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1.
The role of H+-ATPase in proximal tubule cell pH regulation was studied by microperfusion techniques and by confocal microscopy. In a first
series of experiments, proximal S3 segments of rabbit kidney were perfused ``in vitro' while their cell pH was measured by
fluorescence microscopy after loading with BCECF. In Na+- and Cl−-free medium, cell pH fell by a mean of 0.37 ± 0.051 pH units, but after a few minutes started to rise again slowly. This
rise was of 0.17 ± 0.022 pH units per min, and was significantly reduced by bafilomycin and by the Cl− channel blocker NPPB, but not by DIDS. In a second series of experiments, subcellular vesicles of proximal tubule cells of
S3 segments of mouse kidney were studied by confocal microscopy after visualization by acridine orange or by Lucifer yellow.
After superfusion with low Na+ solution, which is expected to cause cell acidification, vesicles originally disposed in the basolateral and perinuclear
cell areas, moved toward the apical area, as detected by changes in fluorescence density measured by the NIH Image program.
The variation of apical to basolateral fluorescence ratios during superfusion with NaCl Ringer with time was 0.0018 ± 0.0021
min−1, not significantly different from zero (P > 0.42). For superfusion with Na+0 Ringer, this variation was 0.081 ± 0.015 min−1, P < 0.001 against 0. These slopes were markedly reduced by the Cl− channel blocker NPPB, and by vanadate at a concentration that has been shown to disrupt cytoskeleton function. These data
show that the delayed alkalinization of proximal tubule cells in Na+-free medium is probably due to a vacuolar H+-ATPase, whose activity is stimulated in the presence of Cl−, and dependent on apical insertion of subcellular vesicles. The movement of these vesicles is also dependent on Cl− and on the integrity of the cytoskeleton.
Received: 11 April 2000/Revised: 14 August 2000 相似文献
2.
Summary Electrophysiologic and tracer experiments have shown that Cl– entersNecturus proximal tubule cells from the tubule lumen by a process coupled to the flow of Na+, and that Cl– entry is electrically silent. The mechanism of Cl– exit from the cell across the basolateral membrane has not been directly studied. To evaluate the importance of the movement of Cl– ions across the basolateral membrane, the relative conductance of Cl– to K+ was determined by a new method. Single-barrel ion-selective microelectrodes were used to measure intracellular Cl– and K+ as a function of basolateral membrane PD as it varied normally from tubule to tubule. Basolateral membrane Cl– conductance was about 10% of K+ conductance by this method. A second approach was to voltage clamp the basolateral PD to 20 mV above and below the spontaneous PD, while sensing intracellular Cl– activity with the second barrel of a double-barrel microelectrode. An axial wire electrode in the tubule lumen was used to pass current across the tubular wall and thereby vary the basolateral membrane PD. Cell Cl– activity was virtually unaffected by the PD changes. We conclude that Cl– leavesNecturus proximal tubule cells by a neutral mechanism, possibly coupled to the efflux of Na+ or K+. 相似文献
3.
Osmotically driven water flow, u (cm/s), between two solutions of identical osmolarity, co (300 mM in mammals), has a theoretical isotonic maximum given by u = j/co, where j (moles/cm2/s) is the rate of salt transport. In many experimental studies, transport was found to be indistinguishable from isotonic.
The purpose of this work is to investigate the conditions for u to approach isotonic. A necessary condition is that the membrane salt/water permeability ratio, ε, must be small: typical
physiological values are ε = 10−3 to 10−5, so ε is generally small but this is not sufficient to guarantee near-isotonic transport. If we consider the simplest model
of two series membranes, which secrete a tear or drop of sweat (i.e., there are no externally-imposed boundary conditions
on the secretion), diffusion is negligible and the predicted osmolarities are: basal = co, intracellular ≈ (1 + ε)co, secretion ≈ (1 + 2ε)co, and u ≈ (1 − 2ε)j/co. Note that this model is also appropriate when the transported solution is experimentally collected. Thus, in the absence
of external boundary conditions, transport is experimentally indistinguishable from isotonic. However, if external boundary
conditions set salt concentrations to co on both sides of the epithelium, then fluid transport depends on distributed osmotic gradients in lateral spaces. If lateral
spaces are too short and wide, diffusion dominates convection, reduces osmotic gradients and fluid flow is significantly less
than isotonic. Moreover, because apical and basolateral membrane water fluxes are linked by the intracellular osmolarity,
water flow is maximum when the total water permeability of basolateral membranes equals that of apical membranes. In the context
of the renal proximal tubule, data suggest it is transporting at near optimal conditions. Nevertheless, typical physiological
values suggest the newly filtered fluid is reabsorbed at a rate u ≈ 0.86 j/co, so a hypertonic solution is being reabsorbed. The osmolarity of the filtrate cF (M) will therefore diminish with distance from the site of filtration (the glomerulus) until the solution being transported
is isotonic with the filtrate, u = j/cF.With this steady-state condition, the distributed model becomes approximately equivalent to two membranes in series. The
osmolarities are now: cF ≈ (1 − 2ε)j/co, intracellular ≈ (1 − ε)co, lateral spaces ≈ co, and u ≈(1 + 2ε)j/co. The change in cF is predicted to occur with a length constant of about 0.3 cm. Thus, membrane transport tends to adjust transmembrane osmotic
gradients toward εco, which induces water flow that is isotonic to within order ε. These findings provide a plausible hypothesis on how the proximal
tubule or other epithelia appear to transport an isotonic solution. 相似文献
4.
G. Radhakrishnamacharya Peeyush Chandra M. R. Kaimal 《Bulletin of mathematical biology》1981,43(2):151-163
The hydrodynamical problem of flow in proximal renal tubule is investigated by considering axisymmetric flow of a viscous,
incompressible fluid through a long narrow tube of varying cross-section with reabsorption at the wall. Two cases for reabsorption
have been studied (i) when the bulk flow,Q, decays exponentially with the axial distancex, and (ii) whenQ is an arbitrary function ofx such thatQ-Q
0 can be expressed as a Fourier integral (whereQ
0 is the flux atx=0). The analytic expressions for flow variables have been obtained by applying perturbation method in terms of wall parameter
ε. The effects of ε on pressure drop across the tube, radial velocity and wall shear have been studied in the case of exponentially
decaying bulk flow and it has been found that the results are in agreement with the existing ones for the renal tubules. 相似文献
5.
Herbert Sherman 《Bulletin of mathematical biology》1960,22(4):417-424
The theory of measurement of flow and volume by indicator dilution techniques is given in conditions of time-variable flow
rates. It is shown that the usual Hamilton (1932,Am. J. Physiol.,99, 534–551) methods can be misleading if the flow changes at a rate of close to that of the transport function.
Operated with support from the United States Army, Navy, and Air Force. 相似文献
6.
Peter B. Bright 《Bulletin of mathematical biology》1967,29(1):123-138
Previous papers by F. M. Snell (Jour. Theor. Biol.,8, 469–479, 1965) and M. A. Fox and H. D. Landahl (Bull. Math. Biophysics,27, Spec. Issue, 183–190, 1965) have found that the formulation by previous authors for the oxygen flow rates through hemoglobin
solution as a function of pressure determined by E. Hemmingsen and P. F. Scholander (Science,132, 1379–1381, 1960) did not give a satisfactory quantitative fit of the curve for constant pressure difference. The suggestion
of Fox and Landahl that the Bohr effect involving the shift in acidity accompanying the oxidation of Hb should give rise to
voltage and pH differences in oxyhemoglobin transport is examined in more detail. In this paper, the previous expressions
for the total oxygen flow rate in terms of the end point concentrations are extended to include the effects of the electrical
field. Estimates of the potential difference shows it to be negligible. A derivation of a voltage-pH relation shows that the
Nernst relation does not apply and a negligible voltage difference does not preclude a pH shift which is the more probable
explanation of the discrepancies observed. Several other predictions suitable for experimental testing are made. 相似文献
7.
K+ channels in the renal proximal tubule play an important role in salt reabsorption. Cells of the frog proximal tubule demonstrate
an inwardly rectifying, ATP-sensitive K+ conductance that is inhibited by Ba2+, GBa. In this paper we have investigated the importance of phosphorylation state on the activity of GBa in whole-cell patches. In the absence of ATP, GBa decreased over time; this fall in GBa involved phosphorylation, as rundown was inhibited by alkaline phosphatase and was accelerated by the phosphatase inhibitor
F−(10 mM). Activation of PKC using the phorbol ester PMA accelerated rundown via a mechanism that was dependent on phosphorylation.
In contrast, the inactive phorbol ester PDC slowed rundown. Inclusion of the PKC inhibitor PKC-ps in the pipette inhibited
rundown. These data indicate that PKC-mediated phosphorylation promotes channel rundown. Rundown was prevented by the inclusion
of PIP-2 in the pipette. PIP-2 also abrogated the PMA-mediated increase in rundown, suggesting that regulation of GBa by PIP-2 occurred downstream of PKC-mediated phosphorylation. G-protein activation inhibited GBa, with initial currents markedly reduced in the presence of GTPγs. These properties are consistent with GBa being a member of the ATP-sensitive K+ channel family. 相似文献
8.
D. Claveau I. Pellerin M. Leclerc M.G. Brunette 《The Journal of membrane biology》1998,165(3):265-274
In the rabbit as well as the rat, a Na+/H+ exchanger is expressed in the apical membrane of both the proximal and distal tubules of the renal cortex. Whereas the isoform
derived from the proximal tubule has been extensively studied, little information is available concerning the distal luminal
membrane isoform. To better characterize the latter isoform, we purified rabbit proximal and distal tubules, and examined
the ethylpropylamiloride (EIPA)-sensitive 22Na uptake by the luminal membrane vesicles from the two segments. The presence of 100 μm EIPA in the membrane suspension decreased the 15 sec Na+ uptake to 75.70 ± 4.70% and 50.30 ± 2.23% of the control values in vesicles from proximal and distal tubules, respectively.
The effect of EIPA on 35 mm Na+ uptake was concentration dependent, with a IC50 of 700 μm and 75 μm for the proximal and distal luminal membranes. Whereas the proximal tubule membrane isoform was insensitive to cimetidine
and clonidine up to a concentration of 2 mm, the 35 mm Na+ uptake by the distal membrane was strongly inhibited by cimetidine (IC50 700 μm) and modestly inhibited by clonidine (IC50 1.6 mm).
The incubation of proximal tubule suspensions with 1 mm (Bu2) cAMP decreased the 15-sec EIPA-sensitive Na+ uptake by the brush border membranes to 24.1 ± 2.38% of the control values. Unexpectedly, the same treatment of distal tubules
enhanced this uptake by 46.5 ± 10.3%. Finally, incubation of tubule suspensions with 100 nm phorbol 12-myristate 13-acetate (PMA) decreased the exchanger activity to 58.6 ± 3.04% and 79.7 ± 3.21% of the control values
in the proximal and distal luminal membranes, respectively. In conclusion, the high sensitivity of the distal luminal membrane
exchanger to various inhibitors, and its stimulation by cAMP-dependent protein kinase A, indicate that this isoform differs
from that of the proximal tubule and probably corresponds to isoform 1.
Received: 6 March 1998/Revised: 6 July 1998 相似文献
9.
Brush border membrane vesicles, BBMV, from eel intestinal cells or kidney proximal tubule cells were prepared in a low osmolarity
cellobiose buffer. The osmotic water permeability coefficient P
f
for eel vesicles was not affected by pCMBS and was measured at 1.6 × 10−3 cm sec−1 at 23°C, a value lower than 3.6 × 10−3 cm sec−1 exhibited by the kidney vesicles and similar to published values for lipid bilayers. An activation energy E
a
of 14.7 Kcal mol−1 for water transport was obtained for eel intestine, contrasting with 4.8 Kcal mol−1 determined for rabbit kidney proximal tubule vesicles using the same method of analysis. The high value of E
a
, as well as the low P
f
for the eel intestine is compatible with the absence of water channels in these membrane vesicles and is consistent with
the view that water permeates by dissolution and diffusion in the membrane. Further, the initial transient observed in the
osmotic response of kidney vesicles, which is presumed to reflect the inhibition of water channels by membrane stress, could
not be observed in the eel intestinal vesicles. The P
f
dependence on the tonicity of the osmotic shock, described for kidney vesicles and related to the dissipation of pressure
and stress at low tonicity shocks, was not seen with eel vesicles. These results indicate that the membranes from two volume
transporter epithelia have different mechanisms of water permeation. Presumably the functional water channels observed in
kidney vesicles are not present in eel intestine vesicles. The elastic modulus of the membrane was estimated by analysis of
swelling kinetics of eel vesicles following hypotonic shock. The value obtained, 0.79 × 10−3 N cm−1, compares favorably with the corresponding value, 0.87 × 10−3 N cm−1, estimated from measurements at osmotic equilibrium.
Received: 28 January 1999/Revised: 15 June 1999 相似文献
10.
Ting Liu Jialing Yuan Caihong Dai Jinxian Xu Shude Li Benjamin D. Humphreys Daniel T. Kleven Jian-Kang Chen 《Journal of cellular physiology》2020,235(12):9958-9973
Nephron loss stimulates residual functioning nephrons to undergo compensatory growth. Excessive nephron growth may be a maladaptive response that sets the stage for progressive nephron damage, leading to kidney failure. To date, however, the mechanism of nephron growth remains incompletely understood. Our previous study revealed that class III phosphatidylinositol-3-kinase (Pik3c3) is activated in the remaining kidney after unilateral nephrectomy (UNX)-induced nephron loss, but previous studies failed to generate a Pik3c3 gene knockout animal model. Global Pik3c3 deletion results in embryonic lethality. Given that renal proximal tubule cells make up the bulk of the kidney and undergo the most prominent hypertrophic growth after UNX, in this study we used Cre-loxP-based approaches to demonstrate for the first time that tamoxifen-inducible SLC34a1 promoter-driven CreERT2 recombinase-mediated downregulation of Pik3c3 expression in renal proximal tubule cells alone is sufficient to inhibit UNX- or amino acid-induced hypertrophic nephron growth. Furthermore, our mechanistic studies unveiled that the SLC34a1-CreERT2 recombinase-mediated Pik3c3 downregulation inhibited UNX- or amino acid-stimulated lysosomal localization and signaling activation of mechanistic target of rapamycin complex 1 (mTORC1) in the renal proximal tubules. Moreover, our additional cell culture experiments using RNAi confirmed that knocking down Pik3c3 expression inhibited amino acid-stimulated mTORC1 signaling and blunted cellular growth in primary cultures of renal proximal tubule cells. Together, both our in vivo and in vitro experimental results indicate that Pik3c3 is a major mechanistic mediator responsible for sensing amino acid availability and initiating hypertrophic growth of renal proximal tubule cells by activation of the mTORC1–S6K1–rpS6 signaling pathway. 相似文献
11.
The proximal tubule Na+-HCO−
3 cotransporter is located in the basolateral plasma membrane and moves Na+, HCO−
3, and net negative charge together out of the cell. The presence of charge transport implies that at least two HCO−
3 anions are transported for each Na+ cation. The actual ratio is of physiological interest because it determines direction of net transport at a given membrane
potential. To determine this ratio, a thermodynamic approach was employed that depends on measuring charge flux through the
cotransporter under defined ion and electrical gradients across the basolateral plasma membrane. Cells from an immortalized
rat proximal tubule line were grown as confluent monolayer on porous substrate and their luminal plasma membrane was permeabilized
with amphotericin B. The electrical properties of these monolayers were measured in a Ussing chamber, and ion flux through
the cotransporter was achieved by applying Na+ or HCO−
3 concentration gradients across the basolateral plasma membrane. Charge flux through the cotransporter was identified as difference
current due to the reversible inhibitor dinitro-stilbene disulfonate. The cotransporter activity was Cl− independent; its conductance ranged between 0.12 and 0.23 mS/cm2 and was voltage independent between −60 and +40 mV. Reversal potentials obtained from current-voltage relations in the presence
of Na+ gradients were fitted to the thermodynamic equivalent of the Nernst equation for coupled ion transport. The fit yielded a
cotransport ratio of 3HCO−
3:1Na+.
Received: 19 January 1996/Revised: 24 April 1996 相似文献
12.
A study of nephron function in normal tropical residents using the creatinine and lithium clearances
S. K. Arthur A. K. Nyarko J. Y. Asaku A. Y. Akyeampong 《International journal of biometeorology》1997,41(1):40-43
The kidney bears the brunt of the demands of a tropical climate for water and electrolyte homeostasis. We hypothesised that
a tropical climate may cause adaptive changes in the entire organism leading to altered renal function in our subjects. Hence
renal function data for residents of a temperate climate may not be applic- able to tropical residents. We therefore sought
to elucidate renal function in subjects residing in a tropical climate. We used lithium clearance, C
Li, a non-invasive tool for assessing proximal tubular function in humans, and endogenous creatinine clearance, C
Cr, to estimate proximal tubular function and glomerular function, respectively, in our subjects. We did this in order to establish
whether or not nephron function in our subjects differs from that for residents of a temperate climate. Nineteen male and
12 female Ghanaian subjects aged between 15 and 48 years were studied. The estimated G
Cr
was 117.3±6.6 ml/min for male subjects and 97±6.4 ml/min for female subjects. C
Li was 20.3±1.6 ml/min for male and 19.1±0.4 ml/min for female subjects, respectively. The estimated absolute reabsorption rate
of fluid of proximal tubules was 97.0±6.0 ml/min for males and 78.1±6.0 ml/min for females. The percentage proximal fluid
reabsorption for male and female subjects was 81.2±1.4 and 79.5±1.6, respectively. The differences between male and female
values (mean±SEM) were not statistically significant. The data suggest that the proximal tubule in residents of a tropical
climate may reabsorb more fluid compared to that in residents of a temperate climate. Our values for proximal tubular reabsorption
are higher than those reported for residents of a temperature climate. Our estimate of glomerular filtration, however, is
similar to published data for Caucasians. The difference in proximal tubular function may reflect possible renal adaptation
to a hot, humid climate. We conclude that renal function of tropical residents differs from that of residents of a temperate
climate. This difference may be due to renal adaptation to the hot, tropical climate.
Received: 1 July 1996 / Revised: 22 December 1996 / Accepted: 8 January 1997 相似文献
13.
During normal kidney function, there are routinely wide swings in proximal tubule fluid flow and proportional changes in Na+ reabsorption across tubule epithelial cells. This “glomerulotubular balance” occurs in the absence of any substantial change
in cell volume, and is thus a challenge to coordinate luminal membrane solute entry with peritubular membrane solute exit.
In this work, linear optimal control theory is applied to generate a configuration of regulated transporters that could achieve
this result. A previously developed model of rat proximal tubule epithelium is linearized about a physiologic reference condition;
the approximate linear system is recast as a dynamical system; and a Riccati equation is solved to yield the optimal linear
feedback that stabilizes Na+ flux, cell volume, and cell pH. The first observation is that optimal feedback control is largely consigned to three physiologic
variables, cell volume, cell electrical potential, and lateral intercellular hydrostatic pressure. Parameter modulation by
cell volume stabilizes cell volume; parameter modulation by electrical potential or interspace pressure act to stabilize Na+ flux and cell pH. This feedback control is utilized in a tracking problem, in which reabsorptive Na+ flux varies over a factor of two, in order to represent a substantial excursion of glomerulotubular balance. The resulting
control parameters consist of two terms, an autonomous term and a feedback term, and both terms include transporters on both
luminal and peritubular cell membranes. Overall, the increase in Na+ flux is achieved with upregulation of luminal Na+/H+ exchange and Na+–glucose cotransport, with increased peritubular Na+–3HCO3− and K+–Cl− cotransport, and with increased Na+, K+–ATPase activity. The configuration of activated transporters emerges as a testable hypothesis of the molecular basis for
glomerulotubular balance. It is suggested that the autonomous control component at each cell membrane could represent the
cytoskeletal effects of luminal flow. 相似文献
14.
John Z. Hearon 《Bulletin of mathematical biology》1953,15(2):111-119
On the basis of a previous general formulation (Bull. Math. Biophysics,15, 21–29, 1953a) a discussion is given of the error in the approximation method of N. Rashevsky. This error, inherent in the
method when the metabolic rate is different at each point in the cell, is discussed in detail and numerical values are presented
for two particular cases: the rate proportional to the concentration and the rate a prescribed function of the spatial coordinates.
It is shown that the formulation for the first case also applies to several other cases, that the error is negligible provided
the rate is sufficiently small, and that the error is fairly sensitive to the cell size. If the rate depends upon the coordinatesalone a small rate is not sufficient to insure a negligible error. The relations between the exact method, the standard approximate
method, an earlier approximate method (Physics,7 260, 1936), and a more recent refinement (Bull. Math. Biophysics,10, 201, 1948) of the standard method are discussed.
This work was performed while the author was a research participant, Oak Ridge Institute of Nuclear Studies, assigned to the
Mathematics Panel, Oak Ridge National Laboratory. 相似文献
15.
Summary. In the kidney the proximal tubule is responsible for the uptake of amino acids. This occurs via a variety of functionally
and structurally different amino acid transporters located in the luminal and basolateral membrane. Some of these transporters
show an ion-dependence (e.g. Na+, Cl− and K+) or use an H+-gradient to drive transport. Only a few amino acid transporters have been cloned or functionally characterized in detail
so far and their structure is known, while little is known about a majority of amino acid transporters. Only few attempts
have been untertaken looking at the regulation of amino acid transport. We summarized more recent information on amino acid
transport in the renal proximal tubule emphasizing functional and regulatory aspects.
Received August 8, 1999; Accepted April 20, 2000 相似文献
16.
Cnida discharge in the actinian Anemonia starts with the extrusion of the capsule from the cnidocyte followed by the eversion of the tubule. As the tubule everts, it maintains a tightly closed tip until fully everted. This is considered to be essential for a capsule to discharge as a result of an increase in intracapsular pressure. Venom volumes were measured in 3 types of nematocyst: 408 µm3, 98 µm3, and 9 µm3. Venom flow rates were estimated to range from >43 to 324 µm3 s–1. It is suggested that the intracapsular pressures required for these flow rates range from 9.7 × 105 to 1.9 x 106 Pa. 相似文献
17.
Joshua R Edwards Evangelos A Diamantakos Jacob D Peuler Peter C Lamar Walter C Prozialeck 《BMC physiology》2007,7(1):1
Background
Ethidium homodimer is a cell-membrane impermeant nuclear fluorochrome that has been widely used to identify necrotic cells in culture. Here, we describe a novel technique for evaluating necrosis of epithelial cells in the proximal tubule that involves perfusing ethidium homodimer through the intact rat kidney. As a positive control for inducing necrosis, rats were treated with 3.5, 1.75, 0.87 and 0.43 mg/kg mercuric chloride (Hg2+, intraperitoneal), treatments which have previously been shown to rapidly cause dose-dependent necrosis of the proximal tubule. Twenty-four h after the administration of Hg2+, ethidium homodimer (5 μM) was perfused through the intact left kidney while the animal was anesthetized. The kidney was then removed, placed in embedding medium, frozen and cryosectioned at a thickness of 5 μm. Sections were permeabilized with -20°C methanol and then stained with 4',6-diamidino-2-phenylindole (DAPI) to label total nuclei. Total cell number was determined from the DAPI staining in random microscopic fields and the number of necrotic cells in the same field was determined by ethidium homodimer labeling. 相似文献18.
Serge Carrière 《The Yale journal of biology and medicine》1978,51(3):307-313
Kidney weight, length of superficial and juxtamedullary proximal tubules, glomerular diameter, kidney filtration rate and PAH clearance, sodium excretion and intrarenal distribution of filtration (with 14C-ferrocyanide) were measured in the remaining hypertrophic kidneys of dogs 10 days after unilateral nephrectomy. Whereas kidney weight increased to 75 percent of the original total renal mass, proximal tubule length and mean glomerular diameter remained unchanged. PAH and creatinine clearance, and absolute, but not fractional, sodium excretion, rose significantly. The ratio superficial/juxtamedullary filtration rate remained unchanged, indicating parallel increases of filtration in both cortical regions of hypertrophied kidneys. 相似文献
19.
M. S. Gabri 《Journal of morphology》1983,175(2):131-142
The proximal, intermediate, and distal convoluted tubules of the neprhon of Podarcis (= Lacerta) taurica were examined by electron microscopy. Proximal tubule cells have large, apical cytoplasmic protrusions and microvilli interpreted to function in urate secretion. Adjacent cells are bound apically by tight junctions and desmosomes but interdigitate in their basal region. This situation is repeated in the other tubules with significant differences in intercellular space width. The basal surfaces bear numerous cytoplasmic processes. The intermediate tubule has proximal and distal segments each with dark, ciliated, and light cells, the cuboidal dark cells with dense cytoplasm constituting the main bulk of the wall. As the cells of the proximal and distal segments resemble those of the proximal and distal convoluted tubules, respectively, the intermediate tubule is considered as a transition region. The ciliated cell body has two broad processes extending from the lumen, one to the basement membrane and one to a foot process of a light cell. The light cell is surrounded by dark and ciliated cells. It does not reach the lumen, but contacts the basement membrane through a process running below a ciliated cell to form a mushroom-shaped structure in tubule cross-section, the light cell process forming the stalk and a ciliated cell the cap. The cilia probably propel the glomerular filtrate towards the distal convoluted tubule. This latter tubule has initial, middle, and terminal zones, all nonciliated but with different lumen widths and cell shapes. 相似文献
20.
Alejandro Toro-Labbé Soledad Gutiérrez-Oliva Jane S. Murray Peter Politzer 《Journal of molecular modeling》2009,15(6):707-710
The reaction force F(R) and the position-dependent reaction force constant κF(R) are defined by F(R)=-∂V(R)/∂R and κ(R)=∂2V(R)/∂R2, where V(R) is the potential energy of a reacting system along a coordinate R. The minima and maxima of F(R) provide a natural division of the process into several regions. Those in which F(R) is increasing are where the most dramatic changes in electronic properties take place, and where the system goes from activated
reactants (at the force minimum) to activated products (at the force maximum). κ(R) is negative throughout such a region. We summarize evidence supporting the idea that a reaction should be viewed as going
through a transition region rather than through a single point transition state. A similar conclusion has come out of transition state spectroscopy. We describe this region as a chemically-active, or electronically-intensive,
stage of the reaction, while the ones that precede and follow it are structurally-intensive. Finally, we briefly address the
time dependence of the reaction force and the reaction force constant. 相似文献