro in phosphate buffered saline (PBS) and fetal calf serum and in vivo by longitudinal scintigraphic imaging of radiolabeled 131I-rhBMP-2 and gamma counting of dissected implants. In vitro release from pretreated discs was higher during the first day. For both formulations, release in PBS was limited compared to release in serum. In vivo release considerably exceeded in vitro release. In vivo release kinetics showed no significant difference of half-lives between pretreated and control discs. Both formulations showed sustained release during at least 4 weeks. Ex vivo gamma counting of retrieved samples confirmed scintigraphic results and showed that the capsule and surrounding tissues only contained a minor fraction rhBMP-2. We conclude that 1. scintigraphy of 131I-labeled rhBMP-2 provides a reliable method for longitudinal measurement of rhBMP-2 release kinetics in vivo. 2. albumin pretreatment of porous Ca-P cement does not results in relevant increase of initial release of rhBMP-2 in vivo, and 3. preset rhBMP-2 loaded porous Ca-P cement discs exhibit one phase exponential release kinetics in the rat ectopic model, characterized by a retention of 20-30% after 4 weeks.

Kinetic Studies of Calcium-Induced Calcium Release in Cardiac Sarcoplasmic Reticulum Vesicles

Fast Ca 2+ release kinetics were measured in cardiac sarcoplasmic reticulum vesicles actively loaded with Ca 2+. Release was induced in solutions containing 1.2 mM free ATP and variable free [Ca 2+] and [Mg 2+]. Release rate constants ( k) were 10-fold higher at pCa 6 than at pCa 5 whereas Ryanodine binding was highest at pCa ≤5. These results suggest that channels respond differently when exposed to sudden [Ca 2+] changes than when exposed to Ca 2+ for longer periods. Vesicles with severalfold different luminal calcium contents exhibited double exponential release kinetics at pCa 6, suggesting that channels undergo time-dependent activity changes. Addition of Mg 2+ produced a marked inhibition of release kinetics at pCa 6 (K 0.5 = 63 μM) but not at pCa 5. Coexistence of calcium activation and inhibition sites with equally fast binding kinetics is proposed to explain this behavior. Thimerosal activated release kinetics at pCa 5 at all [Mg 2+] tested and increased at pCa 6 the K 0.5 for Mg 2+ inhibition, from 63 μM to 136 μM. We discuss the possible relevance of these results, which suggest release through RyR2 channels is subject to fast regulation by Ca 2+ and Mg 2+ followed by time-dependent regulation, to the physiological mechanisms of cardiac channel opening and closing.

Development and evaluation of verapamil resinates-loaded controlled release microcapsules using a binary polymer system in drug release rate modulation

The application of polystyrene-​divinylbenzene sulfonic acid-​based ion exchange resins as a carrier system for the sustained delivery of verapamil hydrochloride was primarily evaluated. A large degree of variation in the loading efficiencies (drug loaded: 19.17-​48.86​%)​, was obsd. between the different drug-​resin complexes depending on resin type and drug​/resin ratio. Upon comparing different carrier resins at drug​/resin ratio of 1:1, verapamil release in either 0.1 N HCl or phosphate buffer (pH 7.4) was in the order of Amberlite MB-​1 > Amberlite IRP-​69 (Na+) ≥ Amberlite IR-​120 (Na+) > Dowex-​50 W (Na+)​. For further retardation of the drug release rate, microencapsulation of the strongly acidic cation exchange resin (Amberlite IR-​120 (Na+)​) loaded with verapamil hydrochloride (drug​/resin ratio of 1:1, resinate drug content: 48.86​%, core​/coat ratio of 1:2) was carried out by means of a modified emulsion-​solvent evapn. / extn. technique (ESE​/E) using different coating polymers, namely Et cellulose (EC)​, cellulose acetate phthalate (CAP)​, cellulose acetate butyrate (CAB) as well as CAB​/polystyrene (PS) binary polymer systems. The effect of polymer type, polystyrene concn. and core​/coat ratio on the yield, size distribution as well as release characteristics and surface topog. of the microcapsules were investigated. The results obtained revealed that polystyrene utilization as a complementary wall material at a particular compn. of 70:30 (​%) of CAB to PS was found to improve greatly the microcapsule yield, reduce the av. microcapsule size and modulate the in-​vitro release of the entrapped drug. On the other hand, the entrapment efficiencies increased and the release rate decreased with increasing microcapsule size and​/or theor. drug loading of CAB​/PS (30​%) - microcapsules. Kinetic assessment of the release data using different math. models showed that the drug release from CAB or CAB​/PS (7.5​%)​-​microcapsules (core​/coat ratio of 1:2) was found to be best explained by a Fickian-​diffusion kinetics (a diffusion-​controlled model for a planar matrix)​, whereas the calcd. exponential release exponents (n values) of the empirical equation (Mr/M∞ = Ktn) indicated that the release behavior of CAB​/PS (30​%) - microcapsules was a non-​Fickian-​diffusion kinetics, confirming that a diffusion / chain relaxation-​controlled release mechanism was operative. Overall, this study demonstrated that the prepd. CAB​/PS microcapsules were capable of releasing their drug content gradually for an extended period of time, irresp. of variations in the pH of the gastrointestinal tract and exhibited slower release rates as compared with the com. sustained-​release product (Isoptin(SR) tablets)​.

An approach to analysis of pigment release from microcapsules with size distribution

We have studied the release curve for microcapsules with size distribution. On the basis of an analogy to the relaxation phenomena with multiple characteristic times, we propose a stretched exponential release curve for the system with size distribution and relate the release curve to the size distribution function of the microcapsule. This method was successfully applied to the transfer of azo-pigments from inner medium of dioctyl phthalate to dispersing medium of methanol through poly(ureaurethane) microcapsule membrane.

Preparation and evaluation of sustained release carbamazepine matrix tablets using Eudragit RS 100 and Tragacanth

Carbamazepine is a widely prescribed anticonvulsant antiepileptic drug which shows variable absorption and variable bioavailability. It is advisable to prep. it in sustained release dosage form to improve patient compliance and to perform const. blood level with min. side effects. Problems may be encountered with carbamazepine tablets prepd. by wet granulation if the granules are not thoroughly dried before processing. Therefore, the present study deals with the application of direct compression technique to prep. sustained release carbamazepine matrix tablets. Different levels (0 to 25 % wt.​/wt.) of Eudragit RS 100 and Tragacanth were tested to select the best level of matrix forming material that provides the most release prolonging effect, by direct compression technique using Avicel PH 102 and anhyd. lactose as directly compressible vehicles. Directly compressed matrix tablets of carbamazepine were found to be complied with the USP​/NF 23 requirements for the uniformity of wt., diam., and acceptable mech. properties with respect to tensile strength and friability. Examn. of the release characteristics of the drug from the directly compressed tablets as well as the detn. of the release mechanism were carried out. It was found that, as the percentage of the polymer (either Eudragit or Tragacanth) in all formulations increased from 3.125​% to 18.75​% or 25​% wt.​/wt., the drug release decreased significantly. Involvement of Eudragit RS 100 in the directly compressed matrix tablets gave more sustaining release of carbamazepine than inclusion of Tragacanth in both acidic medium (pH 1.2) and phosphate buffer medium (pH 6.8)​. This was due to the difference between the physicochem. properties of natural polymer Tragacanth and synthetic polymer Eudragit. Comparatively, the formula of carbamazepine tablet contg. 25 % wt.​/wt. Tragacanth showed similar release behavior to that of the com. Tegretol tablets. The calcd. exponential release exponents (n values) indicated that release behavior of carbamazepine matrix tablets was anomalous (non-​Fickian) diffusion kinetics.

The estimation of yearly probability gain for seismic statistical model

Based on the calculation method of information gain in the stochastic process presented by Vere-Jones, the relation between information gain and probability gain is studied, which is very common in earthquake prediction, and the yearly probability gain for seismic statistical model is proposed. The method is applied to the nonstationary Poisson model with whole-process exponential increase and stress release model. In addition, the prediction method of stress release model is obtained based on the inverse function simulation method of stochastic variable.

Antithrombogenic Coating of Stents Using a Biodegradable Drug Delivery Technology

To reduce the thrombogenic properties of coronary artery stents, a biodegradable polylactic acid (PLA) stent coating with an incorporated thrombin inhibitor and a platelet aggregation inhibitor has been developed. In an ex vivo human stasis model, its effect on platelets, plasmatic coagulation and its release characteristics were studied using whole blood. Bare steel and bare gold-surface stents were compared to steel and gold-surface stents coated with PLA (30 kDa) containing 5% polyethyleneglycol (PEG)-hirudin and 1% iloprost, with an empty tube as control. Markers of activated coagulation (prothrombin fragment F1-2 and thrombin-antithrombin III complex, TAT), were assayed and the release of drugs from the coating was assessed by aPTT and collagen-induced platelet aggregation. Bare steel and gold stents were completely covered by a blood clot, and high levels of coagulation markers (F1-2 fragment and TAT) were detected. No differences in the thrombogenic properties were found between bare gold or steel stents. Coated stents were free of blood clots and only minor elevations of markers were detected. Release data from in-vitro studies over 90 days showed a gradual release of the drugs with an initial exponential release characteristic for PEG-hirudin, slow release of iloprost and a 10% degradation of the PLA carrier. This drug releasing biodegradable coating effectively reduced thrombus formation independent of the metallic surface.

DEVELOPMENT AND EVALUATION OF A PROLONGED-RELEASE MATRIX TABLETS OF DICLOFENAC SODIUM RESINATE

The preparation of a potential prolonged-release matrix tablets (IER-tablets) containing diclofenac sodium – anion exchange resin (Dowex I-X8.Cl-) complexes has been performed by direct compression of the drug-resin complex (drug resinate) with various release-regulating excipients (Avicel PH 101, lactose, sodium carboxymethylcellulose, and compritol). All the formulations produced tablets with good mechanical properties. The in-vitro release rate of diclofenac sodium from the drug-resin complex samples with different loaded concentrations and from the prepared IER-tablets (Kh = 0.92-5.52 mg/min-0.5) was determined using a rotating paddle dissolution apparatus. The results obtained showed that drug release from these formulations was slower than from the tabletted diclofenac sodium alone (Kh = 6.55-15.10 mg.min-0.5) or a commercial sustained-release tablet formulation of diclofenac sodium (Kh = 8.23 mg.min-0.5) and that IER-tablets containing sodium carboxymethylcellulose or compritol exhibited the lowest release rates (Kh = 0.92 and 1.27 mg/min-0.5, respectively). The kinetics of the drug release from resinate was evaluated on the basis of the mass low. In most cases, the profiles for both the effect of drug/resin ratio and sodium ion concentration upon drug release from the resonates exhibited two release rate processes: rapid release during the initial period, followed by a decreasing release rate, suggesting a particle diffusion release process. The release rate data of the tablets were investigated by using zero-order and the matrix-diffusion-controlled kinetics. The calculated exponential release exponents (n-values) revealed that release behaviour of all IER-tablets was a Fickian-diffusion kinetics (i.e., The Higuchi-linear square root of time relationship), confirming that a matrix diffusion-controlled mechanism was operative. To evaluate the feasibility of the drug resinate tablet system, the optimum IER-tablet formulations, drug resonates, sustained-release commercial tablet product and plain drug were examined for their ulcerogenic activity in rabbits. The results proved the superiority of the drug resinate and IER-tablets containing Avicel PH 101 over the plain drug, commercial product or IER-tablets containing compritol or sodium carboxymethylcellulose. Overall, this study demonstrated the significance of using ionic complex systems in offering a simple gastro-protected tablets and showed the characteristics of the matrix materials and their influence on the drug activity as well as tablet performance in-vivo.

Transmitter release in inner hair cell synapses: a model analysis of spontaneous and driven rate properties of cochlear nerve fibres

The inner hair cell (IHC) synapse is one of the stages of cochlear processing that determine the relation between sound pressure level and spike rate in auditory nerve fibres. Transmitter released in the non-stimulated condition is held responsible for the wide range of spontaneous spike rates (SR) observed in these fibres. Properties of stimulated spike activity in auditory nerve fibres, including rate threshold and operating range of a fibre, are known to systematically vary with SR. This paper presents a model analysis of the relation between IHC transmembrane potential and transmitter release rate as becoming manifest in these spontaneous and driven rate properties. A previously developed computational model is used to identify those transfer properties of its synapse section which lead to reproduction of the variation of rate thresholds, shapes of rate-intensity functions and maximal driven rate with SR known from the literature. First a simple additive release model, in which driven transmitter release depends linearly on IHC potential, is elaborated. Its results lead to the hypothesis that the true release function is non-linear and variable across synapses generating different SR. An exponential release function is then introduced, with parameters varying across SR in a physiologically dictated way. This approach leads to adequate reproduction of the variation in rate thresholds and rate-intensity functions with SR. Finally, the model is applied in an inverse way to directly estimate the release function from given rate-intensity functions. The conclusion of both forward and inverse model analyses is that transmitter release is a non-linear function of IHC potential which, by the systematic variation of its parameters across SR, effectively leads to the physiological variation in dynamic range across fibres of different SR. Possible relations of these results with ultrastructural morphology and basic physiology of IHC synapses are discussed.

Propagation of blast waves with exponential heat release and internal heat conduction and thermal radiation

The problem of reactive blast waves in a combustible gas mixture, where the heat release at the detonation front decays exponentially with the distance from the center, is analyzed. The central theme of the paper is on the propagation of reactive blast into a uniform, quiescent, counterpressure atmosphere of a perfect gas with constant specific heats. The limiting cases of Chapman-Jouguet detonation waves are considered in the phenomenon of point explosion. In order to deal with this problem, the governing equations including thermal radiation and heat conduction were solved by the method of characteristics using a problem-specific grid and a series expansion as start solution. Numerical results for the distribution of the gas-dynamic parameters inside the flow field are shown and discussed.

A new in vitro technique for the evaluation of drug release profile from colloidal carriers - ultrafiltration technique at low pressure

A new in vitro technique, ultrafiltration at low pressure, is proposed for the evaluation of drug release profile from colloidal carriers such as submicron emulsions and nanoparticles. A thorough description is given after having reviewed the various available techniques for drug release evaluation from colloidal carriers. The results yielded by the new technique clearly show that miconazole release from either the submicron emulsion or a marketed micellar aqueous solution was very rapid under perfect sink conditions. This fact raised doubts and practically rules out the possibility of using submicron emulsions as a controlled-release drug delivery system for i.v. administration. The in vitro indomethacin release profile from PLA or PLGA nanoparticles was found to be biphasic with an immediate release of 40–80% of the initial drug concentration, followed by a slower exponential release which lasted a few hours. It was deduced from the overall kinetic results that the ultrafiltration method at low pressure was versatile, sensitive and capable of distinguishing between different kinetic behaviors. The technique offers the possibility of identifying the underlying release mechanism of drug from colloidal carriers.

Defect Relaxation Dynamics in Amorphous Silicon

Using transient capacitance and transient spin techniques, we have the determined the manner in which the mobility gap energy of the D defect is altered following a change in its charge state. This relaxation process gives rise to a power law rather an exponential thermal release of defect electrons with time and also causes the charge emission and spin transients to obey a scaling law. We also deduce that the D°/D+ transition rate depends on the tenure of the proceeding D-/D° transition. This last aspect of the D defect emission behavior implies that it must be treated as a non-Markovian process. Such relaxation dynamics have profound consequences for the steady state distribution of D defect energies. Using the relaxation parameters determined by the transient measurements we have been able to solve a set of coupled differential equations under steady-state conditions to provide the energy distributions of both the D° and DD+ defect sub-bands. The results of these calculations agree remarkably well with the experimental distributions determined by modulated photocurrent and steady-state capacitance measurements. This implies that the statistical variations in the occupation history of the defect may be the dominant factor determining both distributions.

Binding of Tissue Plasminogen Activator to Vascular Grafts

Tissue plasminogen activator labeled with radioactive iodine (125I-tPA) was immobilized on vascular prostheses chemically modified with a thin coating of water-insoluble surfactant, tridodecylmethylammonium chloride (TDMAC). Surfactant-treated Dacron, polytetrafluoroethylene (PTFE), silastic, polyethylene and polyurethane bound appreciable amounts of 125I-tPA (5-30 micrograms 125I-tPA/cm2). Upon exposure to human plasma, the amount of 125I-tPA bound to the surface shows an initial drop during the first hour of incubation, followed by a slower, roughly exponential release with a t 1/2 of approximately 75 hours. Prostheses containing bound tPA show fibrinolytic activity as measured both by lysis of clots formed in vitro, and by hydrolysis of a synthetic polypeptide substrate. Prior to incubation in plasma, tPA bound to a polymer surface has an enzymic activity similar, if not identical to that of the native enzyme in buffered solution. However, exposure to plasma causes a decrease in the fibrinolytic activity of both bound tPA and enzyme released from the surface of the polymer. These data demonstrate that surfactant-treated prostheses can bind tPA, and that these chemically modified devices can act as a slow-release drug delivery system with the potential for reducing prosthesis-induced thromboembolism.

ADP release from myosin in permeabilized smooth muscle

The purpose of this study was to determine the nucleotide bound to myosin and its rate of release under relaxed and activated conditions in permeabilized rabbit portal veins. Incubation of the muscles in a relaxing solution containing [3H]-ATP resulted in the formation of 60-70 microM radiolabeled ADP in the muscle whether or not the myosin light chains had been thiophosphorylated. This value was similar to the estimate of the concentration of myosin subfragment 1. Upon transfer of the muscles to a chase solution containing no labeled ATP, there was a very slow loss of labeled ADP when the light chains were unphosphorylated, but a much faster release occurred when the light chains were thiophosphorylated. The results suggest that smooth muscle myosin exists primarily in a complex with ADP under both relaxed and phosphorylated conditions and that phosphorylation of all of the light chains results in a large increase in the rate of release of the products of ATP splitting from all of the myosin. Interestingly, the exponential release of ADP in relaxed muscle shows two components, one of which contains about two-thirds of the total ADP and is 5- to 10-fold faster than the other. If the difference in rates of ADP release observed in relaxed muscle persists when the myosin is phosphorylated, then it is possible that there is a 5- to 10-fold difference in rates of cycling for different phosphorylated cross bridges in smooth muscle.

Inside-to-outside growth and turnover of the wall of gram-positive rods

Gram-positive, rod-shaped bacteria, given a pulse of peptidoglycan precursors, first exhibit a lag before the second or turnover phase of peptidoglycan commences. This is because new material is inserted on the inner face of the wall and gradually displaced through the wall. Based on this experimental observation, a mathematical model was constructed and compared with experimental data obtained in several laboratories for the first and second phases of wall turnover of Bacillus subtilis. The model allows the parameters of the process to be estimated for experiments with any labeling time. According to the surface stress theory the wall which is layed down immediately outside the cytoplasmic layer is in an unextended conformation. As subsequent additions of murein occur, the wall moves outward, becomes stretched, and bears the stress due to hydrostatic pressure. Ultimately, peptide and glycosyl bonds become cleaved. At the end of the lag phase the cleavage becomes so extensive that wall fragments are liberated into the medium. This strategy permits rod-shaped growth. In some experimental situations the half-life of wall radioactivity in this second phase roughly equals the doubling time; consequently, the exponential release probably does not represent random turnover but instead is the result of expansion of the underlying wall that continues to create strain which favors autolysis action. The slower turnover of the third phase, where there is a much slower loss, is also included in the analysis.

A Family of Epidermoid Lung Cancer Models

A method of sustained release implantation has been developed whereby Silastic cylinders, impregnated with benzo[ a]pyrene (BP) or methylcholanthrene (MCA) each at 2% (low dose) and 10% (high dose) concentrations, were inserted into the bronchus intermedius of hamsters. High-dose BP and MCA, and low-dose MCA had first-order exponential release rates: the half-time of release was 40 days for high-dose BP, 30 days for high-dose MCA, and 165 days for low-dose MCA. Release rate of low-dose BP was a second-order function: half-time of release was 40 days. Atypical squamous metaplasia was noted by 4 weeks in more than 65% of hamsters after insertion of each high-dose carcinogen but in less than 30% with the low-dose carcinogens. Carcinoma in situ was noted approximately 8 weeks after high-dose BP and 19 weeks after low-dose BP. At about 15 to 17 weeks after a high-dose carcinogen, 64% of animals had invasive epidermoid cancer, whereas after a low-dose carcinogen, only 21% did. After 25 weeks of exposure to a high-dose carcinogen, more than 85% of hamsters had invasive epidermoid cancer; up to 52 weeks were required for invasive epidermoid cancer to develop in 30% after a low-dose carcinogen. Measured by image analysis, nuclear deoxyribonucleic acid content of cells with severe atypical squamous metaplasia was greater than tetraploid (mean ± standard deviation [SD], 3.77 ± 1.4), whereas cells with invasive epidermoid cancer were suprahexaploid (mean ± SD, 6.48 ± 3.6). These differences are significant ( p < 0.05). We conclude that the developmental continuum of epidermoid lung cancer can now be reproduced in controlled, predictable fashion in hamsters.