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At least 1,351 records · Page 75Linked to original sources
PubMed2000

Using phase space diagrams to interpret multiple frequency drive sonoluminescence

The recent experimental results of J. Holzfuss, M. Ruggeberg, and R. Mettin [Phys. Rev. Lett. 81, 1961 (1998)] in which a second harmonic drive system was used to generate sonoluminescence (SL) have been analyzed in the context of the dissociation hypothesis (DH) of D. Lohse and S. Hilgenfeldt [J. Chem. Phys. 107, 6986 (1997)]. The second harmonic introduces two more variables that are under experimental control: a phase and an additional pressure term to the acoustic drive pressure. Diffusive equilibrium curves for a fixed gas concentration were calculated as was the Mach criterion. A phase space diagram was constructed to permit the prediction of regions of stable SL, unstable SL, stable non-SL, and unstable non-SL. These were compared to Holzfuss' experimental observations, and excellent quantitative agreement was seen. The results provide further evidence that the underlying assumptions of DH are sound. They also indicate the utility of DH for determining appropriate experimental conditions to achieve SL and for optimizing an experimental system.

Journal Article↗
PubMed1988

Identification and characterization of guinea pig angiotensin II ventricular and atrial receptors: coupling to inositol phosphate production.

Angiotensin II elicits a receptor-mediated positive inotropic response in cardiac tissue from most mammalian species by activating voltage-sensitive slow Ca2+ channels. In the guinea pig, we confirm there is no in vitro contractile force development in bioassay systems using isolated hearts or left atrial tissue in response to angiotensin peptides. However, 125I-angiotensin II binding sites that have the characteristics of a membrane receptor were identified in ventricular (myocardial) and atrial membrane preparations from guinea pigs. In ventricles, saturation-binding data yielded an optimal fit to a two-site model with a high-affinity site Kd1 = 3.6 +/- 0.7 nM and a low-affinity site Kd2 = 433 +/- 126 nM and binding capacities of 66 +/- 10 and 821 +/- 49 fmol/mg protein, respectively. In atria, saturation binding data yielded an optimal fit to a two-site model with a high-affinity site Kd1 = 1.6 nM and a low-affinity site Kd2 = 300 nM and capacities of 145 and 752 fmol/mg protein, respectively. The ventricular binding of 125I-angiotensin II was stimulated approximately twofold in the presence of the divalent cations calcium and magnesium (10 mM). Nonhydrolyzable analogues of guanosine triphosphate increased the dissociation rate of the bound 125I-angiotensin II and decreased hormone binding to the receptor at equilibrium. Competition for 125I-angiotensin II binding by an agonist-antagonist analogue series correlated with previous studies obtained in the rabbit, a mammal in which inotropic responses to angiotensin II were demonstrated. The data indicate the presence of angiotensin II myocardial and atrial receptors and a G-type coupling protein in guinea pig. Although this species lacks an inotropic response to angiotensin peptides, there is a dose-dependent increase in inositol-1-phosphate production in response to angiotensin II, and this response is blocked by the selective angiotensin II antagonist [Sar1,Ile8]angiotensin II.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Catalytic properties of Escherichia coli polyphosphate kinase: an enzyme for ATP regeneration.

Catalytic properties of Escherichia coli polyphosphate kinase (EC 2.7.4.1), a promising enzyme for use in ATP regeneration (Hoffman, et al., 1988, Biotechnol. Appl. Biochem. 10, 107-117), are reported here. E. coli polyphosphate kinase (PPK) is broadly active in the pH range 5.5 to 8.5, having an optimal Vmax at pH 7.2. The Km values for the substrates, ADP and polyphosphate (Pn), change little in the same pH range. The optimal concentration range for the Mg2+ activator is 1-20 mM, with an activity maximum at 10 mM Mg2+. In addition to Mg2+, Mn2+ and Co2+ can serve as activators of E. coli PPK, whereas Zn2+ and Cu2+ are highly inhibitory. E. coli PPK is most active with Pn substrates of chain length greater than 132 phosphoryl units. The enzyme activity decreases with decreasing Pn chain length and approaches zero (less than 1%) at a chain length less than or equal to 5. Equilibrium yields of ATP of greater than 85% are readily attained at substrate concentrations below 1 mM. An operational equilibrium constant for the PPK reaction, defined as [ATP]/[ADP][Pn], was determined to be 7.5 (+/- 3.4) x 10(5) M-1. The data presented here serve as a base of information from which assessments of the suitability of E. coli PPK for specific ATP regeneration applications can be made.

Adenosine Triphosphate↗
PubMed1980

Characterization of the receptor for epidermal growth factor-urogastrone in human placenta membranes.

The binding of mouse epidermal growth factor-urogastrone (EGF-URO) to membranes from term human placenta is peptide-specific, saturable (about 20 pmol of EGF-URO bound maximally/mg of protein), reversible, and of high affinity (KD about 400 pM). Optimal binding is observed at pH 7.6. At low pH (3.5 to 5.0). EGF-URO can be reversibly dissociated from the receptor; however, exposure to pH < 3 irreversibly inactivates the receptor. The binding, which does not exhibit ligand cooperativity, exhibits an association rate constant of 6.1 x 10(-4) s-1 and a dissociation rate constant of 6.1 x 10(-4) s-1. The dissociation constant determined from the rate constants, 240 pM, is in reasonable agreement with the constant estimated by equilibrium methods. Both monovalent and divalent cations augment EGF-URO binding 2- to 3-fold. Although in general, divalent cations enhance binding at lower concentrations (optimum, 5 mM) than do monovalent cations (optimum, approximately 80 mM), there is no cation-specific effect. Neither guanine nor adenine nucleotides affect EGF-URO binding. Whereas the proteolytic enzymes (trypsin, chymotrypsin, papain, and pepsin) inactivate the receptor, neuraminidase and phospholipases A2, C, and D augment EGF-URO binding. Neuraminidase increases the number of available sites without affecting ligand affinity. Wheat germ agglutinin, concanavalin A, and phytohemagglutinin all compete for the binding of EGF-URO. The data complement previous observations of EGF-URO binding obtained in intact cells and provide a basis for the solubilization, characterization, and isolation of this receptor from a rich tissue source.

Animals↗
PubMed1997

Analysis of an optimal control model of multi-joint arm movements.

In this paper, we propose a model of biological motor control for generation of goal-directed multi-joint arm movements, and study the formation of muscle control inputs and invariant kinematic features of movements. The model has a hierarchical structure that can determine the control inputs for a set of redundant muscles without any inverse computation. Calculation of motor commands is divided into two stages, each of which performs a transformation of motor commands from one coordinate system to another. At the first level, a central controller in the brain accepts instructions from higher centers, which represent the motor goal in the Cartesian space. The controller computes joint equilibrium trajectories and excitation signals according to a minimum effort criterion. At the second level, a neural network in the spinal cord translates the excitation signals and equilibrium trajectories into control commands to three pairs of antagonist muscles which are redundant for a two-joint arm. No inverse computation is required in the determination of individual muscle commands. The minimum effort controller can produce arm movements whose dynamic and kinematic features are similar to those of voluntary arm movements. For fast movements, the hand approaches a target position along a near-straight path with a smooth bell-shaped velocity. The equilibrium trajectories in X and Y show an "N' shape, but the end-point equilibrium path zigzags around the hand path. Joint movements are not always smooth. Joint reversal is found in movements in some directions. The excitation signals have a triphasic (or biphasic) pulse pattern, which leads to stereotyped triphasic (or biphasic) bursts in muscle control inputs, and a dynamically modulated joint stiffness. There is a fixed sequence of muscle activation from proximal muscles to distal muscles. The order is preserved in all movements. For slow movements, it is shown that a constant joint stiffness is necessary to produce a smooth movement with a bell-shaped velocity. Scaled movements can be reproduced by varying the constraints on the maximal level of excitation signals according to the speed of movement. When the inertial parameters of the arm are altered, movement trajectories can be kept invariant by adjusting the pulse height values, showing the ability to adapt to load changes. These results agree with a wide range of experimental observations on human voluntary movements.

Arm↗
PubMed1987

Binding of leukotriene B4 and its analogs to human polymorphonuclear leukocyte membrane receptors.

LTB4-induced proinflammatory responses in PMN including chemotaxis, chemokinesis, aggregation and degranulation are thought to be initiated through the binding of LTB4 to membrane receptors. To explore further the nature of this binding, we have established a receptor binding assay to investigate the structural specificity requirements for agonist binding. Human PMN plasma membrane was enriched by homogenization and discontinuous sucrose density gradient purification. [3H]-LTB4 binding to the purified membrane was dependent on the concentration of membrane protein and the time of incubation. At 20 degrees C, binding of [3H]-LTB4 to the membrane receptor was rapid, required 8 to 10 min to reach a steady-state and remained stable for up to 50 min. Equilibrium saturation binding studies showed that [3H]-LTB4 bound to high affinity (dissociation constant, Kd = 1.5 nM), and low capacity (density, Bmax = 40 pmol/mg protein) receptor sites. Competition binding studies showed that LTB4, LTB4-epimers, 20-OH-LTB4, 2-nor-LTB4, 6-trans-epi-LTB4 and 6-trans-LTB4, in decreasing order of affinity, bound to the [3H]-LTB4 receptors. The mean binding affinities (Ki) of these analogs were 2, 34, 58, 80, 1075 and 1275 nM, respectively. Thus, optimal binding to the receptors requires stereospecific 5(S), 12(R) hydroxyl groups, a cis-double bond at C-6, and a full length eicosanoid backbone. The binding affinity and rank-order potency of these analogs correlated with their intrinsic agonistic activities in inducing PMN chemotaxis. These studies have demonstrated the existence of high affinity, stereoselective and specific receptors for LTB4 in human PMN plasma membrane.

Binding, Competitive↗
PubMed1995

[3H]1-aminocyclopropanecarboxylic acid, a novel probe for strychnine- insensitive glycine receptors.

[3H]1-Aminocyclopropanecarboxylic acid (ACPC) exhibits high affinity, specific binding to strychnine-insensitive glycine receptors. In extensively washed rat forebrain membranes, the specific binding of [3H]ACPC was optimal at 25 degrees C in the presence of 10 mM MgCl2. Comparable levels of specific [3H]ACPC binding were obtained using centrifugation and filtration for separation of bound from free radioligand. [3H]ACPC labels two sites with Kdl and Bmax1 values of 129 +/- 34 nM and 2.30 +/- 0.37 pmol/mg protein and Kd2 and Bmax2 values of 7.26 +/- 1.69 microM and 20.6 +/- 2.2 pmol/mg protein for the high and low affinity sites, respectively. The Kd of [3H]ACPC (66 nM) estimated under non-equilibrium conditions (koff = 8.91 +/- 0.78 x 10(-3) s-1; kon = 1.35 x 10(-4) nM-1 s-1) was similar to the value obtained for the high affinity site obtained by equilibrium binding. The Kd1 of[3H]ACPC is in good agreement with the previously reported Ki values of ACPC to inhibit the binding of other glycinergic ligands including [3H]glycine, [3H]5,7-dichlorokynurenic acid (5,7-DCKA) and [3H]L-689,560 ((+/-)-4-(trans)-2-carboxy-5,7-dichloro-4- phenylaminocarbonylamino-1,2,3,4-tetrahydroquinoline). Moreover, the potencies of a series of glycine site ligands, including glycine. ACPC, 1-aminocyclobutanecarboxylic acid (ACBC), 5,7-DCKA, 7-chlorokynurenic acid (7-CKA), R(+)-3-amino-1-hydroxy-2- pyrrolidine (HA-966) and D-serine, to inhibit [3H]ACPC binding were highly correlated with their potencies to inhibit [3H]glycine and [3H]5,7-DCKA binding (r2 = 0.98-0.51). These results demonstrate that [3H]ACPC is a useful tool for examining the neurochemical and pharmacological properties of strychnine-insensitive glycine receptors.

Amino Acids↗
PubMed2006

Synchronous fluorescence, UV-visible spectrophotometric, and voltammetric studies of the competitive interaction of bis(1,10-phenanthroline)copper(II) complex and neutral red with DNA.

Constant wavelength synchronous fluorescence spectroscopy (CW-SFS), UV-visible absorption spectroscopy, and cyclic and differential pulse voltammetry were applied to investigate the competitive interaction of DNA with the bis(1,10-phenanthroline)copper(II) complex cation ([Cu(phen)(2)](2+)) and a fluorescence probe, neutral red dye (NR), in a tris-hydrogen chloride buffer (pH 7.4). The results show that both the [Cu(phen)(2)](2+)and the NR molecules can intercalate competitively into the DNA double-helix structure. The cyclic voltammetry method showed that both anodic and cathodic currents of [Cu(phen)(2)](2+) decreased on addition of the DNA and the intercalated [Cu(phen)(2)](2+)-DNA complex formed (beta = (4.14 +/- 0.24) x 10(3)). CW-SFS measurements were facilitated by the use of the three-way resolution of the CW-SFS for NR, [Cu(phen)(2)](2+), and NR-DNA. The important constant wavelength (CW) interval, Deltalambda, was shown to vary considerably when optimized (135, 58, and 98 nm for NR, NR-DNA, and [Cu(phen)(2)](2+), respectively). This approach clearly avoided the errors that otherwise would have arisen from the common assumption that Deltalambda is constant. Furthermore, a chemometrics approach, parallel factor analysis (PARAFAC), was applied to resolve the measured three-way CW-SFS data, and the results provided simultaneously the concentration information for the three reaction components, NR, [Cu(phen)(2)](2+), and NR-DNA, for the system at each equilibrium point. The PARAFAC analysis indicated that the intercalation of the [Cu(phen)(2)](2+) molecule into the DNA proceeds by exchanging with the NR probe and can be attributed to two parallel reactions. Comprehensive information was readily obtained; the replacement of the intercalated NR commenced immediately on introduction of [Cu(phen)(2)](2+), approximately 50% of NR was replaced by [Cu(phen)(2)](2+) at a concentration of 0.45 x 10(-5) mol L(-1), and nearly all of the NR was replaced at a [Cu(phen)(2)](2+) concentration of 2.50 x 10(-5) mol L(-1). This work has the potential to improve extraction of information from the fluorescence intercalator displacement (FID) assay.

Binding, Competitive↗
PubMed2003

Rapid sample-mixing technique for transient NMR and photo-CIDNP spectroscopy: applications to real-time protein folding.

We describe the development and application of a novel rapid sample-mixing technique for real-time NMR (nuclear magnetic resonance) spectroscopy. The apparatus consists of an insert inside a conventional NMR tube coupled to a rapid injection syringe outside the NMR magnet. Efficient and homogeneous mixing of solutions in the NMR tube is achieved with a dead time of tens of milliseconds, without modification of the NMR probe or additional hardware inside the magnet. Provision is made for the inclusion of an optical fiber to allow in situ laser irradiation of samples, for example to generate photo-CIDNP (chemically induced dynamic nuclear polarization). An NMR water suppression method has been implemented to allow experiments in H(2)O as well as in deuterated solvents. The performance of the device has been tested and optimized by a variety of methods, including sensitive detection of residual pH gradients and the use of NMR imaging to monitor the extent of mixing in real time. The potential utility of this device, in conjunction with the sensitivity and selectivity of photo-CIDNP, is demonstrated by experiments on the protein hen lysozyme. These measurements involve the direct detection of spectra during real-time refolding, and the use of CIDNP pulse labeling to study a partially unfolded state of the protein under equilibrium conditions. Magnetization transfer from this disordered state to the well-characterized native state provides evidence for the remarkable persistence of nativelike elements of structure under conditions in which the protein is partially denatured and aggregation prone.

Animals↗
PubMed1997

Model Predictive Impedance Control: A Model for Joint Movement.

Impedance control has been suggested as the strategy employed by the central nervous system to control human postures and movements. A realization of this strategy is presented that uses a model predictive control algorithm as a higher motor controller. External disturbances are explicitly included in the model. The combination of 3 key factors-joint impedance control, model predictive controller, and external disturbance input-forms the basis for the generality of this model. The model was applied to 3 different types of joint movements: a tracking movement with an unpredicted disturbance, a rhythmic movement, and an unstable biped model of human walking. Computer simulation results showed excellent performance of the model in all 3 cases for optimal values of active joint impedances and an exact match between the musculoskeletal system and the model internal to the model predictive controller. The controller was also able to maintain acceptable performance in the presence of a 25% mismatch between the musculoskeletal system and its internal model.

equilibrium-point hypothesis↗
PubMedDate not supplied

Analytical model of stemwood growth in relation to nitrogen supply.

We derived a simplified version of a previously published process-based model of forest productivity and used it to gain information about the dependence of stemwood growth on nitrogen supply. The simplifications we made led to the following general expression for stemwood carbon (c(w)) as a function of stand age (t), which shows explicitly the main factors involved: c(w)(t) = eta(w)G*/ micro (w)(1 - lambdae(- micro (w)t) - micro (w)e(-lambdat)/lambda - micro (w)), where eta(w) is the fraction of total carbon production (G) allocated to stemwood, G* is the equilibrium value of G at canopy closure, lambda describes the rate at which G approaches G*, and micro (w) is the combined specific rate of stemwood maintenance respiration and senescence. According to this equation, which describes a sigmoidal growth curve, c(w) is zero initially and asymptotically approaches eta(w)G*/ micro (w) with the rate of approach dependent on lambda and micro (w). We used this result to derive corresponding expressions for the maximum mean annual stem-wood volume increment (Y) and optimal rotation length (T). By calculating the quantities G* and lambda (which characterize the variation of carbon production with stand age) as functions of the supply rate of plant-available nitrogen (U(o)), we estimated the responses of Y and T to changes in U(o). For a plausible set of parameter values, as U(o) increased from 50 to 150 kg N ha(-1) year(-1), Y increased approximately linearly from 8 to 25 m(3) ha(-1) year(-1) (mainly as a result of increasing G*), whereas T decreased from 21 to 18 years (due to increasing lambda). The sensitivity of Y and T to other model parameters was also investigated. The analytical model provides a useful basis for examining the effects of changes in climate and nutrient supply on sustainable forest productivity, and may also help in interpreting the behavior of more complex process-based models of forest growth.

Journal Article↗
PubMed1976

Kinetics of Hormone-induced H Excretion.

A study has been made of the kinetics of hormone-induced H(+) excretion from peeled Avena coleoptile sections using a new, simple technique involving direct application of the pH electrode to the surface of the section. Hormone-induced H(+) excretion begins after lags and occurs at rates which are consistent with a role of H(+) in regulating cell elongation. With fusicoccin, H(+) excretion begins within the 1st minute, and an external pH of 5 (optimal for wall loosening) is reached in 5 to 8 minutes, while with auxin the lag averages 14 minutes and pH 5 is reached in 20 to 30 minutes. KCN, which inhibits cell elongation in 3 to 5 minutes, stops H(+) excretion in less than 1 minutes, whereupon the external pH rises sharply. Cycloheximide stops auxin-induced H(+) excretion in 3 to 8 minutes, and the pH then rises slowly. In the absence of hormones, the pH of the extracellular solution comes to equilibrium at 5.6, but the actual pH of the wall solution is probably about 0.3 unit below this due to Donnan effects.

Journal Article↗
PubMed1997

Blood gas transport at high altitude.

As a model of human hypoxia, exposure to high altitude causes a number of ventilatory, circulatory and hemopoietic adaptations. A review of the literature on blood gas transport responses to hypoxia indicates that they are influenced not only by altitude, but also by factors related to acclimatization. In addition, it appears that the need to oxygenate tissues conflicts with the need to maintain H+ homeostasis. Thus, the final situation represents a compromise between the respiratory adjustment aimed at increasing blood alkalosis in order to optimize the oxygen transport system, and the metabolic readjustment aimed at reestablishing normal blood pH. There are factors like red cell 2,3-diphosphoglycerate, a compound that decreases the hemoglobin affinity for oxygen, that can influence that balance by affecting arterial oxygen saturation through mechanisms independent of respiration.

2,3-Diphosphoglycerate↗
PubMed1999

Effect of ligand and DNA binding on the interaction between human transcription intermediary factor 1alpha and estrogen receptors.

Hormonal regulation of gene activity is mediated by nuclear receptors acting as ligand-activated transcription factors. To achieve efficient regulation of gene expression, these receptors must interact with different type of molecules: 1) the steroid hormone, 2) the DNA response element, and 3) various proteins acting as transcriptional cofactors. In the present study, we have investigated how ligand and DNA binding influence the in vitro interaction between estrogen receptors (ERs) and the transcription intermediary factor hTIF1alpha (human transcriptional intermediary factor 1alpha). We first optimized conditions for the coactivator-dependent receptor ligand assay to lower ED50, and we then analyzed the ability of various natural and synthetic estrogens to allow the binding of the two types of proteins. Results were compared with the respective affinities of these ligands for the receptor. We then developed a protein-protein-DNA assay allowing the quantification of cofactor-ER-estrogen response element (ERE) complex formation in the presence of ligand and used measurements of fluorescence anisotropy to define the equilibrium binding parameters of the interaction. We demonstrated that the leucine-charged domain of hTIF1alpha is sufficient to interact with ERE-bound ERalpha in a ligand-dependent manner and showed that binding of ERalpha onto DNA does not significantly affect its hormone-dependent association with TIF1alpha. Finally, we show that, mainly in the absence of hormone, hTIF1alpha interacts better with ERbeta than with ERalpha independently of the presence of ERE.

Binding Sites↗
PubMed1984

Chlorotetracycline fluorescence is a quantitative measure of the free internal Ca2+ concentration achieved by active transport. In situ calibration and application to bovine cardiac sarcolemmal vesicles.

Chlorotetracycline (CTC) fluorescence is shown to be a competent and quantitative measure of the free internal calcium concentration, [Ca2+]i, obtained by ATP supported active uptake by bovine cardiac sarcolemmal (SL) vesicles. The fluorescence response of CTC to [Ca2+]i is calibrated by pre-equilibrating the vesicles with known Ca2+ concentrations and then diluting into a Ca2+-free medium containing CTC. The experiments show that CTC comes into equilibrium with the internal Ca2+ more rapidly than the latter can passively leak from the vesicles. The amplitude of the fluorescence increase is proportional to the Ca2+ concentration with which the vesicles are pre-equilibrated. This constitutes a calibration procedure for the use of CTC fluorescence as a quantitative measure of the free internal Ca2+ concentrations achieved in active transport. This method is applied to the determination of the average free Ca2+ concentrations achieved in ATP-energized uptake with sarcolemmal vesicles. Under optimal conditions an initial rate of 13 mM/min (37 nmol/mg/min) is observed. Uptake reaches a maximum corresponding to 70 mM (179 nmol/mg). Half-maximal values are obtained after 5 min of reaction. The mechanism of the CTC response to free internal Ca2+ concentration is discussed and is compared with measurements of vesicle-associated 45Ca2+.

Animals↗
PubMed1981

Alkaline protease from Neurospora crassa. Purification and partial characterization.

A simple purification procedure has been developed for the extracellular alkaline protease from Neurospora crassa. Key steps in the purification were: 1) the choice of gelatin as the protein inducer, which induces optimally at a much lower concentration than other commonly employed protein inducers; 2) heat treatment, during which the inducer is digested by the protease; and 3) a concentration step that eliminates the usual precipitation procedures and removes much of the digested protein inducer. These procedures were followed by routine ion exchange chromatography and gel filtration. The preparation was homogeneous, as determined by gel electrophoresis and ultracentrifugal analyses. A molecular weight of approximately 30,500 was determined by amino acid analysis, gel electrophoresis, and sedimentation equilibrium. The protease has 100% activity from pH 6.0 to 10.0, is heat labile above 45 degrees C, and susceptible to autodigestion. Hydrolysis of the beta chain from insulin indicates a preferential cleavage on the carboxyl group side of neutral and aromatic amino acids.

Amino Acid Sequence↗
PubMed2007

Impact of antinucleants on transdermal delivery of testosterone from a spray.

The goal was to explore whether the incorporation of antinucleant polymers into a testosterone spray formulation could stabilize a putative supersaturated state and improve the delivery of the drug across the skin. Several antinucleants were screened using differential scanning calorimetry (DSC) and two candidates showed particular promise: a cyclodextrin derivative (RAMEB) and a vinylpyrrolidone/vinyl acetate copolymer (Kollidon VA64). These agents also improved significantly the long-term stability of saturated solutions of the drug. Further, using the method of mixed cosolvents, it was possible to create, in the presence of 5% w/v antinucleant polymer, supersaturated ethanol/propylene glycol/water (4:1:1 v/v) solutions of the drug with degrees of saturation between 1.4 and 2.6; however, these metastable systems existed only transiently under carefully controlled conditions and had reverted back to equilibrium solubilities of the drug within 6 h. When the same solutions were administered to hairless rat skin in vitro from mechanical sprays, no improvement in testosterone delivery, relative to a nonstabilized control, was observed. It appears, therefore, that the in situ crystallization process of the drug is more complex and incompletely understood (and cannot be predicted from DSC experiments). The complicated evaporation/volatilization process, which takes place when a spray is pulverized, requires better characterization before the use of supersaturation for testosterone delivery can be optimized.

Administration, Cutaneous↗
PubMed2002

Non-equilibria in small metapopulations: comparing the deterministic Levins model with its stochastic counterpart.

In this paper, we examine, for small metapopulations, the stochastic analog of the classical Levins metapopulation model. We study its basic model output, the expected time to metapopulation extinction, for systems which are brought out of equilibrium by imposing sudden changes in patch number and the colonization and extinction parameters. We find that the expected metapopulation extinction time shows different behavior from the relaxation time of the original, deterministic, Levins model. This relaxation time is therefore limited in value for predicting the behavior of the stochastic model. However, predictions about the extinction time for deterministically unviable cases remain qualitatively the same. Our results further suggest that, if we want to counteract the effects of habitat loss or increased dispersal resistance, the optimal conservation strategy is not to restore the original situation, that is, to create habitat or decrease resistance against dispersal. As long as the costs for different management options are not too dissimilar, it is better to improve the quality of the remaining habitat in order to decrease the local extinction rate.

Animals↗