共查询到20条相似文献,搜索用时 15 毫秒
1.
Terence P. McGonigle 《Mycorrhiza》2001,10(5):249-254
For the regression of sigmoid-shaped responses with time t of colonization C of roots by arbuscular mycorrhizal fungi, C=C
p/1+[e
−k(t−ti)
] is the most useful form of the logistic equation. At the time of inflection t
i the slope is maximal and directly proportional to the product of the colonization plateau C
p and the abruptness k of the curve. Coefficient k has a high value when the curve rises abruptly following and preceding long shallow phases. The logistic equation has a curve
that is symmetrical about t
i such that C=C
p/2 at inflection. Although the logistic equation can generate a good fit to many data sets for changes in colonization with
time, there are cases that are not sigmoid and the logistic equation does not apply. For sigmoid curves, the lag in the development
of colonization is directly related to both t
i and k but not to the plateau and not to the value of the maximum slope. Higher values of k or t
i reflect longer lag. When considered alone, t
i and k do not fully summarize the lag in colonization, and so a numerical method to combine them is presented here which allows
lag to be compared between curves. In this method, the lag is evaluated by calculating the time during early colonization
when the slope equals half of the value of the maximum slope. In summary, use of the logistic equation for regression of sigmoid
curves of colonization with time allows numerical comparison between curves of the lag, the period of steep ascent, and the
plateau. The logistic equation does not model directly the fundamental processes at work in the development of the mycorrhizae.
Instead, it can be used as described here to gain insight into the colonization process by comparing the dynamics of that
colonization for different species under various conditions.
Accepted: 20 October 2000 相似文献
2.
Solutions are presented for N + 1 sequential and reversible first-order reactions for which the magnitude of the reverse rate constant, kb, for all steps except the last is identical. Also the magnitude of the forward rate constant, kf, for all steps except the first and last is identical. The initial and final steps are nucleation reactions; therefore, the initial and final kf are modified by the factors σ′ and γ respectively. The final kb is modified by the factor γ σ. The ratio kb/kf is defined as s, which has the same meaning as s in the Zimm-Bragg theory. The mathematical model is intended to apply to polymeric molecules of N segments and allows the calculation of the mole fraction of molecules in state i at any time t, Ci(t). A molecule in state i has i unreacted segments and N – i reacted ones. Because the reactions are sequential, all reacted segments are contiguous. Our numerical results show that when σ′ is much less than unity and the forward reaction is favored, the relaxation curve is sigmoidal. If, however, the forward and reverse reactions are equally favored (i.e., s ? 1) the relaxation curve is a straight line. When s and σ′ are near unity, the curve is exponential for a considerably large fraction of the reaction. Further, in the exponential for a considerably large fraction of the reaction. Further, in the exponential phase of the reaction, the relaxation time is proportional to N2 for highly cooperative systems (i.e., Nσ ? 1). As found by Pipkin and Gibbs, if N is sufficiently large and s is less than unity (e.g., N ? 50 and s ?0.9) the relaxation curve is largely linear with a slope inversely proportional to N. Applications are given for the unwinding of double-helical poly(A·U) and the order–order transition in poly-L -proline. 相似文献
3.
Rates of change in final summer densities of two desert annuals, Eriogonum abertianum and Haplopappus gracilis, as constrained by their initial winter germination densities were estimated with regression quantiles and compared with mechanistic fits based on a self‐thinning rule proposed by Guo et al. (1998); Oikos 83: 237–245). The allometric relation used was equivalent to S=Nf (Ni)?1=cf (Ni)?1, where S is the ratio of final to initial densities (survivorship), cf is a constant that is a final density specific to the species and environment, Ni is the initial plant density, and Nf is final plant density. We used regression quantiles to estimate cf assuming the exponent of ?1 was fixed (model 1, Nf (Ni)?1=cf (Ni)?1) and also obtained estimates by treating the exponent as a parameter to estimate (model 2, Nf (Ni)?1=cf (Ni)λ). Regression quantiles allow rates of change to be estimated through any part of a data distribution conditional on some linear function of covariates. We focused on estimates for upper (90–99th) quantiles near the boundary of the summer density distributions where we expected effects of self‐thinning to operate as the primary constraint on plant performance. Allometric functions estimated with regression quantiles were similar to functions fit by Guo et al. (1998) when the exponent was constrained to ?1. However, the data were more consistent with estimates for model (2), where exponents were closer to ?0.4 than ?1, although model fit was not as good at higher initial plant densities as when the exponent was fixed at ?1. An exponential form (model 3, Nf (Ni)?1=cf (Ni)λ eγNi) that is a generalization of the discrete logistic growth function, where estimates of λ were ?0.23 to ?0.28 and estimates of γ were ?0.003 to ?0.006, provided better fit from low to high initial germination densities. Model 3 predictions were consistent with an interpretation that final summer densities were constrained by initial germination densities when these were low (<40 per 0.25 m2 for Eriogonum and <100 per 0.25 m2 for Haplopappus) and were constrained by the self‐thinning process at higher germination densities. Our exponential model (3) estimated with regression quantiles had similar form to the mechanistic relation of Guo et al. (1998) when plotted as a survivorship function, but avoided the unrealistic assumption that all populations attained a similar final density, and was based on a statistical model that has formal rules for estimation and inference. 相似文献
4.
Calcium homeostasis and mitochondrial oxidative metabolism interact closely in brain and both processes are impaired during hypoxia. Since the regulation of the pyruvate dehydrogenase complex (PDHC) may link these two processes, the relation of cytosolic free calcium ([Ca2+]i) to the activation state of PDHC (PDHa) was assessed in isolated nerve terminals (i.e. synaptosomes) under conditions that alter [Ca2+]i. K+ depolarization elevated [Ca2+]i and PDHa and both responses required external calcium. Treatment with KCN, an in vitro model of hypoxia decreased ATP and elevated [Ca2+]i and PDHa. Furthermore, in the presence of KCN, PDHa became more sensitive to K+ depolarization as indicated by larger changes in PDHa than in [Ca2+]i. The calcium ionophore Br-A23187 elevated [Ca2+]i, but did not affect PDHa. K+ depolarization elevated [Ca2+]i and PDHa even if [Ca2+]i was elevated by prior addition of ionophore or KCN. Previous in vivo studies by others show that PDHa is altered during and after ischemia. The current in vitro results suggest that hypoxia, only one component of ischemia, is sufficient to increase PDHa. These data also further support the notion that PDHa is regulated by [Ca2+]i as well as by other factors such as ATP. Our results are consistent with the concept that PDHa in nerve endings may be affected by [Ca2+]i and that these two processes are clearly linked.Abbreviations (PDHa)
Activation state of the pyruvate dehydrogenase complex
- ([Ca2+]i)
cytosolic free calcium concentrations
- (MOPS)
3(N-morpholino)propanesulfonic acid
- (fura-2AM)
fura-2 acetoxymethyl ester
- (AABS)
p-(p-aminophenylazo)benzene sulfonic acid
- (PDHC)
pyruvate dehydrogenase complex
- (TES)
N-tris{[hydroxymethyl]methyl}-2-amino-ethanesulfonic acid
- (SNK-test)
Student-Newman-Keuls test 相似文献
5.
The difference equation f
b
:[0,1]–[0,1] defined by f
b
(x)=b x(1–x) is studied. In particular complete qualitative information is obtained for the parameter value b=3.83. For example the number of fixed points of (f
b
)i is given by
Ni = 1 + ( \frac1 + ?5 2 )i + ( \frac1 - ?5 2 )iN_i = 1 + \left( {\frac{{1 + \sqrt 5 }}{2}} \right)^i + \left( {\frac{{1 - \sqrt 5 }}{2}} \right)^i 相似文献
6.
G. Pagani Zecchini M. Paglialunga Paradisi I. Torrini G. Lucente E. Gavuzzo F. Mazza G. Pochetti M. Paci M. Sette A. Di Nola G. Veglia S. Traniello S. Spisani 《Biopolymers》1993,33(3):437-451
In order to induce a β-turn conformation into the chemotactic linear tripeptide N-formyl-L -methionyl-L -leucyl-L -phenylalanine (fMLP), the new analogue N-formyl-L -methionyl-ΔZleucyl-L -phenylalanine methyl ester [ ΔZLeu]2f MLP-OMe ( 1 ) has been synthesized. The conformational and biochemical consequences of this chemical modification have been determined. Analogue 1 has been synthesized by using N-carboxy-(Z)-α,β-didehydroleucine anhydride as key compound to introduce the unsaturated residue at the central position of the tripeptide 1 . The x-ray analysis shows that 1 adopts in the crystal a type II β-turn conformation in which the new residue occupies the (i + 2) position, and an intramolecular H bond is formed between the formylic oxygen and the Phe NH. 1H-nmr analysis based on nuclear Overhauser effect measurements suggests that the same folded conformation is preferred in CDCl3 solution; this finding is also supported by molecular dynamics simulation. The biological activity of 1 has been determined on human neutrophils (polymorphonuclear leukocytes) and compared to that shown by f MLP-OMe. Chemotactic activity, granule enzyme release, and superoxide anion production have been determined. Analogue 1 is practically inactive as chemoattractant, highly active in the superoxide generation, and similar to the parent in the lysozyme release. The conformational restriction imposed on the backbone by the presence of the unsaturated residue is discussed in relation with the observed bioselectivity. © 1993 John Wiley & Sons, Inc. 相似文献
7.
Statistics are collected and analyzed for the possibility of hydrogen bonding in the secondary structures of globular proteins,
based on geometric criteria. Double and bifurcated bonds are considered as pairs of admissible H-bonds with two proton donors
or two proton acceptors, respectively. Most of such bonds belong to peptide groups in α-helices, with O
i
…N
i + 3 nearly as frequent as O
i
…N
i + 4; in contrast, most of the 3/10-helical segments are too short to have any. Alternating double and bifurcated bonds in α-helices
form an apparently cooperative network structure. A typical α-helical segment perhaps carries two stretches of the H-bond
network broken in the middle. The constituent H-bonds are nonlinear: the hydrogen atom is off the straight line connecting
the proton donor and proton acceptor atoms. This deflection is larger for H
i + 3 vs. bond line O
i
−N
i + 3 than for H
i + 4 vs. O
i
−N
i + 4, and though the two kinds of bond have about the same length (exceeding those typical of low-molecular compounds), O
i
…N
i + 4 must be stronger than O
i
…N
i + 3. Double/bifurcated bonds are also not coplanar, i.e., hydrogen atoms are beyond the N…O…N (or O…N…O) plane.
The text was submitted by the authors in English. 相似文献
8.
To investigate how excess excitation energy is dissipated in a ribulose-1,5-bisphospate carboxylase/oxygenase activase antisense
transgenic rice with net photosynthetic rate (P
N) half of that of wild type parent, we measured the response curve of P
N to intercellular CO2 concentration (C
i), electron transport rate (ETR), quantum yield of open photosystem 2 (PS2) reaction centres under irradiation (Fv′/Fm′), efficiency of total PS2 centres (ΦPS2), photochemical (qP) and non-photochemical quenching (NPQ), post-irradiation transient increase in chlorophyll (Chl) fluorescence (PITICF), and
P700+ re-reduction. Carboxylation efficiency dependence on C
i, ETR at saturation irradiance, and Fv′/Fm′, ΦPS2, and qP under the irradiation were significantly lower in the mutant. However, NPQ, energy-dependent quenching (qE), PITICF, and P700+ re-reduction were significantly higher in the mutant. Hence the mutant down-regulates linear ETR and stimulates cyclic electron
flow around PS1, which may generate the ΔpH to support NPQ and qE for dissipation of excess excitation energy. 相似文献
9.
Abstract: The identity and role of G proteins in coupling adenosine receptors to effectors have been studied to a limited degree. We have identified the G proteins whose GTPase activity is stimulated by adenosine receptor agonists in neuronal membranes. (R)-Phenylisopropyladenosine, 2-chloroadenosine, and N-ethylcarboxamideadenosine produced a concentration-dependent stimulation of GTPase. At 10?5M, the increase above basal GTPase in frontal cortex was 25 ± 4, 20 ± 3, and 8 ± 1%, respectively, and in the cerebellum 55 ± 2, 41 ± 4, and 22 ± 2%, respectively. The effects of (R)-phenylisopropyladenosine and 2-chloroadenosine were inhibited by (1) A1 antagonists (76–96% reduction), (2) pretreatment with pertussis toxin (90–100% reduction), and (3) antibodies raised against the α-subunit of Gi and Go (55–57% reduction by each), suggesting that A1 receptors interact equally with Gi and Go. (R)-Phenylisopropyladenosine increased the binding of a nonhydrolyzable analogue of GTP to membranes in a pertussis toxin-sensitive manner, indicative of activation of Gi or Go. Previously, (±)-Bay K 8644 enhanced GTP hydrolysis by Go but not Gi. Now we report a profound synergistic stimulation of GTPase in the presence of (R)-phenylisopropyladenosine and (±)-Bay K 8644 (10?7 to 10?5M). (±)-Bay K 8644 had no effect on nucleotide exchange and, thus, cannot activate Go. It appears that a positive cooperative stimulation of Go occurs when it is first activated by A1 receptors and subsequently interacts with the L-type Ca2+ channel. 相似文献
10.
The effects of the selective adenosine (ADO) A3receptor agonist IB-MECA (N6-(3-iodobenzyl)adenosine-5′-N-methylcarboxamide) on cultured newborn rat cardiomyocytes were examined in comparison with ADO, the ADO A1receptor-selective agonistR-PIA (N6-R-phenylisopropyladenosine), or the ADO A3selective antagonist MRS 1191 (3-ethyl-5-benzyl-2-methyl-6-phenyl-4-phenylethynyl-1,4-(±)-dihydropyridine-3,5 dicarboxylate), using digital image analysis of Feulgen-stained nuclei. At high concentration, IB-MECA (10 μM ) and ADO (200 μM) induced apoptosis; however,R-PIA or MRS 1191 did not have any detectable effects on cardiac cells. In addition, DNA breaks in cardiomyocytes undergoing apoptosis following treatment by IB-MECA were identifiedin situusing the nick end labeling of DNA (“TUNEL”-like) assay. In the presence of 10 μM IB-MECA, disorder in the contraction waves appeared, and a decrease in the frequency of beats was observed. Analysis with light microscopy revealed that the number of contracting cells decreased in a concentration-dependent manner. The A3receptor agonist IB-MECA caused an increase in intracellular free calcium concentration ([Ca2+]i). The drug produced a rapid rise followed by a sustained increase in [Ca2+]i, which lasted for 40–60 s. Finally, cessation of beating and Ca2+transients were observed. Full recovery of contractile activity and rhythmical Ca2+transients were observed 15–20 min after IB-MECA treatment. The induction of apoptosis in the cardiocytes by IB-MECA led to the appearance of features of apoptotic nuclei: the onset of condensation, compacting, and margination of nuclear chromatin. These effects were accompanied by the disintegration of the structural framework of the nucleus and nuclear breakdown. The results suggest that activation of the A3adenosine receptor may participate in the process of apoptosis in cardiomyocytes. 相似文献
11.
Masae Shiyomi 《Population Ecology》1967,9(1):53-61
The plant-to-plant movement of insects in one of the factors determining the distribution of individuals in insect populations. In this report the movement of barley aphids was analyzed by a statistical model. The model is represented as the convolution of three probability functions:
12.
Dermenkephalin, H-Tyr-(D ) Met-Phe-His-Leu-Met-Asp-NH2, a highly potent and selective δ-opioid peptide isolated from frog skin, was studied in DMSO-d6 solution by two-dimensional nmr spectroscopy, including the determination of NH temperature coefficients, the evaluation of 3J coupling constants from phase-sensitive correlated spectroscopy (COSY) and the volumes of nuclear Overhauser effect (NOE) correlations. The two-dimensional NOE spectroscopy (NOESY) spectrum of dermenkephalin revealed sequential, medium-, and long-range effects. To put this information on a quantitative basis, special attention was devoted to J cross-peak suppression, quantification of the NOE volumes and analysis of the overlaps, normalization of the NOEs against diagonal peaks and Hββ′ geminal interactions. Although most of the dihedral angles deduced from the 3JNα coupling constants together with several Niαi and αiNi + 1 NOEs pointed to a partially extended peptide backbone, several Ni Ni + 1 NOEs and βi Ni + 1 interactions argued in favor of a folded structure. Moreover, several long-range correlations of strong intensities were found that supported a close spatial proximity between the side chains of D -Met2 and Met6, Tyr1 and His4, Tyr1 and Asp7, and His4 and the C-terminal amide group. In Phe, the g? rotamer in the side chain is deduced from the 3Jαβ coupling constants and αβ and Nβ NOE correlations. Whereas the amide proton dependency was not indicative of stable hydrogen bonds, the nonuniform values of the temperature coefficient may reflect an equilibrium mixture of folded and extended conformers. The overall data should provide realistic starting models for energy minimization and modelization studies. © 1993 John Wiley & Sons, Inc. 相似文献
13.
Fujita H Kato T Watanabe N Takahashi T Kitagawa S 《Archives of biochemistry and biophysics》2011,(1):121-60
Calpain inhibitors induce pertussis toxin (PTx)-sensitive chemotaxis in human neutrophils and monocytes. Here, we show that various calpain inhibitors (PD150606, PD151746, N-acetyl-Leu-Leu-Nle-CHO [ALLN], N-acetyl-Leu-Leu-Met-CHO [ALLM], and calpeptin) and γ-secretase inhibitor I induced PTx-sensitive increase in cytoplasmic free Ca2+ ([Ca2+]i) in human neutrophils and neutrophil migration. HEK-293 cells stably expressing human formyl peptide receptor (hFPR) or hFPR-like 1 (hFPRL1) displayed stimulus-specific increase in [Ca2+]i in response to calpain inhibitors (PD150606, PD151746, ALLN, ALLM, MG-132, and calpeptin), γ-secretase inhibitor I, and N-formyl-Met-Leu-Phe. Parent HEK-293 cells also displayed PTx-sensitive increase in [Ca2+]i in response to calpeptin and γ-secretase inhibitor I, whereas they displayed PTx-resistant increase in [Ca2+]i in response to MG-132. MDL-28170 induced neither an increase in [Ca2+]i in neutrophils and HEK-293 cells nor neutrophil migration. Ionomycin-induced cleavage of talin (a substrate of calpain) in neutrophils was inhibited by all inhibitors used here. These findings suggest that potent calpain inhibitors could stimulate phagocyte functions via activation of hFPR, hFPRL1 and/or other G-protein coupled receptors depending on the inhibitors used. 相似文献
14.
Dean M. Young Danny W. Turner Virgil R. Marco 《Biometrical journal. Biometrische Zeitschrift》1987,29(6):721-730
Let population Πi, i = 1,2, be characterized by a multivariate normal density function, N(μi,∑i), i = 1, 2, respectively. This paper provides conditions under which simple conditional error rates may be computed for the quadratic discriminant function with known population parameters. Also, a simple bound on the overall error rate is derived. Examples are given which demonstrate the proposed methods. 相似文献
15.
Fujita H Kato T Watanabe N Takahashi T Kitagawa S 《Archives of biochemistry and biophysics》2011,516(2):121-127
Calpain inhibitors, including peptide aldehydes (N-acetyl-Leu-Leu-Nle-CHO and N-acetyl-Leu-Leu-Met-CHO) and α-mercapto-acrylic acid derivatives (PD150606 and PD151746), have been shown to stimulate phagocyte functions via activation of human formyl peptide receptor (hFPR) and/or hFPR-like 1 (hFPRL1). Using the homology modeling of the receptors and the ligand docking simulation, here we show that these calpain inhibitors could bind to the putative N-formyl-Met-Leu-Phe (fMLF) binding site on hFPR and/or hFPRL1. The studies with HEK-293 cells stably expressing hFPR or hFPRL1 showed that the concentrations of calpain inhibitors required to induce an increase in cytoplasmic free Ca2+ ([Ca2+]i) was much higher (>100 folds) than those of fMLF and Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm). HEK-293 cells expressing hFPR or hFPRL1 with the mutated fMLF binding site never exhibited the [Ca2+]i response to calpain inhibitors. When the optimal concentrations of each stimulus were used, pretreatment of cells with fMLF or WKYMVm abolished an increase in [Ca2+]i induced by calpain inhibitors as well as the same stimulus, whereas pretreatment of cells with calpain inhibitors significantly suppressed, but never abolished, the [Ca2+]i response induced by fMLF or WKYMVm, suggesting that the binding affinity of the inhibitors to the putative fMLF binding site may be lower than that of fMLF or WKYMVm. 相似文献
16.
L. M. Kovrizhnykh 《Plasma Physics Reports》2010,36(8):665-675
It was shown earlier that, in the range of rare collisions, transport equations for stellarators allow steady discontinuous
solutions for the ambipolar electric field and for the plasma density and temperature gradients. Moreover, such solutions
are non-single-valued; that is, their explicit form depends on the initial values of the ambipolar electric field. The time-independent
transport equations are derived under the conventional quasineutrality condition; i.e., it is assumed that the electron and
ion densities, N
e
and N
i
, are related by the relationship N
e
= ZN
i
(where Z is the ion charge number). In other words, the plasma charge density is assumed to be much less than the product e
i
N
i
. Under typical conditions, the corresponding inequality is satisfied by a large margin. However, if the electric field E has discontinuities, then it can be seen from the equation ▿·E = 4πρ that, at the discontinuity points, the charge density
becomes infinite and the relationship N
e
= ZN
i
fails to hold, so it is necessary to replace it with N
e
= ZN
i
+ ρ/e
e
. In the transport equations, this latter replacement produces additional terms, proportional to the second radial derivative
of the field E. With these additional terms, the steady solutions are modified substantially. First, the ambipolar field and the derivatives
of the density and temperatures all become continuous functions of the coordinates, a result that seems to be quite obvious.
The second, not-so-obvious result is that the steady solutions become single-valued, i.e., independent of the initial values
of the ambipolar electric field. It turns out that, in this case, two regimes are possible, depending on the values of the
plasma parameters. In the first regime, the solution is unique and is independent of the initial conditions. In the second
regime, two steady solutions can exist, depending on the initial conditions. One of the solution is similar to that obtained
in the first regime, and the other differs from the first one both in the ambipolar field profile and in the dependence of
the density and temperatures on the minor plasma radius. It cannot be excluded that different plasma confinement modes revealed
in experiments are associated with the existence of such solutions. 相似文献
17.
Marc Artzrouni 《Journal of mathematical biology》1986,24(4):405-422
In an age-structured population that grows exponentially, each age groupP
i(t) at periodt is asymptotically equivalent tox
0
t
for some positive number x0. In this paper we show that the speed at which the ith age group reaches its exponential state of equilibrium can be measured by the rate at which the ratio vi(t)=Pi(t)/pi(t–1) converges tox
0. The age specific rate of convergence is determined by considering a quantityr satisfyingv
i(t)-x
0
¦ r
t
whent is large;R
i=Infr (over all initial populations,r satisfying the above inequality) is the R-factor used in numerical analysis to measure the rate at which the sequencev
i
(t) converges tox
0;S
i
=- In Ri is then defined as the rate of convergence to stability of the ith age group. The case of constant net maternity rates is studied in detail; in this contextS
0 is compared to the population entropyH, which was proposed by Tuljapurkar (1982) as a measure of the rate of convergence to stability. 相似文献
18.
Peter B. Simpson Stefan R. Nahorski R. A. John Challiss 《Journal of neurochemistry》1996,67(1):364-373
Abstract: The mechanisms involved in Ca2+ mobilization evoked by the muscarinic cholinoceptor (mAChR) agonist carbachol (CCh) and N-methyl-d -aspartate (NMDA) in cerebellar granule cells have been investigated. An initial challenge with caffeine greatly reduced the subsequent intracellular Ca2+ concentration ([Ca2+]i) response to CCh (to 45 ± 19% of the control), and, similarly, a much reduced caffeine response was detectable after prior stimulation with CCh (to 27 ± 6% of the control). CCh-evoked [Ca2+]i responses were inhibited by preincubation with thapsigargin (10 µM), 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ; 25 µM), ryanodine (10 µM), or dantrolene (25 µM). BHQ pretreatment was found to have no effect on the sustained phase of the NMDA-evoked [Ca2+]i response. Both CCh (1 mM) and 1-aminocyclopentane-1S,3R-dicarboxylic acid (ACPD; 200 µM) evoked a much diminished increase in [Ca2+]i in granule cells pretreated with CCh for 24 h compared with vehicle-treated control cells (CCh, 23 ± 14%; ACPD, 27 ± 1% of respective control values). In contrast, a 24-h CCh pretreatment decreased the subsequent inositol 1,4,5-trisphosphate (InsP3) response to CCh to a much greater extent compared with responses evoked by metabotropic glutamate receptor (mGluR) agonists; this suggests that the former effect on Ca2+ mobilization represents a heterologous desensitization of the mGluR-mediated response distal to the pathway second messenger. Furthermore, [Ca2+]i responses to caffeine and NMDA were unaffected by a 24-h pretreatment with CCh. This study indicates that ryanodine receptors, as well as InsP3 receptors, appear to be crucial to the mAChR-mediated [Ca2+]i response in granule cells. As BHQ apparently differentiates between the CCh- and NMDA-evoked responses, it is possible that the directly InsP3-sensitive pool is physically different from the ryanodine receptor pool. Also, activation of InsP3 receptors may not contribute significantly to NMDA-evoked elevation of [Ca2+]i in cerebellar granule cells. A model for the topographic organization of cerebellar granule cell Ca2+ stores is proposed. 相似文献
19.
Summary The ionic dependencies of the transepithelial and intracellular electrical parameters were measured in the isolated frog cornea. In NaCl Ringer's the intracellular potential differenceV
sc measured under short-circuit conditions depolarized by nearly the same amount after either increasing the stromal-side KCl concentration from 2.5 to 25mm or exposure to 2mm BaCl2 (K+ channel blocker). With Ba2+ the depolarization of theV
sc by 25mm K+ was reduced to one-quarter of the control change. If the Cl-permselective apical membrane resistanceR
o remained unchanged, the relative basolateral membrane resistanceR
i, which includes the lateral intercellular space, increased at the most by less than twofold after Ba2+. These effects in conjunction with the depolarization of theV
sc by 62 mV after increasing the stromal-side K+ from 2.5 to 100mm in Cl-free Ringer's as well as the increase of the apparent ratio of membrane resistances (a=R
o/Ri) from 13 to 32 are all indicative of an appreciable basolateral membrane K+ conductance. This ratio decreased significantly after exposure to either 25mm K+ or Ba2+. The decline ofR
o/Ri with 25mm K+ appears to be anomalous since this decrease is not consistent with just an increase of basolateral membrane conductance by 25mm K+, but rather perhaps a larger decrease ofR
o thanR
iAlso an increase of lateral space resistance may offset the effect of decreasingR
i with 25mm K+. In contrast,R
o/Ri did transiently increase during voltage clamping of the apical membrane potential differenceV
o and exposure to 25mm K+ on the stromal side. This increase and subsequent decrease ofR
o/Ri supports the idea that increases in stromal K+ concentration may produce secondary membrane resistance changes. These effects onR
o/Ri show that the presence of asymmetric ionic conductance properties in the apical and basolateral membranes can limit the interpretative value of this parameter. The complete substitution of Na+ withn-methyl-glucamine in Cl-free Ringer's on the stromal side hyperpolarized theV
sc by 6 mV whereas 10–4
m ouabain depolarized theV
sc by 7 mV. Thus the basolateral membrane contains K+, Na+ and perhaps Cl– pathways in parallel with the Na/K pump component. 相似文献
20.
Several problems were discussed in relation toMacDonald' s method (MacDonald , 1957) for estimating the survival rate of a natural population of adults with varying survival rates and unstable age structure.
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