Anion-sensitive sodium conductance in the apical membrane of toad urinary bladder

Membrane potentials and the electrical resistance of the cell membranes and the shunt pathway of toad urinary bladder epithelium were measured using microelectrode techniques. These measurements were used to compute the equivalent electromotive forces (EMF) at both cell borders before and after reductions in mucosal Cl- concentration ([Cl]m). The effects of reduction in [Cl]m depended on the anionic substitute. Gluconate or sulfate substitutions increased transepithelial resistance, depolarized membrane potentials and EMF at both cell borders, and decreased cell conductance. Iodide substitutions had opposite effects. Gluconate or sulfate substitutions decreased apical Na conductance, where iodide replacements increased it. When gluconate or sulfate substitutions were brought about the presence of amiloride in the mucosal solution, apical membrane potential and EMF hyperpolarized with no significant changes in basolateral membrane potential or EMF. It is concluded that: (a) apical Na conductance depends, in part, on the anionic composition of the mucosal solution, (b) there is a Cl- conductance in the apical membrane, and (c) the electrical communication between apical and basolateral membranes previously described is mediated by changes in the size of the cell Na pool, most likely by a change in sodium activity.     

Microelectrode studies in toad urinary bladder epithelium. effects of Na concentration changes in the mucosal solution on equivalent electromotive forces

Microelectrode techniques were employed to measure membrane potentials, the electrical resistance of the cell membranes, and the shunt pathway, and to compute the equivalent electromotive forces (EMF) at both cell borders in toad urinary bladder epithelium before and after reductions in mucosal sodium concentration. Basal electrical parameters were not significantly different from those obtained with impalements from the serosal side, indicating that mucosal impalements do not produce significant leaks in the apical membrane. A decrease in mucosal Na concentration caused the cellular resistance to increase and both apical and basolateral EMF to depolarize. When Na was reduced from 112 to 2.4 mM in bladders with spontaneously different baseline values of transepithelial potential difference (Vms), a direct relationship was found between the change in Vms brought about by the Na reduction and the base-line Vms before the change. A direct relationship was also found by plotting the change in EMF at the apical or basolateral border caused by a mucosal Na reduction with the corresponding base-line EMF before the change. These results indicate that resting apical membrane EMF (and, therefore, resting apical membrane potential) is determined by the Na selectivity of the apical membrane, whereas basolateral EMF is at least in part the result of rheogenic Na transport. These results are consistent with data of others that suggested a link between the activity of the basolateral Na pump and apical Na conductance.     

Pathways for movement of ions and water across toad urinary bladder

Summary Mucosal hypertonicity, produced by the addition of NaCl, KCl, mannitol, urea, sucrose or raffinose, reduced the electrical resistance of toad urinary bladder and induced bullous deformations (blisters) of the most apical junctions of the mucosal epithelium: the smaller solutes were most effective in eliciting both phenomena. Study of the effect of addition and subsequent removal of mannitol from the mucosal medium indicated that both the electrical and morphologic changes are reversible and follow the same time course. Mucosal hypertonicity induced comparable changes in the tissue in the presence or absence of inhibition of active sodium transport by replacement of sodium by choline, or by addition of ouabain or amiloride. Dilution of the tonicity of the serosal medium similarly reduced the tissue resistance and induced blisters within the epithelium, demonstrating that the osmotic gradient, rather than the mucosal hypertonicity itself is the cause of the osmotically-induced resistance change. The data indicate, therefore, that the osmotic gradient reduces the electrical resistance of the tissue primarily by deforming the apical junctions.The simplest interpretation of the data is that the apical tight junctions are considerably more permeable to water and small solutes than had previously been thought. Addition of solute to the mucosal medium leads to the diffusion of solute into the junctions: the subsequent transfer of water from the lateral intercellular spaces and/or the adjacent cellular cytoplasm, deforms these structures and reduces the resistance to the passage of ions across the tissue. The results suggest that the apical junctions constitute the rate-limiting permeability barrier of the putative parallel shunt pathway across toad bladder.     

The active ion transport properties of canine lingual epithelia in vitro. Implications for gustatory transduction

The electrophysiological properties of the dorsal and ventral canine lingual epithelium are studied in vitro. The dorsal epithelium contains a special ion transport system activated by mucosal solutions hyperosmotic in NaCl or LiCl. Hyperosmotic KCl is significantly less effective as an activator of this system. The lingual frenulum does not contain the transport system. In the dorsal surface it is characterized by a rapid increase in inward current and can be quantitated as a second component in the time course of either the open-circuit potential or short-circuit current when the mucosal solution is hyperosmotic in NaCl or LiCl. The increased inward current (hyperosmotic response) can be eliminated by amiloride (10(-4) M). The specific location of this transport system in the dorsal surface and the fact that it operates over the concentration range characteristic of mammalian salt taste suggests a possible link to gustatory transduction. This possibility is tested by recording neural responses in the rat to NaCl and KCl over a concentration range including the hyperosmotic. We demonstrate that amiloride specifically blocks the response to NaCl over the hyperosmotic range while affecting the KCl response significantly less. The results suggest that gustatory transduction for NaCl is mediated by Na entry into the taste cells via the same amiloride-sensitive pathway responsible for the hyperosmotic response in vitro. Further studies of the in vitro system give evidence for paracellular as well as transcellular current paths. The transmural current-voltage relations are linear under both symmetrical and asymmetrical conditions. After ouabain treatment under symmetrical conditions, the short-circuit current decays to zero. The increase in resistance, though significant, is small, which suggests a sizeable shunt pathway for current. Flux measurements show that sodium is absorbed under symmetrical conditions. Mucosal solutions hyperosmotic in various sugars also induce an amiloride-sensitive inward current. In summary, this work provides evidence that the sodium taste receptor is most probably a sodium transport system, specifically adapted to the dorsal surface of the tongue. The transport paradigm of gustation also suggests a simple model for electric taste and possible mechanisms for sweet taste.     

Passive Electrical Properties of Toad Urinary Bladder Epithelium : Intercellular Electrical Coupling and Transepithelial Cellular and Shunt Conductances

The electrical resistances of the transcellular and paracellular pathways across the toad urinary bladder epithelium (a typical "tight" sodium-transporting epithelium) were determined by two independent sets of electrophysiological measurements: (a) the measurement of the total transepithelial resistance, the ratio of resistance of the apical to the basal cell membrane, and cable analysis of the voltage spread into the epithelium; (b) the measurement of the total transepithelial resistance and the ratio of resistances of both cell membranes before and after replacing all mucosal sodium with potassium (thus, increasing selectively the resistance of the apical membrane). The results obtained with both methods indicate the presence of a finite transepithelial shunt pathway, whose resistance is about 1.8 times the resistance of the transcellular pathway. Appropriate calculations show that the resistance of the shunt pathway is almost exclusively determined by the zonula occludens section of the limiting junctions. The mean resistance of the apical cell membrane is 1.7 times that of the basal cell membrane. The use of nonconducting materials on the mucosal side allowed us to demonstrate that apparently all epithelial cells are electrically coupled, with a mean space constant of 460 µm, and a voltage spread consistent with a thin sheet model.     

The electrical potential profile of gallbladder epithelium.

In this study the relative ionic permeabilities of the cell membranes of Necturus gallbladder epithelium have been determined by means of simultaneous measurement of transmural and transmucosal membrane potential differences (PD) and by ionic substitution experiments with sodium, potassium and chloride ions. It is shown that the mucosal membrane is permeable to sodium and to potassium ions. The baso-lateral membrane PD is only sensitive to potassium ions. In both membranes chloride conductance is negligible or absent. The ratio of the resistances of the mucosal and baso-lateral membranes, RM/RS, increases upon reducing the sodium concentration in the mucosal solution. The same ratio decreases when sodium is replaced by potassium which implies a greater potassium than sodium conductance in the mucosal membrane. The relative permeability of the shunt for potassium, sodium and chloride ions is: PK/PNa/PCl=1.81:1.00:0.32. From the results obtained in this study a value for the PK/PNa ratio of the mucosal membrane could be evaluated. This ratio is 2.7. From the same data the magnitude of the electromotive forces generated across the cell membranes could be calculated. The EMF's are -15mV across the mucosal membrane and -81mV across the baso-lateral one. Due to the presence of the low resistance shunt the transmucosal membrane PD is -53.2mV (cell inside negative) and the transmural PD is +2.6mV (serosal side positive). The change in potential profile brought about by the low resistance shunt favors passive entry of Na ions into the cell across the mucosal membrane. Calculations show that this passive Na influx is maximally 64% of the net Na flux estimated from fluid transport measurements. The C-1 conductive of the baso-lateral membrane is too small to allow electrogenic coupling of C1 with Na transport across this membrane. Experiments with rabbit gallbladder epithelium indicate that the membrane properties in this tissue are qualitatively similar to those of Necturus gallbladder epithelium.     

Electrical properties of the cellular transepithelial pathway in Necturus gallbladder. II. Ionic permeability of the apical cell membrane.

Microelectrode techniques were employed to study the ionic permeability of the apical cell membrane of Necturus gallbladder epithelium. Results obtained from continuous records in single cells, and from several cellular impalements shortly after a change in solution, were similar and indicate that both the apical membrane equivalent electromotive force (Va) and electrical resistance (Ra) strongly depend on external [K]. Cl substitutions produced smaller effects, while the effects of Na substitutions with N-methyl-D-glucamine on both Va and Ra were minimal. These results indicate that the permeability sequence of the apical membrane is PKgreater thanPClgreater than PNa. From the calculated absolute value of PNa it is possible to estimate the diffusional Na flux from the mucosal solution into the cells (from the cell potential and an assumed intracellular Na concentration). The calculated flux is roughly three orders of magnitude smaller than the measured net transepithelial flux in this tissue and in gallbladders of other species. Thus, only a minimal portion of Na entry can be attributed to independent diffusion. From estimations of the electrochemical potential gradient across the apical membrane, Cl transport at that site must be active. At the serosal cell membrane, Na transport takes place against both chemical and electrical potentials, while a significant portion of the Cl flux can be passive, if this membrane has a significant Cl conductance. The changes in shunt electromotive force and in transepithelial potential after mucosal substitutions were very similar, indicating that transepithelial bi-ionic potentials yield appropriate results on the properties of shunt pathway.     

The effect of high sodium concentration on the action potential of the skate heart

It already has been well documented that the maximum rate of depolarization and amplitude of action potentials are directly dependent on [Na+]o in the vertebrate myocardium. Almost all studies have been carried out at low sodium concentration ranges by substituting NaCl for other substances. Action potentials should be demonstrable in higher sodium concentrations, but cells are inevitably damaged by osmotic changes. The blood of elasmobranchs is nearly isosmotic with sea water, but NaCl accounts for 54.5% of the osmotic pressure and 38.7% of it is maintained by urea molecules. Utilizing this special situation in elasmobranchs, the effect of high sodium concentration was studied up to 170% of normal sodium concentration, while still retaining isosmotic condition. The rate of depolarization, amplitude, and duration of the myocardial action potential all increased in direct proportion to [Na+]o, and no depressant effect on transmembrane action potentials was observed in solutions of high sodium concentration. With regard to depolarization rate, the regression curve fitted by the least squares method passed through zero within two standard errors. At high sodium levels, the overshoot changed as expected theoretically, but at lower ranges it deviated from the theoretical values. [Na+]i and [K+]i in this tissue have been determined, and these data are explained on the basis of the Na theory.     

Increased Ca++ or mg++ concentration reduces relative tight-junction permeability to Na+ in the cortical thick ascending limb of Henle's loop of rabbit kidney

We have tested whether increased Ca++ and Mg++ concentrations have an effect on transepithelial voltage (PDte) and transepithelial resistance (Rte) in isolated perfused cortical thick ascending limbs (cTAL) of rabbit kidney. The divalent cations added at 2.5, 5.0 and 10.0 mmol.l-1 to the lumen or peritubular bath perfusate led to a concentration-dependent increase in Rte. The maximal response in Rte was observed between 5 and 10 mmol.l-1. No significant change in active transepithelial potential difference (PDte) was observed. The increase in Rte still occurred when the transcellular current was reduced by Ba++ (3 mmol.l-1) added to the lumen perfusate. This suggests that the increase in Rte caused by Ca++ and Mg++ is due to a modification of the paracellular shunt pathway. In the absence of active transport, i.e. when furosemide (5.10(-5) mol.l-1) was added to the lumen perfusate. Ca++ and Mg++ reduced the transepithelial diffusion potential generated by a NaCl gradient established across the epithelium, and thus produced a reduction of the relative permeability for Na+ over Cl- (PNa+/PCl-) of the paracellular shunt pathway. This indicates that divalent cations increase Rte by reducing the sodium permeability of the tight junctions. The observed Ca++ and Mg++ induced reduction of the sodium permeability of the paracellular pathway corresponds to a decrease in net Na+ reabsorption by 5-10%. Since it has been demonstrated that peptide hormones such as parathyrin (PTH) modulate divalent cation and NaCl reabsorptions, in a second series of experiments we tested the effects of PTH (2-20 USP.l-1) and dbcAMP (10(-3) mol.l-1) on PDte and Rte of isolated perfused cTAL segments of rabbit nephron. Neither Rte nor PDte were affected by PTH or dbcAMP.     

Passive ionic properties of frog retinal pigment epithelium.

The isolated pigment epithelium and choroid of frog was mounted in a chamber so that the apical surfaces of the epithelial cells and the choroid were exposed to separate solutions. The apical membrane of these cells was penetrated with microelectrodes and the mean apical membrane potential was --88 mV. The basal membrane potential was depolarized by the amount of the transepithelial potential (8--20 mV). Changes in apical and basal cell membrane voltage were produced by changing ion concentrations on one or both sides of the tissue. Although these voltage changes were altered by shunting and changes in membrane resistance, it was possible to estimate apical and basal cell membrane and shunt resistance, and the relative ionic conductance Ti of each membrane. For the apical membrane: TK approximately equal to 0.52, THCO3 approximately equal to 0.39 and TNa approximately equal to 0.05, and its specific resistance was estimated to be 6000--7000 omega cm2. For the basal membrane: TK approximately equal to 0.90 and its specific resistance was estimated to be 400--1200 omega cm2. From the basal potassium voltage responses the intracellular potassium concentration was estimated at 110 mM. The shunt resistance consisted of two pathways: a paracellular one, due to the junctional complexes and another, around the edge of the tissue, due to the imperfect nature of the mechanical seal. In well-sealed tissues, the specific resistance of the shunt was about ten times the apical plus basal membrane specific resistances. This epithelium, therefore, should be considered "tight". The shunt pathway did not distinguish between anions (HCO--3, Cl--, methylsulfate, isethionate) but did distinguish between Na+ and K+.     

Insulation of the conduction pathway of muscle transverse tubule calcium channels from the surface charge of bilayer phospholipid

Functional calcium channels present in purified skeletal muscle transverse tubules were inserted into planar phospholipid bilayers composed of the neutral lipid phosphatidylethanolamine (PE), the negatively charged lipid phosphatidylserine (PS), and mixtures of both. The lengthening of the mean open time and stabilization of single channel fluctuations under constant holding potentials was accomplished by the use of the agonist Bay K8644. It was found that the barium current carried through the channel saturates as a function of the BaCl2 concentration at a maximum current of 0.6 pA (at a holding potential of 0 mV) and a half-saturation value of 40 mM. Under saturation, the slope conductance of the channel is 20 pS at voltages more negative than -50 mV and 13 pS at a holding potential of 0 mV. At barium concentrations above and below the half-saturation point, the open channel currents were independent of the bilayer mole fraction of PS from XPS = 0 (pure PE) to XPS = 1.0 (pure PS). It is shown that in the absence of barium, the calcium channel transports sodium or potassium ions (P Na/PK = 1.4) at saturating rates higher than those for barium alone. The sodium conductance in pure PE bilayers saturates as a function of NaCl concentration, following a curve that can be described as a rectangular hyperbola with a half-saturation value of 200 mM and a maximum conductance of 68 pS (slope conductance at a holding potential of 0 mV). In pure PS bilayers, the sodium conductance is about twice that measured in PE at concentrations below 100 mM NaCl. The maximum channel conductance at high ionic strength is unaffected by the lipid charge. This effect at low ionic strength was analyzed according to J. Bell and C. Miller (1984. Biophysical Journal. 45:279-287) and interpreted as if the conduction pathway of the calcium channel were separated from the bilayer lipid by approximately 20 A. This distance thereby effectively insulates the ion entry to the channel from the bulk of the bilayer lipid surface charge. Current vs. voltage curves measured in NaCl in pure PE and pure PS show that similarly small surface charge effects are present in both inward and outward currents. This suggests that the same conduction insulation is present at both ends of the calcium channel.     

Procaine effects on the sodium transport in frog skin

A study on the influence of procaine on the sodium transport properties in frog skin was carried out. The application of procaine hydrochloride on either the mucosal or the serosal sides of the isolated frog skin has opposite effects. When added to the mucosal compartment, the procaine (as well as two procaine based drugs: Gerovital H3 and Aslavital) biphasically increase the short-circuit current (Isc) with a noticeable "recline" phenomenon, and decrease the slope resistance, as given by the I-V curves. When applied in the serosal compartment, Isc is decreased and the slope resistance of the epithelium is increased. The procaine effect on the apical membranes shows a pronounced dependence on the external sodium concentration. The shift of the E2 inflection point (which indicates the critical intensity of the electric field at which the epithelial conductance changes), with respect to the transepithelial open-circuit potential, shows a rapid and quasi-exponential increase following the application of 25 mM procaine in addition to the different mucosal Na concentrations.     

Size and shape of charged micelles of ionic surfactants in aqueous salt solutions

Light-scattering has been measured on aqueous NaCl solutions of dodecyldimethylammonium chloride and sodium dodecyl sulfate. From molecular weight determination it is confirmed that spherical micelles are formed at low NaCl concentrations, but at high NaCl concentrations the small micelles formed at the critical micelle concentration further associate to form large rod-like micelles with increasing micelle concentration. The reduction of repulsion between charged groups induces the sphere-rod transition of micelle shape. The dependence of molecular weight on ionic strength can be expressed by double logarithmic relations, which are dependent on the micelle shape. While dodecyldimethylammonium chloride dissolves even in 4.00 M NaCl, sodium dodecyl sulfate solutions exhibit some XXX in angular dissymmetry at NaCl concentrations higher than 0.50 M at low temperatures.     

Ultrastructural changes in chloroplasts resulting from fluctuations in NaCl concentration: freeze-fracture of thylakoid membranes in Dunaliella salina.

The structure of chloroplasts isolated from Dunaliella salina has been studied with respect to changing concentrations of sodium chloride in the culture medium. Freeze-fracture replicas and thin sections of intact chloroplasts do not exhibit any noticeable changes in structure at concentrations ranging between 3.5 and 25% NaCl. Chloroplasts isolated from algal cells that have been acclimatized to the higher salt concentration show a change in the thylakoid membranes. The thylakoid membranes appear compressed over a major portion of the membrane surface, with only the end of the thylakoid membranes unappressed. The number of particles per unit area on the B face is also altered by the salt concentration. The chloroplasts acclimatized to 25% NaCl have about 3 times the number of particles per unit area on a B face of end-membranes as on a comparable face of thylakoid membranes acclimatized to low (3.5% NaCl) salt concentration. These morphological changes can be reversed if the chloroplasts acclimatized to high or low salt concentrations are returned to a medium of different salt concentration prior to freeze-fracturing.     

Electrical properties of the cellular transepithelial pathway in Necturus gallbladder. I. Circuit analysis and steady-state effects of mucosal solution ionic substitutions.

Microelectrode techniques were employed to measure the electrical resistance of the cell membranes and the shunt pathway, and the equivalent electromotive forces (EMF's) at both cell borders in Necturus gallbladder epithelium. The cell is, on the average, 57 mV negative to the mucosal solution and 59 mV negative to the serosal solution. The transepithelial potential (Vms) ranges from 0.5 to 5 mV, serosal solution positive. Assuming that the shunt EMF (Vs) is zero with standard Ringer's bathing oth sides of the tissue, both cell membrane EMF's are oriented with the negative pole toward the cell interior and are 39.9 +/- 3.6 mV (apical, Va), and 69.4 +/- 1.8 mV (basal-lateral, Vb)...     

Effect of salts on D-glycerate dehydrogenase kinetic behavior.

Bovine liver D-glycerate dehydrogenase (D-glycerate:NAD (NADP) oxidoreductase, EC 1.1.1.29) adapts its kinetic behaviour to a sequential mechanism. The presence of NaCl causes an appreciable variation in the Km and V values. relative to the both substrates in the hydroxypyruvate/D-glycerate dehydrogenase/NADH system, which does not happen in the D-glycerate/D-glycerate dehydrogenase/NAD system. The former system is inhibited by high concentrations of NaCl and activated by low salt concentrations. The hydroxypyruvate concentration causing substrate inhibition increases as the concentration of NaCl increases; excess NADH inhibition is independent of the salt concentration. The variation of the initial rates of both systems, in the presence of chlorides having monovalent and divalent cations, or sodium halides, Na2SO4 and NaNO3 (at constant ionic strength) suggests that the anions have a specific action on the enzyme. An increase in the NaCl concentration causes a displacement of the optimum D-glycerate dehydrogenase pH (with hydroxypyruvate and NADH as substrates) towards the acid area. The enzyme stability, at varying pH, varies with the salt concentration.     

Evidence for an apical membrane effect in the regulation of the hexose monophosphate shunt pathway in toad bladder studies with amiloride

Aldosterone stimulates Na⁺ transport in toad bladder and, simultaneously with a coincident dose-response relationship, inhibits the hexose monophosphate shunt pathway. Amiloride, an acylguanidine diuretic, inhibits sodium transport when applied to the apical surface of the bladder. In this study amiloride was found to partially reverse the inhibitory effect of aldosterone on the hexose monophosphate shunt pathway. The amiloride effect upon glucose metabolism was detected when it was applied to both surfaces of the bladder simultaneously, in flask experiments, and when it was applied to the apical surface. No effect of amiloride on the shunt pathway was detected when it was applied to the serosal surface only, even at very high concentrations. It may be, but has not been proven, that the effects of aldosterone and amiloride on the hexose monophosphate shunt pathway are mediated by a common site at the apical membrane.     

Direct measurement of uptake of sodium at the outer surface of the frog skin

A combination of the methods described by Schultz et al. (6) and by Ussing and Zerahn (9) was used to measure directly the unidirectional uptake of sodium from the outside solution into the frog skin, under short-circuit conditions. The sodium uptake was determined at six sodium concentrations ranging from 3.4 to 114 mM. NaCl was replaced by choline chloride in the solutions bathing both sides of the skin. Sodium uptake is not a linear function of sodium concentration but appears to be composed of two components, a saturating one and one that varies linearly with concentration. The sodium uptake is inhibited by the addition of lithium to the outside solution. The effect appears to be primarily on the saturating component and has the characteristics of competitive inhibition. In addition, lithium uptake by the skin is inhibited by sodium. The effects of lithium cannot be ascribed to changes in electrical potential difference. Measurements with microelectrodes indicate that under short-circuit condition there is no change in the intracellular potential when lithium chloride is added to the outside solution.     

Movement of Sodium Across the Mucosal Surface of the Isolated Toad Bladder and its Modification by Vasopressin

Studies have been made on the isolated urinary bladder of the toad, Bufo marinus, in an attempt to evaluate gradients of chemical activity across the mucosal surfaces of the epithelial cells which would serve to maintain a net movement of sodium from the mucosal medium into the cells. The likelihood of such chemical gradients has been established by the demonstration of lower contents of sodium within the tissue, expressed as microequivalents per gram of tissue water, than of concentrations of sodium in the mucosal medium at all levels of the latter examined. The transepithelial transport of sodium and the sodium content of the tissue were found to increase rapidly with rise in concentration of sodium in the mucosal medium up to values of 30 to 60 meq per liter. Further increase in concentration of the medium above this value failed to induce further stimulation of sodium transport or increase in the sodium content of the tissue. Vasopressin increased the rate of transport of sodium at every concentration of sodium in the mucosal medium without altering this relationship. Although entry of sodium across the mucosal surface of the epithelial cells may be passive it is not by free diffusion but involves some considerable interaction with the mucosal surface of the bladder and constitutes the major determinant of the rate of transepithelial transport of sodium. Vasopressin acts to enhance this initial step in the transport of sodium.     

NaCl胁迫对马齿苋光合作用及叶绿素荧光特性的影响

以马齿苋为材料,采用温室盆栽法研究了14 d NaCl胁迫处理对其幼苗生长、光合作用和叶绿素荧光特性的影响.结果显示:(1)马齿苋幼苗的鲜重和株高在25 mmol·L-1 NaCl胁迫时与对照无显著差异,但其随着NaCl浓度的继续增加均显著降低,且其生物量受到的抑制早于株高.(2) NaCl胁迫下,马齿苋幼苗叶片净光合速率(Pn)降低,胞间二氧化碳浓度(C1)增大,且两者的变化幅度随着NaCl浓度增加而增大.(3)NaCl胁迫下,马齿苋幼苗叶片的初始荧光(F0)、最大荧光(Fm)、可变荧光(Fv)、恒态荧光(Fs)、恒态荧光与初始荧光差值(△F0)、PSⅡ潜在光化学效率(Fv/Fo)和PSⅡ最大光化学效率(Fv/Fm)均降低,叶片光化学荧光猝灭系数(qP)也在NaCl胁迫下降低,而非光化学荧光猝灭系数(NPQ)则上升;在0～50 mmol·L-1NaCl胁迫下,幼苗叶片各荧光参数下降幅度小于其他高浓度NaCl胁迫.研究表明,在NaCl胁迫条件下,马齿苋幼苗叶片的光合作用受光抑制伤害,但在低浓度NaCl下能够较多地将光能用于光化学反应,光抑制程度较低,保持了较高的净光合速率,明显减轻盐胁迫对植株生长的影响,表现出一定的耐盐性.