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1.
Influence of Transepithelial Potential Difference on the Sodium Uptake at the Outer Surface of the Isolated Frog Skin 总被引:8,自引:4,他引:4
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The unidirectional uptake of sodium across the outer surface of the isolated frog skin (J12Na) was measured in the presence of transepithelial potential difference (Δψ) ranging from +100 to -100 mV. With a sodium concentration of 115 mM in the bathing solutions J12Na increases significantly when the spontaneous Δψ is reduced to zero by short-circuiting the skin. With an Na concentration of 6 mM a progressive increase J12Na can be observed when Δψ is decreased in several steps from +100 to -100 mV (serosal side positive and negative, respectively). The observed change J12Na amounts to a fraction only of that predicted from the shift in Δψ. The results suggest that under open circuit conditions the potential step across the outside surface is at most one half of Δψ and that the resistance across the outside and inside barrier of the skin is ohmic. This is in agreement with measurements of intracellular potentials in the frog skin and with resistance measurements carried out in the toad skin. The data strongly support the view that the saturating component of Jψ proceeds via a charged carrier system. Exposure to negative values of Δψ of 50 mV or more for times of 24 min or more result in a marked reduction of J12Na which shows only partial or no reversibility. 相似文献
2.
Energetics of Sodium Transport in Frog Skin : II. The effects of electrical potential on oxygen consumption 总被引:5,自引:3,他引:2
Studies were made of the dependence of the rate of oxygen consumption, Jr, on the electrical potential difference, Δψ, across the frog skin. After the abolition of sodium transport by ouabain the basal oxygen consumption was independent of Δψ. In fresh skins Jr was a linear function of Δψ over a range of at least ±70 mv. Treatment with aldosterone stimulated the short-circuit current, Io, and the associated rate of oxygen consumption, Jro, and increased their stability; linearity was then demonstrable over a range of ±160 mv. Brief perturbations of Δψ (±30–200 mv) did not alter subsequent values of Io. Perturbations for 10 min or more produced a "memory" effect both with and without aldosterone: accelerating sodium transport by negative clamping lowered the subsequent value of Io; positive clamping induced the opposite effect. Changes in Jro were more readily detectable in the presence of aldosterone; these were in the same direction as the changes in Io. The linearity of Jr in Δψ indicates the validity of analysis in terms of linear nonequilibrium thermodynamics—brief perturbations of Δψ appear to produce no significant effect on either the phenomenological coefficients or the free energy of the metabolic driving reaction. Hence it is possible to evaluate this free energy. 相似文献
3.
Diurnal changes in volume and solute transport coefficients of phaseolus roots 总被引:1,自引:0,他引:1
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Fiscus EL 《Plant physiology》1986,80(3):752-759
Volume (Jv) and solute (Js) fluxes through Phaseolus root systems were observed over a 24-hour period. The volume flux was varied in a pressure chamber by altering the hydrostatic pressure in 10 steps, from 0 to 0.41 megapascals. All root systems showed strong diurnal peaks in volume flux. The five transport coefficients (σ, ω, Js*, Lp, and π*) were estimated from a nonlinear least squares algorithm. Analysis of the data revealed that all the coefficients exhibited a diurnal rhythm. When the total differential of the volume flux was considered it was possible to show that the diurnal changes in volume flux were due to a complex interaction between the diurnally shifting coefficients with the role of each highly dependent on the level of volume flux. At low volume fluxes, ω, Js*, and π* accounted for nearly all the diurnal change in volume flux. At high volume fluxes, however, the major influence shifted to Lp and π*, while ω and Js* became relatively unimportant. Thus, π* was the only coefficient of interest across the entire range of Jv and appeared to be the single most important one in determining the diurnal rhythm of Jv under conditions of a constant applied pressure. 相似文献
4.
Yuan-Heng Tai Jehan-F. Desjeux Giustina Danisi Peter F. Curran 《The Journal of membrane biology》1977,31(1):189-208
Summary Na and Cl fluxes and short-circuit current (I
sc) in rabbit ileum have been studied as a function of ionic concentrations in HCO3-free solutions. Both net Na flux (J
net
Na
) andI
sc show similar saturation functions of [Na] at fixed [Cl]. They show no significant difference between zero and 112mm Na but at 140mm NaI
sc is significantly greater than theJ
net
Na
. Net Cl transport, secretion, is observed only at 140mm Na and is approximately equivalent to the difference between theI
sc andJ
net
Na
. The transcellular mucosa-to-serosa Na fluxes measured at 140 and 70mm Na do not differ significantly from the correspondingI
sc. The net Cl flux varies with [Cl] at fixed [Na] whileI
sc is virtually not affected by [Cl]. These results suggest that the absorptive Na transport process is electrogenic and responsible for theI
sc and that the secretory fluxes of Na and Cl are coupled, require high [Na], vary with [Cl], and do not contribute toI
sc. K-free solution abolishes theI
sc after a prolonged lag. Finally, the effect of a low resistance shunt pathway on active Na absorption is examined with a four-compartment model.Deceased (October 16, 1974). 相似文献
5.
Na transport and electrical properties of Necturus renal proximal tubules were analyzed, in vivo, by a voltage clamp method which utilizes an axial electrode in the tubule lumen for passage of current and simultaneous determination of net fluid (or Na) flux by the split droplet method. When the average spontaneous transepithelial potential difference of –8 mv (lumen negative) was reduced to zero by current passage, net Na flux doubled from a mean of 107 to 227 pmoles/cm2 per sec. The relationship between flux and potential over the range –25 to +10 mv was nonlinear, with flux equilibrium at –15 mv and droplet expansion at more negative values. Calculated Na permeability at flux equilibrium was 7.0 x 10–6 cm/sec. Voltage transients, similar to those caused by intraepithelial unstirred layers, were observed at the end of clamping periods. Tubular electrical resistance measured by brief square or triangle wave pulses (<100 msec) averaged 43 ohm cm2. The epithelial current-voltage relationship was linear over the range –100 to +100 mv, but displayed marked hysteresis during low frequency (<0.04 Hz) triangle wave clamps. The low transepithelial resistance and large opposing unidirectional ion fluxes suggest that passive ionic movements occur across extracellular shunt pathways, while the voltage transients and current-voltage hysteresis are consistent with the development of a local osmotic gradient within epithelium. 相似文献
6.
Ouabain switches Na+ transport to Na+/Na+ equivalent exchange in normal and apoptotic lymphoid cells
A. A. Rubashkin 《Biophysics》2013,58(5):660-663
A theory of change of the ionic fluxes in the lymphoid cells in their transition from normal to apoptosis we have developed previously is applied to the analysis of Na+/Na+ exchange fluxes in human lymphoid cells U937 exposed to ouabain. We solve a system of equations describing changes in the intracellular concentrations of Na+, K+ and Cl?, membrane potential and cell volume. It is shown that the Na+ influx (I Na/Na) and output flux through the Na+/Na+ tract increased 4 times in 8 h after disconnecting Na+/K+-ATPase for normal cell U937. These fluxes increased 2.6 times for apoptotic cells. The value of I Na/Na after 8 h off pump by ouabain is 97% of the total Na+ input for both cell types. It is concluded that ouabain not only inhibits the Na+/K+-ATPase, but also increases Na+ exchange fluxes through the Na+/Na+ tract, thereby switching sodium transport across the membrane of lymphoid cells to Na+/Na+ equivalent exchange. 相似文献
7.
Charges and Potentials at the Nerve Surface : Divalent ions and pH 总被引:57,自引:33,他引:24
Bertil Hille 《The Journal of general physiology》1968,51(2):221-236
The voltage dependence of the voltage clamp responses of myelinated nerve fibers depends on the concentration of divalent cations and of hydrogen ions in the bathing medium. In general, increases of the [Ca], [Ni], or [H] increase the depolarization needed to elicit a given response of the nerve. An e-fold increase of the [Ca] produces the following shifts of the voltage dependence of the parameters in the Hodgkin-Huxley model: m∞, 8.7 mv; h∞, 6.5 mv; τn, 0.0 mv. The same increase of the [H], if done below pH 5.5, produces the following shifts: m∞, 13.5 mv; h∞, 13.5 mv; τn, 13.5 mv; and if done above pH 5.5: m∞, 1.3 mv; h∞, 1.3 mv; τn, 4.0 mv. The voltage shifts are proportional to the logarithm of the concentration of the divalent ions and of the hydrogen ion. The observed voltage shifts are interpreted as evidence for negative fixed charges near the sodium and potassium channels. The charged groups are assumed to comprise several types, of varying affinity for divalent and hydrogen ions. The charges near the sodium channels differ from those near the potassium channels. As the pH is lowered below pH 6, the maximum sodium conductance decreases quickly and reversibly in a manner that suggests that the protonation of an acidic group with a pKa of 5.2 blocks individual sodium channels. 相似文献
8.
Frog sartorius muscles were made Na-rich by immersion in K-free sulfate Ringer's solution in the cold. The muscles were then loaded with Na24 and the extracellular space cleared of radioactivity. When such Na-rich muscles were transferred to lithium sulfate Ringer's solution at 20°C, Na efflux was observed to increase with time, to reach a maximum about 15 minutes after the transfer of the muscles to Li2SO4, and then to decline. The decline in efflux from these muscles was proportional to ([Na]i)8 over a considerable range of [Na]i. The membrane potential of Na-rich muscles was about -48 mv in K-free sulfate Ringer's at 4°C but changed to -76 mv in the same solution at 20°C and to -98 mv in Li2SO4 Ringer's at 20°C. By contrast, muscles with a normal [Na]i showed a fall in membrane potential when transferred from K-free sulfate Ringer's to Li2SO4 Ringer's solution. The general conclusions from this study are (a) that Na extrusion is capable of generating an electrical potential, and (b) that increases in [Na]i lead to reversible increases in PNa of muscle fibers. 相似文献
9.
Summary Net influxes of Na and Cl and effluxes of K and H (J
Na,J
Cl,J
K andJ
H) and volume flowJ
v
across isolated open-circuited toad skins were measured using rotating chambers and a small volume of external solution, the ion fluxes being determined by chemical analysis of the external solution, in the range of 0.2 to 5.0mm external Na concentration. In this concentration range, with skin potential varying with (Na)
e
,J
Na is a linear function of the Na electrochemical potential difference across the skin, as expected on irreversible thermodynamic grounds. TheL
Na coefficient calculated asJ
Na/
Na is equal to 5.5×10–12 mole2 joule–1 cm–2 min–1, which is similar to values obtained for the same species in the short-circuited state and in higher ranges of (Na)
e
. A positive correlation is observed betweenJ
Na andJ
K whenJ
Na varied with (Na)
e
and also whenJ
Na varies in randomly selected skins. Antidiuretic hormone stimulatesJ
Na,J
K andJ
v
in the range of 0.2 to 5.0mm (Na)
e
and lowers the Na concentration in the equivalent solution absorbed by the skin (calculated asJ
Na/J
v
). Substitution of external Cl by SO4 has no effect onJ
Na,J
K andJ
H and also in the skin potential in the range of (Na)
e
studied. Substitution of external Na by K abolishesJ
Cl and reverses the skin polarity, the external solution now being positive to the internal one. Na removal from the external solution also reducesJ
K almost to zero.J
H is significantly reduced in this condition; however, a basal secretion still persists with (Na)
e
equal to zero. The results of these experiments can be tentatively interpreted in terms of electrical coupling between ion fluxes, since only the procedures that result in alterations of skin potential are followed by changes in the rates of ion transport. The existence of other coupling mechanisms cannot be ruled out. 相似文献
10.
Mathematical models were developed to characterize the physiological bases of the responses of tomato (Lycopersicon esculentum Mill. cv T5) seed germination to water potential (ψ) and abscisic acid (ABA). Using probit analysis, three parameters were derived that can describe the germination time courses of a seed population at different ψ or ABA levels. For the response of seed germination to reduced ψ, these parameters are the mean base water potential (¯ψb, MPa), the standard deviation of the base water potential among seeds in the population (σψb, MPa), and the “hydrotime constant” (θH, MPa·h). For the response to ABA, they are the log of the mean base ABA concentration ([unk]ABAb, m), the standard deviation of the base ABA concentration among seeds in the population (σABAb, log[m]), and the “ABA-time constant” (θABA, log[m]·h). The values of ¯ψb and [unk]ABAb provide quantitative estimates of the mean sensitivity of germination rate to ψ or ABA, whereas σψb and σABAb account for the variation in sensitivity among seeds in the population. The time constants, θH and θABA, indicate the extent to which germination rate will be affected by a given change in ψ or ABA. Using only these parameters, germination time courses can be predicted with reasonable accuracy at any medium ψ according to the equation probit(g) = [ψ - (θH/tg) - ¯ψb]/σψb, or at any ABA concentration according to the equation probit(g) = [log[ABA] - (θABA/tg) - log[[unk]ABAb]]/σABAb, where tg is the time to radicle emergence of percentage g, and ABA is the ABA concentration (m) in the incubation solution. In the presence of both ABA and reduced ψ, the same parameters can be used to predict seed germination time courses based upon strictly additive effects of ψ and ABA in delaying the time of radicle emergence. Further analysis indicates that ABA and ψ can act both independently and interactively to influence physiological processes preparatory for radicle growth, such as the accumulation of osmotic solutes in the embryo. The models provide quantitative values for the sensitivity of germination to ABA or ψ, allow evaluation of independent and interactive effects of the two factors, and have implications for understanding how ABA and ψ may regulate growth and development. 相似文献
11.
The anion channel protein from Clavibacter michiganense ssp. nebraskense (Schürholz, Th. et al. 1991, J. Membrane Biol. 123: 1-8) was analyzed at different concentrations of KCl and KF. At 0.8 M KCl the conductance G(Vm) increases exponentially from 21 pS at 50 mV up to 53 pS at Vm = 200 mV, 20°C. The concentration dependence of G(Vm) corresponds to a Michaelis-Menten type saturation function at all membrane voltage values applied (0-200 mV). The anion concentration K0.5, where G(Vm) has its half-maximum value, increases from 0.12 M at 50 mV to 0.24 M at 175 mV for channels in a soybean phospholipid bilayer. The voltage dependence of the single channel conductance, which is different for charged and neutral lipid bilayers, can be described either by a two-state flicker (2SF) model and the Nernst-Planck continuum theory, or by a two barrier, one-site (2B1S) model with asymmetric barriers. The increase in the number of open channels after a voltage jump from 50 mV to 150 mV has a time constant of 0.8 s. The changes of the single-channel conductance are much faster (<1 ms). The electric part of the gating process is characterized by the (reversible) molar electrical work ΔGθel = ρZgFVm ≈ -1.3 RT, which corresponds to the movement of one charge of the gating charge number |Zg| = 1 across the fraction ρ = ΔVm/Vm = 0.15 of the membrane voltage Vm = 200 mV. Unlike with chloride, the single channel conductance of fluoride has a maximum at about 150 mV in the presence of the buffer PIPES (≥5 mM, pH 6.8) with K0.5 ≈ 1 M. It is shown that the decrease in conductance is due to a blocking of the channel by the PIPES anion. In summary, the results indicate that the anion transport by the Clavibacter anion channel (CAC) does not require a voltage dependent conformation change of the CAC. 相似文献
12.
Toru Ishikawa Yoshinori Marunaka Daniela Rotin 《The Journal of general physiology》1998,111(6):825-846
The epithelial Na+ channel (ENaC), composed of three subunits (α, β, and γ), is expressed in several epithelia and plays a critical role in salt and water balance and in the regulation of blood pressure. Little is known, however, about the electrophysiological properties of this cloned channel when expressed in epithelial cells. Using whole-cell and single channel current recording techniques, we have now characterized the rat αβγENaC (rENaC) stably transfected and expressed in Madin-Darby canine kidney (MDCK) cells. Under whole-cell patch-clamp configuration, the αβγrENaC-expressing MDCK cells exhibited greater whole cell Na+ current at −143 mV (−1,466.2 ± 297.5 pA) than did untransfected cells (−47.6 ± 10.7 pA). This conductance was completely and reversibly inhibited by 10 μM amiloride, with a Ki of 20 nM at a membrane potential of −103 mV; the amiloride inhibition was slightly voltage dependent. Amiloride-sensitive whole-cell current of MDCK cells expressing αβ or αγ subunits alone was −115.2 ± 41.4 pA and −52.1 ± 24.5 pA at −143 mV, respectively, similar to the whole-cell Na+ current of untransfected cells. Relaxation analysis of the amiloride-sensitive current after voltage steps suggested that the channels were activated by membrane hyperpolarization. Ion selectivity sequence of the Na+ conductance was Li+ > Na+ >> K+ = N-methyl-d-glucamine+ (NMDG+). Using excised outside-out patches, amiloride-sensitive single channel conductance, likely responsible for the macroscopic Na+ channel current, was found to be ∼5 and 8 pS when Na+ and Li+ were used as a charge carrier, respectively. K+ conductance through the channel was undetectable. The channel activity, defined as a product of the number of active channel (n) and open probability (P
o), was increased by membrane hyperpolarization. Both whole-cell Na+ current and conductance were saturated with increased extracellular Na+ concentrations, which likely resulted from saturation of the single channel conductance. The channel activity (nP
o) was significantly decreased when cytosolic Na+ concentration was increased from 0 to 50 mM in inside-out patches. Whole-cell Na+ conductance (with Li+ as a charge carrier) was inhibited by the addition of ionomycin (1 μM) and Ca2+ (1 mM) to the bath. Dialysis of the cells with a pipette solution containing 1 μM Ca2+ caused a biphasic inhibition, with time constants of 1.7 ± 0.3 min (n = 3) and 128.4 ± 33.4 min (n = 3). An increase in cytosolic Ca2+ concentration from <1 nM to 1 μM was accompanied by a decrease in channel activity. Increasing cytosolic Ca2+ to 10 μM exhibited a pronounced inhibitory effect. Single channel conductance, however, was unchanged by increasing free Ca2+ concentrations from <1 nM to 10 μM. Collectively, these results provide the first characterization of rENaC heterologously expressed in a mammalian epithelial cell line, and provide evidence for channel regulation by cytosolic Na+ and Ca2+. 相似文献
13.
Sodium, potassium, and chloride unidirectional fluxes have been studied in the mature mouse egg. Their relationship to cell membrane potential and conductance has been investigated. Unidirectional Na efflux is composed of a ouabain sensitive component, presumably representing an active Na efflux, an external Na-dependent component and a diffusional component. The data indicate that the external Na-dependent component represents a Na:Na exchange mechanism. There also exists an ouabain-sensitive component of K influx. The stoichiometry of the ouabain-sensitive fluxes is approx. 2.7:1 (Na to K). From the diffusional components of Na and K flux, the membrane permeability to these cations has been estimated. PNa and PK are 1.2 × 10−7 cm sec−1 and 0.8 × 10−7 cm sec−1 respectively. These permeabilities, in conjunction with the internal exchangeable fractions of Na and K and the external concentrations, predict an egg membrane potential of −11 mV (inside negative). Microelectrode measurements yield an egg membrane potential of −14 ± 0.4 mV, indicating that the cell membrane potential is predominantly a result of the Na and K permeabilities and distributions. Internal exchangeable Cl is 67 ± 3 mM in standard medium, as determined from 36Cl distribution. The chloride equilibrium potential is therefore −15 mV, which is not significantly different from the egg membrane potential. This suggests that Cl distributes passively across the egg membrane, reflecting the egg membrane potential. Hyperpolarization of the egg membrane potential to −27 ± 1.5 mV by reduction of external Na results in an exchangeable internal Cl of 49 ± 8 mM. This yields a Cl equilibrium potential of −24 mV, indicating that the Cl distribution shifts in the predicted manner upon a change in cell membrane potential. Tracer flux data indicate that Cl conductance comprises the bulk of the total membrane conductance with Na and K sharing the remainder in approximately equal amounts. 相似文献
14.
Propagation Speed in Myelinated Nerve: I. Experimental Dependence on External Na+ and on Temperature
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W. L. Hardy 《Biophysical journal》1973,13(10):1054-1070
Conduction speed (θ) in single myelinated Rana pipiens sciatic nerve fibers has been precisely measured using intracellular recording and on-line digital computer techniques. The dependence of relative speed on external Na concentration at 15°C has been found to be ln(θ1/θ2) = 0.524 (±0.018) ln ([Na+]1/[Na+]2) + 0.003. Thus θ has very close to a square root dependence on [Na+]0 for these fibers. This experimental finding is not in complete agreement with a theoretical prediction based on a solution of the Hodgkin-Huxley (H.H.) equations. The effect of small temperature variations around 15°C on θ has also been measured for Rana fibers in Ringer's solution. θ has close to an exponential dependence on T and a Q10 of 2.95 has been estimated. 相似文献
15.
Discussions of active transport usually assume stoichiometry between the rate of transport J+ and the metabolic rate Jr. However, the observation of a linear relationship between J+ and Jr does not imply a stoichiometric relationship, i.e., complete coupling. Since coupling may possibly be incomplete, we examine systems of an arbitrary degree of coupling q, regarding stoichiometry as a limiting case. We consider a sodium pump, with J+ and Jr linear functions of the electrochemical potential difference, -X+, and the chemical affinity of the metabolic driving reaction, A. The affinity is well defined even for various complex reaction pathways. Incorporation of a series barrier and a parallel leak does not affect the linearity of the composite observable system. The affinity of some region of the metabolic chain may be maintained constant, either by large pools of reactants or by regulation. If so, this affinity can be evaluated by two independent methods. Sodium transport is conveniently characterized by the open-circuit potential (Δψ)I=0 and the natural limits, level flow (J+)X+=0, and static head X0+ = (X+)J+=0. With high degrees of coupling -X0+/F approaches the electromotive force ENa (Ussing); -X0+/F cannot be identified with ((RT/F) ln f)X+=0, where f is the flux ratio. The efficiency η = -J+X+/JrA is of significance only when appreciable energy is being converted from one form to another. When either J+ or -X+ is small η is low; the significant parameters are then the efficacies εJ+ = J+/JrA and εX+ = -X+/JrA, respectively maximal at level flow and static head. Leak increases both J+ and εJ+ for isotonic saline reabsorption, but diminishes -X0+ and εX♀. Electrical resistance reflects both passive parameters and metabolism. Various fundamental relations are preserved despite coupling of passive ion and water flows. 相似文献
16.
Carbon Dioxide and Light Responses of Photosynthesis in Cowpea and Pigeonpea during Water Deficit and Recovery 总被引:1,自引:0,他引:1
Greenhouse-grown pigeonpea (Cajanus cajan, [L.] Millsp.; cultivar UW-10) and cowpea (Vigna unguiculata, [L.] Walp.; cultivar California No. 5) were well-watered (control) or subjected to low water potential by withholding water to compare their modes of adaptation to water-limited conditions. Leaf CO2 exchange rate (CER), leaf diffusive conductance to CO2 (gl), and CO2 concentration in the leaf intercellular air space (Ci) were determined at various CO2 concentrations and photon flux densities (PFD) of photosynthetically active radiation (400 to 700 nanometer). In cowpea, gl declined to less than 15% of controls and total water potential (ψw) at midafternoon declined to −0.8 megapascal after 5 days of withholding water, whereas gl in pigeonpea was about 40% of controls even though midafternoon ψw was −1.9 megapascal. After 8 days of withholding water, ψw at midafternoon declined to −0.9 and −2.4 megapascals in cowpea and pigeonpea, respectively. The solute component of water potential (ψs) decreased substantially less in cowpea than pigeonpea. Photosynthetic CER at saturation photon flux density (PFD) and ambient external CO2 concentration (360 microliters per liter) on day 5 of withholding decreased by 83 and 55% in cowpea and pigeonpea, respectively. When measured at external, CO2 concentration in bulk air of 360 microliters per liter, the CER of cowpea had fully recovered to control levels 3 days after rewatering; however, at 970 microliters per liter the PFD-saturated CERs of both species were substantially lower than in controls, indicating residual impairment. In stressed plants of both species the CER responses to Ci from 250 to 600 microliters per liter indicated that a substantial nonstomatal inhibition of CER had occurred. Although the sensitivity of gl to water limitation in cowpea suggested a dehydration avoidance response, parallel measurements of CER at various Ci and PFD indicated that photosynthetic activity of cowpea mesophyll was substantially inhibited by the water-limited treatment. 相似文献
17.
Studies on the electrical potential profile across rabbit ileum. Effects of sugars and amino acids on transmural and transmucosal electrical potential differences 总被引:21,自引:8,他引:13
When isolated strips of mucosal rabbit ileum are bathed by physiological electrolyte solution the electrical potential difference (PD) across the brush border (ψmc) averages 36 mv, cell interior negative. Rapid replacement of Na in the mucosal solution with less permeant cations, Tris or choline, results in an immediate hyperpolarization of ψmc. Conversely, replacement of choline in the mucosal solution with Na results in an abrupt depolarization of ψmc. These findings indicate that Na contributes to the conductance across the brush border. The presence of actively transported sugars or amino acids in the mucosal solution brings about a marked depolarization of ψmc and a smaller increase in the transmural PD (Δψms). It appears that the Na influx that is coupled to the influxes of amino acids and sugars is electrogenic and responsible for the depolarization of ψmc. Under control conditions Δψms can be attributed to the depolarization of ψmc together with the presence of a low resistance transepithelial shunt, possibly the lateral intercellular spaces. However, quantitatively similar effects of amino acids on ψmc are also seen in tissues poisoned with metabolic inhibitors or ouabain. Under these conditions Δψmc is much smaller than under control conditions. Thus, the depolarization of ψmc might not account for the entire Δψms, observed in nonpoisoned tissue. An additional electromotive force which is directly coupled to metabolic processes might contribute to the normal Δψms. 相似文献
18.
Cody D. Smith Cameron A. Schmidt Chien-Te Lin Kelsey H. Fisher-Wellman P. Darrell Neufer 《The Journal of biological chemistry》2020,295(48):16207
Compensatory changes in energy expenditure occur in response to positive and negative energy balance, but the underlying mechanism remains unclear. Under low energy demand, the mitochondrial electron transport system is particularly sensitive to added energy supply (i.e. reductive stress), which exponentially increases the rate of H2O2 (JH2O2) production. H2O2 is reduced to H2O by electrons supplied by NADPH. NADP+ is reduced back to NADPH by activation of mitochondrial membrane potential–dependent nicotinamide nucleotide transhydrogenase (NNT). The coupling of reductive stress-induced JH2O2 production to NNT-linked redox buffering circuits provides a potential means of integrating energy balance with energy expenditure. To test this hypothesis, energy supply was manipulated by varying flux rate through β-oxidation in muscle mitochondria minus/plus pharmacological or genetic inhibition of redox buffering circuits. Here we show during both non-ADP– and low-ADP–stimulated respiration that accelerating flux through β-oxidation generates a corresponding increase in mitochondrial JH2O2 production, that the majority (∼70–80%) of H2O2 produced is reduced to H2O by electrons drawn from redox buffering circuits supplied by NADPH, and that the rate of electron flux through redox buffering circuits is directly linked to changes in oxygen consumption mediated by NNT. These findings provide evidence that redox reactions within β-oxidation and the electron transport system serve as a barometer of substrate flux relative to demand, continuously adjusting JH2O2 production and, in turn, the rate at which energy is expended via NNT-mediated proton conductance. This variable flux through redox circuits provides a potential compensatory mechanism for fine-tuning energy expenditure to energy balance in real time. 相似文献
19.
Christopher?M. Stanley Dominique?G. Gagnon Adam Bernal Dylan?J. Meyer Joshua?J. Rosenthal Pablo Artigas 《Biophysical journal》2015,109(9):1852-1862
Cardiac cells express more than one isoform of the Na, K-ATPase (NKA), the heteromeric enzyme that creates the Na+ and K+ gradients across the plasmalemma. Cardiac isozymes contain one catalytic α-subunit isoform (α1, α2, or α3) associated with an auxiliary β-subunit isoform (β1 or β2). Past studies using biochemical approaches have revealed minor kinetic differences between isozymes formed by different α-β isoform combinations; these results make it difficult to understand the physiological requirement for multiple isoforms. In intact cells, however, NKA enzymes operate in a more complex environment, which includes a substantial transmembrane potential. We evaluated the voltage dependence of human cardiac NKA isozymes expressed in Xenopus oocytes, and of native NKA isozymes in rat ventricular myocytes, using normal mammalian physiological concentrations of Na+o and K+o. We demonstrate that although α1 and α3 pumps are functional at all physiologically relevant voltages, α2β1 pumps and α2β2 pumps are inhibited by ∼75% and ∼95%, respectively, at resting membrane potentials, and only activate appreciably upon depolarization. Furthermore, phospholemman (FXYD1) inhibits pump function without significantly altering the pump’s voltage dependence. Our observations provide a simple explanation for the physiological relevance of the α2 subunit (∼20% of total α subunits in rat ventricle): they act as a reserve and are recruited into action for extra pumping during the long-lasting cardiac action potential, where most of the Na+ entry occurs. This strong voltage dependence of α2 pumps also helps explain how cardiotonic steroids, which block NKA pumps, can be a beneficial treatment for heart failure: by only inhibiting the α2 pumps, they selectively reduce NKA activity during the cardiac action potential, leading to an increase in systolic Ca2+, due to reduced extrusion through the Na/Ca exchanger, without affecting resting Na+ and Ca2+ concentrations. 相似文献
20.
Shi Liu Ron Jacob David Piwnica-Worms Melvyn Lieberman 《Molecular and cellular biochemistry》1989,89(2):147-150
We have recently reported the presence of an electroneutral (Na + K + 2 Cl) cotransport mechanism that is bumetanide-sensitive and maintains Cli above its electrochemical equilibrium in cultured chick heart cells. In steady state, (Na + K + 2 Cl) cotransport is inwardly directed and so contributes to the Na influx that must be counterbalanced by the activity of the Na/K pump to maintain Nai homeostasis. We now show that manipulating (Na + K + 2 Cl) cotransport by restoring Clo to a Cl-free solution indirectly influences Na/K pump activity because the bumetanide-sensitive recovery of a
infNa
supi
to its control level and the accompanying hyperpolarization could be blocked by 10–4M ouabain. In another protocol, when the Na/K pump was reactivated by restoring Ko (from 0.5 mM to 5.4 mM) and removing ouabain, the recovery of aNa was attenuated by 10–4M bumetanide. The relatively slow rate of ouabain dissociation coupled with the activation of Na influx by (Na + K + 2 Cl) cotransport clearly establishes the interaction of these transport mechanisms in regulating Nai. Although (Na + K + 2 Cl) cotransport is electroneutral, secondary consequences of its activity can indirectly affect the electrophysiological properties of cardiac cells. 相似文献