共查询到20条相似文献,搜索用时 24 毫秒
1.
L. Goldman 《European biophysics journal : EBJ》1989,16(6):321-325
The time constant of the process producing the delay in Na inactivation development as determined by the two pulse method (delay) was extracted and compared to that of the slowest Na activation process 3 for the I
Na during the conditioning pulse of that same determination. delay and two pulse inactivation c values were computer generated using a nonlinear least squares algorithm. h and single pulse inactivation h values were independently generated for each determination also with the aid of the computer using the same non-linear least squares algorithm. In one determination at 2 mV, c was 4.68 and delay 0.494 ms while h was 4.70 and 3 0.491 ms for a c/h of 0.996 and a delay/3 of 1.006. Mean delay/3 from five determinations in four axons, both Cs and K perfused, and spanning a potential range of-27 to 2mV was 1.068. The precursor process to inactivation is channel opening. Some fraction of channels presumably inactivate via another route where prior channel opening is not required. 相似文献
2.
Quantification of the time course and amplitude of endplate currents (EPC) was made with respect to dispersion of quanta secretion and to changes in the exponential decay of miniature endplate currents (mepc). The relationship between RPC amplitude and mepc follows a double-exponential curve with 1= 0.3 ms and 2 = 6 ms. If the amplitude of fully synchronised EPC is taken as 100%, then the loss of EPC amplitude is already 42% with physiological parameters of dispersion (the half-rise and decay constant of distribution of secretion probability = 0.5 ms, mepc =1 ms). This loss is even more substantial if secretion is more dispersed or miniature endplate currents decay faster.
Correspondence to: F. Vyskocil 相似文献
3.
Nicholas F. Hughes 《Environmental Biology of Fishes》1992,33(3):299-305
Synopsis Length-frequency data suggest Nile perch, Lates niloticus, from the Nyanza Gulf grew to a total length of 9 cm by age 118 days and 23 cm by age 287 days. A modified von Bertalanffy growth curve t = 1.35·L(1-e–K(t-t
o)) with the parameters L = 93.1, K = 0.272 and to = 0.046, is suggested to describe growth up to 5 years of age and the relationship t = 1.35·(31.96 + 7.681t) for fish aged 6 years and above. Length-weight relationships were = 0.0234·-gt2.74 for fish between 7 and 15.9 cm total length, = 0.0151·2.94 for fish between 16 and 45.9 cm total length, and = 0.0023·3.44 for fish between 46 and 120 cm total length. Male Nile perch first matured between 50 and 55 cm total length when they were probably 2 years old; female Nile perch first matured between 80 and 85 cm total length when they were probably 4 years old. Small males were common, large males were rare, with the reverse holding for females. Sex change, from male to female, is a possible explanation for this size dimorphism. 相似文献
4.
N. B. Cherfas Y. Peretz N. Ben-Dom B. Gomelsky G. Hulata 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1994,89(2-3):281-286
The results of a series of experiments conducted in our laboratory on the ornamental common carp (koi), aimed at optimizing heat-shock chromosome-set manipulation procedures, are described. The timing of heat-shock initiation was expressed in the relative unit of embryological age (0) in order to standardize this parameter, the absolute time for heat-shock initiation being calculated from duration of one 0 at two different pre-treatment water temperatures. Heat shocks were applied within the periods of 0.05–0.60 0 and 1.20–2.20 0 which, respectively, cover the successive phases of the 2nd meiotic division and the 1st cleavage. The highest production of diploid gynogenetic offspring was observed when heat shocks were initiated at 0.15–0.25 0 and at 1.5 0, after insemination, corresponding to anaphase of meiosis-II, and metaphase of the 1st cleavage, respectively. Similar results were obtained irrespective of the different pre-treatment water temperatures, thus confirming the possibility of standardizing heat-shock timing by 0. 相似文献
5.
Evoked and spontaneous end-plate currents (EPC) were studied in normal voltage-clamped frog sartorius muscle fibers and 2 weeks after application of colchicine to the nerve innervating the muscle to block axoplasmic transport in its fibers. Application of colchicine was found to reduce the rate of rise and to prolong decay of EPC without affecting the amplitude of the EPC and miniature EPC, the quantum composition of EPC, and the frequency of miniature EPC. The histogram of distribution of the time constant () of EPC decay under normal conditions follows the normal law, but after application of colchicine to the nerve it is shifted to the right, with separation of two modes (1 and 2). Three types of synapses can be distinguished from the character of EPC decay: monoexponential decay with 1 (44%), biexponential decay with 1 and 2 (39%), and monoexponential decay with 2 (19%). An increase in of EPC decay is accompanied by strengthening of the dependence of this process on the clamping voltage. The current-voltage characteristic and reversal potential of EPC are unchanged. It is suggested that the change in character of EPC decay after application of colchicine to the motor nerve is due to the appearance of acetylcholine-activated ionic channels in the muscle membrane with a longer duration of the open state and with potential-dependence of the open state similar to that taking place after muscle denervation.S. V. Kurashov Medical Institute, Ministry of Health of the RSFSR, Kazan'. Translated from Neirofiziologiya, Vol. 17, No. 2, pp. 204–211, March–April, 1985. 相似文献
6.
Structure–dynamics interrelationships are important in understanding protein function. We have explored the empirical relationship between rotational correlation times (c and the solvent accessible surface areas (SASA) of 75 proteins with known structures. The theoretical correlation between SASA and c through the equation SASA = Krc
(2/3) is also considered. SASA was determined from the structure, c
calc was determined from diffusion tensor calculations, and c
expt was determined from NMR backbone13 C or 15N relaxation rate measurements. The theoretical and experimental values of c correlate with SASA with regression analyses values of Kr as 1696 and 1896 m2s-(2/3), respectively, and with corresponding correlation coefficients of 0.92 and 0.70. 相似文献
7.
Müh Frank Schulz Claudia Schlodder Eberhard Jones Michael R. Rautter Joachim Kuhn Matthias Lubitz Wolfgang 《Photosynthesis research》1998,55(2-3):199-205
The effects of detergents on the electronic structure of the oxidized primary donor P+ and the time constant AP of the P+Q
A
–
charge recombination at ambient temperatures have been investigated in native and mutant reaction centers (RCs) from Rhodobacter sphaeroides. It is shown that N-lauryl-N,N-dimethyl-3-ammonio-1-propane sulfonate (SB12) induces a transition to a second distinct conformation of the RC. In the case of the wild type and the mutant FY(M197), in which a hydrogen bond is introduced to the 2-acetyl group of the dimer half of P that is associated with the M-subunit of the RC, the conformational change causes a more asymmetric spin density distribution between the two bacteriochlorophyll moieties of P+ in favor of the L-half. For both types of RCs the time constant AP depends on the SB12/RC ratio as does the position of the long-wavelength band of P, max. The increase of AP by 30 ms and the shift of max from 866 nm to 851 nm are indicative for the conformational change. In addition, a smaller linear increase of AP with increasing SB12/RC ratio is superimposed on the variation of AP due to the conformational change. Similar effects of SB12 on the optical spectra as well as on AP are also observed for the two heterodimer mutants HL(L173) and HL(M202), in which one of the bacteriochlorophylls of P is replaced by a bacteriopheophytin. There are no clear indications for a correlation of AP with the localization of the positive charge in P+. Furthermore, it is concluded from the dependence of AP on the SB12/RC ratio that the single-site mutations do not affect the standard free energy difference of the two conformations to a measurable extent. 相似文献
8.
The effect of correlation between the kinetics of acetylcholine (ACh) quanta release and the kinetics of the postsynaptic membrane channel activity on end-plate currents (EPCs) was studied in model experiments. The presynaptic process was described by a transmitter secretion probability distribution (SPD), and the postsynaptic process by characteristicsof miniature EPC (MEPC) with standard amplitude and variable decay duration (MEPC). The SPD was represented by a curve with an S-like rise and exponential decay. The main portion of the data was analyzed after being adjusted to 20°C. The effect of the EPC amplitude loss (the difference between the EPC amplitudes in cases of synchronous and non-synchronous release of the ACh quanta) due to temporal dispersion of the transmitter secretion process was 43% at MEPC=1.0 msec and 20% at MEPC=4–5 msec under conditions when acetylcholinesterase (AChE) was inhibited.When MEPC varied over a wide range, the effect of the EPC amplitude loss could be described by a curve with steeply and gently sloping portions; the boundary between these two portions corresponded to approximately 1.0 msec MEPC, which is nearly the channel lifetime at resting membrane potential. The rise time of the EPC only slightly related to changes in MEPC. The rate of EPC decay exceeded that in MEPC only when the lifetimes of the ACh-activated ionic channels were lower than their physiological values. In this case, the kinetics of transmitter release became a factor determining the EPC decay time course.Three variants of an increase in temporal dispersion of the ACh quanta secretion were modeled for constant MEPC: one resulting from an increase in the SPD rising phase; a second resulting from equal increases in the SPD rising and declining phases, and a third resulting from an increase in the SPD declining phase. The results of the first variant corresponded most closely to experimental data on changes in EPC during a short-term period of rhythmical activity. This was also characterized by minimal effect of EPC amplitude loss.It has been suggested that the interrelation between the parameters of SPD and MEPC is an important factor regulating the amplitude and duration of the postsynaptic signal. The maintenance of this interrelation at a constant level ensures reliability of synaptic transmission.Translated from Neirofiziologiya, Vol. 25, No. 2, pp. 126–132, March–April, 1993. 相似文献
9.
The effective diffusivity of glucose in porous glass beads was determined using a transient method. Predictions for the intraparticle and surface concentrations were made by an analytical solution of the mass balance. The value of the diffusivity was expected to be lower than the value of the corresponding diffusion coefficient in water, but the opposite was observed. This effect results from intraparticle fluid flow, leading to high values of the apparent effective glucose diffusivity. To measure diffusion only and to prevent any internal convection during the diffusion experiment, the pores of the porous glass beads were filled with Ca-alginate gel. For these glass beads (internal porosity, , equal to 0.56), we found an effective glucose diffusivity of 2.2×10–10 m2/s at 30°C. Using the relationship to effective intraparticle diffusivity (Deff)=effective diffusivity in 1% Ca-alginate beads (Dgel) / (with the tortuosity factor) this gives =1.7. For known and measuring by the method described, the Deff can be calculated for other porous materials or diffusing substances. Knowledge of the exact value of the effective diffusivity is a necessity in bioreactor modelling and was demonstrated by prediction of the residence time distribution profiles in a packed-bed bioreactor containing immobilized yeast cells. 相似文献
10.
N. Shiragami 《Bioprocess and biosystems engineering》1994,10(1):47-51
A new method for testing the strength of cells against fluid shear stress by using a long capillary column was proposed. The trajectories of cells in the column were simulated by introducing the Brownian motion model. The Brownian motion was performed by the generation of random numbers. The mean exposure time to shear stress and the mean shear stress acting on the surface of cells were discussed by the result of computer simulation. The mean shear stress acting on the surface of cells flowing in the capillary column was estimated as 4/3-fold of the shear stress at the column wall provided that the ratio of the cell radius to the column radius does not exceed 0.08. The effectiveness of this new method for testing the strength of cells against fluid shear stress was shown.List of Symbols
a m
radius of cell
-
c
constant
-
E
distribution function
-
L m
length of capillary column
-
M
number of division
-
N
number of division
-
p
probability
-
Q m3/s
flow rate
-
R m
radius of capillary column
-
r m
radial position
-
t s
time
-
T s
exposure time
-
T
m s
mean exposure time
-
T
0 s
mean residence time
-
m/s
axial velocity
-
u
m
m/s
cross-sectional flow velocity
-
z m
axial position
-
s–1
shear rate
-
w
s–1
shear rate at wall
-
Pa s
viscosity
-
spherical coordinate
-
spherical coordinate
-
Pa
shear stress
-
m
Pa
mean shear stress
-
w
Pa
shear stress at wall 相似文献
11.
Study of neuronal gain in a conductance-based leaky integrate-and-fire neuron model with balanced excitatory and inhibitory synaptic input 总被引:2,自引:0,他引:2
Neurons receive a continual stream of excitatory and inhibitory synaptic inputs. A conductance-based neuron model is used to investigate how the balanced component of this input modulates the amplitude of neuronal responses. The output spiking rate is well described by a formula involving three parameters: the mean and variance of the membrane potential and the effective membrane time constant Q. This expression shows that, for sufficiently small Q, the level of balanced excitatory-inhibitory input has a nonlinear modulatory effect on the neuronal gain. 相似文献
12.
Laser-velocimetry was applied in order to study the effect of light on the velocity of protoplasmic streaming (pps) in Characean cells. A change from dark to light (= 6 W · m–2) leads to an acceleration of streaming by about 15–30% with a time-constant of approx. 300 s. The transition from light to dark causes a transient decrease of velocity below the original dark level. This response occurs with a time constant of about 500 s. It returns to its initial value with a time-constant of about 2000 s. This may indicate that a control loop of cytosolic homeostasis takes a decrease in pCa more seriously than an increase. A possible involvement of temperature effects caused by illumination was excluded by measuring the influence of temperature. Steady-state velocity of streaming changed by 5% per 1° C. Irradiation with infra-red light ( > 780 nm) did not cause a change in velocity. The absence of a light effect on streaming velocity in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) shows that photosynthesis and not phytochrome is involved. The role of light-induced changes of pCa is discussed, especially with respect to the hypothesis of Vanselow and Hansen (1989, J. Membr. Biol. 110, 175–187) that photosynthesis acts on the plasmalemma K+-channel via light-induced uptake of Ca2+ into the chloroplasts.Abbreviations and Symbols ASF
auto structure function
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- pps
protoplasmic streaming
- L, D, C
time-constants of the light and dark responses, and of a putative Ca-control system
Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. The first author was granted a scholarship by the state of Schleswig-Holstein. We are indebted to Prof. Dr. G. Pfister for technical advice and helpful discussions and to Mrs. E. Götting for drawing the figures. 相似文献
13.
Conformational and dynamic properties of the anticodon loop of yeast tRNAPhe were investigated by analyzing the time resolved fluorescence of wybutine serving as a local structural probe adjacent to the anticodon GmAA on its 3 side. The influence of Mg2+, important for stabilizing the tertiary structure of tRNA, and of the complementary anticodon s2UUC of E. coli tRNA
2
Glu
were investigated.Fluorescence lifetimes and anisotropies were measured with ps time resolution using time correlated single photon counting and a mode locked synchronously pumped and frequency doubled dye laser as excitation source. From the analysis of lifetimes () and rotational relaxation times (
R
) we conclude that wybutine occurs in various structural states: (i) one stacked conformation where the base has no free mobility and the only rotational motion reflects the mobility of the whole tRNA molecule (=6 ns,
R
=19 ns), (ii) an unstacked conformation where the base can freely rotate (=100 ps,
R
= 370 ps) and (iii) an intermediary state (=2 ns,
R
= 1.6 ns).Under biological conditions, i. e. in the presence of Mg2+ and neutral salts, wybutine is found in a stacked and immobile state which is consistent with the crystallographic picture. In the presence of the complementary codon however, as exemplified by the E. coli-tRNA
2
Glu
anticodon, our analysis indicates that the codon-anticodon complex exists in an equilibrium of structural states with different rotational mobility of wybutine. The conformation with wybutine freely mobile is the predominant one and suggests that this conformation of the codon-anticodon structure differs from the canonical 3–5 stack. 相似文献
14.
It was shown on the example of chick embryo that the number 0 (
n
/0) can be recommended as a measure of biological time and, for this purpose, the duration of the minimal mitotic cycle during synchronous cleavage divisions should be determined (in minutes) in various avian species. Based on the preliminary data, one can propose the comparability and similarity of the temporal programs of gastrulation and somitogenesis in the chick embryo and embryos of some fish and amphibians. 相似文献
15.
Summary The kinetics of DNA chain breakage in solution induced by 2 µs pulses of 15 MeV electrons were investigated by light scattering. On irradiating native calf thymus DNA at room temperature the decrease of light scattering intensity (LSI) - due to double strand ruptures - shows a fast decay with a half life
1/2 of about 30 ms as well as a slow decay with
1/2 of about 10 s. With increasing temperature (20–40° C) both the total degree of degradation and the fraction of the fast decay increase due to the facilitated melting of segments between two single strand breaks on alternate strands forming a double strand break. Above 40° C a third mode of LSI decay with
1/2 of 5–10 s arises, indicating detachment of relatively long segments.The total relative decrease of LSI after irradiation A, which can be taken as a measure of the degree of degradation, follows the square of the absorbed dose in the case of native DNA, whereas on irradiating denatured DNA A rises linearly with dose. The decay of LSI due to the degradation of denatured DNA is much faster than that of native DNA with
1/2 down to 150 µs, depending on the absorbed dose. The half lives are interpreted in terms of the separation of fragments by diffusion and of the melting of double strand segments between two single strand breaks. 相似文献
16.
The role of postysynaptic potentiation (PSP) and asynchronous secretion of acetylcholine (ACh) in the generation of multiquantal currents and end plate currents (EPC) was investigated under voltage clamp conditions in transected sartorius muscle of the frog before and after 4-aminopyridine (4-AP) treatment. Compared with miniature EPC (MEPC), showing an average quantum content of 249, multiquantal EPC has a larger amplitude, longer rise-time, and longer decay-time (epc). Magnesium ions (6–10 mM) reduce the amplitude and spec of EPC without affecting its rise-time. Rhythmical stimulation (10 Hz for 60 sec) results in reduced amplitude and but increased rise-time of EPC. D-turbocurarine (5×10–7 M) and -bungarotoxin (1×10–5 gm/ml) diminishes the difference between epc and mepc. In the presence of 4-AP, all these effects are much more pronounced. It is proposed that asynchronous secretion of ACh from motor nerve teminals causes prolongation of the rise-time and reduction of the amplitude of EPC but has little or no effect upon the decay rate of EPC. The slow decay of multiquantal EPC, both in the absence and in the presence of 4-AP, is almost entirely due to postynaptic interaction of ACh quanta, i.e., PSP.Kurashov Meidcal Institute, Kazhan. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 48–56, January–February, 1991. 相似文献
17.
John A. White Carson C. Chow Jason Rit Cristina Soto-Treviño Nancy Kopell 《Journal of computational neuroscience》1998,5(1):5-16
We study some mechanisms responsible for synchronous oscillations and loss of synchrony at physiologically relevant frequencies (10–200 Hz) in a network of heterogeneous inhibitory neurons. We focus on the factors that determine the level of synchrony and frequency of the network response, as well as the effects of mild heterogeneity on network dynamics. With mild heterogeneity, synchrony is never perfect and is relatively fragile. In addition, the effects of inhibition are more complex in mildly heterogeneous networks than in homogeneous ones. In the former, synchrony is broken in two distinct ways, depending on the ratio of the synaptic decay time to the period of repetitive action potentials (s/T), where T can be determined either from the network or from a single, self-inhibiting neuron. With s/T > 2, corresponding to large applied current, small synaptic strength or large synaptic decay time, the effects of inhibition are largely tonic and heterogeneous neurons spike relatively independently. With s/T < 1, synchrony breaks when faster cells begin to suppress their less excitable neighbors; cells that fire remain nearly synchronous. We show numerically that the behavior of mildly heterogeneous networks can be related to the behavior of single, self-inhibiting cells, which can be studied analytically. 相似文献
18.
Interrelation between HeLa-S3 cell transfection and hemolysis in red blood cell suspension using pulsed ultrasound of various duty cycles 总被引:1,自引:0,他引:1
We have studied the in vitro transfection of a plasmid DNA with the lacZ gene to HeLa-S3 cells and hemolysis in a red blood cell (RBC) suspension under pulsed ultrasound with duty cycles of 10, 20 and 30% using a digital sonifier at a frequency of 20 kHz and an intensity of 6.2 W/cm2 on the surface of a horn tip. Cultured HeLa-S3 cells in suspension were exposed to pulsed ultrasound for an apparent exposure time t from 0 to 60 s. HeLa-S3 viability decreased as a single exponential function of the total exposure time t=t with a common time constant =3.8 s for three duty cycles. Transfection was evaluated by counting the number of -galactosidase(-Gal)-positive cells relative to the total number of cells. Pulsed ultrasound provided an enhanced transfer of the -Gal plasmid to HeLa-S3 cells, 3.4-fold as compared with that in the case of the control. The optimal transfection efficiencies were 0.75, 0.80 and 0.74% near t= with =10, 20 and 30%, respectively. The number ratio of -Gal-positive cells to the surviving cells after exposure increased with t according to a modified logistic equation. The degree of hemolysis also increased exponentially with t at a time constant =0/ for the RBC suspension in physiological saline at a hematocrit concentration of 0.5% with 0=0.9 s. Thus the total exposure time for the optimal transfection efficiency was , that is, nearly four times of 0. Hemolysis in the RBC suspension may be a useful model for determining optimal transfection by pulsed ultrasound of various duty cycles. 相似文献
19.
Summary Activation kinetics of single high-threshold inactivating (HTI orN-type) calcium channels of cultured dorsal root ganglion cells from mouse embryos was studied using a patchclamp method. Calcium channels displayed bursting activity. The open-time histogram was single exponential with an almost potential-independent mean open time op. The closed-time histogram was multicomponent; at least three of the components were associated with the activation process. The fast exponential component with the potential-independent time constant
cl
f
included all intraburst gaps, while two slower ones with potential-dependent time constants
cl
vs
described shut times between bursts and between clusters of bursts. The burst length histogram was biexponential. The fast component with a relatively potential-independent time constant
bur
f
described short, isolated channel openings while the slow component characterized real bursts with a potential-dependent mean life time. The waiting-time histogram could be fitted by a difference of two exponentials with time constants being the same as
cl
s
and
cl
vs
. The data obtained were described in the frame of a 4-state sequential model of calcium channel activation, in which the first two stages are formally attributed to potential-dependent transmembrane transfer of two charged gating particles accompanying the channel transitions between three closed states, and the third one to fast conformational changes in channel protein leading to the opening of the channel. The rate constants for all transitions were defined. The validity of the proposed model for both low-threshold inactivating (LTI orT-type) and high-threshold noninactivating (HTN orL-type) calcium channels is discussed. 相似文献
20.
Joël Mispelter Claudine Lefèvre Élisabeth Adjadj Éric Quiniou Vincent Favaudon 《Journal of biomolecular NMR》1995,5(3):233-244
Summary Dynamics of the backbone and some side chains of apo-neocarzinostatin, a 10.7 kDa carrier protein, have been studied from 13C relaxation rates R1, R2 and steady-state 13C-{1H} NOEs, measured at natural abundance. Relaxation data were obtained for 79 nonoverlapping C resonances and for 11 threonine C single resonances. Except for three C relaxation rates, all data were analysed from a simple two-parameter spectral density function using the model-free approach of Lipari and Szabo. The corresponding C–H fragments exhibit fast (e < 40 ps) restricted libration motions (S2=0.73 to 0.95). Global examination of the microdynamical parameters S2 and e along the amino acid sequence gives no immediate correlation with structural elements. However, different trends for the three loops involved in the binding site are revealed. The -ribbon comprising residues 37 to 47 is spatially restricted, with relatively large e values in its hairpin region. The other -ribbon (residues 72 to 87) and the large disordered loop ranging between residues 97–107 experience small-amplitude motions on a much faster (picosecond) time scale. The two N-terminal residues, Ala1 and Ala2, and the C-terminal residue Asn113, exhibit an additional slow motion on a subnanosecond time scale (400–500 ps). Similarly, the relaxation data for eight threonine side-chain C must be interpreted in terms of a three-parameter spectral density function. They exhibit slower motions, on the nanosecond time scale (500–3000 ps). Three threonine (Thr65, Thr68, Thr81) side chains do not display a slow component, but an exchange contribution to the observed transverse relaxation rate R2 could not be excluded at these sites. The microdynamical parameters (S2, e and R2ex) or (S
infslow
sup2
, S
inffast
sup2
and slow) were obtained from a straightforward solution of the equations describing the relaxation data. They were calculated assuming an overall isotropic rotational correlation time e for the protein of 5.7 ns, determined using standard procedures from R2/R1 ratios. However, it is shown that the product (1–S2)× e is nearly independent of e for residues not exhibiting slow motions on the nanosecond time scale. In addition, this parameter very closely follows the heteronuclear NOEs, which therefore could be good indices for local fast motions on the picosecond time scale. 相似文献