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1.
Illumination of an Aplysia giant neuron evokes a membrane hyperpolarization which is associated with a membrane conductance increase of 15%. The light response is best elicited at 490 nM: the neuron also has an absorption peak at this wavelength. At the resting potential (-50 to -60 mV) illumination evokes an outward current in a voltage-clamped cell. This current reverses sign very close to EK calculated from direct measurements of internal and external K+ activity. Increases in external K+ concentration shift the reversal potential of the light-evoked response by the same amount as the change in EK. Decreases in external Na+ or Cl- do not affect the response. Therefore, the response is attributed to an increase in K+ conductance. Pressure injection of Ca2+ into this neuron also hyperpolarizes the cell membrane. This effect is also due largely to an increase in K+ conductance. The light response after Ca2+ injection does not appear to be altered. Pressure injection of EGTA abolished or greatly reduced the light response. The effect was reversible. We suggest that light acts upon a single pigment in this neuron, releasing Ca2+ which in turn increases K+ conductance, thereby hyperpolarizing the neuronal membrane. 相似文献
2.
The aim of this work was to examine the effects of changes in external K+ concentration (K
o
) around its physiological value, of various K+ channels blockers, including internal Cs+, of vacuolar H+-ATPase inhibitors and of the protonophore CCCP on the resting potential and the voltage-dependent K+ current of differentiated neuroblastoma x glioma hybrid NG108-15 cells using the whole-cell patch-clamp technique. The results are as follows: (i) under standard conditions (K
o
=5 mm) the membrane potential was –60±1 mV. It was unchanged when K
o
was decreased to 1 mm and was depolarized by 4±1 mV when Ko was increased to 10 mm. (ii) Internal Cs+ depolarized the membrane by 21±3 mV. (iii) The internal application of the vacuolar H+-ATPase inhibitors N-ethylmaleimide (NEM), NO
3
–
and bafilomycin A1 (BFA) depolarized the membrane by 15±2, 18±2 and 16±2 mV, respectively, (iv) When NEM or BFA were added to the internal medium containing Cs+, the membrane was depolarized by 45±1 and 42±2 mV, respectively. (v) The external application of CCCP induced a transient depolarization followed by a prolonged hyperpolarization. This hyperpolarization was absent in BFA-treated cells. The voltage-dependent K+ current was increased at negative voltages and decreased at positive voltages by NEM, BFA and CCCP. Taken together, these results suggest that under physiological conditions, the resting potential of NG108-15 neuroblastoma cells is maintained at negative values by both voltage-dependent K+ channels and an electrogenic vacuolar type H+-ATPase.This work was supported by a grant from INSERM (CRE 91 0906). 相似文献
3.
The membrane potential (Em) of sartorius muscle fibers was made insensitive to [K+] by equilibration in a 95 mM K+, 120 mM Na+ Ringer solution. Under these conditions a potassium-activated, ouabain-sensitive sodium efflux was observed which had characteristics similar to those seen in muscles with Em sensitive to [K+]. In addition, in the presence of 10 mM K+, these muscles were able to produce a net sodium extrusion against an electrochemical gradient which was also inhibited by 10?4 M ouabain. This suggests that the membrane potential does not play a major role in the potassium activation of the sodium pump in muscles. 相似文献
4.
Energization of potassium uptake in Arabidopsis thaliana 总被引:1,自引:0,他引:1
Plant roots accumulate K+ from micromolar external concentrations. However, the absence of a firm determination of the trans-plasma-membrane electrochemical gradient for K+ in these conditions has precluded an assessment of whether K+-accumulation requires energization in addition to the driving force provided by the inside-negative membrane electrical potential (Em). To address this question unequivocally, we measured Em, and the cytosolic and external K+-activities in root cells of Arabidopsis thaliana (L.) Heynh. cv. Columbia in conditions in which net K+-accumulation occurs at low external K+ (10 M). In these conditions, net K+-uptake was about 0.1 mol · (g FW)-1 · h-1, Em varied between-153 and -129 mV and the cytosolic K+-activity, determined with K+-selective electrodes, was 83 ± 4 mM. These values yield an outwardly-directed driving force on K+ of at least 6.5 kJ · mol-1. Only if external potassium is raised to the region of 1 mM does Em become sufficient to drive net K+-accumulation. It is therefore concluded that at micromolar external K+-activities which prevail in most soils, K+-uptake cannot be solely energized by Em — as exemplified by a channel-mediated mechanism. The nature of the energization mechanism is discussed in relation to processes operating in fungal and algal cells.Abbreviations and Symbols AAS
atomic absorption spectrometry
- Em
membrane potential
-
electrochemical potassium gradient
- F
Faraday constant (96500 C · mol-1)
We thank Peter Barraclough, Roger Leigh, David Walker and Tony Miller (Rothamsted Experimental Station, Harpenden, UK) for helpful discussions. Financial support was provided by the Agricultural and Food Research Council (Grant PG87/529). 相似文献
5.
Heinke B Clauss W 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1999,169(2):148-156
The patch-clamp technique was employed in whole cells to analyze K+ conductances of amphibian colonic cells. Xenopus laevis colonic epithelium was dissected, and single epithelial cells were isolated using Ca2+-free solution and mild enzyme treatment. Vital epithelial cells had a round shape, and a distinction between apical and basolateral
poles was no longer possible. Their epithelial origin was, however, verified by antibodies against keratin. The average resting
potential of the colonocytes was −37.6 ± 1 mV (n = 220) and the resulting membrane current was strongly potassium selective. Further characterization of this conductance
was achieved by current-voltage relationship in the presence and absence of various K+ channel blockers. Barium and cesium showed pronounced voltage-dependent blockage, with interaction at about 35% inside the
pores. Lidocain, as well as quinine and quinidine also blocked, but with different kinetics and binding characteristics. Both
TEA and verapamil were ineffective. We also explored the effects of extra- (pHo) and intracellular pH (pHi) on the K+ conductance. An increase of pHo, as well as pHi, caused membrane hyperpolarization, and the shift of the current-voltage relationship indicates a stimulation of K+ channels by decreasing external and/or internal H+ concentration. The results provide the first whole-cell measurements on isolated amphibian colonic epithelial cells and demonstrate
the presence of various K+ channel types in this preparation.
Accepted: 15 January 1999 相似文献
6.
Boron induces hyperpolarization of sunflower root cell membranes and increases membrane permeability to k 总被引:5,自引:1,他引:4
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Although many studies have alluded to a role for boron (B) in membrane function, there is little evidence for a direct effect of B on the plasmalemma of higher plant cells. These studies were conducted to demonstrate, by electrophysiological techniques, a direct effect of B on the membrane potential (Em) of sunflower (Helianthus annuus [L.], cv Mammoth Grey Stripe) root tip cells and to determine if the response to B occurs rapidly enough to account for the previously observed effects of B on ion uptake. By inserting a glass microelectrode into an individual cell in the root tip, the Em of the cell was determined in basal salt medium (BSM), pH 6.0. The perfusion solution surrounding the root tissue was then changed to BSM + 50 micromolar H3BO3, pH 6.0. The exposure to B induced a significant plasmalemma hyperpolarization in sunflower root cells within 20 minutes. After just 3 minutes of exposure to B, the change in Em was already significantly different from the negligible change in Em observed over time in root cells never exposed to B. Membrane hyperpolarization could be caused by a stimulation of the proton pump or by a change in the conductance of one or more permeable ions. Since B has been shown to affect K+ uptake by plants, the electrophysiological techniques described above were used to determine if B has an effect on membrane permeability to K+, and could thereby lead to an increased diffusion potential. When sunflower root tips were pretreated in 50 micromolar B for 2 hours, cell membranes exhibited a significantly greater depolarization with each 10-fold increase in external [K+] than minus-B cells. Subsequent studies demonstrated that the depolarization due to increased external [K+] was also significantly greater when tissue was exposed to B at the same time as the 10-fold increase in [K+], indicating that the effect of B on K+ permeability was immediate. Analysis of sunflower root tips demonstrated that treatment in 50 micromolar B caused a significantly greater accumulation of K+ after 48 hours. The B-induced increase in K+ uptake may cause a subsequent stimulation of the H+-ATPase (proton pump) and lead to the observed hyperpolarization of root cell membranes. Alternatively, B may stimulate the proton pump, with the subsequent hyperpolarization resulting in an increased driving force for K+ influx. 相似文献
7.
G.A. Volkov 《BBA》1973,314(1):83-92
Transient hyperpolarization of the external cytoplasmatic membrane may be observed on rapid illumination of the Nitella flexilis cell. Several important properties of that response make the latter similar to a considerable degree to the excitation response.The condition for transient hyperpolarization is the normal functioning of the electron transport chain conjugated with non-cyclic photophosphorylation.The value of the membrane potential at the moment of hyperpolarization of the external cytoplasmic membrane, is determined by the difference in the electrochemical potential of HCO3? or H+. This state of the plasmalemma supplements the two other known states: normal and depolarized (excited), when the main ions determining membrane potential are K+ and Cl?. 相似文献
8.
The resting membrane potential (RMP) of Aplysia neurons is very temperature-dependent, and in some cells increases with increasing temperature by as much as 2 mv/°C. RMP at room temperature may significantly exceed the potassium equilibrium potential, which can be determined by measurement of the equilibrium point of the spike after potential. The hyperpolarization on warming is completely abolished by ouabain, replacement of external Na+ by Li+, removal of external K+, and by prolonged exposure to high Ca++, while it is independent of external chloride but is increased by cocaine (3 x 10-3
M). In an identified cell that shows a marked temperature dependence of RMP, both the potassium equilibrium potential and the membrane resistance were found to be relatively independent of temperature. The hyperpolarization on warming, which may increase RMP by as much as 50%, can most reasonably be ascribed to the activity of an electrogenic Na+ pump. 相似文献
9.
《Developmental biology》1986,118(2):371-378
Immediately after dissection, the ovulated oocyte of the prawn Palaemon serratus had a resting potential Em of −42 ± 2 mV and a membrane resistance Rm of 15 ± 5 MΩ; the membrane was more permeable to Cl− than to K+. The oocyte spontaneously hyperpolarized and Em gradually reached −70 mV 20–30 min after removal of the oocyte from the female, due to increased membrane permeability to K+. However, the hyperpolarization occured only if Mg2+ was present in the seawater; external Ca2+ was not required. Long-term incubation without external Mg2+ depolarized the membrane and increased membrane resistance. After preincubation in Mg2+-free ASW, oocytes transferred to standard artificial seawater (ASW) transiently hyperpolarized and then repolarized, before gradually hyperpolarizing to a sustained value of −62 ± mV. The respective roles of external Mg2+ and fertilization in eliciting the electrical response of the prawn egg at natural spawning are discussed. 相似文献
10.
Summary When a bundle of cardiac muscle cells is hyperpolarized, membrane current declines with time. Voltage clamp experiments on sheep and cat ventricular bundles showed that the magnitude of inward current depended on the external K+ concentration. Following prolonged hyperpolarization, membrane current near the resting potential was generally outward. The half-time of decay of this outward current was approximately 2.5 sec at –60 mV. The potential measured in the absence of externally supplied current was generally more negative than it would have been without conditioning hyperpolarization.The half-time of recovery of the current response following hyperpolarization was also approximately 2.5 sec at –60 mV, a factor of approximately 3.7 slower than the preceding decline of inward current. The rate of recovery has only a slight temperature dependence (Q
101.2).The experimental results are consistent with the idea that during hyperpolarization K+ is depleted from approximately 3% of the total muscle volume, and that the replenishment of K+ occurs primarily by K+ diffusion from a much larger fraction of the extracellular space. 相似文献
11.
Julio C. Chávez José L. de la Vega-Beltrán Jessica Escoffier Pablo E. Visconti Claudia L. Trevi?o Alberto Darszon Lawrence Salkoff Celia M. Santi 《PloS one》2013,8(4)
Unlike most cells of the body which function in an ionic environment controlled within narrow limits, spermatozoa must function in a less controlled external environment. In order to better understand how sperm control their membrane potential in different ionic conditions, we measured mouse sperm membrane potentials under a variety of conditions and at different external K+ concentrations, both before and after capacitation. Experiments were undertaken using both wild-type, and mutant mouse sperm from the knock-out strain of the sperm-specific, pH-sensitive, SLO3 K+ channel. Membrane voltage data were fit to the Goldman-Hodgkin-Katz equation. Our study revealed a significant membrane permeability to both K+ and Cl− before capacitation, as well as Na+. The permeability to both K+ and Cl− has the effect of preventing large changes in membrane potential when the extracellular concentration of either ion is changed. Such a mechanism may protect against undesired shifts in membrane potential in changing ionic environments. We found that a significant portion of resting membrane potassium permeability in wild-type sperm was contributed by SLO3 K+ channels. We also found that further activation of SLO3 channels was the essential mechanism producing membrane hyperpolarization under two separate conditions, 1) elevation of external pH prior to capacitation and 2) capacitating conditions. Both conditions produced a significant membrane hyperpolarization in wild-type which was absent in SLO3 mutant sperm. Hyperpolarization in both conditions may result from activation of SLO3 channels by raising intracellular pH; however, demonstrating that SLO3-dependent hyperpolarization is achieved by an alkaline environment alone shows that SLO3 channel activation might occur independently of other events associated with capacitation. For example sperm may undergo stages of membrane hyperpolarization when reaching alkaline regions of the female genital tract. Significantly, other events associated with sperm capacitation, occur in SLO3 mutant sperm and thus proceed independently of hyperpolarization. 相似文献
12.
The responses of freshly isolated hippocampal pyramidal neurons to rapid, elevations of the external potassium concentration
([K+]
out
) were investigated using the whole-cell variation of a patch-clamp technique. An elevation of [K+]
out
induced a two-phase inward current at the membrane potentials more negative than the reversal potential for K ions. This
current consisted of a leakage, current and a time-dependent current (τ=40–50 msec at 21°C), the latter designated below asI
ΔK. It displayed first-order activation kinetics that showed neither voltage, nor concentration dependence. The amplitude of
this current was determined by the external K+ concentration and increased with hyperpolarization. Voltage dependence ofI
ΔK measured within the range from −20 to −120 mV was similar to that for inward rectifier. Activation ofI
ΔK was utterly dependent on Na+; substitution of extracellular Na+ with choline chloride almost completely depressedI
ΔK.I
ΔK was absent in the cells freshly dissociated from the nodosal and dorsal root ganglia. This suggests that this earlier unrecognized
current is instrumental in preserving densely packed hippocampal pyramidal neurons from sudden increases in [K+]
out
and following spontaneous over-excitation. It prevents the neurons from responding to K+-induced depolarizations by slowing down potassium influx. 相似文献
13.
A. Wenning R. L. Calabrese 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,168(1):53-61
Summary The nephridial nerve cells of the leech, Hirudo medicinalis, 34 sensory cells, each associated with one nephridium, are sensitive to changes in extracellular Cl- concentration, an important factor in ion homeostasis. Using single-electrode current- and voltage clamp and ion substitution techniques, the specificity and mechanism of Cl- sensitivity of the nephridial nerve cell was studied in isolated preparations. Increase of the normally low external Cl- concentration leads to immediate and sustained hyperpolarization, decrease of the frequency of bursts and decrease of membrane conductance. The response is halogen specific: Cl- can be replaced by Br–, but not by organic mono- or divalent anions or inorganic divalent anions.At physiological Cl- concentrations (36mM extra-cellular Cl-), the nephridial nerve cell has a high resting conductance for Cl- and the membrane potential is governed by Cl-. In high extracellular Cl- concentrations (110–130 mM), membrane conductance is low, most likely due to the gating off of Cl- channels. Under these conditions, membrane potential is dominated by the K+ distribution and the nephridial nerve cell hyperpolarizes towards EK.Abbreviations
NNC
nephridial nerve cell
-
V
m
membrane potential
-
E
Cl(k)
equilibrium potential for Cl (K)
-
IV-curve
current-voltage relationship 相似文献
14.
Gibberellic acid (GA3) stimulates K+ efflux from the barley (Hordeum vulgare L. cv. Himalaya) aleurone. We investigated the mechanism of K+ flux across the plasma membrane of aleurone protoplasts using patch-clamp techniques. Potassium-ion currents, measured over the entire surface of the protoplast plasma membrane, were induced when the electrochemical gradient for K+ was inward (into the cytoplasm). The magnitude and voltage-dependence of this inward current were the same in protoplasts treated with GA3 and in control protoplasts (no GA3). Inward currents activated by negative shifts in the membrane potential (EM) from the Nernst potential for K+ (EK) showed membrane conductance to be a function of the electrochemical gradient (i.e. EM-EK). Single-channel influx currents of K+ were recorded in small patches of the plasma membrane. These channels had a single-channel conductance of 5–10 pS with 100 mM K+ on the inside and 10 mM K+ on the outside of the plasma membrane. Single-channel currents, like whole-cell currents, were the same in protoplasts treated with GA3 and control protoplasts. Voltage-gated efflux currents were found only in protoplasts tha thad been incubated without GA3. We conclude that K+ influx in the aleurone is mediated by channels and these membrane proteins are not greatly effected by GA3.Abbreviations and symbols FK
Nernst potential for K+
- EM
membrane potential
- Erev
reversal potential
- GA3
gibberellic acid
- Ki
concentration of K+ inside the cell
- Ko
concentration of K+ outside the cell
- R
gas constant
- S
conductance (siemens)
- T
temperature (oK)
-
i
ionic activity coefficient for internal (cytoplasmic) solution
-
o
ionic activity coefficient for external medium 相似文献
15.
Leonard Levine 《Journal of cellular physiology》1966,67(1):107-123
Comparison has been made between innervated and chronically denervated frog sartorius muscle fibers for resting potentials and a number of features of the action potential. Muscles were obtained from force-fed frogs maintained at room temperature for periods up to one year, and were studied with intracellular microelectrodes. Denervated muscles increased in sensitivity to acetylcholine by 100–400-fold. Studies were made in normal Ringer's solution, and in media in which concentrations of K+, Na+, Ca++, and Cl? were altered. The only significant differences noted between the denervated and the innervated fibers were a reduction in the maximum rate of fall of the action potential (ca. 20%) and an increase in the fall time of the active membrane potential (ca. 25%). These differences were present in normal Ringer's solution and remained when the bathing medium was modified. The resting membrane potential of denervated and innervated muscles varied with log [K+]o in exactly the same manner, and followed the theoretical relation proposed by Hodgkin (Proc. Roy. Soc., B, 148: 1–37, ′58), with the term representing the ratio of the sodium to potassium permeabilities assigned a value of 0.01. The results suggest that (a) the resting sodium and potassium permeabilities are reduced proportionately after denervation, since it is known that denervated frog muscle has a smaller potassium permeability, and (b) the mechanism controlling the increase in potassium conductance during the action potential is less available after denervation. Data indicate that the system controlling the sodium permeability is capable of activation to the same extent as in innervated muscles. Muslces which had been allowed to reinnervate did not show the differences presented by the denervated muscles. Innervated and denervated muscles did not show any significant changes in maximum rates of rise or fall of the action potential, nor of the active membrane potential amplitude over a 30 mV range of resting membrane potentials, indicating that the sodium and potassium permeability systems are fully available in frog muscle at membrane potentials larger than ?80 mV. 相似文献
16.
The mechanisms of the hyperpolarizing and depolarizing actions of cesium were studied in cardiac Purkinje fibers perfused in vitro by means of a microelectrode technique under conditions that modify either the Na+-K+ pump activity or If. Cs+ (2 mM) inconsistently increased and then decreased the maximum diastolic potential (MDP); and markedly decreased diastolic depolarization (DD). Increase and decrease in MDP persisted in fibers driven at fast rate (no diastolic interval and no activation of If). In quiescent fibers, Cs+ caused a transient hyperpolarization during which elicited action potentials were followed by a markedly decreased undershoot and a much reduced DD. In fibers depolarized at the plateau in zero [K+]o (no If), Cs+ induced a persistent hyperpolarization. In 2 mM [K+]o, Cs+ reduced the undershoot and suppressed spontaneous activity by hyperpolarizing and thus preventing the attainment of the threshold. In 7 mM [K+]o, DD and undershoot were smaller and Cs+ reduced them. In 7 and 10 mM [K+]o, Cs+ caused a small inconsistent hyperpolarization and a net depolarization in quiescent fibers; and decreased MDP in driven fibers. In the presence of strophanthidin, Cs+ hyperpolarized less. Increasing [Cs+]o to 4, 8 and 16 mM gradually hyperpolarized less, depolarized more and abolished the undershoot. We conclude that in Purkinje fibers Cs+ hyperpolarizes the membrane by stimulating the activity of the electrogenic Na+-K+ pump (and not by suppressing If); and blocks the pacemaker potential by blocking the undershoot, consistent with a Cs+ block of a potassium pacemaker current. 相似文献
17.
Summary Microscopic observations of isotonic suspensions of human red blood cells demonstrate that cell shape is unaltered when the transmembrane electrical potential, orE
m
, is set in the range –85 to +10 mV with valinomycin at varied external K+, or K
o
.E
m
was measured with the fluorescent potentiometric indicator, diS-C3(5), as calibrated by a pH method. Repeating Glaser's experiments in which echinocytosis was attributed to hyperpolarization, we found that at low ionic strength the pH-dependent effects of amphotericin B appear to be unrelated toE
m
. The effects of increased intracellular Ca2+, or Ca
o
, on echinocytosis and onE
m
are separable. With Ca ionophore A23187 half-maximal echinocytosis occurs at greater Ca
o
than that which induces the half-maximal hyperpolarization associated with Ca-induced K+ conductance (Gardos effect). Thus, cells hyperpolarized by increased Ca
o
remain discoidal when Ca is below the threshold for echinocytosis. With A23187 and higher Ca
o
, extensive echinocytosis occurs in cells which are either hyperpolarized or at their resting potential. The Ca-activation curve for echinocytosis is left-shifted by low K
o
, a new observation consistent with increased DIDS-sensitive uptake of45Ca by hyperpolarized cells. These results support the following conclusions: (1) the shape and membrane potential of human red blood cells are independent under the conditions studied; (2) in cells treated with A23187, the Gardos effect facilitates echinocytosis by increasing Ca. 相似文献
18.
Stephen K. Roberts Mark Tester 《The Plant journal : for cell and molecular biology》1995,8(6):811-825
In an attempt to understand the processes mediating ion transport within the root, the patch clamp technique was applied to protoplasts isolated from the cortex and stele of maize roots and their plasma membrane conductances investigated. In the whole-cell configuration, membrane hyperpolarization induced a slowly activating inwardly rectifying conductance in most protoplasts isolated from the root cortex. In contrast, most protoplasts isolated from the stele contained a slowly activating outwardly rectifying conductance upon plasma membrane depolarization. The reversal potential of the inward current indicated that it was primarily due to the movement of K+; the outwardly rectifying conductance was comparatively less selective for K+. Membrane hyperpolarization beyond a threshold of about ?70 mV induced inward currents. When EK was set negative of this threshold, inward currents activated negative of EK and no outward currents were observed positive of EK. Outward currents in the stelar protoplasts activated at potentials positive of ?85 mV. However, when EK was set positive of ?85 mV a small inward current was also observed at potentials negative (and slightly positive) of the equilibrium potential for K+. Inwardly and outwardly rectifying K+ channels were observed in outside-out patches from the plasma membrane of cortical and stelar cells, respectively. Characterization of these channels showed that they were likely to be responsible for the macroscopic ‘whole-cell’ currents. Inward and outward currents were affected differently by various K+ channel blockers (TEA+, Ba2+ and Cs+). In addition, Ca2+ above 1 mM partially blocked the inward current in a voltage-dependent manner but had little effect on the outward current. It is suggested that the inwardly rectifying conductance identified in protoplasts isolated from the cortex probably represents an important component of the low-affinity K+ uptake mechanism (mechanism II) identified in intact roots. The outwardly rectifying conductance identified in protoplasts isolated from the stele could play a role in the release of cations into the xylem vessels for transport to the shoot. 相似文献
19.
The electrical membrane properties of cultured human cytotrophoblast were examined by means of a standard electrophysiological technique. The mean values of the membrane potential (Em) and the membrane resistance in a physiological medium were around ?49 mV and 12 MΩ, respectively. The membrane potential was dependent, to a large extent, on the external Ca2+ concentration ([Ca2+]0). Deprivation of external Ca2+ reduced membrane potential to about ?20 mV, and an increase in [Ca2+]0 caused a hyperpolarization in a saturable manner. The Ca2+-dependency of membrane potential was affected remarkably by [K+]0, but not by [Na+]0 or [Cl?]0. The intracellular Ca2+ injection hyperpolarized the membrane in a Ca2+-free medium. A Ca2+ channel blocker, verapamil, completely abolished the Ca2+-dependent Em. The Ca2+-dependent Em was also suppressed by cooling or by the application of metabolic inhibitors. It is suggested that the Ca2+-dependent Em in cultured human cytotrophoblast is caused by a Ca2+ influx which, in turn, increases the K+ conductance of the cell membrane, presumably due to stimulation of Ca2+-activated K+ channel. 相似文献
20.
Summary The ratio of valinomycin-mediated unidirectional K+ fluxes across the human red cell membrane, has been determined in the presence of the protonophore carbonylcyanidem-chlorophenylhydrazone, CCCP, using the K+ net efflux and42K influx. The driving force for the net efflux (V
m
–E
K
+) has been calculated from the membrane potential, estimated by the CCCP-mediated proton distribution and the Nernst potential for potassium ions across the membrane. An apparent driving potential for the K+ net efflux has been calculated from the K+ flux ratio, determined in experiments where the valinomycin and CCCP concentrations were varied systematically. This apparent driving force, in conjunction with the actual driving force calculated on basis of the CCCP estimated membrane potential, is used to calculate a flux ratio exponent, which represents an estimate of the deviation of valinomycin-mediated K+ transport from unrestricted electrodiffusion, when protonophore is present.In the present work, the flux ratio exponent is found to be 0.90 when the CCCP concentration is 5.0 m and above, while the exponent decreases to about 0.50 when no CCCP is present. The influence of CCCP upon the rate constants in the valinomycin transport cycle is discussed. The significance of this result is that red cell membrane potentials are overestimated, when calculated from valinomycin-mediated potassium isotope fluxes, using a constant field equation. 相似文献